Practical Electrical Wiring Standards - AS 3000:2018
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Practical Electrical Wiring Standards AS 3000:2018
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Contents
1
Introduction to AS/NZS 3000:2018
1.1
1.2
1.3
1.4
1.5
1.6
1.7
2
3
4
5
Objectives of the course
Need for rules and regulations
Background and evolution of AS/NZS 3000
Scope of AS/NZS 3000 Wiring Rules
General exceptions and exclusions
Other related regulations and standards
Summary
1
1
2
4
6
9
9
11
Electrical Distribution Systems
13
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
13
15
17
18
20
23
24
25
27
28
29
Evolution of electrical distribution systems
Ac systems and dc systems
Poly-phase ac circuits
Vectorial representations
Advantages of three phase systems
Ac system connections
HV and LV distribution systems
Importance of earthing
Importance of testing and verification
Distribution systems in special locations
Summary
Design of Electrical Equipment and Safety
31
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
31
32
33
35
40
45
45
46
47
47
Objectives of safe design
Preventing electric shocks
Importance of insulation in electrical safety
Importance of enclosures in ensuring safety
Prevention of adverse thermal effects
Isolation arrangements
Earthing and interlocks
Equipment selection
Role of codes and standards in installation safety
Summary
Earthing of Electrical Systems
50
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
50
54
56
56
64
64
70
72
73
Unearthed and earthed systems
Supply system (source) earthing or functional earthing
Protective earthing
Prevention of shock hazards
Protective earth conductors
Earthing practices in low voltage consumer installations
Earth fault loop impedance
Sensing of earth faults
Summary
Section 1 – Scope, Application and Fundamental Principles 75
5.1
Scope and application areas (clauses 1.1 and 1.2)
75
5.2
5.3
5.4
5.5
5.6
5.7
5.8
6
Section 2 – General Arrangement, Control and Protection
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
6.15
6.16
6.17
7
Definitions (clause 1.4)
Protection for safety (clause 1.5)
Design of an electrical installation (clause 1.6)
Selection of electrical equipment (clause 1.7)
Verification (inspection and testing) (clause 1.8)
Compliance requirements (clause 1.9)
Summary
76
81
84
84
86
86
87
90
Selection of switchgear and controlgear (clause 2.1)
90
Arrangement of electrical installation (clause 2.2)
91
Electrical equipment control requirements (clause 2.3)
93
Isolation
93
Main isolation switches (clause 2.3.3)
95
Other main switches (clause 2.3.4)
96
Emergency switching (clause 2.3.5)
96
Switches for maintenance (clause 2.3.6)
97
Functional (control) switching (clause 2.3.7)
97
Fault protection and protective devices (clauses 2.4 and 2.5) 98
Discrimination and coordination of protective devices
106
Simplified protective device selection (appendix C, C3)
108
Residual current devices for protection (clause 2.6)
110
Automatic disconnection time (appendix B)
112
Protection against voltage effects (clauses 2.7 and 2.8)
113
Switchboards (clause 2.9)
114
Summary
118
Section 3 – Selection and Installation of Wiring Systems
121
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
121
123
123
125
129
133
133
134
137
140
140
141
Types of wiring systems (clause 3.2)
Selection criteria (clause 3.1)
External influences (clause 3.3)
Sizing of conductors
Installation requirements of wiring systems
Enclosure of cables (clause 3.10)
Cables installed in conduits (appendix C, clause C6)
Underground wiring systems (clause 3.11)
Aerial wiring systems (clause 3.12)
Cables supported by a catenary (clause 3.13)
Earth sheath return systems (clause 3.16)
Summary
ii
8
9
10
Section 4 – Selection and Installation of Appliances and
Accessories
143
8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8
8.9
8.10
8.11
8.12
8.13
143
144
145
146
147
145
151
152
154
155
156
156
156
Section 5 – Earthing Arrangements and Earthing
Conductors
159
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
159
160
162
165
167
168
169
171
175
175
Requirements of earthing system (clauses 5.1 and 5.2)
Multiple Earthed Neutral system (clause 5.3)
Selection of earthing conductor (clauses 5.3.2-5.3.3)
Earthing connections (clauses 5.3.4-5.3.6)
Equipment earthing (clause 5.4)
Earthing arrangements (clause 5.5)
Equipotential bonding (clause 5.6)
Earth fault loop impedance (clause 5.7 and appendix B)
Earthing requirements for other systems (clause 5.8)
Summary
Section 6 – Damp Situations
177
10.1
10.2
177
10.3
10.4
10.5
10.6
10.7
10.8
11
Selection and installation criteria (clause 4.1)
Protection against thermal effects (clause 4.2)
Connection of electrical equipment (clause 4.3)
Socket-outlets (clause 4.4)
Lighting equipment and accessories (clause 4.5)
Cooking appliances and heating systems
Electricity converters (clause 4.12)
Motors (clause 4.13)
Transformers (clauses 4.14-4.16)
Capacitors (clause 4.15)
Airconditioning and heat pump systems
Lifts
Summary
Basic requirements (clause 6.1)
Locations of baths, showers or fixed water containers
(clause 6.2)
Swimming pools, paddling pools and spa pools/tubs
(clause 6.3)
Fountains and water features (clause 6.4)
Locations containing sauna heaters (clause 6.5)
Refrigeration rooms (clause 6.6)
Sanitization and general hosing down areas (clause 6.7)
Summary
178
185
189
191
193
193
194
Section 7 – Special Electrical Installations
196
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
196
197
200
203
206
207
208
210
210
Applicable areas (clause 7.1)
Safety services (clause 7.2)
Electricity generation systems (clause 7.3)
Electrical separation (clause 7.4)
Extra-low voltage electrical installations (clause 7.5)
High voltage electrical installations (clause 7.6)
Hazardous areas (clause 7.7)
Other specific installations (clause 7.8)
Supplies for electric vehicles (NZ only)
iii
12
11.10 Summary
210
Section 8 – Verification
213
12.1
12.2
12.3
12.4
12.5
213
214
215
217
223
Basic objectives and requirements (clause 8.1)
Safety precautions and testing devices
Visual inspection (clause 8.2)
Mandatory tests (clause 8.3)
Summary
Appendix 1 Determination of Maximum Demand for
Electrical Installations
225
Appendix 2 Wiring Systems Classification
234
Appendix 3 IEC Classification of Supply Systems based
on Earthing Practices
239
Appendix 4 Earthing Regulations and Practices from
Other National Codes
247
Appendix 5 Practical Exercises
255
Appendix 6 Answers to Exercises
271
Appendix 7 Reducing the Impact of Power Supply Outages 286
Appendix 8 Electrical Conduits
289
Appendix 9 Installation of Arc Fault Detection Devices
292
Appendix 10 Guidance for Installation of Electrical Vehicle Socket –
Outlets and Charging Stations
295
Appendix 11 Dc Circuit Protection Application Guide
iv
298
1
Introduction to AS/NZS 3000:2018
The latest edition of AS/NZS 3000 Wiring Rules was published in the year 2018. This standard,
approved by the council of standards in Australia and New Zealand, defines the regulations to be
adopted in selection, design and installation of Electrical distribution systems mainly falling under
low voltage category. In this chapter we will go through the history of this standard and the
objectives with which this standard had been framed along with a review on the needs and benefits
of such regulations. The chapter also includes an overview on the scope of this standard with brief
introduction to the various topics covered in the standard and how they are organized in the latest
publication before going into a more detailed discussion on each of the sections and their
importance in the subsequent chapters.
Learning objectives
•
•
•
•
•
•
•
•
1.1
Objectives of the course
Need for regulations
Background and evolution of AS/NZS 3000
Objectives of the AS/NZS 3000 regulations
Scope of Wiring Rules
General exclusions
Other related standards and regulations
Arrangement of AS/NZS 3000
Objectives of the course
The objective of this course is to familiarize the participants with the Australian / New Zealand
Wiring Rules (Fifth edition) published as Australia / New Zealand standard AS/NZS 3000: 2018.
This standard covers the requirements to be adopted for electrical installations of nominal voltages
up to and including 1000V ac and 1500V dc. It contains stipulations covering issues of safety,
selection, installation testing and verification of electrical equipment in common areas as well as in
special locations. The topics contained in the standard are of interest and relevance to all
practitioners of electrical technology whether they are designers of electrical installations, erection
engineers or personnel responsible for operation and maintenance of the installations.
In order to appreciate the requirements and stipulations contained in the standard better, it is
necessary to have a clear understanding of the fundamental principles that the standard aims to
address. In preparing this manual an attempt has been made to give the reader an insight into the
relevance of the stipulations of the standard by first touching upon the basics of the relevant aspects
of electrical theory before going on to study the requirements contained in the standard. As such
the chapters of this manual cover the following main objectives of this course.
2 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
•
Review the basic electrical theory related to selection of equipment and circuit connections in low
voltage systems and the importance of the regulations defining the rules for safe use of electrical
equipment and systems
Review the important and basic safety aspects related to electrical systems covering insulation,
enclosure, earthing, etc.
Review the various sections and requirements of AS/NZS:3000: 2018
Areas of applications of these regulations in Electrical installation
Understand the simple calculations for these systems related to conductor sizes, main and earthing
conductor sizes, maximum demand, etc. to ensure safe design and reliable operation
Review on the recommended checks and tests to be carried out in an electrical installation before
energisation and also periodically to ensure its longevity.
1.2
•
•
•
•
•
•
•
•
•
Need for rules and regulations
Today the rules and regulations have become necessary to ensure that the people in the society get
fair deal in every aspect of their daily life. These can range from the basic need for food to specific
needs related to the travel, dresses, etc. With electricity becoming an important commodity in the
modern society it is necessary to ensure that it is used in a safe and reliable manner. The electrical
system today comprises of power generation limited to specific locations in a country but its
transmission, distribution and consumption being extended to all the parts and remote corners, in an
extensive and elaborate manner. Hence it is absolutely essential that the distribution systems and
consumer equipments are designed and installed with features that would ensure safe consumption
of electricity and also help regular maintenance, revamping, additions/ alterations, etc in a safe
way. With due consideration for the same, regulations have been brought in to specify the kind of
features essential for the safe use and also strictly enforced as rules to fulfill the following.
To ensure uniform practices in the construction/ installation for enabling interchangeability for identical
equipment when needed and also for avoiding interchangeability when not desired to meet the
fundamental safety aspects.
To ensure a stable source of supply and loads in the system for longevity in service of the equipment as
well as for the electrical systems.
To define the aspects that are to be considered in the design of the systems such as characteristics of
available supply, nature of demand, emergency supplies for safety services, environmental
considerations, conductors to be used, type of wiring, protective equipment, emergency control,
isolation and switching, accessibility for operation etc in order achieve the earlier two.
To ensure availability of correct and dependable electrical equipment to suit the voltage and frequency
of the supply system followed in the country.
To enable the designers in selecting correct and matching devices that shall be uniformly available in
the market satisfying basic safety needs
To make certain that the manufacturers, designers and installation contractors follow uniform
regulations for the safety of people, properties and livestock coming in close contact with these systems
in day to day life
To support quality and reliability of installations and equipments for safe and continuous service.
To ensure safety against shocks, and also against failures or damages due to thermal effects consequent
to over current, fault current and over voltage.
To aid equipment erection using good workmanship and proper materials, use of conductors with proper
sizing and rating, proper jointing and connections at terminal points, installations in a manner not to
cause temperatures in excess of the design temperatures and verification and testing of equipment
periodically to avoid degradation.
We will illustrate few of these major objectives further.
1.2.1 Ease of Interfacing
Imagine for a moment, that there are no standards for electrical appliances. The result would be that
each product manufacturer might choose a different voltage rating for his product. It would mean
that we will have electrical heaters, ovens, toasters etc. each of different voltage, different plugs,
Introduction to AS/NZS 3000:2018
3
etc that cannot work on a common electrical system but would require tailor made system to serve
their purpose. Your power supply company might have a distribution voltage that is unsuitable for
any or all of your gadgets. This obviously is not helpful and not desirable.
Thus a standard has to be established and its adherence made mandatory within a national or
geographical entity so that generation, transmission, distribution and utilization of electrical energy
are done at stipulated voltages and frequency which will vary only within acceptable bands
specified for each parameter.
Such a standard enables the designer of an appliance to choose a suitable voltage and frequency at
which the appliance can function and also the variations of these parameters which have to be taken
care of in the design for the operational range of the appliance. It also allows the designer to select
appropriate conductors and configuration of power supply connectors forming part of the
appliance.
In turn, this enables people to buy an off-the shelf appliance and connect it to the electrical outlet at
home and use it without worrying too much about the suitability of the appliance for the electric
supply provided by the power company. Anyone who has traveled with a device made in one
country and tried to use in another where different standards prevail would certainly appreciate the
convenience which uniform standards provide us with.
Also the use of standards reduces the number of variant appliance designs a manufacturer has to
plan and manufacture; an issue which will have adverse cost implications to the manufacturer and
hence to the buyer. Low cost mass production is thus a direct result of standards benefitting
millions across the globe.
1.2.2
Ensuring Quality of supply, equipment and installation
Equipment and installations have to deliver functionalities for which they are designed without any
undue hazards to the users or the environment for their entire design life under varying operating
conditions. The provisions of a standard therefore define the parameters for functionality, safety
and maintainability. They also contain stipulations that lay down the tests that the device has to
withstand to either prove a design (by what are called Type tests conducted on prototypes or
selected samples) or ensure that the output and quality parameters are met (by Routine Tests done
on each piece manufactured). Thus when you buy an appliance or equipment that is declared as
conforming to a particular standard, you have an assurance that it will perform under conditions
defined by the standard, is safe to use and will deliver the output or functionalities which the
manufacturer furnishes in accordance with the standard for the period it is expected to serve.
An installation standard has a similar objective too. When an installation is carried out in
accordance with a standard, it has to follow the methodologies stipulated in the standard using
recommended accessories which, in turn will ensure that the installation achieves the intended
quality minima, is safe for personnel and environment, and will have adequate provisions for
maintainability. An installation standard also usually lays down the procedures for initial inspection
and testing for certifying that the installation is fit to be put in service and the periodicity and detail
of subsequent inspections and testing to ensure that it is fit to remain in service till the next
scheduled inspection.
1.2.3 Ensuring safety
The regulations also specify some of the finer aspects related to the practices to be adopted right from
basic design, during selection, installation and also tests/ verifications to be carried out periodically so
that the people and livestock in close proximity to these systems are protected against various hazards
commonly prevailing in such systems. The following are some of the main regulations defined in the
standards and are expected to be followed by the system designers so that safety becomes part of the
design, selection and installation when the systems are put into use.
• Regulations for protection against direct and indirect contacts that otherwise could lead to shocks
4 Practical Electrical Wiring Standards - AS 3000:2018
• Regulations for enclosure and equipment design features for protection against thermal effects,
over current, fault current and over voltage that otherwise could lead to fire accidents or
equipment failures.
• Methods to be followed for selection of electrical equipment with guidelines for properly sizing
the conductors, providing emergency services, safe isolation and switching, accessibility, etc
• Erection methods to be followed to ensure minimum quality with consistent workmanship by use
of proper materials, proper jointing and connections and methods to safeguard against high
surface temperatures, etc
• Regulations and mandatory procedures for inspection and testing of equipment by competent
personnel to ensure that the installed equipments meet some basic characteristics desired in the
regulations and also are having provisions for taking out the failed equipments from service
without impacting the healthy equipments.
1.3
Background and evolution of AS/NZS 3000
This Standard was prepared by the Joint Standards Australia/Standards New Zealand Committee EL001, represented by the following agencies/ authorities.
• Australian Building Codes Board
• Australian Energy Council
• Australian Industry Group
• Communications, Electrical and Plumbing Union - Electrical Division
• Consumer New Zealand
• Consumers Federation of Australia
• Electrical Contractors Association of New Zealand
• Electrical Regulatory Authorities Council
• Electrical Safety New Zealand
• Electrical Workers Registration Board
• ElectroComms & Energy Utilities Industries Skills Council
• Energy Efficiency & Conservation Authority of New Zealand
• Energy Networks Australia
• Engineers Australia
• Institute of Electrical Inspectors
• Master Electricians Australia
• National Electrical and Communications Association
• National Electrical Switchboard Manufacturers Association
• New Zealand Manufacturers and Exporters Association
• NSW Department of Industry, Skills and Regional Development
• Wellington Electrical Association
• WorkSafe New Zealand
The earlier 2000 edition superseded Australian standard AS 3000:1991, Electrical installations –
Buildings, structures and premises (known as SAA Wiring Rules). In New Zealand the 2000 edition had
superseded selected parts of NZS 3000:1997 Electrical installations – Buildings, structures and premises
(known as the NZS Wiring Rules). The 2000 edition was further updated with Amendments No. 1
(September 2001), No. 2 (April 2002) and No. 3 (July 2003).
The development of the 2018 edition of the standard had been based on the following considerations by
the council of standards.
a)
new technology, new equipment and improved installation techniques;
b)
industry feedback regarding readability and compliance;
c)
identification and clarification of normative (mandatory) requirements and informative
guidance material throughout the document; and
d) experience gained in the application of the previous edition as expressed to Standards
Australia and Standards New Zealand.
Introduction to AS/NZS 3000:2018
5
During preparation of this Standard, reference was made to IEC 60364, Electrical installations of
buildings (all parts) and acknowledgment is made of the assistance received from this source. The 2007
edition had been published on 12 November 2007 after being approved on behalf of the Council of
Standards Australia on 19 October 2007 and on behalf of the Council of Standards New Zealand on 9
November 2007. This Standard was superseded by AS/NZS 3000: 2010 from its date of publication.
The edition was improved with additional diagrammatic representation of concepts and by including
more practical examples adopted in the user installations as desired by the industry. The edition is
divided into two parts with Part 1 (Section-1) covering Scope, application and fundamental principles of
safe electricity use and is generally made complete in itself without cross-referencing to Part 2. The
edition also establishes the ‘deemed to comply’ status of AS/NZS 3018 relating to simple domestic
applications.
National requirements
Certain provisions of the Standard have a different application in Australia and New Zealand. The
following symbols appearing in the outer margin indicate that the identified Section or Clause is:
(i) Applicable in Australia only.
(ii) Applicable in New Zealand only.
Informative appendices
An informative appendix is for information or guidance only. Informative appendices provide additional
information intended to assist in the understanding or use of the Standard.
Deemed to comply
The term ‘deemed to comply’ means that a requirement can be met by following a specified Standard or
method. So, where an installation is carried out in accordance with the specified Standard or method,
within the text of this Standard, the installation is ‘deemed to comply’ with the requirements of this
Standard. Conformance to a deemed to comply Standard may exceed the minimum requirements of this
Standard.
Objectives of AS/NZS 3000
The main objective of regulations for electrical installations in any country is to provide the rules for the
design and erection of electrical installations for safety and proper functioning. It is also necessary that
when use of a new material or invention in an installation results in deviation from one or more
stipulations already prevailing in the regulations, the degree of safety shall not get compromised by such
deviation. The fact of such use shall also be recorded on the electrical installation certificate as
reference for anyone who is concerned with the safe functioning of the installation. Keeping these basic
objectives and to enable ease of understanding of the regulations, the AS/NZS 3000 Wiring rules
standard is divided into two main parts – Part 1 and Part 2.
This Standard comprises two parts, as set out below, with both parts bound as one document.
Part 1 provides uniform essential elements that constitute the minimum regulatory requirements for a
safe electrical installation.
Part 1 also provides an alternative regulatory vehicle for Australian and New Zealand regulators seeking
to move from the present prescription of AS/NZS 3000 in electrical installation safety and licensing
legislation.
Part 1 satisfies the following objectives:
• To allow its content to be called up in regulation as a separate Part or together with Part 2.
• To be generally complete in itself to avoid cross-referencing to Part 2.
• To provide high level safety performance outcomes/conditions without prescriptive work
methods that demonstrate means of compliance (which are in Part 2).
6 Practical Electrical Wiring Standards - AS 3000:2018
• To establish an enforcement link to Part 2. Failure to comply with a work method in Part 2
would breach the requirements of Part 1 unless an alternative method is used.
• To establish the ‘deemed to comply’ status of Part 2, confirming that installations that comply
with Part 2 comply with the requirements of Part 1.
• To maintain alignment with IEC 60364, Low voltage electrical installations (series),
developments at the level of essential safety.
• To provide a mechanism for acceptance of alternative design and installation practices that
are not addressed in, or are inconsistent with those given in the ‘deemed to comply’ Part 2.
This mechanism is intended to apply where departures from the methods in Part 2 are
significant rather than minor aspects that remain within the flexibility of Part 2.
• To detail requirements for designers or installers seeking to apply an alternative method to
the ‘deemed to comply’ methods contained in Part 2.
Part 2 provides installation practices that are deemed to comply with the essential safety
requirements of Part 1.
Part 2 satisfies the following objectives:
• To allow it to be called up in regulation, in addition to Part 1, to reflect a range of regulatory
adoption options.
• To incorporate and elaborate on all requirements of Part 1 with additional requirements and
recommendations to clarify and support compliance.
• To restore certain requirements, recommendations and examples of typical, effective
compliant solutions from previous editions.
• To emphasize common, practicable and cost-effective methods that achieve safety
compliance, fitness for purpose and a level of good practice rather than overly conservative or
obscure measures.
• To make greater use of figures and examples to promote understanding of common or
difficult aspects, e.g. line diagrams, alternative overcurrent device locations, ingress
protection (IP) rating and switchboard access.
1.4
Scope of AS/NZS 3000 Wiring Rules
This Standard sets out requirements for the design, construction and verification of electrical
installations, including the selection and installation of electrical equipment forming part of such
electrical installations.
These requirements are intended to protect persons, livestock, and property from electric shock,
fire and physical injury hazards that may arise from an electrical installation that is used with
reasonable care and with due regard to the intended purpose of the electrical installation.
In addition, guidance is provided so that the electrical installation will function correctly for the
purpose intended and takes into account mitigating the foreseeable adverse effects of disruption to
supply.
Changes to AS/NZS 3000:2018 include the following:
Section 1:
1.
2.
3.
4.
5.
6.
7.
8.
New and revised definitions are indicated in Clause 1.4 by an asterisk (*) in the left margin.
The definition of mains supply has been removed.
‘Direct contact’ and ‘indirect contact’ are now designated ‘basic protection’ and ‘fault protection’.
IP ratings revised to suit local environmental conditions.
Requirements for conductors with green/yellow insulation are specified.
References to AS/NZS 3018 have been replaced with references to other Standards.
Requirements for alterations and repairs have been clarified and expanded.
New Part 1 solutions have been added along with details on where they may be used.
Section 2:
Introduction to AS/NZS 3000:2018
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Operating characteristics of switchgear, control gear and switchboards have been added.
Origin requirements of sub-mains and final subcircuits have been added.
Requirements for main switch operations have been added.
Positions of overload protective devices have been clarified.
Requirements for alternate positions of short circuit protective devices have been updated.
Discrimination/selectivity of protective devices has been expanded.
Protection requirements for switchboard internal arcing faults have been enhanced.
Requirements for RCD protected circuits in domestic, non-domestic, non-residential and medical
installations have been added, and RCD requirements for alterations and repairs clarified.
Illustration of basic clearances for switchboard access has been updated.
New clause on arc fault detection devices and their installation requirements has been added.
Requirements for switchboard installations at 800 A or greater have been enhanced, including
access and egress, switchroom door sizes and minimum clearances around switchboards in
switchrooms.
Further clarification has been provided regarding rising mains tee-offs.
Section 3:
1. Improved installation safety requirements for cables that pass through bulk thermal insulation.
2. Colour identification of active, neutral and earth conductors further clarified.
3. Requirements for wiring systems installed in positions where they are likely to be disturbed have
been clarified.
4. Requirements have been clarified for cables of different electrical installations in common
enclosures and for segregation of cables.
5. Requirements for segregation of cables of different voltage levels have been clarified.
Section 4:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Revised figures identify where IP rated equipment is to be installed.
The requirements for installation wiring connected via an installation coupler have been revised.
Electric vehicle socket-outlet requirements now included.
Requirements for lighting equipment and accessories have been revised.
Requirements for the safe installation of recessed luminaires have been enhanced, and an updated
list of luminaire classifications added.
Requirements for cooking appliance switching devices clarified for improved safety outcomes.
Gas appliances and equipment isolation requirements clarified.
Further clarification of isolator requirements for airconditioning and heat pump systems.
A new clause and figures have been added relating to electrical equipment installed in locations
requiring protection from the weather.
Installation and location requirements for socket-outlets for electric vehicle charging stations
have been added.
Clearance requirements for socket-outlets and switches from open gas or electric cooking
appliances have been added.
Requirements for isolating switches to be installed adjacent to all fixed wired water heaters have
been added.
Requirements on hazardous areas presented by gas relief vent terminals have been added.
Requirements for airconditioners and heat pumps where the internal unit (or units) are supplied
from a switchboard or circuit separate to that of the compressor, and new exceptions have been
added.
Requirements for lifts installed for general use and that are not emergency lifts (safety services)
have been added.
Section 5:
1. MEN system further defined for clarity.
7
8 Practical Electrical Wiring Standards - AS 3000:2018
2.
3.
4.
5.
MEN connection requirements have been added regarding location in an accessible position.
Acceptable earth electrodes types have been updated.
Earthing requirements for SELV and PELV systems have been updated.
Equipotential bonding requirements have been expanded and clarified through enhanced
requirements for showers, bathrooms, pools and spas.
6. Earthing of conductive building materials in combined outbuildings.
7. Earthing requirements for individual outbuildings and combined outbuildings.
8. Earthing requirements for conductive switchboard enclosures associated with unprotected
consumer mains.
9. Earthing of conductive reinforcing in combined outbuildings that contain showers or baths.
10. Conductive pool structures and the bonding connection point required to be installed and
bonded to the installation earthing system regardless of other specified requirements.
11. Figure showing bonding arrangements for pools and spas has been added.
12. Requirements on conductive fixtures and fittings installed within arm’s reach of the pool edge,
and that are in contact with the general mass of earth, either directly or indirectly, have been
added.
Section 6:
1. Additional content applying to water containers into which persons do not normally put a part or
all of their body.
2. Installation requirements for deluge showers have been clarified.
3. Showers Zone 1 has been clarified for different shower head configurations.
4. Fixed water container size reduced from 45 L to 40 L.
5. A figure for showers with a hinged door has been included.
6. Specified capacity for spa pools or tubs has been increased from 500 L to 680 L.
7. Electricity generation systems, including inverters have been excluded from being installed in
classified zones.
8. Clause excluding pools and spas from being located in areas containing electrical equipment
owned by the electricity distributor, that result in such electrical equipment being incorporated
into any classified zone.
Section 7:
1. Clause 7.2, Safety services, has been restructured.
2. Installation requirements for electricity generation systems have been reviewed and clarified in
line with applicable Standards.
3. Electric vehicle charging system requirements have been added.
4. Clause 7.8, Standards for specific electrical installations, has been revised.
Section 8:
1. A number of clauses split into subclauses to differentiate between general, application, visual
inspection, test requirements and accepted values.
2. Extra low voltage installation testing requirements have been relocated to Section 8 from Section
7.
3. Clarification of RCD testing and EFLI testing.
4. The date of initial energization is now required to be recorded at the installation switchboard.
Appendices:
1. Appendix A—Now a single list of referenced Standards.
2. Appendix B—Table from FAQ34 (voltage drop and EFLI values comparison) added for further
guidance.
3. Appendix C—Expanded and the information provided on maximum demand has been clarified and
updated.
Introduction to AS/NZS 3000:2018
9
4. Appendix D—Revised to provide more comprehensive guidance information for the construction of
private aerial lines.
5. Appendix E—Updates incorporated and building classifications Class 1 and Class 10 have been added.
6. Appendix F—A recent update carried out by Committee EL-024 on protection against lightning.
7. Appendix K—Switchboard equipment summary has been added to provide a checklist of requirements
for switchboards.
8. Appendix L—Appendix deleted. Formerly on first aid in Australia.
9. Appendix M—Formerly on first aid in New Zealand. This content was deleted and a new Appendix on
reducing the impact of power supply outages has been added to provide guidance on continuity of
supply and back up plans.
10. Appendix N—New Appendix to provide guidance on the types and variations of conduit available for
electrical installations.
11. Appendix O—New Appendix to provide guidance on the installation of Arc Fault Detection Devices
(AFDDs).
12. Appendix P—New Appendix to provide guidance for circuits intended to supply energy to electric
vehicles.
13. Appendix Q—New Appendix to provide guidance for the selection of circuit protection and switching
devices when being operated on a d.c.supply that would be deemed to meet the design, equipment
selection and installation criteria of this Standard.
1.1 General exceptions and exclusions
Italic print in the Code indicates exceptions or variations to requirements. Exceptions generally give
specific examples where the requirements do not apply or where they are varied for certain
applications. They may contain requirements. Examples are also presented in italic text. As
applicable in any country, the wiring rules does NOT cover the requirements for
design/manufacture of electrical equipment but limits itself to their selection and application
in electrical installations.
1.2 Other related regulations and standards
Appendix A of the standard provides detailed list of other regulations and standards that are
referenced in the rules. Table 1.1 tries to identify some of the important standards that are to be
additionally reviewed by the practitioners to ensure compliance with the stipulations in AS/NZS 3000.
10 Practical Electrical Wiring Standards - AS 3000:2018
Table 1.1
Partial list of standards/ regulations referred in AS/NZS 3000
Standard
AS 2067
AS 60269
AS 60947
AS 60947.2
AS 60947.4.1
AS 60947.8
AS/NZS 2430
AS/NZS 3008
AS/NZS 3008.1.1
AS/NZS 3439
AS/NZS 3439.1
AS/NZS 3439.2
AS/NZS 3439.5
AS/NZS 3820
AS/NZS 5000
AS/NZS 5000.1
AS/NZS 5000.2
AS/NZS 61009
ABCA and NZBC
Title
Switchgear assemblies and ancillary equipment for
alternating voltages above 1 kV
Low-voltage fuses
Low-voltage switchgear and controlgear
Part 2: Circuit-breakers
Part 4.1: Contactors and motor-starters—Electro
mechanical contactors and motor-starters
Part 8: Control units for built-in thermal protection (PTC)
for rotating electrical machines
Classification of hazardous areas
Electrical installations—Selection of cables—Cables for
alternating voltages up to and including 0.6/1 kV
Part 1.1: Typical Australian installation conditions
Low-voltage switchgear and controlgear assemblies
Part 1: Type-tested and partially type-tested assemblies
Part 2: Particular requirements for busbar trunking
systems (busways)
Part 5: Particular requirements for assemblies intended
to be installed outdoors in public places—Cable
distribution cabinets (CDCs) for power distribution in
networks
Essential safety requirements for low voltage electrical
equipment
Electric cables—Polymeric insulated
Part 1: Electric Polymeric insulated cables for working
voltages up to and including 0.6/1 (1.2) kV
Part 2: Electric Polymeric insulated cables for working
voltages up to and including 450/750 V
Residual current operated circuit-breakers with integral
overcurrent protection for household and similar uses
(RCBOs)
Building Code of Australia (ABCA) and the New Zealand
Building Code (NZBC)
A number of other standards covering fire protection systems, storage battery systems, hoists,
elevators, etc are also listed for further reference and guidance, which are not covered in this table.
Introduction to AS/NZS 3000:2018 11
Summary
The regulations are needed to ensure uniform practices adopted in all equipment and installation practices
for safety and reliability of the installations. AS/NZS 3000 standard covers regulations to be followed for
design, selection and installation of LV electrical systems of common and special premises in Australia and
New Zealand. The 2018 year edition had been updated with many illustrations and worked out examples
compared to the earlier edition based on the feedback from industry and the end users. The standard is
divided into two parts. Part-1 of the standard provides basic compliance requirements to be met in the
design, selection and installation of the systems with an objective to achieve high level of safety in the
systems without referencing part-2. The second part is divided into a number of sections and outlines
guidelines and procedures to be adopted by the designers and installation contractors for achieving the high
level safety objectives of part-1 in specific application areas.
This book is not intended to replace the AS/NZS Wiring Rules as a work of reference but is merely an
introduction to it. As all of us are aware, the standards are dynamic in nature in the manner that they
continuously undergo amendments and revisions to match the pace of the growth in the technology. In case
further information is required it is recommended that the participants shall directly refer the standard as
well as other references such as the reference documents identified in appendix A of the standard. A lot of
published literature is available on these topics by industry bodies and reputed manufacturers of electrical
equipment as well as on the Internet and can be referred for assistance in solving specific problems one may
come across.
12 Practical Electrical Wiring Standards - AS 3000:2018
2
Electrical Distribution Systems
Electrical distribution is a specialized subject and in this chapter we will see how the electrical
distribution systems evolved over the last two centuries with power demand running into millions
of watts. A review will be made on the features of ac and dc systems and also the importance of
poly-phase distribution. We will spend some time in understanding why three phase ac system is
the preferred choice for generation, transmission and distribution. We will also cover the vectorial
representation followed for defining ac parameters and the methods of connections employed in
three phase ac installations. A review will also be made on the importance of earthing and testing/
verifications on these systems.
Learning objectives
•
•
•
•
•
•
•
•
•
•
2.1
Evolution of electrical distribution systems
Ac systems and dc systems
Polyphase ac circuits
Vectorial representations
Advantages of three phase systems
Ac system connections
HV and LV distributions systems
Importance of Earthing
Importance of Testing and verification
Distribution systems in special locations
Evolution of electrical distribution systems
When Michael Faraday invented the first electricity generator, he could have hardly imagined that
an entire new technology will follow his invention and electricity would become an integral part of
our daily life. Today, it is difficult to carry on with most of our routine activities without electricity
being available. This has naturally given rise to an extensive electrical network in all parts of the
globe for generation and distribution of electric power.
In its most simple form, an electrical circuit (figure 2.1) consists of:
• A source
• A load
• Conductors that carry the load current from the source to the load
14 Practical Electrical Wiring Standards - AS 3000:2018
Conductors
S
Source
Figure 2.1
A Simple Electrical Circuit
The source could be a primary or secondary battery, a generator driven by a prime mover or a
photovoltaic cell. The load is an energy consuming device which converts electricity into some
other form of energy. It can be a motor which converts electrical to motive energy, a lamp which
converts electricity into light energy, a heating element converting electricity into heat or a
chemical reactor such as an electrolytic cell. Conductors are materials which carry the electrical
current from the source to the load and back thus completing the electrical circuit and are made of
materials such as copper or aluminium. Though most metals conduct electricity to some extent
their electrical resistance is much higher than the above two materials.
This simple circuit is not a very practical system and needs other devices to work properly. Rarely
a source feeds just a single load. Also the loads need to be connected and disconnected as and when
required. So we now have an improved version of the basic circuit (Figure 2.2).
Figure 2.2
An Improved Version of Electrical Circuit
The source needs to be isolated from the distribution system too. And also the distribution system
needs to have a single point control. With these facilities added, the system now looks like the one
in figure 2.3, which can be assumed to almost represent a system connected by a single source.
Electrical Distribution Systems 15
Figure 2.3
A More Practical Version of the Basic Circuit
Today, the electrical systems in any country comprise of hundreds of large power sources and
millions of consumer and industry loads separated by long distances with most of their feeding
substations interconnected with more than one source to ensure that a load connected from the
substation can have continuity of power supply. For this purpose all the substations are
interconnected with provisions either to feed another substation or to receive power from another
substation incorporating interlocks and changeover provisions, as needed. All these requirements
add a lot of complexity to the distribution systems, which is outside the purview of this manual.
However we can conclude that a simple system shown in figure 2.3 is no longer adequate and we
have distribution systems that comprise of multiple voltage levels and multiple types of energy
sources to meet the ever growing power needs.
2.2
AC systems and DC systems
The systems we saw above show the loads being directly connected to the source. In other words,
the source and loads operate (nearly) at the same voltage. When the quantity of power to be
handled increases, the conductors which carry this power have to operate at higher and higher
currents (as per the formula Power = Voltage × Current assuming a pure resistance to be the load)
the voltage being a constant value decided by the consuming appliances.
Electrical power that is used in everyday life is broadly divided into two main categories viz., ac
(Alternating Current) power and dc (Direct Current) power. In the case of dc power, the electrons
flow in one direction only. In the case of ac power, the electrons oscillate back and forth at a
defined frequency. Edison's inventions, from the light bulb to the electric fan, were based on dc
electricity.
Though dc can be generated using dc machines with rotating armature and stationary field
windings, the capacity is limited because of the need for a commutator/brush gear within the
machine. Further, transmission of dc power over long distances cannot be as easily achieved as ac
power. In today’s world dc power is mostly derived from stationary batteries of Lead acid type,
Nickel Cadmium type, etc and naturally the sizes of these dc sources become unmanageable for
high power applications. Hence the use of dc power is limited to standby power/ emergency
generator starting applications with a reasonably large sizes of dc sources. Small gadgets like
mobile phones, cameras, etc. use smaller dc sources.
Another factor to be considered is the efficiency of transmission. Any electrical conductor has a
critical value of current beyond which the power loss in the conductor (computed by the formula P
= I × I × R, I being the current and R the resistance of the conductor) will cause the conductor to
attain excessive temperatures. The equilibrium temperature that the conductor attains is decided by
the following factors.
16 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
Power loss I2R converted to heat. (It is worth noting that the resistance of the
conductor material is not constant but will increase with temperature according to the
temperature coefficient of resistance for the material).
Heat dissipation from the conductor to the environment through conduction,
convection and radiation decided by the conductor geometry. In the case of insulated
conductors and cables, the heat dissipation will have to be done through the layers of
insulation.
Capacity of the conductor to store the heat (decided by the specific heat of the
material and the mass of the conductor).
The temperature attained by the conductor is limited by the value which, the insulating material
used to insulate the conductor can tolerate without suffering failure of insulation (or in the case of
bare conductors to support it). An insulating material has a negative coefficient of resistance and
beyond a critical temperature the insulation can become conductive. It may also lose mechanical
strength in the process of heating up. The resulting short circuit faults would cause much higher
currents. In extreme cases, the conductor itself can attain temperatures close to its melting point
and melt away. The ways to prevent excessive conductor heating is to increase the conductor size
and put a number of conductors in parallel. This again has physical limits beyond which the
conductor capacity cannot be increased.
Alternatively, the system voltage can be increased so that the current value for a given quantum of
power transmitted will reduce in inverse proportion. This will necessitate use of thicker insulation
in the appliances to withstand the higher voltages, which in turn will make the appliances more
expensive. Also there are practical limits beyond which voltage cannot be increased without
compromising economy and safety. The ideal solution is therefore to use a mix of voltages so that
transmission, distribution and consumption each adopts an optimum voltage value decided by
economic considerations appropriate to the application.
AC systems give us an easy way to get these mixed voltages using a transformer, which can either
step up or step down the voltage as required. Transformers step up the voltage for transmission of
power over long distances and near the loads, the voltage is stepped down again to a value
convenient for consuming appliances.
Generators are capable of generating various waveforms but the requirement to be able to
transform voltage magnitudes leads to the choice of a sinusoidal wave. This is the only waveform
which will transform into another sinusoid. The sinusoidal primary current produces a sinusoidal
magnetic field within the transformer which induces an emf in the secondary which is proportional
to the rate of change of the magnetic field (i.e. the differential). The differential of a sine wave is
another sine wave - just shifted by 90 degrees.
Within a short span time since the invention of electricity, the advantages of ac power became
apparent. Ac power requires simpler and robust generator design and is also very easy to transmit
over long distances. Although dc power continues to be used in equipment, it is invariably obtained
by conversion of ac power could be readily converted to run dc appliances—which is another
advantage offered by the ac power.
Since transmission of power is more economical at higher voltages, ac power transmission and
distribution systems deploying transformers have become the norm in power industry.
Transformers are also very useful as components within equipment where they are utilized to
derive lower voltages to suit the application requirements. In modern power systems, transmission
of power at high dc voltages has been found to possess specific advantages and is being used
increasingly in specific segments. But this is more of an exception than the norm and is yet to attain
the pre-eminent position of ac systems in the power industry. The discussion of the same is beyond
the scope of this manual.
Ac systems have thus become a standard all over the world for electrical generation and
distribution systems. Figure 2.4 illustrates the simplified basic configuration of an ac system. In
Electrical Distribution Systems 17
most cases, transmission as well as distribution is done at more than one voltage to make the
system more efficient (in other words, reducing the power loss in the conductors). This is done
depending on the quantum of power transferred and the distances over which it is done
Figure 2.4
A simplified configuration of ac system
2.3
Poly-phase AC circuits
As you would have noticed, the circuits that are shown in the earlier paragraphs have two
conductors, one going to the load and one returning from the load. This is called a single phase ac
system. A single-phase supply is suited to certain types of loads such as Lighting and heating. But
when it comes to rotary electro-dynamic equipment, such as a motor, it has some disadvantages.
This prompted the use of a different type of a system where more than two conductors are used.
A
D
C
N
E
S
Note
N and S are the poles of the rotor magnetic
System
A and B are terminals on the stator winding
One pole pitch apart
C is the mid-point of the winding
D and E divide the winding into three
Equal parts
Figure 2.5
Theory of Poly-phase generator
B
18 Practical Electrical Wiring Standards - AS 3000:2018
A simple generator consists of a stator winding, a rotor and a field winding. Figure 2.5 shows the
winding (on the stator or the fixed portion) of a simple electrical generator and two adjacent poles
on the rotor (rotating part) of the generator. Please take note of the location points A, B, C, D and E
in the windings which are referred in the following section to indicate the voltage generated.
The first ac generators with a single set of windings and a rotating magnet generated single-phase
voltage. Generators with spatially displaced windings generating poly-phase ac voltage were a later
development. A single-phase power system results in higher current for transferring a given amount
of energy, which increases the size of generators and also the conductors required to carry the
current over long distances. The advantages of three-phase power and the economy achieved in
generation and transmission of electricity were then evident and it became the norm for all ac
electrical systems.
Ac power consumed today is divided into single phase and three-phase power, though generation is
with three phases. Though single phase power is used today both in industries and
commercial/residential applications, their usage is limited to final distribution circuits for low
capacity devices such as lighting, small pumps, small capacity air conditioners, computers, etc.
Again, this single-phase power is actually derived out of the three phase system and does not
require any special equipment / devices for separation of three phase power to single phase, when
needed.
2.4
Vectorial Representations
The ac generator in a power plant is coupled to a prime mover, which is made to rotate (move) by
its primary energy source, which can be a liquid fuel, gas, water (in hydroelectric power stations)
etc. The prime mover drives the rotor of the generator at the required speed. This in turn produces
an alternating voltage at the generator’s three output terminals in sinusoidal form, which is the most
commonly followed system for generation and transmission. The waveform in one of the phases
will be as shown in figure 2.6, with time on X-axis and the voltage value on the Y-axis. The arrow
lines by the side of the waveform basically indicate its angular position at that particular time and
the instantaneous value of the wave form is given by the vertical line length from the arrow end.
Figure 2.6
AC sinusoidal voltage and vector representation
The above variation in the magnitude and the relative angle from 0 to 3600 is termed as one cycle
and the voltage in one phase is represented by a vector line, which makes 3600 rotation for one full
cycle. AC generators produce this kind of sinusoidal voltages at 50 or 60 cycles per second (known
as the frequency of the electrical source and expressed in cycles/second or Hertz) in its three phases
with B phase lagging A phase by 1200 and leading phase C by 1200. Though the arrows in the
figure 2.6 are shown as moving in clockwise direction (from 0 to peak), it is generally a practice to
show the vector traveling in anti clock direction. These three phases A, B and C are represented as
three rotating vectors as shown in figure 2.7.
Electrical Distribution Systems 19
Figure 2.7
Regular anticlockwise phase voltage rotation
Referring to the single phase generator shown in the earlier figure 2.5, the voltage generated across
A and B is the resultant of the voltages in each turn whose phasors lie along the semicircle with
diameter AB and can be vectorially shown as in figure 2.8.
Figure 2.8
Voltage phasors and relationship of a generator
The generator is provided with three independent windings in such a way as to produce the
voltages in the above fashion. The windings at one end are brought to terminals A, B and C (this is
different from A, B, C identified in figure 2.5) and the other ends are interconnected to form the
neutral end of the generator. Though the windings can also be connected in Delta form, it is not
followed in generators. It is also an established practice to connect the neutral terminal to the
ground through a resistance called Neutral Grounding Resistor (NGR) to limit the fault currents in
20 Practical Electrical Wiring Standards - AS 3000:2018
the generator during earth fault. Further, it is to be noted that when multiple generators are
connected to supply a common bus, the phase angles of voltages at terminals A, B and C of all
generators shall be exactly same as otherwise it would lead to severe short circuit conditions. The
other major parameter to be matched is the magnitude of the voltage of parallel-operated generators
at these terminals, which shall be almost equal (V A , V B , V C in figure 2.9).
The three-phase system is adopted for use in electrical systems all over the world. Even when
single-phase loads are to be fed they are essentially fed from taking the supply from any one phase
of the three-phase system. The three-phase systems are represented by phasors and waveforms as
shown in figure 2.9.
Figure 2.9
Voltage Phasors and Wave Forms in a 3-Phase System
2.5
Advantages of three phase systems
One of the main advantages of a three-phase system is the simplicity of motors designed to run on
it. This requires an understanding on the basic principle of single phase motor and three phase
motor. A single phase motor can be electrically represented by the figure 2.10. The winding is in
fact a fixed coil energized with an ac voltage.
Electrical Distribution Systems 21
Figure 2.10
Single Phase Motor
The input voltage produces a magnetic field which is alternating too as represented by the wave
form in figure 2.11. The intensity of the magnetic field varies in magnitude and direction along the
axis of the coil. When a rotor is placed in such a field, it does not experience any torque. Thus a
single phase ac motor is not self starting. Of course, one can use commutators and split windings to
make the motor self-starting but these arrangements introduce complexities in the system.
Figure 2.11
Single Phase Motor-Magnetic Field Waveform
On the other hand, a three phase motor can be represented by three coils 120 degree apart each
energized by one phase of the supply system which are also 120 degree apart electrically. It can be
shown that this arrangement produces a magnetic field which is of constant magnitude and rotates
in the physical space at a speed decided by the supply system frequency and the number of virtual
poles in the winding. A magnetic rotor placed in the system will lock into the rotating field and
rotate with it. This makes a three phase motor self-starting without adding any complexities.
22 Practical Electrical Wiring Standards - AS 3000:2018
A three phase system thus has the following advantages:
• For a given size of a motor power output is higher than a single phase system.
• Similarly, when a given amount of power is transmitted by a three system at a given
voltage over a given distance, the least amount conductor material is used for
achieving the same efficiency.
• Single phase motors are not self starting but three phase motors are. Also three phase
motors produce uniform torque which is ideal for the loads being run from such a
system.
• It is difficult to operate single phase alternators in parallel since the synchronizing
torque is much lower.
Today’s power plants invariably generate three-phase ac power at hundreds and thousands of
Megawatts. The box below shows a comparison of power handled for single phase, two-phase, 3phase and m-phase systems.
It can be seen that the power output of a two phase system is 41.4% higher than that of the single
phase system and for the three phase system it is 50% higher than the single phase system. Beyond
this, the increase obtained by increasing the no. of phases to higher values becomes marginal. Even
for an infinite number of phases, the additional output is only 7% higher than the 3-phase system.
Electrical Distribution Systems 23
A generator thus works at a power output near the theoretical maximum value when it feeds a
three-phase system.
2.6
AC system connections
Two types of connections are possible in a three-phase system as illustrated in figure 2.12. This is
applicable both for a source and a load. Possible combinations of source and load are shown in
figure 2.13 (a 1 –a 2 , b 1 –b 2 and c 1 –c 2 are the end of the windings of each phase interconnected
among them to form Star or Delta connection as shown in the figures).
Fig. 2.12
Star and Delta Configurations
Most systems use the star connection at the source, which gives them a flexibility to feed both three
phase and single-phase loads. This is a de-facto distribution system standard in most parts of the
world for low voltage distribution systems and unless otherwise stated, will be the one that will
figure in our discussions further. A 3-phase 4-wire system is necessary when single-phase loads
(loads across one phase and neutral) are supplied by the system. In this case the neutral of the
source and the load will have to be connected by a neutral conductor. When the current drawn from
each of the phases have equal magnitude and same phase angle, the system is said to have a
balanced load. In a balanced 3-phase 4-wire system the neutral current is zero. In case only three
phase equipment such as motors are fed from the system, then the neutral conductor can be
dispensed with. In such case, the system becomes a 3-phase 3-wire system. A source may feed a
combination of loads requiring a 3-wire system and 4-wire system.
24 Practical Electrical Wiring Standards - AS 3000:2018
Figure 2.13
Three Phase System Connections
2.7
HV and LV distribution systems
Any electrical system intended to serve many consumers, whether in a plant or a township or a
country, primarily consists of equipment with different voltage ratings mostly based on economic
considerations. This is mainly made possible by the use of transformers, which are the backbones
of efficient electrical distribution at minimum losses.
The use of equipment at different voltage levels have led to the need for demarcation of type of
equipment based on their normal operating voltage, which is termed as Nominal operating voltage
of the equipment. The main reason for demarcation is to limit the cost of equipment to be used at a
Electrical Distribution Systems 25
particular voltage and the current to be transmitted over the conductors. The ac operating voltages
are broadly divided to three main categories Viz.,
• Low Voltage (LV) referring to voltages up to 1kV but generally the operating voltage
seldom is expected to be between 500 to 1000V under this category.
• High Voltage (HV) referring to all voltages above 1000V. However per ANSI
standards, voltages above 69kV are termed as HV. It is also a general practice to refer
this range (>69kV) as Extra High Voltage (EHV) in Europe and Asian countries,
though EHV term is not commonly adopted in all countries including Australia.
• Medium Voltage (MV) is another term referring to equipment above 1kV up to and
including 69kV as per ANSI. In some parts of the world, MV is generally termed up
to 3.3kV, beyond which the term HV is applied.
In Australia ac voltages up to 1000V and dc voltages upto 1500V come under low voltage
category. AS/NZS-3000 primarily specifies rules for the installations that are connected at these
specific voltages. The low voltage distribution is the most common system and almost everyone is
in contact in day to day life on these low voltage systems and equipments ranging from simple
lighting system under which you may be reading this chapter to different types of switches and
switchboards controlling such systems. The LV systems are mainly intended for small power loads
whose ratings may range from fractional to a few hundreds of HP.
2.8
Importance of earthing
Earthing is one of the important considerations in safety of electrical installations and in this
chapter we will try to have brief fundamental principles on the concept of earthing and a separate
chapter provides more details and requirements on the same. The earliest electrical distributions
systems were unearthed ones with no connection to earth at all. Even though such systems still
exist in specific areas, they are the exceptions rather than the rule. By and large some form of
earthing is adopted for all power systems, as they provide some important functions for safety and
reliability of system. We review below the reasons for earthing.
We know that the insulating layer around the current carrying conductors in electrical systems is
prone to deterioration. When a failure of insulation takes place due to aging and/or external factors
such as electrical, mechanical or thermal stresses, it is necessary to detect the point of failure so
that repairs can be undertaken. In a system that has no earth reference, it is not at all easy to
correctly pinpoint the faulted location. Refer to Figure 2.14, which shows such a system. It can be
seen that due to the absence of a conducting path through earth, there will be no current flow for a
protective device to sense and isolate the faulty circuit. However if a second fault occurs in the
unaffected lines at some other point in the system it can cause a shorting path to be made available
resulting in the flow of high magnitude fault currents that can be detected by protective devices.
Fig 2.14
Fault in Unearthed System
26 Practical Electrical Wiring Standards - AS 3000:2018
To detect and isolate the first faulted circuit as soon as the fault develops without waiting for a
second fault to happen, we need to connect one of the two poles of the source S to the earth as
shown in figure 2.15. In three phase electrical system, it is the center point of the star connected
winding that is usually earthed. It would be of interest to note that the connection of the system to
the earth in figure 2.15 is only at the source. The return current from the load flows through the
neutral conductor back to the source. For this reason, the neutral needs to be always insulated
usually to the same degree as the line conductor. When there is an insulation failure in the line
conductor, high current flows through the electrical circuits and through the earth path back to the
source. The magnitude of this current depends on the resistance of the earth path (called the earth
loop resistance). The current flow in this path can be detected by appropriate protective equipment.
Figure 2.15
Effect of Earthing the System
Thus one of the primary purposes of earthing the source is to permit easy detection of faults in
electrical systems by providing a path for the flow of currents from the fault point through the earth
(and sometimes the earth mass) back to the source. This earthing is referred as functional earthing
in AS/NZS 3000.
Now let us take a step further and see as to why a separate earth reference is necessary at the
consumer point or the load side equipment. While Figure 2.15 shows that the source is earthed, it
does not indicate another point of connection to earth. However, in practical systems, the fact that a
failure of insulation takes place does not mean that a earth connection is automatically established.
This can only be done if the point of failure is connected to earth using low resistance earth path.
Such a path is created using a reference earth bus at the consumer end and connecting the metallic
housing of all electrical equipment to this bus (Refer Figure 2.16).
Figure 2.16
Fault Current Flow in an earthed System
Electrical Distribution Systems 27
It should be noted that the neutral of the electrical load is isolated from the earth and the connection
between neutral and earth is still at the source point only. Thus the earth reference at the consumer
or load end fulfils the primary function of providing a metallic return path to allow for earth fault
current to flow with a minimum of earth loop impedance. This allows the earth fault currents to
attain sufficient magnitudes that can be detected easily without resorting to any special sensitive
protective equipment. Hence it can be concluded that earthing primarily serves the following
purposes with respect to the connected systems and the source.
• It ensures the contact surfaces of the electrical systems are at or near zero potential to
guarantee basic safety under normal conditions (Protective earthing)
• To permit easy detection of faults using it as a path for the flow of currents from the
fault point through the earth back to the source for disconnection of source to avoid
catastrophic accidents
As we have seen earlier there are different types of connections possible at the source and the load
ends. The earthing methods are suitably chosen matching the system connections to detect faults
and to minimize fault currents. The different types of earthing and the earthing practices adopted/
recommended will be discussed in detail later. It is essential to have a clear understanding on the
following important definitions related to earthing, as we proceed further.
• Earth: The conductive mass of the Earth whose electric potential at any point is
conventionally taken as zero.
• Protective Earthing: Connection of exposed conductive parts of an installation to the
earth through earthing terminals.
• Functional Earthing: Earth connection for proper functioning of equipment like
suppression of noise signals in instrumentation circuits and not for enabling safety.
• Earthing Terminal: The terminal or bar provided for the connection of protective
conductors and functional earthing conductors to the earth.
2.9
Importance of testing and verification
The continuity of power distribution depends on the reliability of the electrical equipment in a
system. While the reliability of many equipment have increased manifold during the last century, it
is not recommended to connect any finished equipment to a system directly from the manufacturing
place, unless its performance is proven. The manufacturers in the initial days had to think of many
ways to prove the worthiness and reliability of their equipment. Nevertheless due to various
reasons, manufacturers duplicating proven equipment also gained entry into the market. This had
led to claims and counterclaims by the sellers with the consumers and end users getting confused.
However the concepts have changed and bringing equipment under common umbrella to prove
their performance have slowly become the practice in every country. Each country had established
committees and organizations to ensure the uniformity and performance of electrical equipment in
an orderly way. This had led to the release of electrical standard in each country for all the
electrical equipment. The major content of most of these cover the minimum tests that are to be
conducted on an equipment in an environment, which may be severe than normal the operating
conditions, in terms of their voltage and current levels.
With the sharing of knowledge among the intellectuals from different regions and with the
globalization leading to use of electrical equipment from different parts of the world, a common
way to establish the capability of equipment had been accepted leading to mandatory testing of
electrical equipment before being put into use. The tests and the methods to be followed are
covered in all Electrical standards.
It can be concluded that testing electrical equipments is necessary
• To prove the performance of an equipment before being put into service
• To ensure that the equipment are assessed on a common basis with respect to their
technical capabilities
28 Practical Electrical Wiring Standards - AS 3000:2018
•
•
To give confidence to the end user about the capability and performance of the
equipment where it is to be used.
To establish an assurance showing that the distribution system as well as the
equipment will not cause any damage to the property and personnel, when they are
put into service.
It is to be noted that most of the tests are carried out in a manufacturer’s works under some specific
conditions. However when the equipment is connected to a system it is not possible to assume that
the system conditions will be matching the equipment requirements. Hence it is vital that a
complete testing and verification of the distribution system is carried out by authorized personnel in
a step by step method to verify that the system complies with all the established standards. The
quality of an electrical installation and ensuring safety of personnel who operate and maintain the
installation are important issues. Carrying out the design and construction of an installation as per
applicable standards, regulations and codes of practice is crucial in ensuring the quality, safety and
integrity of the installation, since standards and codes put must emphasis on matters pertaining to
safety. An installation must be inspected for conformity with the applicable regulations and for
safety on completion of erection and thereafter periodically.
AS/NZS Wiring rules stipulate various requirements to achieve these objectives. Planning, design
and erection of an electrical system need extreme care in order to ensure that the installations are
safe for the personnel who use, operate and maintain them. Proper planning using the methods of
systematic assessment given in Section 8 of the Wiring Rules will ensure that the installations
function as intended and are not unduly affected by the presence of external influences. Proper
design of the system and selection of equipment, which form part of the installation, ensure that the
system is safe and remains safe over its entire intended life. Proper erection ensures that the
equipment operates and meets the functional requirements as intended. Inspection verifies the
compliance with regulations and safety requirements.
AS/NZS 3000 standard specifies the verifications to be adopted in an installation and it is also
mandatory that the system is not energized unless the verifications are completed and certified by
licensed personnel. This ensures that the system will be safe to be put into service thereby serving
the objectives of the standard.
It is also necessary that periodical inspection and verification is carried out on the systems. This is
because of the ageing factors of the systems and also due to environmental conditions like
temperature, dust, etc affecting the performance of the equipment resulting in systems deterioration
over a period of time. Periodical verifications ensure identifying defective parts for rectifications
and replacements, as required. It is also helpful for adding new loads and/or doing alterations in the
systems as a part of refurbishment and new technological innovations.
Hence it can be concluded that the inspection serves the following objectives.
• To ensure that a new installation is safe to energize, operate and maintain
• To ensure that the installation remains safe during its operation without deterioration
• To ensure that additions/modifications to an existing installation do not impair its
safety.
2.10 Distribution systems in special locations
Special locations may be classified as those that require additional precautions to ensure proper
operation of the equipment and safety for the persons coming or working close to such locations.
These are also locations that require special attention for maintaining continuous power supply to
large areas. AS/NZS 3000 requirements cover additional precautions needed for design, selection
and installation of distribution systems in such special installations. Some of the special
installations covered by the standard are given below:
• Electrical systems for safety services (emergency supply systems)
• Electricity generating stations
Electrical Distribution Systems 29
•
•
•
Locations storing explosive substances or process areas that generate explosive gases
so the operation of electrical equipment can lead to arcing, fire and explosions unless
special precautions are taken.
Systems operating at high voltages
Construction/ demolition sites, shows/ carnivals, outdoor sites under heavy
conditions, etc.
In locations like bathrooms and swimming pools the system insulation can be affected by moisture
and water entering the electrical equipment and wiring systems. These locations also demand
special attention to maintain safety and reliability. The requirements to be complied in these
installations are covered in a separate section of the standard. We will discuss the specific
guidelines needed in the electrical distribution systems in all such special installations in
subsequent chapters.
2.11 Summary
Over the years the electrical systems evolved with incorporation of isolation switches at source and
load ends for better control and reliability of the ever growing systems with many interconnections
added to improve reliability and continuity of power supply at consumer ends. Though dc and
single phase systems were the beginning of electrical distribution, the power distribution moved to
three phase ac systems in today’s world due to specific advantages related to these systems. The
major advantages with three phase systems are the reduction in transmission and distribution losses
compared to single phase systems including other economical advantages like easy and efficient
voltage transformation, optimum conductor sizing, etc. The ac system parameters that vary in
magnitude with time can be represented by straight line vectors. The angular positions of respective
vectors show the relative phase shift between different voltages and currents at any instant. The
three phase ac systems are connected in the form of star or delta to meet specific distribution needs
with star + neutral combination being commonly adopted in low voltage systems to enable single
phase distribution at still lower voltages
AS/NZS 3000 primarily covers the LV distribution systems adopted in almost all installations to
distribute power supply to lighting, sockets, fans, etc. Earthing is an important requirement to
detect faults in electrical distribution systems. Protective earthing of equipment maintains safety for
personnel coming close to or in contact with surfaces enclosing live conductors by ensuring all
contact surfaces are at zero potential.
Testing of the equipment at manufacturers’ works and again at field prior to energisation is
necessary to ensure that the equipments meet the specific purposes and operating conditions for
which they are designed and manufactured. Verification and testing of an installation before putting
into service is a mandatory requirement as per AS/NZS 3000 and the specific guidelines will be
reviewed in a subsequent chapter.
30 Practical Electrical Wiring Standards - AS 3000:2018
3
Design of Electrical Equipment
and Safety
The safety of an electrical system depends on its application from the design stage onwards.
AS/NZS: 3000 Wiring rules covers the safety requirements for design, selection, installation of
electrical equipments and distribution systems in detail. In this chapter we will understand the
common electrical hazards and the importance of safe design to overcome these hazards. Since
insulation and enclosures play major roles in providing safety, we will review the salient
requirements and classification methods of these two features in electrical designs. A brief
presentation on the prevention methods followed for isolation under fault conditions and fire
hazards is also provided with typical details of the protective devices adopted for the same. We will
also review the common practices followed for isolation and interlocking to ensure safety in the
electrical systems.
Learning objectives
•
•
•
•
•
•
•
•
•
3.1
Objectives of safe design
Insulation and its role in safety
Types and classification of insulation
Enclosures for safety
IP Classification for enclosures
Adverse thermal effects and prevention
Prevention of hazards by Isolation and interlocks
Equipment selection
Role of standards in safety
Objectives of safe design
The rules and standards for electrical systems stipulate guidelines that are mostly focused towards
safety of electrical equipment and installations. Many of such requirements for safety in respect of
electrical equipment and installations must be addressed at the design and planning stage itself.
Any unsafe situations arising from faulty design or incorrect selection cannot be easily rectified
later, and, if at all it is possible to do so, the cost of rectifications or modifications as well as time
required to make the installation safe are likely to be prohibitively high. Enough attention is
therefore needed at the design and planning stage itself to ensure that the equipment or installation
is safe to operate and maintain, and remains so during its entire service life.
32
Practical Electrical Wiring Standards - AS 3000:2018
Safety through design has the primary objective of eliminating hazards that can arise from the use
of equipment both under normal circumstances as well as abnormal situations. The most common
hazards associated with electrical equipment are:
• Electric shock and internal organ damage due to passage of electricity through human
body
• Burns on skin at point of contact and injuries by electric shock combined with fall
• Temperature hazards during operation
• Arc flash causing external burns and injuries by explosive expansion of air due to the
arc.
In addition, fire hazard through combustible components of electrical equipment or from materials
stored in the vicinity of electrical equipment, mechanical hazards from motive equipment and from
impact of parts dislodged due to an arc fault and corrosion/explosion hazards from electro-chemical
equipment are also likely to occur. Thus, the following basic safety aspects need to be addressed
while designing electrical equipment.
• Electric shocks
• Arcing due to breakdown of insulation
• Mechanical failures and injury to personnel
• Burns due to high surface temperature.
• Fire in nearby combustible materials
• Features to ensure isolation and prevent accidental switching
• Earthing facilities and interlocking to prevent accidents due to incorrect operations
We will deal with these aspects in further detail in the ensuing sections of this chapter.
3.2
Preventing electric shocks
Shock hazard is possible as a result of direct contact or indirect contact. Direct contact hazard can
be prevented by using appropriate insulation, by proper enclosure of live parts, by placing obstacles
for contact with exposed live parts or by placing live parts out of reach. The last named is
applicable for all outdoor installations where proper clearances are provided to ensure that there
could be no direct contact during normal operations in the vicinity of live conductors. The other
method of preventing danger by direct (or indirect) contact is by the use of Separated Extra Low
Voltage (SELV) systems. The voltage in these systems is so low that body current through direct
contact can be withstood without harm for indefinitely long periods. Such use may be limited only
for specific cases such as certain types of hand-tools, hand-lamps or other body-worn electrical
gear and will not be further discussed here.
Indirect contact occurs when a fault takes place in electrical equipment (usually as a result of
insulation failure) between live parts or between live parts and an exposed metallic enclosure.
Minimizing the hazard from indirect contact is probable by:
• Limiting the voltage of contact (touch/step/transferred) surfaces and
• Limiting the time of contact
Limiting the voltage of contact is largely through adopting proper earthing practices as we
discussed in earlier chapter. Limiting the time of contact under faults is possible through properly
designed protective relaying that can sense the fault conditions. The importance of proper design of
earthing and protection systems need not therefore be overemphasized. But prevention being better
than cure, all attempts must be made to avoid the occurrence of a fault in the first place for which
insulation is a fundamental choice.
We can ascertain from the foregoing discussion that insulation plays an important part in avoiding
dangers from both direct and indirect contact. Enclosures mainly act as a protective barrier against
direct contact with live parts. By virtue of properly earthing the enclosures, that ensures early
detection of faults, indirect hazard can also be minimized. The enclosures also offer safety against
arc faults and protect live parts from ingress of dust and water, thus reducing the possibilities of
insulation failures.
Safety aspects in electrical equipment design and selection 33
3.3
Importance of insulation in electrical safety
No electrical equipment can be designed, constructed or used without insulating materials in some
form or other, as they are needed to maintain the potential difference between the active parts of an
installation for its proper functioning. Insulation materials can be solid such as insulators used for
support of live conductors, insulating materials applied over windings and so on; they can be
liquid, such as the dielectric oils used in transformers, or they can be in gaseous state, an example
being the sulphur hexa-fluoride (SF 6 ) gas used in HV switchgear and circuit breakers. Even
vacuum behaves as an insulating medium as there are no molecules that can conduct electricity.
Naturally gaseous and liquid dielectrics have to be used in combination with solid dielectrics in any
practical installation. Thus solid insulating materials play a crucial role in safety, in particular,
when the insulating materials are applied over conductors.
Insulation helps to prevent short circuit between live conductors, and between live conductor and
the enclosures of equipment. Both of these are important from a safety point of view. A short
circuit between live conductors can result in excessive currents flowing through the system
resulting in extreme thermal and mechanical stresses in the conductors and in the system as a
whole. There is also a danger of an arc flash developing because of such a fault. On the other hand,
a fault between a live conductor and an equipment enclosure will cause the exposed metallic
enclosure of faulty equipment to become live and cause an indirect contact hazard.
Insulation materials have certain specific properties which decide their suitability for any specific
application. One such property is their voltage withstand rating and the other is their operating
temperature limit. The voltage withstand rating is expressed usually as kV/mm or some other such
unit of thickness, and electrical stress beyond this limit, may result in a breakdown of the insulating
material. Once a material thus breaks down, it may show signs of burning and becomes a good
conductor; it is no longer useful as insulation. Similarly, each insulating material has a temperature
limit beyond which it will get destroyed and is not usable as insulation. The composition of the
material used as insulation decides the temperature withstanding capability. Insulating materials
used for windings of electrical machinery are classified based on their temperature rating which is
generally as per table 3.1.
Table 3.1
Insulation classes of adopted in Electrical Machinery
Class of
Insulation
A
Limiting hot spot
temperature
105 deg C
Common materials/ combination
E
120 deg C
Dry chemical, Halon, CO 2
B
130 deg C
Mica, asbestos, glass fiber, and similar
inorganic materials with suitable bonding,
impregnating or coating substances.
F
155 deg C
Mica, asbestos or glass fiber base with a
high temperature resistant binding organic
or inorganic material
H
180 deg C
Silicone elastomer and Mica, asbestos, or
glass fiber base with a high temperature
resistant binding material like silicone
resins
Cotton, silk, paper, and similar organic
materials, impregnated or immersed in oil,
and enamel applied on wires.
34
Practical Electrical Wiring Standards - AS 3000:2018
The properties of any insulating material generally deteriorate with age until it is no longer able to
serve as insulation at the original ratings. This leads to sudden insulation failures even when the
material is operating within its stated ratings. It should also be noted that prolonged operation at
higher than limit values of temperature will cause faster ageing and lead to early insulation failure.
High and repeated voltage and mechanical stresses can also cause breaking of insulating material
and will lead to failures.
Insulation failures are caused by the following conditions:
• Temperature in excess of the limiting value (most common)
• Voltage stress beyond its rated withstand value
• Ageing of insulation
• Excessive mechanical stresses
Preventing unforeseen insulation failures is therefore important from the viewpoint of electrical
safety. The following steps during the design phase will help to reduce failures and obtain longer
life from insulation.
3.3.1
Preventing temperature related failures
The approach here is to operate the insulation within its rated temperature under all operating
conditions.
• Design with suitable class of insulating material depending on anticipated operating
temperatures
• Consider maximum expected ambient temperature
• Design on the basis of maximum internal temperature (hot spot)
• Consider final temperature after a short circuit fault where required
• Provide adequate safety margin or choose higher class
• Provide protective devices to isolate a circuit in the event of abnormalities so that
abnormal temperatures are detected well in advance and prevented
While specifying electrical equipment, it is essential to state the ambient temperatures appropriate
to the installation. In critical equipment, it is usual to also specify a higher safety margin for
operating temperature by specifying a higher class of insulation and stipulate temperature
restriction corresponding to a lower class. For example, a turbo generator stator may be specified
with class F insulation but with design temperature corresponding to class B. The trade-off here is
between optimizing the cost of the equipment against the cost of a failure during operation and
consequential losses.
3.3.2
Preventing voltage stress related failures
Failure due to voltage stress can not be a common occurrence. The design withstand value of an
insulation is governed by the Basic Impulse Level of the system which is usually quite high
compared to the operating voltage (Example: a 10 kV system may have a BIL of 75 kV or even 90
kV). This is to prevent failures during voltage surges occurring due to lightning strike or switching.
Design approach here should be:
• Design with required thickness/layers of material
• Design for maximum expected voltage
• Provide protection against unexpected over-voltage by external surge protection
devices (SPD)
Surge protection is not a mandatory requirement per AS/NZS 3000. Appendix F of the standard
provides informative details on selection and installation of Surge protection devices (SPD) in low
voltage systems that can limit transient overvoltages caused either by power line disturbances or by
natural events like lightning strikes on exposed conductors. SPDs should be installed after the main
switch but prior to any RCD devices. Type of surge protection shall be selected to meet the kind of
protection needed as noted below.
Safety aspects in electrical equipment design and selection 35
•
•
•
•
Primary SPDs should be installed near the origin of the electrical installation or in the
main switchboard.
Secondary protection in the form of plug-in surge filters or distribution board
protection may be warranted for premises containing sensitive electronic equipment.
Such secondary protection should be coordinated with the upstream SPDs in line with
the manufacturer’s instructions.
For most domestic single-phase supplies in urban environments, a maximum surge
rating of 40 kA per phase for an 8/20 µs impulse and a minimum working voltage of
275 V AC is suitable.
In the case of installations in exposed locations, e.g. high lightning areas, long
overhead service lines, industrial and commercial premises, it may be prudent to
install SPDs with a higher surge rating, typically 100 kA per phase for an 8/20 µs
impulse.
3.3.3 Preventing ageing related failures
An insulating material cannot work indefinitely. Ageing is more rapid with exposures to constant
high temperatures. To obtain a long life from insulation, it should not be allowed to operate at
temperatures higher than its rated value. The design should aim at an optimum point between initial
cost and life obtainable.
3.3.4 Preventing failures due to mechanical stresses
Mechanical stresses develop in insulation due to various reasons. One such reason is stress due to
centrifugal forces caused by rotation or by electrodynamic forces. Such failures are normally
observed in the overhang portion of the windings of electrical rotating machinery. Mechanical
stresses can also be caused due to electromagnetic forces occurring during short circuits. In busbar
systems, these can result in damages to the support insulators as well as to the conductors. Such
failures can be prevented by proper mechanical design of equipment with adequate supports.
3.3.5 Insulators for transmission systems and outdoor yards
In addition to the classes of insulation discussed above, other solid insulating materials such as
ceramics (porcelain) are used as insulating materials for conductor support in overhead lines and
switchyards. The temperature limit of these insulators is very high. Failures in this type of
insulators are either due to voltage stress or mechanical damage (which sometimes follows a
voltage break down). These insulators bear the brunt of surges due to a direct lightning strike or
induced surges resulting from a direct strike on nearby conductors. Since lines do not have any
surge protection devices, a failure can take place across an insulator, thereby creating an earth fault.
Failures can be prevented by the following measures:
• Design for appropriate clearance/creepage distances for the concerned system voltage
• Heavy duty/longer creepage insulators to be selected in areas prone to dust and
corrosion.
• Switchyard equipment shall be protected using surge arrestors in points of line
termination and near major equipment such as transformers
• Provide surge grading for multi-element insulators
• Provide on-line washing equipment for critical installations
3.4
Importance of enclosures in ensuring safety
Enclosures serve many important functions in electrical equipment. The first is naturally that of
providing protection against direct contact by preventing access. Another function of enclosures is
protection of electrical equipment from the ingress of water and dust, which can result in
equipment failures. Enclosures also serve to contain and divert an arc flash within the equipment
and provide safety to operating personnel from burns and the explosive effects of faults. Enclosures
are characterized by their degree of protection which is expressed by using the International
Protection (IP) classification.
36
Practical Electrical Wiring Standards - AS 3000:2018
3.4.1
IP Classification
IP Classification is an alphanumeric code consisting of the following characters:
• Prefix of the letters IP followed by a 2- digit group denoting protection offered
against entry of solid objects and liquids
• First digit: Protection against entry of solid objects (Numerals used 0 to 6, or letter
X), with the 0 depicting NIL protection and 6 providing the highest possible
protection against dust.
• Second digit: Protection against entry of liquid (Numerals 0 to 8) with 0 depicting
NIL protection and 8 denoting the highest possible protection from water like
submerged in water for a long time.
• A character X in place of the numerals means that the particular protection
characteristic is not relevant in the given situation or it may be because it is defined in
the subsequent group
The above method of classification with two numerals the most common adopted for equipments
meeting IEC, European and Australian standards. USA follows different method of enclosure
classification as specified by NEMA (National Electrical Manufacturers Association) and is
generally applied for equipments used within North America.
Tables 3.2 and 3.3 give the explanation of the first and second numerals adopted in IP category in
accordance with AS 60529, which is given in AS/NZS 3000 appendix G.
Table 3.2
IP Classification – Meaning of the first digit (Source: AS/NZS 3000, Figure G1a)
First
Protection offered against solid
Numeral
particles
0
No protection
Protection of persons
against access with
Non-protected
1
Full penetration ≥ 50 mm diameter sphere
Back of hand
2
Full penetration ≥ 12.5 mm dia sphere
Finger
3
Access probe penetration ≥ 2.5 mm dia
Tool
4
Access probe penetration ≥ 1.0 mm dia
Wire
5
Limited ingress of dust (no harmful deposit)
Wire
6
Totally protected against ingress of dust
Wire
Safety aspects in electrical equipment design and selection 37
Table 3.3
IP Classification – Meaning of the second digit (Source: AS/NZS 3000, Figure G1b)
Second
Numeral
Protection offered against Water
Protection from
effects of
0
No protection
Non-protected
1
Vertically falling drops with limited ingress
Vertically dripping
2
Vertically falling drops with enclosure tilted
15° from the vertical. Limited ingress
permitted
Dripping up to 15° from
the vertical
3
Sprays at 60° from the vertical with limited
ingress
Limited spraying
4
Splashed from all directions with limited
ingress
Splashing
5
Jets of water with limited ingress
Hosing jets in all
directions
6
Strong jets with limited ingress permitted
Strong hosing jets from
all directions
7
Immersion between 15 cm and 1 m
Temporary immersion
8
Long periods of immersion under pressure
Continuous immersion
Though, not strictly followed by many users and industries, it is also an optional requirement to
specify two additional alphabets as the third and fourth characters after the first two numeric
values. The first additional alphabet (one of the four - A, B, C or D) specifies the protection offered
by the enclosures from entry of objects like tool, wire, etc. The second/supplementary alphabet
(one of the four – H, M, S or W) defines the operation suitability of the enclosure under certain
applications like HV, specific weather conditions, etc. Tables 3.4 and 3.5 provide ready reference
for the meanings of these alphabets.
The IP rating shall suit the environmental conditions and the relevant mounting position as
specified by the manufacturer.
NOTE: This applies in particular to parts of enclosures that might serve as—
(a) a floor; or
(b) a surface where objects on surrounding surfaces may be displaced into openings.
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Practical Electrical Wiring Standards - AS 3000:2018
Table 3.4
IP Classification – Meaning of the first additional alphabet (Source: AS/NZS 3000, Figure G1c)
First additional
alphabet
Protection offered
Protection of
persons against
access with
Back of hand
A
Penetration of 50 mm diameter sphere up
to barrier do not contact hazardous parts
B
Test finger penetration to a maximum of 80
mm must not contact hazardous parts
Finger
C
Wire of 2.5 mm diameter × 100 mm long
must not contact hazardous parts when
spherical stop face is partially entered
Tool
D
Wire of 1.0 mm diameter × 100 mm long
must not contact hazardous parts when
spherical stop face is partially entered
Wire
Table 3.5
IP Classification – Meaning of the supplementary alphabet (Source: AS/NZS 3000, Figure G1d)
Supplementary
alphabet
H
Protection offered
Example
High voltage apparatus
M
Tested for harmful effects from the ingress
of water when the movable parts of the
equipment are in motion
The rotor of a
rotating machine
S
Tested for harmful effects from the ingress
of water when the movable parts of the
equipment are stationary
The rotor of a
rotating machine
W
Suitable for use under specified weather
conditions when provided with additional
protective features or processes
A typical example of enclosure specification as per IP Classification is given below by way of
illustration:
I P 56 D W stands for an enclosure which is:
• Dust proof (First numeral 5)
• Water tight (Second numeral 6)
• Protected against access with a wire of 1mm diameter and 100 mm long (Additional
letter D)
• Suitable for use under specified weather conditions (Supplementary letter W)
While on this subject, it may be recalled that it is also a practice to achieve safety against direct
contact by using obstacles. An obstacle is designed to prevent:
• Unintentional approach to a live part and
• Unintentional contact with a live part while operating on/near energized equipment
Safety aspects in electrical equipment design and selection 39
A part used as an obstacle must fulfill the following criteria:
• Obstacles shall be secured to prevent unintentional removal
• Removal should be possible only through the use of a tool or key
• An obstacle should have a minimum protection rating of IP XX B.
It must be noted that the door of a switchgear cubicle, which is opened using screwed wing-knobs,
will not qualify as an obstacle. On the other hand, a bus chamber protected with a bolted cover
would be deemed to have adequate protection against direct contact.
Therefore, an IP classification serves to ascertain that an enclosure serves as an obstacle for direct
contact with live parts as well as providing protection against the ingress of water and dust. As far
as protection against arc fault hazard is concerned, an enclosure must ensure that it can withstand
the explosive effect of a fault in the equipment within, without incurring physical damage. It must
not have any openings through which flames can pass, which could harm the operator. Preferably,
it should have some kind of explosion venting arrangement, which will help to deflect the arc gases
in a safe manner away from operating personnel. Examples of such arrangements can be found in
phase segregated terminal boxes of HV motors, which are provided with an explosion diaphragm at
the lower part to release arc products. Many designs of HV metal clad switchgear provide hinged
vent flaps at the top, which will open and release arc gases if there is an arc fault in the bus, breaker
or cable compartment. Safety of the operator who will be normally standing in front of the cubicle
is therefore ensured.
Enclosures for electrical equipment for use in areas containing hazardous gases will have to be
designed in accordance with relevant regulations to withstand an explosion of gases within the
enclosure, in addition to meeting the protection against dust and water. These designs will require
appropriate testing and certification for safety.
3.4.2
Placing live parts beyond ‘arm’s reach’
When work is carried out near exposed live parts, a safe clearance must be maintained with
reference to the live parts, so that the worker does not accidentally come into contact with these
parts. It must be remembered that in the case of HV equipment, the intense electric field generated
around the live parts mat, causes the surrounding air to break down and conduct so that one may
get electrocuted even without directly touching a live part. This danger does not exist in LV
systems. Exposed conducting parts of a system must be located in such a way that during normal
work in the vicinity of live equipment, safe clearances are always maintained appropriate to the
working voltage. This is achieved by placing the live part out of ‘arm's reach’ from the envelope of
minimum safe clearance for:
• An exposed conductive part
• An extraneous conductive part
• A bare live part of any other system
The same will have to be modified for HV systems by adding the minimum clearance for the
specific operating voltage. The voltage dependent clearances will thus have to be calculated from
the arm’s reach positions. For example, if the minimum safe clearance for 220 kV is stipulated as
say 3000 mm, the minimum clearance from earth should be 5500 mm to place the conductor out of
arm’s reach for an operator standing on the ground with his arm fully extended. In case there are
any structures beneath the conductor on which the operator may stand, the clearance should be
appropriately increased.
A similar approach is also applied in AS/NZS 3000, which is explained in a subsequent chapter.
Note that ‘arm’s reach’ defined above assumes that no metallic tools, long conducting objects (such
as ladders) etc. are being held by the worker.
Figure 3.1 illustrates the principle of ‘arm’s reach’ in all the three axes for LV systems as per IEE
regulations.
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Practical Electrical Wiring Standards - AS 3000:2018
Figure 3.1
Definition of ‘arm’s reach’ (Source IEE Wiring Regulations)
3.5 Prevention of adverse thermal effects
Adverse thermal effects of electrical equipments include:
• Burns due to high surface temperature
• Fire in nearby combustible materials
• Fire originating from electrical equipment
• Arcing due to breakdown of insulation
• Mechanical failures and injury to personnel due to arc faults
One of the main design features for limiting thermal hazards is the protection against overloads
(overcurrents) and short circuit faults. This is obtained through circuit protective devices such as
fuses, protective relays and circuit breakers. This section briefly discusses these devices.
3.5.1 Protection with fuses
Fuses are perhaps the simplest and cheapest device used for interrupting an electrical circuit under
short circuit or excessive overload, current magnitudes. It is used for overload and short circuit
protection in high voltage up to 66kV and low voltage up to 400V installations. The action of the
fuse is based on the heating effect of the electric current. In normal operating conditions, when
current flowing through the circuit is within safe limits, the heat developed in the fuse element
carrying this current is readily dissipated into the surrounding air and therefore the fuse element
remains below a temperature below its melting point. However when a fault such as short circuit
occurs or when load connected in a circuit exceeds its capacity, the current exceeds the limiting
value and the heat generated due to this excessive current cannot be dissipated fast enough. As a
result, the fusible element gets heated, melts and breaks the circuit. In this way, it protects the
equipment or an installation from damage due to excessive current.
Advantages of fuses:
• It needs no maintenance
• Its operation is automatic
• It interrupts enormous short circuit currents without noise, flame, gas or smoke
• The minimum time of operation is made much smaller than that of the circuit breaker
• The smaller sizes of fuse elements impose a current-limiting effect under short circuit
conditions
• Its inverse time current characteristic enables its use for overload protection
Disadvantages with fuses:
• Considerable time is lost in rewiring or replacing fuses after operation
Safety aspects in electrical equipment design and selection 41
•
•
On heavy short circuits, discrimination between fuses in series cannot be obtained
unless there are considerable differences in the relative sizes of the fuses concerned
The current time characteristic of the fuse cannot be correlated with a protective
device
Figure 3.2 shows a high rupturing capacity fuse and a typical fuse element.
Figure 3.2
HRC Fuse and element
3.5.2
Protection with relays
Protection schemes required for the protection of power system components against abnormal
conditions such as fault etc. consists of protective relaying and circuit breakers. The main functions
of protective relaying are the detection of the presence of faults and their locations; to initiate the
speedy removal from service of any element of the power system when it suffers short circuit, or
when it starts to operate in any abnormal manner which might cause damage.
Protective relays function as a sensing device. It senses the fault, determines the location and
finally sends a tripping command to the circuit breaker. After getting the command from the relay,
the circuit breaker disconnects the faulted element. The relays employed for protection against
short circuits are operated by virtue of the current or voltage supplied to them by current
transformers and/ or voltage transformers. Failure in the system is indicated by relative changes in
currents or voltages supplied to the protective relaying equipment. Relays can be of electromechanical type, static type (analog) or numerical type (digital). A complete description of these
devices is beyond the scope of this text.
A circuit breaker is a device which interrupts the abnormal or fault currents and in addition,
performs the function of a switch. Circuit breakers are mechanical devices designed to close or
open a set of fixed and moving contacts, thereby closing or opening an electrical circuit under
normal or abnormal condition. It consists of fixed and moving contacts which touch each other and
carry the current under normal operating condition i.e. when the circuit breaker is closed. When
fault occurs in any parts of the system, the trip coil of a circuit breaker becomes energized through
the protective relay (or a release) and the moving contact is pulled apart by some mechanism,
thereby opening the circuit. The separation of current carrying parts produces an arc. This arc not
only delays current interruption process, but also generates lots of heat, which may cause damage
to the system. To extinguish the arc within the shortest possible time is the main concern of the
circuit breaker. Hence it is necessary to use special extinguishing mediums like vacuum, oil, SF6 in
HV applications while low voltage breakers can be interrupted in air with suitable designs to avoid
spread of arcs to the surrounding materials.
For low voltage applications, air circuit breakers (ACB) and moulded case circuit breakers
(MCCB) are the most commonly used types to open and close a circuit under normal conditions.
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Practical Electrical Wiring Standards - AS 3000:2018
These are having features to trip under abnormal fault conditions of over load and short circuit.
Miniature circuit breakers are basically mini version of moulded breakers and are generally
available for use upto a maximum of around 100 amperes. Single pole and multipole versions are
available for MCB’s whereas the MCCB’s and ACB’s are available in three/ four pole versions for
isolating all three phases + neutral simultaneously. These are usually fitted with internal releases to
sense and open under fault conditions, though external relays are used for ACB’s in major
applications involving higher currents.
3.5.3
Protection against Fire hazards
One of the major hazards of electrical installations is fire. Many electrical types of equipment
contain flammable materials; for example, mineral oil cooled transformers and oil circuit breakers.
Even PVC, which is used as insulation and as a sheathing material of electrical cables, is
combustible. Often, the reason for fire is an electrical short circuit fault. Excessive currents and
arcing can cause fire in combustible equipment components. In some cases, materials stored nearby
can catch fire and cause a further spreading of the fire. Apart from the general destruction to life
and property, electrical fires have other repercussions also. For example, cable fires emit toxic and
corrosive gases since the PVC insulation contains hydrogen and chlorine. The dense black fumes
also limit visibility and impede fire-fighting attempts. Hydrogen chloride emitted by cable fires can
react with water (used in fire fighting) to form hydrochloric acid. This acid can condense on the
surface of electrical components and printed circuit boards in control room panels, causing
irreversible damage, even when the equipments are not directly affected by the fire. Appropriate
measures must be incorporated in the design of an electrical installation. These measures must help
to prevent fires, sense incipient fires should they occur, contain their spread and extinguish fires.
We will briefly discuss these measures below.
Fires can be avoided to a large extent by preventing insulation failures and short circuits through
adequate design. Since fires are often caused by short circuits, the protective devices for clearing a
short circuit fault must be fast acting in order to limit the energy flowing into the faulty equipment.
LV circuits must be protected with high rupturing capacity fuses of appropriate short circuit rating
or by current limiting circuit breakers. Another precaution is to avoid using fire-prone equipment.
For example, mineral oil cooled transformers can be replaced with dry type/cast resin transformers
when indoor use is required. Oil circuit breakers should be replaced by air/vacuum/SF6 circuit
breakers. Cables running close to furnaces or other combustion zones prone to flames must be
given fireproof coating to avoid starting a cable fire. Storage of combustible substances in
substations, cable vaults and in the vicinity of electric equipment enclosures, which may attain high
temperatures during operation, should be avoided.
Many electrical fires start due to arcing faults in branch circuits. Normal circuit breakers with short
circuit protection cannot sense an arcing condition, as the current initially remains low due to the
high impedance of the arc. Only when the intense heat produced by the arc damages the adjacent
insulation, a full short circuit develops, causing the breaker to operate. Normally, by this time, a
fire would already have started.
Containing fires
If a fire occurs, it must be contained within as small a space as possible. Providing fire partitions in
electrical switchgear rooms and cable vaults is a common practice. Similarly, large mineral oilcooled transformers should be segregated using firewalls. By limiting the amount of combustible
substances present within a given enclosed space, the severity of the fire is reduced and fire
fighting can be more localized and therefore effective. Cable faults should use fire proof doors with
certified withstand ratings. Openings through which cables pass from one enclosure to another
must be sealed with approved fire seals. Special care is needed to ensure that ventilation systems do
not cause the spread of fire between compartmentalized fire zones. Suitable interlocking may be
used to stop ventilation to, not only affected areas, but to other areas communicating with the
affected areas through duct work. Fire dampers with fusible elements can be deployed in ducting
and in ventilation openings between different rooms to automatically close in the event of a fire.
Also, the materials used in an installation should not cause fast propagation of the fire. Self-
Safety aspects in electrical equipment design and selection 43
extinguishing insulation, fire-retarding/low smoke emitting materials etc. are used in cables
intended for critical installations.
Sensing fires
Early warning devices within electrical equipment or rooms should be used to give an alarm of
fires should they occur. Examples are infrared sensors and photoelectric sensors to detect
temperature build up and incipient arc faults within enclosed electrical equipment. They will also
be useful in triggering fire-extinguishing systems. Multiple types of detectors should be used in
cable vaults (example: Ionization detector and smoke detector) with cross zoning for positive
sensing of fires. Linear heat detecting cables are useful in cable vaults to supplement conventional
detection systems. Addressable detectors connected to microprocessor based fire alarm and control
equipment, enable precise information to be available regarding the origin and spread of cable
fires.
Fire extinguishing systems
Critical installations should be protected with fire extinguishing arrangements. Appropriate
methods of fire fighting must be used for each type of installation. Portable fire extinguishers may
be provided in smaller electrical installations where elaborate automated arrangements may not be
viable. Figure 3.3 shows the common types of hand-held extinguishers which are provided with
color-coding for easy identification. Water and foam extinguishers are not recommended for use in
electrical fires.
Figure 3.3
Examples of hand-held fire extinguishers
Table 3.6 shows the application of fire-extinguishers for different types of fire. The number of
extinguishers and locations will depend on local codes/guidelines.
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Practical Electrical Wiring Standards - AS 3000:2018
Table 3.6
Application of fire extinguishers
Class
of Fire
A
Nature of fire
Common combustibles: wood, paper,
cloth, rubber, trash and most plastics
Suitable extinguisher
type
Water type
B
Flammable liquids: gasoline, kerosene,
oil, grease, solvents and paint
Dry chemical, Halon, CO 2
C
Live electrical equipment: motors,
computers, wiring and appliances
Dry chemical, Halon, CO 2
In large substations and electrical installations, it is normal practice to provide integrated fire
protection systems with sensors, fire alarms and extinguishing schemes all operating together in
automated fashion. Figure 3.4 shows such a typical integrated fire detection-alarm-extinguishing
system for use in electrical switchgear rooms with carbon-dioxide (CO 2 ) as the extinguishing
medium. The smoke sensor shown is a photocell type and contains a light emitter and a receiver.
When smoke enters the sensor and reflects the light of the emitter on to the sensor, the sensor gives
a signal to the monitoring unit. The other alternative type sensor is the ionization type, with special
gas sensing elements which output a signal when they sense the gases of the combustion.
The monitoring circuit gives audible and visual alarms, allowing the personnel inside the substation
to evacuate to safety. After a preset time delay, the monitoring unit opens gas valves releasing CO 2
or halon into the fire zone. Personnel should evacuate during the alarm period, in order to avoid the
risk of becoming asphyxiated. To avoid false alarms caused by mal-functioning of the sensors, a
voting system is used, whereby at least two sensors need to sense the fire and relay a signal to the
control unit, in order for the control unit to initiate an action.
Figure 3.4
Integrated fire-fighting system using CO 2
It is necessary to check the gas cylinders periodically. Although CO 2 is non-toxic, it causes
asphyxiation and care must be taken to make the system inoperative whilst personnel are in the
room. This is usually achieved by inserting a stop pin into the lever valve to prevent operation.
Control rooms and other high value assets are protected by Halon (or non-CFC equivalent)
flooding systems. Because a low concentration of the gas is adequate to extinguish the fire, there is
Safety aspects in electrical equipment design and selection 45
no danger of asphyxiation as exists with CO 2 . As such, there is no need to evacuate personnel
before the system releases the gas. Of course, evacuation is still necessary from a fire safety point
of view but it need not precede the gas release.
Transformer fire-fighting systems use a fine water/emulsion spray with frangible bulbs mounted on
the transformer tank to sense the fire. These bulbs are made from thin-walled glass: the bulbs break
in the event of a fire and the resultant release of a pressurizing medium senses the fire. Cable fireextinguishing systems use linear detecting cables to detect a fire, with water spray nozzles for
quenching the fire. Linear detection cables run along cableways and can effectively sense a fire and
help to locate the cableway where a fire has taken place.
3.6
Isolation arrangements
It has been well proven that most electrical accidents occur as a result of improper isolation of
equipment before work on a normally live part is taken up. Around 60% of accidents are due to
improper or insecure isolation. Isolation ensures that an equipment is disconnected fully from all
electrical sources and work on that equipment can be safely carried out. Continued isolation must
be ensured until the equipment, which has been previously isolated, is fit to be put back into
service. A sudden re-energisation may cause electrocution, if the people working on the equipment
presume that it is dead (and therefore safe to work on). Accidental re-energisation of drives may
cause unexpected starting of mechanical equipment, which may, in turn, cause injury to those
working on the equipment.
It is important to distinguish between devices used for isolation and those used for functional
switching.
ISOLATION is a function intended to cut off, for reasons of safety, the supply from all or a
discrete section of the installation, by separating the installation from every source of energy.
Examples: Switch, disconnector, circuit breaker
FUNCTIONAL SWITCHING is intended to switch on or off, or vary the supply of electrical
energy to all or part of an installation for normal operating purposes. Example: A contactor.
Isolation is primarily a safety function and generally uses manually operated/controlled equipment.
A switch is a special isolator, which can perform switching of load currents and can also close on
faults. Switches are also usually manually controlled. Functional switching devices are those
intended to perform load control. Functional switching equipments are not normally used as
isolation devices and are often remotely/automatically operated. Certain devices such as circuit
breakers may perform both functions, particularly when mounted on a withdrawable carriage,
which can ensure positive isolation. A circuit breaker, which cannot be withdrawn from the
operating position, should be treated only as a functional device and must be supplemented by
other isolating devices. Once a device is properly isolated, it must be prevented from accidental reenergisation using mechanical interlocks, padlocking of operating handles or other safety features.
Such facilities should be integrated into the equipment design itself.
3.7
Earthing and interlocks
Any equipment, which has been isolated, must also be earthed to prevent accidental energisation.
Earthing is the best means of ensuring safety of personnel. No work should be performed on
equipment unless it is visibly earthed. In case earthing has been applied at a remote point, a
portable earth clamp should be applied close to the work spot. Checking by voltage indicator prior
to earthing is necessary to avoid inadvertent earthing of live conductors, thereby creating a short
circuit fault.
After isolating the equipment or section required to be maintained, it is necessary to connect it to
earth to ensure the safety of personnel. Where earth switches are available (as is the case with
outdoor HV switchyards and GIS substations), they can be used for this purpose. In other cases,
46
Practical Electrical Wiring Standards - AS 3000:2018
approved types of portable earth clamps must be used. Whether switch or clamp, they short circuit
all three phases and connect them to earth at the point of work. This serves two purposes:
• The part where work is carried out is clamped to earth potential. This will also
minimize any voltages that can be induced because of stray magnetic fields
• If there is any inadvertent re-energisation when work is being carried out, it creates a
three-phase metallic short circuit which will cause the circuit protective devices to
operate
In the case of indoor switchgear panels, such earthing is normally carried out at the outgoing
terminals of the feeding cubicle at the time of rendering the circuit dead. But in certain cases, the
work may be carried out in some other place. In that case, a portable earth clamp must be used
close to the point of work, in such a way that it is between the supply side and the point of work.
The correct placement of the clamp involves firstly connecting the earth lead of the clamp,
followed by the line leads one by one. This operation may be performed by first lightly touching
the line clamp to each phase at the point of work, taking due precautions to avoid being exposed to
any arc, in case the point of work is still live.
Where induced voltages may be present, earth clamps should be used at two points to ensure that a
circulating current is set up and the voltage is effectively controlled. The current rating of the
clamp conductor or switch must be adequate to carry the induced current safely with minimum
voltage drop. Otherwise overheating may occur, exposing working personnel to temperature
hazards.
Interlocks operate to remove hazards prior to access. Interlocks disable electrical sources and/or
mechanical hazards. The means for interlocking must be reliable; often, the switch or device must
be cycle-tested. The interlock should only consist of electro-mechanical components and should
not rely on logic circuits or semiconductors. An analysis should show that a single fault cannot
render the interlock circuit inoperable. Should it fail, it must fail in the safe mode (i.e., hazard
locked-out). Other general considerations include:
• All hazards must be removed before the cover can be opened
• The interlock switch cannot be defeated by hand or without a tool
• The door or cover cannot be closed with the switch defeated
Safety interlocks should be fitted to all enclosures to prevent access with conductors LIVE.
Mechanical and electro-mechanical interlocks are preferred for permanent enclosures; the simpler
the method of operation the more effective it is likely to be. Permanent interlock systems should be
positively operated, must be fail safe, and may, as an advantage, have the wiring segregated from
other wiring. For less permanent experiments simple electrical interlocks may be quite adequate.
3.8
Equipment Selection
The common rules for selection and erection of equipment consists of five main criteria:
• Operational conditions and external influence
• Compliance with standard
• Accessibility
• Identification and notices
• Mutual detrimental influences
These requirements are common rules and must be applied to every installation irrespective of its
location and environment. The equipment must, of course, be fit for purpose and suitable for all the
relevant operating conditions as well as for the external influences as shown in table 3.7.
Safety aspects in electrical equipment design and selection 47
Table 3.7
Equipment selection on basis of operation
Aspect
Requirement
Voltage
Equipment shall be suitable for nominal voltage to earth or
between phases as per application.
Current
To be suitable for design current: account to be taken of
capacitive and inductive effects. To be suitable for fault current:
for the duration of the fault.
Frequency
Where frequency affects the characteristic of the equipment, the
rated frequency of the equipment must match that of supply
Power
The power characteristics of the equipment to be suitable for the
duty demands
Compatibility
Equipment not to cause harmful effects to other equipments nor
to the supply
Compliance with standards: In terms of the selection and erection of equipment rule contained in
Regulation, the equipment must be constructed to an acceptable current standard.
The other factors of good installations are:
• Equipment performance: reference to standard specification
• Sound engineering principles and code of practice
• The correct functioning of equipment as specified by user.
The specific guidelines of AS/NZS 3000 in respect of selection of equipment for LV systems are
detailed in a subsequent chapter.
3.9
Role of codes and standards in installation safety
Various standards and codes of practice are available on electrical equipment and installations.
These may be national standards such as ANSI, BS etc., international standards (IEC for example)
or industry-based standards (examples include IEE, IEEE, UL etc.). All such standards and
practices reflect the experience gained over decades of designing and operating electrical
installations and thus attempt to provide guidelines for design and installation of systems, so that
they operate in a trouble-free and safe manner. Apart from these standards, locally applicable codes
and statutory regulations have been put in place by each country, specifically covering their
philosophy and approach to safety in design and installation. By applying these standards and
codes, a designer or installer can ensure satisfactory performance and safe operating conditions.
Appendix-A of AS/NZS 3000 lists out the various codes and regulations to be followed to achieve
the safety objectives of the standard.
3.10 Summary
Many of the requirements for safety in respect of electrical equipment and installations must be
addressed at the design and planning stages, as unsafe situations arising from faulty design or
incorrect selection cannot be easily rectified later. Safety through design has the primary objective
of eliminating hazards that can arise from the use of equipment, both under normal circumstances
as well as in abnormal situations. The main safety features to be incorporated in the design of any
electrical equipment are: preventing electric shock, preventing adverse thermal effects and
providing features to ensure isolation and prevent accidental switching. Insulation plays an
important part in avoiding dangers from both direct and indirect contact. Insulating
materials/components prevent short circuit between live conductors and between live conductors
and the enclosures of equipment. Insulation failures by thermal or voltage stress must be avoided
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Practical Electrical Wiring Standards - AS 3000:2018
by proper design of equipment. External protection devices, such as surge arrestors, must be
provided to prevent failures due to transient over voltages.
Enclosures act as a barrier against direct contact with live parts and by virtue of properly earthing
the enclosures, indirect hazard can be minimized also. They also provide safety against arc faults
and protect live parts from the ingress of dust and water, thereby avoiding insulation failures.
Obstacles to prevent unintentional contact and the placing of a live part out of arm’s reach are ways
in which direct contact can be avoided. Adverse thermal effects are a result of high temperatures
occurring during equipment operation in exposed metallic enclosures and conducting parts.
Adverse thermal effects can be prevented by limiting the temperature of conducting parts as well as
accessible parts, by measures such as the adequate sizing of conductors based on temperature
limits. Harmful thermal effects can further be avoided by limiting the possibility of contact with
parts under high temperature.
One of the major hazards of electrical installations is fire. Appropriate measures must be
incorporated in the design of an electrical installation to prevent, contain, sense and extinguish
fires.
Around 60% of the accidents involving electrical equipment are the results of improper or insecure
isolation. Isolation should ensure that equipment is disconnected fully from all electrical sources
and can be safely maintained. Continued isolation must be ensured until the equipment which has
been previously isolated, is fit to be put back into service. Once a device is properly isolated, it
must be prevented from accidental re-energisation by using mechanical interlocks, padlocking of
operating handles or other safety features. Such facilities should be integrated into the equipment
design itself.
Safety aspects in electrical equipment design and selection 49
4
Earthing of Electrical Systems
In this chapter, we will review the limitations of unearthed systems and the basic theory behind
earthing of electrical installation. The chapter also covers the common earthing methods adopted
at the source end and the load ends and some important safety issues related with earthing like
direct and indirect shock hazards, protective devices operation, thermal capacity of protective
conductors etc. The basic types of earth fault sensing devices and their operations will be
explained. We also will review the common earthing practices recommended and adopted at LV
consumer installations. The chapter will be concluded with the methods adopted for earth fault
detection and the importance of low earth loop impedance, which is one of the important
requirements of AS/NZS 3000.
Learning objectives
•
•
•
•
•
•
•
•
4.1
Unearthed and earthed systems
System Earthing (at the source) and methods adopted
Protective Earthing(at the load ends)
Shock hazards and importance of protective earthing
Protective earth conductors
Common earthing practices in LV consumer ends
Earth loop impedance and its importance.
Detection of earth fault currents
Unearthed and earthed systems
Before going into the earthing methods, we will have a review on unearthed systems and their
limitations in ensuring safety.
4.1.1 Unearthed Systems
As discussed in earlier, providing a reference earth in an electrical system is essential for safe
operation. But there are certain cases in which a system can be operated without such a reference.
By definition, an electrical system which is not intentionally connected to the earth at any point is
an unearthed system. However it should be noted that a connection to earth of sort does exist due to
the presence of capacitances between the live conductors and earth that provides a reference. But
these capacitive reactances are so high that they cannot provide a reliable reference. Figure 4.1
illustrates this point. In some cases the neutral of potential transformer primaries connected to the
system is earthed also thus giving a reference of earth in the system.
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Practical Electrical Wiring Standards - AS 3000:2018
Line
ZL
Load
S
ZN
Neutral
S :
Source of Voltage V
ZL :
Impedance of Line Conductor
to Earth ( Combination of Insulation Resistance and
line to Earth Capacitance )
ZN : Impedance of Neutral Conductor to Earth
Normally : ZL ≈ ZN
Therefore : VL = VN = V / 2
Figure 4.1
A Virtual Earth in an Unearthed System
It may be noted that normally the capacitance values being equal, the lines L1 and L2 are roughly
at a potential equal to half the voltage of the source from the earth. (It is possible to demonstrate
this by measurement of a high impedance device such as an electrostatic type of voltmeter).
The main advantage cited for unearthed systems is that when there is a fault in the system
involving earth the resulting currents are so low that they do not pose an immediate problem to the
system. Therefore the system can continue without interruption which could be important when an
outage will be expensive in terms of lost production or can give rise to life threatening
emergencies.
The second advantage is that one need not invest on elaborate protective equipment as well as
earthing systems, thus reducing the overall cost of the system. (In practice this is however offset
somewhat by the higher insulation ratings which this kind of system calls for due to practical
considerations).
The disadvantages of unearthed systems are as follows.
• In all but very small electrical systems, the capacitances which exist between the
system conductors and the earth can result in the flow of current at the faulted point
which can cause repeated arcing and build up of excessive voltage with reference to
earth. This is far more destructive and can cause multiple insulation failures in the
system at the same instant.
• The second disadvantage in practical systems is to be able to find out the exact
location of the fault, which could take far more time than with earthed systems. This
is because the detection of fault is usually done by means of a broken Delta
connection in the Voltage transformer circuit and to detect exact fault location, a
Earthing of Electrical Systems 51
•
complex system of earth fault protection is required which negates the cost advantage
we originally talked about.
Also a second earth fault occurring in a different phase when one unresolved fault is
present will result in a short circuit in the system.
Due to these overwhelming disadvantages very rarely, if ever, distribution systems are operated as
unearthed. However part of electrical distribution is sometimes done using Separated Extra Low
Voltage systems (SELV) which are essentially unearthed systems of low voltage on a limited scale
with the objective of enhanced safety in vulnerable situations. We will see more about such
systems in later chapters.
4.1.2
Earthed Systems
Figure 4.2 shows the various types of earthing methods that are possible and adopted in ac systems.
System Earthing
UnEarthed
Impedance Earthing
Resistance
Low Resistance
High Resistance
Earthed
Solid Earthing
Reactance
Low Reactance
Tuned High
Reactance
Figure 4.2
Earthing Methods in electrical systems
The diagrammatic representation of these different earthing techniques and the equivalent
impedances from the source to the earth are shown in Figure 4.3.
52
Practical Electrical Wiring Standards - AS 3000:2018
Figure 4.3
Earthing Techniques and Equivalent Impedances
Earthing of Electrical Systems 53
4.2 Supply system (source) earthing or Functional earthing
4.2.1 Solidly earthed
As is evident from the name, a solidly earthed system is one where the neutral of the system is
directly connected to earth without introducing any intentional resistance in the earth circuit.
A solidly earthed system clamps the neutral tightly to earth and ensures that when there is a earth
fault in one phase, the voltage of the healthy phases with reference to earth does not increase to
values appreciably higher than the value under the normal operating conditions.
The advantages of this system are:
• A fault is readily detected and therefore isolated quickly by circuit protective devices.
• It is easy to identify and selectively trip the faulted circuit so that power to the other
circuits or consumers can continue unaffected. (Contrast this with the unearthed
system where a system may have to be extensively disturbed to enable detection of
the faulty circuit).
• No possibility of transient over voltages.
The main disadvantage is that when applied in distribution circuits of higher voltage (5 kV and
above), the very low earth impedance results in extremely high fault currents almost equal to or in
some cases higher than the system’s three phase short circuit currents. This can increase the
rupturing duty ratings of the equipment to be selected in these systems.
Such high currents may not have serious consequences if the failure happens in the distribution
conductors (overhead or cable). But when a fault happens inside a device such as a motor or
generator such currents will result in extensive damage to active magnetic parts through which they
flow to reach the earth.
For these reasons use of solid earthing of neutral is restricted to systems of lower voltage
(380V/400V/480V) used normally in consumer premises. In all the other cases some form of
earthing impedance is always used for reducing damage to critical equipment components.
4.2.2 Earthing through neutral resistance
This is by far the most common type of earthing method adopted in medium voltage circuits. The
system is earthed by a resistor connected between the neutral point and earth. The advantages of
this type of earthing are as follows.
• Reducing damage to active magnetic components by reducing the fault current.
• To minimize the fault energy so that the flash or arc blast effects are minimal thus
ensuring safety of personnel near the fault point.
• To avoid transient over voltages and the resulting secondary failures.
• To reduce momentary voltage dips, which can be caused if, the fault currents were
higher as in the case of a solidly earthed system.
• To have sufficient fault current flow to permit easy detection and isolation of faulted
circuits.
Resistance earthing can again be sub-divided into two categories, viz., high resistance earthing and
low resistance earthing.
High resistance earthing limits the current to about 10 Amps. But to ensure that transient over
voltages do not occur, this value should be more than the system capacitance to earth current. As
such, the applications for high resistance earthing are somewhat limited to cases with very low
tolerance to higher earth fault currents. A typical case is that of large turbine generators which are
directly connected to a high voltage transmission system through a step up transformer. The
capacitance current in generator circuits is usually very low permitting values earth fault currents to
54
Practical Electrical Wiring Standards - AS 3000:2018
be as low as 10 amps. The low current ensures minimal damage to generator magnetic core thus
avoiding expensive factory repairs. Figure 4.4 illustrates a practical case of earthing the neutral of a
generator of this type.
Figure 4.4
Earthing of a Turbine Generator Neutral through a High Resistance
On the other hand, a low resistance earthing is designed for earth fault currents of 100 amps or
more with values of even 1000 amps being common. The value of earth fault current is still far
lower than three phase system fault currents. This method is most commonly used in industrial
medium voltage distribution systems and has all the advantages of transient limitation, easy
detection and limiting severe arc or flash damages from happening.
4.2.3 Earthing with neutral reactance
In this method of earthing, an inductor is used to connect the system neutral to earth. This also
limits the earth fault current which is dependent on the phase to neutral voltage and the neutral
impedance. It is usual to choose the value of the inductor in such a way that the earth fault current
is restricted to a value between 25% and 60% of the three phase fault current to prevent the
possibility of transient over voltages occurring. Even these values of fault current are high if
damage prevention to active parts is the objective.
4.2.4 Resonant earthing using neutral reactor
To avoid the problem of very high earth fault currents with standard/fixed impedances, the method
of resonant earthing can be adopted. Resonant earthing is a variant of reactor earthing with the
reactance value of the neutral reactor chosen such that the earth fault current through the reactor is
equal to the current flowing through the system capacitances under such fault condition. This
enables the fault current to be almost cancelled out resulting in a very low magnitude of current
which is in phase with the voltage. This serves the objectives of low earth fault current as well as
avoiding arcing (capacitive) faults which are the cause of transient over voltages. This type of
earthing is common in systems of 15 kV (primary distribution) range with mainly overhead lines
but is not used in industrial systems where the reactor tuning can get disturbed due to system
configuration changes caused by switching on or off cable feeders (with high capacitive currents)
frequently.
Earthing of Electrical Systems 55
4.3
Protective earthing
Protective earthing refers to the connection of the exposed metal parts (e.g. enclosures of electrical
equipment) as well as extraneous metal parts (like cable support systems) of an installation to earth
usually either in the installation itself or through protective earth conductors to the earthed point of
the supply source system. The basic objectives of protective earthing are as follows.
• To reduce electric shock hazards to personnel.
• To provide a low impedance return path for earth fault currents to the power source
so that the occurrence of fault can be sensed by the circuit protective devices and
faulty circuit can be safely isolated.
• To minimize fire or explosion hazard by providing an earth path of adequate rating
matching the let through energy by circuit protective devices.
• To provide a path for conducting away leakage current (small currents flowing
through electrical protective insulation around live conductors) and for accumulated
static charges.
• To clamp an entire LV installation to as near earth potential as practically possible
when an accidental contact to a higher voltage system occurs (e.g., an internal fault
between MV and LV windings in a distribution transformer). To ensure operation of
protection of the higher voltage system in such an eventuality.
We will discuss the salient aspects of protective earthing in detail below
4.4
Prevention of shock hazards
4.4.1 Impact of shock currents
The human body presents a certain amount of resistance to the flow of electric current. This
however is not a constant value. It depends on factors such as body weight and the manner in
which contact occurs and the parts of the body that are in contact with the earth. Figure 4.5 gives
the resistances of different parts of a human body, through which a current may flow when it
comes in contact with live points inadvertently or under fault conditions.
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Practical Electrical Wiring Standards - AS 3000:2018
RB
RF
RF
RMF
RB : Resistance of human body for current flow between hands,
hand to feet or between feet ( assumed as 1000 ohms )
RF : Resistance of soil below the foot assumed as a circular plate
RMF : Mutual resistance of soil between the feet
Figure 4.5
Resistance of Human Body to Current Flow
If the flow of current through the human body involves the heart muscles, it can produce a
condition known as fibrillation of the heart indicating cardiac malfunction. If allowed to continue,
this can cause death. The value of current flowing in the human body creates different effects based
on its value and duration. Table 4.1 shows typical impacts on humans for specific values of
currents flowing through their bodies.
Earthing of Electrical Systems 57
Table 4.1
Effects of currents through human bodies
Current in mA
Effect on human body
0.5 – 2
Threshold of perception
2 – 10
Painful sensation, increasing with current
10 – 25
Cramp and inability to 'let go'. Increase in blood
pressure. Danger of asphyxiation from respiratory
muscular contraction.
25 – 80
Severe muscular contraction sometimes involving
bone fractures. Increased blood pressure. Loss of
consciousness from heart and/or respiratory
failure.
Over 80
Burns at points of contact. Death from ventricular
fibrillation (uncoordinated contractions of the heart
muscles so that it ceases to pump).
Persons are not normally accidentally electrocuted between phases or phase to neutral but almost
all accidents are phase to earth. Figure 4.6 illustrates the four stages of the effects due to a current
flow through the body:
• Perception – tingling – about 1mA.
• Let-go threshold level – about 10mA.
• Non-let-go threshold level – 16mA.
• Constriction of the therasic muscles – death by asphyxiation and ventricular
fibrillation – about 70 …100mA.
Figure 4.6
Effect of currents through human bodies
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Practical Electrical Wiring Standards - AS 3000:2018
The threshold of time for which a human body can withstand depends on the body weight and the
current flowing through the body. An empirical relation has been developed to arrive at this value.
ts =
SB
IB
2
------------------------------- 4.1
Where,
t s = Duration of exposure in seconds (0.3 to 3 seconds time limits)
I B = RMS current value through the body in amperes
S B = Empirical constant taken as 116 for 50kg and 157 for 70kg body weight
Using the above factors and assuming a normal body weight of 70 kg, it can be calculated that
IB =
0.157
ts
------------------------------- 4.2
Using the above relationship, it can be calculated that an average 70kg body can withstand about
90 milliamperes while for 50kg body it would be about 67 milliaperes both for a maximum
duration of 3 seconds before going to heart fibrillation. However the point to remember is that even
small currents a couple of seconds can lead major accidents which can be common due to touch
potential values increasing to appreciable levels under fault conditions. Hence the current leakage
devices shall be able to sense very low currents in the order of milliamperes and operate very fast
to provide effective protections from shock currents.
A considerable portion of the body resistance is due to the outer skin and any loss of skin due to
burning in contact with electrical conductors can lower the resistance and quickly raise the current
flow to dangerous values. Also, moisture e.g. from bathing also reduces skin resistance by about
half impacting double the current flow for the same voltages. Hence the values of currents given in
this section shall be used with care, as these are based on very normal conditions.
4.4.2
Touch and Step Potential
Electric shock hazard can be either ‘direct’ or ‘indirect’. Direct shock is the condition when a
human body comes into direct contact with a part that is normally live. In this case the current flow
through the body will be the governed by the voltage at the point of contact and earth and
resistance of the human body. Part of the system impedance may also be included in the current
path but this can be neglected since the value is usually negligible in comparison to the body
resistance. Indirect contact is the condition when a potential is applied on a human body by a part
of the system which is not directly in contact with a live part or not expected to be.
First case is when a human body is in contact with an external or extraneous conductive part of an
electrical installation and there is a fault in the system involving earth. For example a person
standing on the earth with his hand touching the earthed metallic enclosure of electrical equipment
and a fault occurs between the live conductor of the equipment and the enclosure.
The other is the case of a potential difference between two points on the earth arising out of an
earth fault in a system which is applied across the two feet when being kept at a distance of about 1
meter. This kind of earth fault usually happens in high voltage electrical switchyards when a live
conductor snaps and falls on the earth. This creates appreciable potential differences which arise
when the high voltage gets dissipated into the soil.
The typical situations explained above may happen in two ways either between two hands or
between one hand and both feet as illustrated in figure 4.7.
Since the human body presents different values of resistance to the flow of electricity in these two
paths, the voltage limits for tolerance of human body are to be calculated individually for both
cases which follow further.
Earthing of Electrical Systems 59
~
~
( a ) Potential applied between two hands
~
( b ) Potential between one hand and both feet
RB
RF
RF
RMF
( c ) EQ. circuit for ( b )
~
RF
~
RMF
RB
RF
( d ) Potential applied between the feet
( e ) EQ. Circuit for ( d )
RB
: Body resistance
RF
: Contact resistance of one foot and ground
RMF : Mutual resistance between both feet
Figure 4.7
Modes of Application of Electric Potential
Case-1: The Resistance of contact with live/exposed conductive part (under fault condition) by
both hands is given by the following relation:
R A = R B + 0.5 (R F + R MF ) -------------------------------- 4.3
Where,
R A = Touch voltage circuit resistance in ohms
R B = Body resistance usually taken as 1000 ohms
R F = Self resistance of each foot to remote earth in ohms
R MF = Mutual resistance between the feet in ohms
Case-2: Resistance offered when the earth contact is through the feet is given by:
R A = RB + 2 ( RF − RMF ) -------------------------------- 4.4
Where, R A is the step voltage circuit resistance in ohms and other factors being same as indicated
under equation 4.3.
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Practical Electrical Wiring Standards - AS 3000:2018
The accidental contact that happens in buildings or other consumer installations is mostly of the
first case. The voltage of tolerance in this mode as calculated using the resistance in case-1 is called
as Touch Potential. The occurrence of the second mode of contact is specific to outdoor electrical
substations with structure mounted equipment. The voltage value arrived at for case-2 is known as
Step Potential.
Since the body resistance is almost fixed under normal conditions, the voltage to which a human
body is subjected is the main factor influencing the current flow through the body in the case of a
direct shock condition. Minimizing of direct shock hazard can be done in two ways
• By using lower voltage systems or
• By making the probability of such contact as low as possible.
The first involves use of extra low voltage systems. The second can be achieved by providing
suitable barriers to prevent accidental contact and by providing adequate clearance between
exposed live parts and work areas so that a person working in the area is not within an ‘arms reach’
of exposed conductors. Where possible, residual current devices sensitive enough to detect
accidental contact (by detecting the leakage current that such a contact causes) can be deployed.
Usually such devices can be put in final circuits feeding low power equipment where possibility of
direct human contact is high (e.g. utility socket outlets, or low capacity domestic circuits) and the
normal leakage current through insulation is negligible.
Indirect shock hazards are minimized by design of commercial, industrial and domestic electrical
installations and their earthing methods with due consideration to touch potential that can arise
during abnormal or fault conditions.
4.4.3
Protective earthing for preventing shock hazards
Protective earthing of electrical equipment is primarily concerned with connecting exposed parts of
the equipment (that are conductive in nature but not normally live) to the earthing system through
conductors known as protective earth conductors. For the earthing to offer effective protection, the
earth fault current flowing (due to internal insulation failure or any other reason) through the
equipment enclosure to the earth return path shall not allow the enclosure voltage to exceed the
Touch Potential. This is also applicable to other parts that normally do not carry electric current.
(Figure 4.8)
Earthing of Electrical Systems 61
Equipment with
Earthed Enclosure
Line
~
Fault
SOURCE
Ig
Neutral
Earthing
Path
with Impedance Zg
VTOUCH
Figure 4.8
Voltage Pattern during Earth Fault
The touch potential to be limited to achieve safety can be calculated by the application of Ohm’s
law i.e. V touch = Ig x Zg, where Ig is the maximum earth fault current that is expected to flow and
Zg is the resistance of the earth circuit. Refer figure 4.9. Ig is dependent on the type of system
earthing adopted and the system voltage.
Figure 4.9
Voltage of enclosure during flow of ground fault current
Hence it is to be understood that for ensuring human safety against indirect shock hazards, the
primary requirement is to limit the value of the voltage appearing on the external conductive parts
62
Practical Electrical Wiring Standards - AS 3000:2018
of electrical equipment. This will ensure that accidental human contact with these enclosures will
not exceed the allowable touch potential which otherwise would result in fatal electrocution or
serious injuries.
4.4.4
Earth potential rise during earth faults
The role of the earth resistance of an installation needs to be properly understood. Most designers
as well as vendors dealing with equipment that are susceptible to electrical noise (such as
computers or communication devices) lay emphasis on low values of earth resistance in
installations. This is not necessarily a factor in deciding the safety of the installation. Let us take for
example, an installation where the source and consumer installation are separated by considerable
distance. In case there is no metallic connection between the protective earth of the consumer and
the source earth point, the earth fault currents tend to flow through the earth mass. This causes an
elevation of earth mass potential at the receiving end. This condition is more relevant to three wire
systems in medium voltage systems where usually metallic earth return paths between source and
receiving equipment are absent, as shown in figure 4.10
Figure 4.10
Earth potential rise during earth faults
However, in most low voltage applications (with some exceptions due to earthing methods) there is
a metallic continuity between the neutral of the source and the protective earth of the installation.
In such cases, the degree of potential rise of the local earth mass will be lower due to the lower
impedance between the enclosure and the Earth. The main point to be remembered (in either case)
is that the potential difference of the enclosure with reference to local earth mass is what essentially
matters in rendering the system safe, regardless of the actual resistance to earth.
The emphasis should be on achieving the lowest loop impedance for earth faults, particularly in LV
installations rather than low earth resistance. This will enable sufficient flow of earth fault currents
and will facilitate their quick detection and isolation as further explained subsequently.
Earthing of Electrical Systems 63
4.5
Protective earth conductors
It is very much important that the protective earth conductor mentioned in the previous paragraph
or any other circuit component, which serves protective function, should be designed to withstand
the resulting fault current without developing excessive temperature or causing sparking at the
joints. This will happen if the joints in the conductor or other bonding connections are improperly
executed. This condition must be true for the magnitude and duration of the current required for
protective devices to operate and isolate the faulty circuit.
It is therefore necessary to ensure good quality workmanship in these installations as otherwise the
high temperatures or sparking may cause fires in the premises where they are installed. Particular
care is needed in the case of installations where hazardous or inflammable materials/mixtures are
present.
Good conductivity and low corrosion are the primary requirements for a good earth conductor. It is
common practice to use copper as earthing conductor, though other materials like aluminium,
galvanized iron, etc are adopted for the protective earthing. AS/NZS 3000 details out calculations
and tables to decide the size of these earth conductors, which will be discussed in a later chapter.
4.6
Earthing practices in low voltage consumer installations
Low Voltage systems supplying to consumer premises are mostly solidly earthed. Protective earth
connection in consumer premises (or extending the supply system’s earth to consumer premises) is
however done in different ways.
4.6.1 Common categories of earthing systems
The common categories of earthing of consumer installations use 2 to 4 alphabets combining the
letters T, N, C and S which denote the following in line with IEC-60364.
• T represents the distribution system is directly connected to earth—at the neutral
point of the supply transformer
• N indicates that the exposed conductive parts are connected to the earthed point of the
distribution system
• C means neutral and protective conductor functions are combined in a single
conductor (the neutral conductor of the distribution system)
• S represents the protective conductor function is separated from the neutral—separate
conductors within the installation.
Accordingly following types of earthing systems are possible in LV consumer installations with
three phase power supply source.
• TN-C system: A system in which a single conductor is provided to have a combined
function of neutral and protective conductor throughout the supply and consumer
installation. Here the neutral is earthed at the source and also along the distribution
points.
• TN-S system: In this system, independent conductors are provided for separate
functions of neutral and protective earth throughout the system.
• TN-C-S system: Here the Supply system combines neutral and earth, but they are
separated out in the installation. A TN-C-S system requires that the PEN conductor
is earthed at multiple points (about every 40m) to reduce the danger created by a
break. In this system it is therefore essential to maintain the connection integrity of
the common neutral-cum-earth conductor.
• TT/IT systems: Here no earth conductor is provided by the supplier and installation
requires its own earth rod (common with overhead supply lines and portable
generator with no earth connection).
Illustrations and more details of these systems are given in a separate appendix of this manual.
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4.6.2
Requirements of TN-C-S system
This is the system commonly adopted by many utility installations including Australia, where it is
named as MEN – multiple earthed neutral – systems. Under the MEN system, the neutral conductor
(PEN) of the distribution system is earthed at the source of supply at regular intervals throughout
the system and at each electrical installation connected to the system. Within the electrical
installation, the earthing system is separated from the neutral conductor and is arranged for the
connection of the exposed conductive parts of equipment.
The MEN system as installed in Australia and New Zealand differs from the IEC system. Both
systems are identical in principle but vary in detail. For further details refer to AS/NZS 61439 and
AS/NZS 3007.
The MEN system and its various parts are illustrated in Figures 4.11 and 4.12. Figure 4.11 shows a
general arrangement and Figure 4.12 an alternative arrangement in an owner or user operated
supply substation installation. IEC 60364 series describes the MEN system as a TN-C-S system
with the letters signifying —
T the distribution system is directly connected to earth—at the neutral point of the supply
transformer;
N the exposed conductive parts are connected to the earthed point of the distribution system—at the
MEN connection;
C the neutral and protective conductor functions are combined in a single conductor (the neutral
conductor of the distribution system);
S the protective conductor function is separated from the neutral— separate conductors within the
installation.
Figure 4.11
Multiple earthed neutral (MEN) system of earthing – General arrangements PEN distribution/TN-C-S
Earthing of Electrical Systems 65
Figure 4.12
Alternative earthing arrangement in an owner or user operated supply substation installation
Like all others systems, TN-C-S does not prevent a fault occurring, but will ensure that the fault
protection device operates quickly when that fault appears. This is a function of the earth loop
resistance. Sufficiently low loop resistance will cause high currents but can be cleared faster. A
lower current could result in a longer duration of fault because of which the energy dissipation
through the point of fault can be quite high and could easily start a fire.
An installation connected to a TN-C-S supply is subject to special requirements concerning the size
of earthing and bonding leads, which are generally larger in cross-section than those for
installations fed by supplies with other types of earthing. Full discussions with the Electricity
Supply Company are necessary before commencing such an installation to ensure that their needs
will be satisfied. Some installations cannot be connected to a TN-C supply due to the danger of
neutral currents flowing from the network to earth via the earthed parts of the installation e.g.
petrol stations or the absence of an equipotential zone e.g. construction sites.
4.6.3
Earth electrodes
The connection to earth mass in an electrical installation is normally achieved by using earth
electrodes which are buried or driven into the earth mass for sufficient depth of not less than 2 to 3
meters. Several types of earth electrodes using different materials, physical configurations and
designs are in wide spread use and follow usually the local standards that govern electrical
installations. A typical earth electrode arrangement is shown in figure 4.13.
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Practical Electrical Wiring Standards - AS 3000:2018
Cover Slab
Ground Level
For water addition
From earth
bus
Brick
Chamber
Soil around
earth pit
Galvanised
steel electrode
3 M Long ; 40 mm Dia
Pit filled with
hygroscopic mixture
( Charcoal + salt )
Figure 4.13
A typical earth electrode used in electrical installations
Several earth electrodes are bonded/ interconnected together to form an earth grid and are usually
provided at substations for achieving satisfactory results. The general requirements that influence
the choice of earth electrodes are as follows.
• The need for achieving minimum acceptable earth resistance appropriate to the
installation involved.
• The need to maintain this resistance all round the year in varying climatic conditions.
• The type of soil where the earthing is carried out.
• Presence of agents that can cause corrosion of elements buried in earth.
• The performance of such electrodes (considering the ground resistance of the
electrode as an indicator) depends on the type of soil, its composition, conductivity,
presence of moisture, soil temperature, etc. Several ground electrodes bonded
together to form a cluster are usually provided for achieving satisfactory results.
The electrode design and methods of installation will be dependent on these requirements. Earth
resistance of an earthing system will vary at different times depending on conditions such as
temperature and moisture content of the soil. Measurement of soil resistivity should be carried out
where values are critical and used in the design of earthing system of an installation and verified by
earth resistance measurements on completion of an installation. In areas of high soil resistivity,
appropriate soil treatment should be undertaken to achieve desirable values of earth resistance.
It will also be evident from the above discussions that being a critical factor in the safety of
installation and personnel, the earthing system will have to be periodically monitored to ensure that
its characteristics do not drift beyond acceptable limits. This forms part of initial verification and
certification of installations as well as periodic inspection.
Earthing of Electrical Systems 67
An appendix of this manual provides further insights to the earthing and earth electrode practices
recommended/ followed in some other National codes.
4.6.4
Equipotential bonding
Equipotential bonding refers to the method of physical interconnections between various exposed–
conductive-parts and extraneous conductive parts to maintain all of them at substantially the same
potential.
Earthed equipotential zone is a zone within which exposed-conductive- parts and extraneousconductive- parts are maintained at substantially the same potential by bonding, such that, under
fault conditions, the difference in potential between simultaneously accessible exposed and
extraneous conductive parts will not cause electric shock.
We have seen earlier in this chapter that when an earth fault takes place in an installation, the
external conducting surfaces of the installation and the earth mass in the vicinity may attain higher
potential with reference to the source earth. There is thus a possibility that a dangerous potential
may develop between the conducting parts of non electrical systems including building structures
and the external conducting parts of electrical installations as well as the surrounding earth. This
may give rise to undesirable current flow through paths that are not normally designed to carry
current (such as joints in building structures) and also cause hazardous situations of indirect shock.
It is therefore necessary that all such parts are bonded to the electrical service earth point of the
building to ensure safety of occupants. This is called equipotential bonding.
Bonding is the practice of connecting all accessible metalwork - whether associated with the
electrical installation (known as exposed-conductive- parts) or not (extraneous- conductive- parts) to the system earth. In a building there are typically a number of services other than electrical
supply that employ metallic connections in their design. These include water piping, gas piping,
HVAC ducting and so on. A building may also contain steel structures in its construction.
There are two aspects to the equipotential bonding; the main bonding where services enter the
building and supplementary bonding within rooms, particularly kitchens and bathrooms. Main
bonding should interconnect the incoming gas, water and electricity service where these are
metallic but can be omitted where the services are run in plastic as is frequently the case nowadays.
Internally, bonding should link any items which are likely to either be at earth potential or which
may become live in the event of a fault and which are sufficiently large that they can contact a
significant part of the body or can be gripped. Small parts, other than those likely to be gripped, are
ignored because the instinctive reaction to a shock is muscular contraction which will break the
circuit.
Supplementary equipotential bonding is normally recommended in the systems as automatic
disconnection by itself cannot ensure safety because the time required for disconnection exceeds
safe limits for humans. The local supplementary equipotential bonding reduces the potential
difference to lower values thus making the system much safer. Figure 4.14 illustrates this
condition.
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Practical Electrical Wiring Standards - AS 3000:2018
Exposed Conductive
Enclosure
Line
Source
Extraneous
Conductive
Part
Ia
Neutral
R
Supplematery Equipotential
Bonding
Main Equipotential Bonding
Rg
Protective Earth Conductor
Figure 4.14
Equivalent circuit with equipotential arrangement
In each electrical installation, main equipotential bonding conductors (earthing wires) are required
to connect the following to the main earthing terminal of the installation.
• metal water service pipes
• metal gas installation pipes
• other metal service pipes and ducting
• metal central heating and air conditioning systems
• exposed metal structural parts of the building
• lightning protection systems
It is important to note that the reference above is always to metal pipes. If the pipes are made of
plastic, they do not call for bonding.
If the incoming pipes are made of plastic, but the pipes within the electrical installation are made of
metal, the main bonding must be carried out. The bonding is being applied on the customer side of
any meter, main stopcock or insulating insert and of course to the metal pipes of the installation.
Such bonding is also necessary between the earth conductors of electrical systems and those of
separately derived computer power supply systems, communication, signal and data systems and
lightning protection earthing of a building. Many equipment failures in sensitive computing and
communication equipment are attributable to the insistence of the vendors to keep them separated
from the electrical service earth. Besides equipment failures such a practice also poses safety
hazards particularly when lightning discharges take place in the vicinity. In such cases large
potential difference can arise for very short periods between metal parts of different services unless
they are properly bonded.
Earthing of Electrical Systems 69
Lightning strikes can lead to flashovers if a lower impedance route to earth is available through the
electrical system. Bonding is therefore required for protection of structures from Lightning.
Sometimes the bonding is reinforced by additional bonding higher up the structure.
Supplementary or additional equipotential bonding (earthing) is required in locations of increased
shock risk. In domestic premises the locations identified as having this increased shock risk are
rooms containing a bath or shower (bathrooms) and in the areas surrounding swimming pools.
However there is no specific requirement to carry out supplementary bonding in domestic kitchens,
wash rooms and lavatories that do not have a bath or shower. That is not to say that supplementary
bonding in a kitchen or wash room is wrong but it is not necessary.
Supplementary bonding is carried out to the earth terminal of equipment within the bathroom with
exposed-conductive part. A supplementary bond is not run back to the main earth. Metal window
frames are not required to be supplementary bonded unless they are electrically connected to the
metallic structure of the building. Metal baths supplied by metal pipes do not require
supplementary bonding if all the pipes are bonded and there is no other connection of the bath to
earth. All bonding connections must be accessible and shall preferably be labeled to caution against
accidental disconnections.
Certain types of non-electrical machinery and equipments can cause a build up of static charge
during their operation and this charge accumulates on the surface of the equipment parts (for
instance, a flat rubber belt around two metal pulleys, which is a very common type of motive
power transmission, generates a lot of static electricity). When a sufficient amount of charge is
built up, a spark-over can occur between the charged part and any grounded body nearby. Such
spark-over carries sufficient energy that can cause explosions in hazardous environments and fires
in case combustible materials are located nearby. It is therefore necessary to provide bonding of the
parts by suitable metallic connections to earth where charge build up can occur. Bonding prevents
such build-up static charges which can also lead to shock hazards.
4.7
Earth fault loop impedance
In the study above, it has been observed that the impedance of the earth system shall be limited to
ensure proper detection of earth fault currents. The impedance of the earth fault current loop
starting and ending at the point of earth fault is termed as the earth fault loop impedance. The earth
fault loop impedance comprises the following starting from the point of fault.
• The circuit protective conductor
• The consumers earthing terminal and earthing conductor
• The metallic or earth return path impedance as applicable
• The path through the earthed neutral point of the transformer
• The transformer winding and
• The phase conductor from the transformer to the point of fault
When a fault to an enclosure takes place in any electrical equipment, the return path through the
earth mass alone is insufficient to operate the protective devices such as over-current release or
fuses. This is so because the impedance between the enclosure and the earth mass is usually high
enough to severely restrict the flow of fault currents, particularly in low voltage systems. In such
cases it is imperative that a low impedance earth return path to the source is available, so that fault
current of adequate magnitudes to cause operation of protective devices is ensured. The circuit
protective conductor fulfills this function of a low impedance connection. Figure 4.15 explains this
point.
70
Practical Electrical Wiring Standards - AS 3000:2018
Equipment
Line
S
V
Neutral
IG1
RGS
RGL
(A) Grounding Path is completed through Earth
Line
G
Neutral
Metallic Grounding
Path (RM)
Ig2
RGL
RGS
(B) Grounding path completed through metallic connection of very low impedance
IG1 = V/(RGL+RGS)
, IG2 = V/ {RM* (RGL+RGS)} / (RM+RGL+RGS)
IG1 will be very low because RGL+RGS may be of the order of a few ohms.
IG2 will be much higher because of much lower parallel
impedance(RM) metallic ground return path
Figure 4.15
Earth Return Path and its importance.
Earthing of Electrical Systems 71
4.8
Sensing of earth faults
Figure 4.16 a to c shows the common methods adopted for sensing earth fault currents. One or
more of these approaches are used depending on the system importance and the descriptions of
these methods are given below.
A
Neutral
B
C
R
(a)
A
Neutral
B
C
R
(b)
A
Neutral
B
C
R
(c)
Figure 4.16
Earth Fault detection circuits
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Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
4.9
In case the power supply source (such as the transformer) is a part of the system, a
current transformer (CT) and relay is provided in the earth connection of the neutral
of the transformer. (Figure 4.16 a). This is possible in substations and large
commercial/industrial distribution systems.
By a single current transformer enclosing all phase and neutral conductors (called as
core balance or zero sequence CT). Such a transformer detects the earth fault currents
and can operate a sensitive relay. (Figure 4.16 b). Its principle is used in residual
current devices recommended in AS/NZS 3000 for LV consumer installations which
will be described later. This device is capable of sensing very low value of earth
fault/earth leakage currents and trips the circuit faster. This device can sense the
current flowing when a human body comes into direct contact with a live part of the
insulation.
By individual current transformer in phase and neutral conductors and providing a
relay in summation circuit to detect unbalance between the sum of phase currents and
the return neutral current. (Figure 4.16 c)
Summary
Though unearthed systems can be adopted, in majority of the installations it is always a practice to
implement earthed systems. There are different types of earthing systems followed at source using
low/ high resistance, reactors, etc as well as solid earthing without any of these devices. The
impedances in the neutral circuits basically limit the earth fault current but however solid earthing
is the preferred choice for LV consumer premises for better safety. It is understood that a small
flow of currents of more than 25 milli-amperes can lead to fatal death for humans and hence it is
very important to ensure shock protection by use of proper earthing methods and devices. The
protective earthing at load ends ensures that the touch potential values of the enclosures are limited
to safe limits under fault conditions and helps to minimize the effects of direct and indirect contacts
and associated shock hazards.
There are different types of earthing practices adopted in LV installations. TN-C-S system with
multiple earthing is usually adopted in utility consumer installations with common protective earth
and neutral (PEN) conductors from the source and independent neutral and earth within the
installation.
The indirect shock hazards can be avoided by minimizing touch potential values with properly
designed protective earthing. Protective earthing is to be carried out using adequately sized
protective earth conductors having good conductivity and non-corrosive characteristics. The sizes
of the earth conductors are usually standardized and given in the regulations for different loads and
these can also be decided using established calculation procedures for known fault current values
and duration. The equipotential bonding of various external parts is also a common practice to
ensure better safety and avoid static charge buildup in addition to keeping low earth impedance.
Earth potential may get elevated during earth fault conditions but it may not be sufficient to drive
the current through the devices which are provided for earth fault detection, if the impedance is
high. Hence there is a need to keep the earth loop impedance as low as possible for better detection
of earth fault currents and to ensure early disconnection. There are established methods to sense
earth faults through direct measurement or by unbalance in the phase and neutral currents and are
incorporated based on distribution system size and its importance.
Earthing of Electrical Systems 73
5
Section 1 - Scope, Application and
Fundamental Principles
This chapter will review the contents covered in section-1 of the AS/NZS 3000. First we will go
through the definitions for a few important electrical terms applicable in the electrical systems
outlined and used in the standard. This will be followed by the discussions on the major hazards
associated with the electrical systems and their effects on the installations and safety of the
consumers. We will also discuss the guidelines given in this section that are to be followed by the
designers and contractors in design, selection and installation of the electrical systems for
preventing the likely dangers associated with these hazards to ensure safety for people and
properties. The applicable clause numbers of AS/NZS 3000 are identified in the respective section
headings (here and in all subsequent chapters) as guidance for quick reference to the standard.
Learning objectives
•
•
•
•
•
•
•
•
5.1
Scope and application areas
Definitions
Protection for safety
Design of an electrical installation
Selection of electrical equipment
Installation of electrical equipment
Verification and testing
Compliance requirements
Scope and application areas (clauses 1.1, 1.2)
Section-1 of AS/NZS standard specifies the basic compliances to be necessarily met by the LV
electrical installations to ensure high level safety for the personnel, livestock and properties in close
proximity to these installations by applying the stipulations given related to the following activities.
• Design of electrical installations
• Selection and installation of electrical equipment forming part of such electrical
installations
• Procedures for construction and verification of electrical installations
The rules in this section are basically intended:
• To protect persons, livestock, and property from hazards likely to arise from an
electrical installation when under use
74 Practical Electrical Wiring Standards - AS 3000:2018
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•
As the guidelines by following which the electrical installation will function correctly
for the desired application
For application through legislative requirements, made in each State and Territory of
Australia and in New Zealand, concerned with the safety of electrical installations by
relevant statutory authorities
The principal application of this Standard is to the electrical installations in use or being established
in all types of premises and land used by electricity consumers. Nevertheless, at the discretion of
competent personnel, these rules can be referenced to or applied through legislative or other
requirements in matters such as
• Safety of work places (like Occupational Health & Safety legislation and associated
codes)
• Safe design and construction of buildings (like building codes of Australia and New
Zealand)
• Electricity generation, transmission and distribution systems (Power plants,
transmission/distribution lines, substations, etc)
• Safe connection to electricity distribution systems. (Like service rules and conditions
stipulated by local electricity distributors for supply of power)
• Qualifications of electricity workers
5.2
Definitions (Clause 1.4)
In the earlier chapters we have come across many terms commonly used in electrical distribution
systems defining the safety, protection and associated features. In section-1 of AS/NZS 3000 most
of the common terms used in the standard are defined for the purpose of better clarity and
understanding by all those who are concerned with electricity distribution and its use. In this part of
the chapter we will have a brief review of these definitions, with further explanations for study/
guidance.
Accessible, readily: Capable of being reached quickly and without climbing over or removing
obstructions, or using a movable ladder, and in any case not more than 2.0 m above the ground,
floor or platform.
Active conductors: Any conductor that is maintained at a difference of potential from the neutral
or earthed conductor. In a system that does not include a neutral or earthed conductor, all
conductors are considered to be active conductors.
Aerial conductor: Any stranded conductor (including aerial bundled conductors) that is supported
by insulators or purpose-designed fittings above the ground and is directly exposed to the
weather.
Appliance: In this standard the term appliance refers to a power consuming device A consuming
device, other than a lamp, in which electricity is converted into heat, motion, or any other form of
energy, or is substantially changed in its electrical character..
• Appliance, fixed: An appliance that is fastened to a support or otherwise secured in a
•
•
•
•
specific location.
Appliance, hand-held: A portable appliance intended to be held in the hand during
normal use, the motor, if any, forming an integral part of the appliance.
Appliance, Portable: Either an appliance that is moved while in operation or an appliance
that can easily be moved from one place to another while connected to the supply.
Appliance, stationary: Either a fixed appliance or an appliance having a mass exceeding
18 kg and not provided with a carrying handle.
Arm's reach: A zone extending from any point on a surface where persons usually stand
or move about, to the limits that a person can reach with the hand in any direction
without assistance (e.g. tools or ladder) Figure 4.1 is the extract from the AS/NZS
Section 1- Scope, Application and Fundamental Principles 75
3000 with dimensions identified in meters defining the arm’s reach zone, without
considering tools that could be used in the hand to reach little further. It can be
noted that these zones are generally matching the IEE regulation definition for the
same. Please take note of the difference among the term “accessible” defined
earlier, “arm’s reach” as well as the three different dimensions 2.5m, 1.25m and
0.75m in the figure based on the possibility of arm’s ability to reach a point in a
particular direction, to have a full understanding on the various possibilities of an
exposure to a hazardous situation under normal conditions.
Figure 4.1
Zone of arm’s reach (Source: AS/NZS: 3000, Figure 1.1)
•
Barrier: A part providing basic protection from any usual direction of access.
o
o
•
Basic insulation (Clause 1.4.73 Insulation system)
Basic protection (Clause 1.4.97 Protection, basic)
Circuit-breaker: A switch suitable for opening a circuit automatically, as a result of
predetermined conditions, such as those of overcurrent or undervoltage, or by some
form of external control.
(In the following definitions the term ‘equipment’ basically refers to electrical equipment operating
within an enclosure. In the standard, the electrical equipments are broadly classified as class I, class
II and class III to imply the kind of protection offered by the respective equipments over and above
the insulation on the live parts)
•
Class I equipment: Equipment in which protection against electric shock does not rely
on basic insulation only, but which includes an additional safety precaution in that
accessible conductive parts are connected to the protective earthing conductor in the
electrical installation in such a way that accessible parts cannot become live in the event
of a failure of the basic insulation.
•
Class II equipment: This type of equipment shall include additional safety
precautions for the live parts like double insulation or reinforced insulation. This
class is further sub divided in the standard as below.
76 Practical Electrical Wiring Standards - AS 3000:2018
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•
•
Insulation-encased Class II equipment: Equipment having durable and substantially
continuous enclosures of insulating material enveloping all metal parts, with the
exception of small parts, such as nameplates, screws and rivets, etc
Metal-encased Class II equipment: Equipment having a substantially continuous
metal enclosure and double insulation used throughout and adopting reinforced
insulation wherever application of double insulation is not viable.
Class III equipment: In this type, protection against electric shock relies on supply at
SELV (separated extra-low voltage - definition follows later) and where voltages
higher than SELV are not generated. It usually refers to equipments operating with
SELV supply that is considered inherently safe against shocks.
Contact, direct (Direct contact): A person getting in contact with a conductor or conductive part
which is live in normal service. Figure 1.2 below is the drawing from the standard explaining this
definition. Basic protection (protective earthing) is mandatory at these parts.
Figure 4.2
Direct contact (Source: Figure 1.2, AS/NZS: 3000 Wiring Rules)
Contact, indirect (indirect contact): Contact with a conductive part which is not normally live but
becomes live under fault conditions (due to insulation failure or some other cause). Figure 4.3
below shows the possible indirect contact with a live part. Faulty protection (of the appropriate
type) is recommended for parts that are expected to face such breakdowns.
Figure 4.3
Indirect contact (Source: Figure 1.3, AS/NZS 3000 Wiring Rules)
Section 1- Scope, Application and Fundamental Principles 77
•
Cord, flexible: Usually refers to a flexible cable but with conductors not exceeding 4
sqmm or 0.31mm diameter and also having not more than 5 cores.
•
Current, overload (Overload current): An overcurrent occurring in a circuit that is
electrically sound.
•
Current, fault: A current resulting from an insulation failure or from the bridging of
insulation.
•
Current, short-circuit: A fault current resulting from a fault of negligible impedance
between live conductors having a difference in potential under normal operating
conditions. The fault path may include the path from active via earth to the neutral.
•
Damp situation: A situation in which moisture is either permanently present, or
intermittently present to such an extent as would be likely to impair the effectiveness
or safety of an electrical installation that complies with this standard for ordinary
situations. Normally refers to locations that are using water or a like substance.
•
Earthed situation: A situation wherein there is a reasonable chance of a person
touching exposed conductive parts and, at the same time, coming into contact with
earth or with any conducting medium that may be in electrical contact with the earth
or through which a circuit may be completed to earth..
•
Earth Fault-loop impedance: The impedance of the earth fault-current loop (active-toearth loop) starting and ending at the point-of-earth fault.
•
Equipotential bonding: Electrical connections intended to bring exposed conductive
parts or extraneous conductive parts to the same or approximately the same potential,
but not intended to carry current in normal service.
•
Exposed conductive part: A conductive part of electrical equipment that—
 can be touched with the standard test finger as specified in AS/NZS 3100; and
 is not a live part but can become live if basic insulation fails.
•
Extraneous conductive part: A conductive part that does not form part of an electrical
installation but that may be at the electrical potential of a local earth.
NOTE: Examples of extraneous conductive parts include the following:
(a) Metal waste, water or gas pipe from outside.
(b) Cooling or heating system parts.
(c) Metal or reinforced concrete building components.
(d) Steel-framed structure.
(e) Floors and walls of reinforced concrete without further surface treatment.
(f) Tiled surfaces, conductive wall coverings.
(g) Conductive fittings in washrooms, bathrooms, lavatories, toilets, etc.
(h) Metallized papers.
•
Main earthing conductor: A conductor connecting the main earthing terminal/
connection or bar to the earth electrode or to the earthing system of the source of
supply.
•
Multiple earthed neutral (MEN) system: A system of earthing in which the parts
of an installation, required under this Standard to be earthed, are connected to the
general mass of earth and, in addition, are connected within the installation to the
neutral conductor of the supply system or the PEN conductor.
78 Practical Electrical Wiring Standards - AS 3000:2018
• Protective earthing conductor: An earthing conductor, other than a main earthing
conductor, intended to carry earth fault currents and connecting any portion of the
earthing system to the portion of the electrical installation or electrical equipment
required to be earthed, or to any other portion of the earthing system.
• Residual current device (RCD) - A device intended to isolate supply to protected
circuits, socketoutlets or electrical equipment in the event of a current flow to earth
that exceeds a predetermined value.
• Safety service: A system or component that operates to identify an emergency, or is
intended to operate during an emergency, and is primarily associated with the safety of
persons evacuating a building, fire-fighting operations or fire suppression.
• SELV (separated extra-low voltage): An extra-low voltage system that is electrically
separated from earth and from other systems in such a way that a single fault cannot
give rise to the risk of electric shock.
•
Switchgear: Equipment for controlling the distribution of electrical energy or for
controlling or protecting circuits, machines, transformers, or other equipment. Usually
incorporates isolating devices like circuit breaker, isolator, etc.
•
Touch current: Electric current which passes through a human body or an animal body
when that body touches one or more accessible parts of electrical equipment or an
electrical installation, under normal or fault conditions (Refer figures 4.2 and 4.3).
•
Touch voltage: Voltage appearing on when two or more accessible parts are touched
simultaneously.
•
Voltage: Differences of potential normally existing between conductors and between
conductors and earth. The voltages are classified as below in AS/NZS 3000.
 Extra-low voltage - not exceeding 50V ac or 120 V ripple-free dc.
 Low voltage - exceeding extra-low voltage, but not exceeding 1000 V ac or 1500 V
dc.
 High voltage - exceeding low voltage.
Section 1- Scope, Application and Fundamental Principles 79
5.3
Protection for safety (Clause 1.5)
The standard identifies three major types of risk associated with use of electricity Viz.,
• Shock current
• Excessive temperatures
• Explosions
Section 1, AS/NZS 3000 stipulates rules that are to be applied in electrical installations to
safeguard the persons, livestock and properties against the above three major risks. We will see the
probable reasons for getting exposed to these risks and hazards connected with them.
Shock current: It represents the current that can accidentally flow in human bodies and live stock
due to either direct contact under normal service or indirect contact under fault conditions, causing
fatal and/or major organ failures.
Excessive temperatures: The excessive temperatures can cause burns, fires and other injurious
effects unless controlled or safeguarded. It is most likely that persons, equipment and materials
adjacent to electrical equipment can be subjected to the following harmful effects because of the
heat developed by electrical equipment or thermal radiation.
• Combustion or degradation of materials
• Burns
• Impairment of the safe function of installed equipment.
Explosions: When electrical equipments are operated in areas where explosive gases or dusts may
be present, the sparking associated can cause ignition of surrounding gases or dusts leading to
explosions that can cause irreparable damage to all persons and materials located close to such
areas.
The main remedy or action needed to safeguard against the above hazards is providing suitable
control and isolation devices that are proven to defend such hazards and can help in maintenance of
electrical equipment, without getting exposed or affected under such hazards. The isolation device
can completely isolate the equipment from all electricity sources external to the equipment, so that
the main cause (power) of any of the three hazards is removed at the point of hazard. Control
devices shall be designed and arranged in such a way to satisfy the following two main
requirements
• They shall be separate/ independent from the control of other equipment
• They can not be unintentionally interrupted by the operation of some other
equipment.
AS/NZS 3000 recommends the following guidelines to be adopted to ensure safe design of the
installation to safeguard the people and installation from these three common hazards of electricity
use.
5.3.1 Shock currents
Shocks are effects felt by humans and live stock when currents flow through their bodies. Shock
currents can flow with direct as well as indirect contacts. The standard recommends the
implementation of the following to prevent direct and indirect contacts.
Direct contact prevention to safeguard against shock: The electrical equipment and systems shall be
provided with either one or any combination of the following methods to prevent direct contact
thereby avoiding shock currents.
• By providing insulation capable of withstanding the mechanical, chemical, electrical
and thermal influences to which they may be subjected in service which can be
removed by destruction only (It is to be noted that though Paints, varnishes, enamels
80 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
or similar products offer some insulation, these are not considered as insulation to
protect against direct contact).
By providing barriers or enclosures that are provided with adequate degree of
protection (IP classification) enclosing the live parts to prevent contact with live
parts. The barriers shall be installed such a way that these can not be unintentionally
removed except by means of a key or a special tool or an interlocking device that can
isolate the live supply before opening the barrier. Barriers and enclosures shall be
firmly secured and shall have adequate stability and strength to withstand any
appreciable distortion that might be caused by the stresses likely to occur in normal
operation, including external influences, in such a way that the required degrees of
protection and separation from live parts are maintained and unaffected.
By providing obstacles between live parts and the accessible parts. However as per
the standard, the obstacles can be acceptable only for installations under direct
control or direct supervision of competent persons as means for safeguarding against
unintentional contacts with live parts. These are not effective for intentional contacts.
By placing the equipment out of reach to prevent direct contact under normal
conditions. This is also recommended only for installations under direct control or
direct supervision of competent persons.
While on this subject, it is once again stressed that RCD’s are not considered to provide basic
protection against direct contacts but only a mean to isolate supply after a direct contact initiates an
earth leakage current.
Protection methods against possible shocks from indirect contacts are basically referred as Fault
protection methods and following are the recommended methods for achieving the same.
• By means of automatic disconnection of mains supply on the occurrence of a fault
that can cause a current flow through a body likely to get in contact with live parts,
equal to or greater than the shock current of around 15 to 30 milli-amperes or when
the touch voltage exceeds 50 V ac or 120 V ripple-free dc. AS/NZS Rules specify a
maximum disconnection time under faults for final subcircuits that supply sockets/
handheld portable equipment for 230/400V applications shall be 0.4 seconds for
effective protection against shocks. RCD’s are accepted means to provide the fault
protection (against indirect contacts).
• By having double or reinforced insulation (Class II equipment) or equivalent
insulation so that the flow of fault current passing through a body can be prevented.
• By electrical separation of the particular circuit to avoid shock current flow under
faults. Live parts of a separated circuit shall not be connected at any point to earth or
to another circuit to achieve this protection.
• By limiting the fault current that can pass through a body to a safe value lower than
the shock current.
Section-1 stipulates that automatic disconnection time in 230/400V systems shall be less than 0.4
seconds for final subcircuits that supply the following:
• socket-outlets having rated currents not exceeding 63 A
• hand-held Class I equipment
• Portable equipment intended for manual movement during use.
The disconnection time can be extended to a maximum of 5 seconds for circuits other than the
above including submains and final subcircuits supplying any fixed or stationary equipment.
Separated extra-low voltage (SELV) or protected extra-low voltage (PELV) systems are also
accepted as means of providing both basic and fault protection against shocks under some specific
conditions like proper segregation from a higher voltage circuits than for which they are rated.
More details about SELV and PELV are given in section-2 of the standard and will be covered
later.
Section 1- Scope, Application and Fundamental Principles 81
5.3.2
Thermal effects
Though heat is generated as part of electricity consumption in most of the equipments, the same
exceeding acceptable limits is a major hazard to the persons, livestock and the installations. Table
5.1 highlights some of the effects of the temperatures/ burns on human bodies based on the
duration.
Table 5.1
Effects of higher temperatures on human bodies
Skin temperature
Time duration
Effect on skin
1100 F
6 hours
Cell breakdown begins
1580 F
1 second
Complete cell destruction
1760 F
0.1 second
Curable burn
2050 F
0.1 second
Incurable burn
Unacceptable thermal effects due to electricity use can be prevented by properly arranging the
equipments like keeping them away from external heat sources and also by providing adequate
ventilation in the areas where such equipments are being operated. These practices will ensure that
design operating temperature of the equipment/ system is never exceeded. Following are some of
the factors recommended for reducing the thermal effects from the electricity.
Preventing fire propagation: Designing and/or executing an installation shall be carried out in such
a way that equipment arrangement inside premises shall NOT
• obstruct escape routes, either directly or by the products of combustion; or
• contribute to, or propagate a fire; or
• attain a temperature high enough to ignite adjacent material; or
• adversely affect means of egress from a structure.
Prevention of high currents: Providing suitable protection from overcurrent and earth fault current
by either limiting such currents and/or ensuring automatic disconnection beyond permissible
currents.
Prevention of voltage effects: Protection against abnormal voltages due to faults with circuits
operating at different voltages and/ or voltages caused due to induction effects. The protection
against thermal effects due to voltages is normally achieved by segregation between different
voltage circuits and also by providing sensing devices that automatically disconnect on detecting
overvoltages. (Protection against overvoltage, like the one originating from lightning or from
switching operations, is not a requirement of this Standard).
Prevention of induction effects: It is also necessary that disconnected, redundant or unused
conductors associated with conductors that run inside the distribution systems/ equipment shall be
terminated and protected at both ends in the same manner as is required for live conductors to
avoid induction effects due to such unused conductors leading to temperature rise in the systems.
5.3.3
Other safety protection methods
AS/NZS 3000 requires that the design and selection of equipment shall also consider the following
protection features for the safety of personnel involved with such installations.
• Protection against injury from normal and unexpected mechanical movements of
equipments, generally by means of emergency isolation switches
82 Practical Electrical Wiring Standards - AS 3000:2018
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5.4
Protection against external influences like exposure to weather, water, flora, fauna,
seismic activity, excessive dampness, corrosive fumes, galvanic action, accumulation
of dust, steam, oil, temperature, explosive atmospheres, vibration or any other
influence to which the electrical installation may be exposed under the conditions of
its use leading to insulation failures, short circuits, etc.
Design of an electrical installation (Clause 1.6)
The design of the installation covering electrical equipments, accessories and their interconnections
shall take care of the following factors.
• Shall protect persons, livestock and property from harmful effects caused by the
electricity under use
• Shall function correctly as intended
• Shall connect, operate safely and be compatible with the electricity distribution
system, or other source of supply to which the electrical installation is to be
connected
• Shall minimize inconvenience to the people in the event of a fault
• Shall facilitate safe operation, inspection, testing and maintenance of the equipment
For reliable, continuous and correct operation during its service life, the equipment design shall
take into account the following supply characteristics, as may be applicable:
• Nature of supply, ac or d.c
• Nature and number of conductors, like Phase, neutral and protective earthing
conductors for ac. and equivalent conductors for dc
• Nominal Voltage and voltage tolerances. (The usual tolerances applicable for
Australia for 230/400 V is +10% to −6% as per AS 60038 and for New Zealand it is
230/400 V +6% to −6% in line with IEC 60038)
• Frequency and frequency tolerances (normally +/- 5%)
• Maximum current that can be supplied (by the source/system/ interconnecting board).
• Prospective short-circuit current (for safe breaking of the circuit under fault
conditions)
• Protective measures inherent in the system installation, e.g. MEN earthing system
• Limits on the use of equipment (Duty cycle, duration, etc)
• Harmonic current or other limitations
The below factors shall be calculated or alternatively assessed in approved manner / established
practices and duly considered in the system design as appropriate to ensure that the installation
shall be able to meet its expected performance requirements.
• Maximum demand of the total installation in amperes or kW to decide the capacity of
conductors, back-up protection, etc
• Voltage drop upto the terminals of the equipment so that same can be limited so as
not to affect the equipment functioning/ usage
During distribution design, it is necessary that every electrical installation shall be divided into a
number of circuits so that following objectives are achieved by such arrangements:
• To avoid danger and minimize inconvenience in the event of a fault; and
• To facilitate safe operation, inspection, testing and maintenance
5.5
Selection and installation of electrical equipment (Clause 1.7)
AS/NZS 3000 recommends that the following aspects shall be duly taken into consideration in the
selection and installation of electrical equipment to be operated in an installation.
• It shall operate in a safe and reliable manner and
• It shall be able to perform under reasonably expected abnormal conditions like
overload, fault or external influences ensuring that these conditions do not cause a
danger like electric shock, fire, high temperature or physical injury to the persons and
live stock
Section 1- Scope, Application and Fundamental Principles 83
•
electrical equipment shall be installed in accordance with the requirements of this
Standard and the additional requirements as specified in the manufacturer’s
instructions
Installation work practice
Electrical equipment shall be installed in accordance with safe and sound work practices,
including the following:
a) Appropriate construction and operating characteristics of equipment to protect against
mechanical, environmental or other external influences
b) Installation instructions provided by the equipment supplier
c) Adequate and safe access or working space is provided to equipment requiring operation
or maintenance
d) Adequate strength and durability of fixings, fastenings and supports
e) Particular needs of the user/operator
f) Installation wiring conductors shall be clearly identified to indicate their intended function
as active, neutral, main earthing, protective earthing or equipotential bonding conductors
Conductors with a green or green/yellow (G/Y) combination core insulation colour or sleeving
colour are strictly reserved for identifying the main earthing conductor, protective earthing (PE)
conductor, or the equipotential bonding (EPB) conductor.
Live conductors shall not be insulated or sheathed with green, yellow or green/yellow
combination colours in installation wiring.
Exception: In New Zealand only, there is no restriction on sheath colour.
g) The polarization of socket-outlets shall be in accordance with the product specification and
shall be consistent throughout the installation
h) Semi-enclosed rewireable fuses shall not be installed
i) Condensation issues—a breathing/pressure equalization valve shall be installed to assist
with changes in humidity and drainage of moisture
j) Electrical equipment shall be installed to manufacturer’s instructions to ensure that the
marked IP rating is maintained
The equipments in an installation are generally sourced from different manufacturers based on cost,
schedule, availability of features, usage requirements, etc. While doing such selections, it shall be
necessary that the selected equipment satisfies the following requirements.
• It shall meet the essential safety requirements for low voltage electrical equipment
specified by AS/NZS 3820
• It shall be manufactured with the safe design, construction, installation and
performance requirements specified by an Australian, New Zealand or
Australian/New Zealand Standard that is appropriate and relevant to the type of
electrical equipment (as per appendix-A of the standard)
• Where an Australian/New Zealand Standard appropriate and relevant to the type of
electrical equipment does not exist, it shall meet the requirements of a recognized
international or national Standard of another country that is appropriate and relevant
to the type of electrical equipment and to the electrical installation conditions in
Australia and New Zealand
It is to be noted that equipment which bears the Regulatory Compliance Mark satisfies the relevant
regulatory requirements for electrical safety and can be presumed to comply with the above
stipulations. Regulatory Authorities may also accept other marks or means to determine whether
particular equipment satisfies the relevant regulatory requirements for electrical safety.
In case of selecting equipments that are expected to be functioning in damp situations, following
conditions shall be studied thoroughly before specifying / choosing the equipment for such areas.
• Its proven capability to operate safely near or within a damp or wet environment (by
established certification and past performance)
84 Practical Electrical Wiring Standards - AS 3000:2018
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Its capability to ensure the required protection against an increased risk of electric
shock in locations due to presence of water or high humidity in these areas; and
The adequacy of such protection offered to take care of any consequent damages that
may be expected from the presence of water or high humidity
It is to be noted that every alteration of, or addition to, an existing electrical installation shall be
considered as a new electrical installation and all relevant provisions of this Standard shall be
applied to every such alteration or addition from design stage upto selection/ installation.
While taking up alterations or additions to an existing electrical installation it shall be ensured that
such additions/ alterations shall not bring up the following issues on any portion of the already
existing electrical installation, or electrical equipment connected thereto.
• Carry currents or sustain voltages in excess of those permitted by this Standard or
• Ultimately used in any manner that is not in accordance with this Standard.
5.6
Verification (Inspection and Testing) (Clause 1.8)
AS/NZS 3000 recommends the following actions in regard to verification and testing of
installations.
• It is necessary that all electrical installations including any alterations, additions and
repairs to electrical installations shall be inspected as far as practicable prior to being
placed in service or use to verify that the installation meets the requirements of this
Standard as appropriate.
• Precautions shall be taken to avoid danger to persons and damage to property/
installed equipment during inspection and testing.
• Where the installation is an extension or alteration of an existing installation, it shall
be verified that such extension or alteration complies with this Standard and does not
impair the safety or performance of the existing installation in any way.
• To verify the correct connection of conductors to protective switching and control
devices as part of inspection.
Section 8 of the standard sets out requirements for the verification and testing of electrical
installations and we will review the same in detail in a subsequent chapter.
5.7
Compliance requirements (Clause 1.9)
Compliance with Part 2 of this Standard
In Australia only, electrical installations that meet all of the relevant requirements of Part 2 of this
Standard are deemed to meet Part 1 of this Standard.
Compliance with the requirements of other standards
Refer to Clause 7.8 for standards applicable to specific electrical installations and Appendix A for
a list of referenced Standards.
Additions, alterations or repairs to an existing installation constructed to a Part 1 design and
installation solution shall not alter the compliance of the existing installation with Part 1.
Repairs to existing electrical installations or parts thereof may be effected using methods that were
acceptable when that part of the electrical installation was originally installed, provided that the
methods satisfy the fundamental safety principles of Part 1 of this Standard.
It is possible that because of some unusual requirements, application or intended use, certain
electrical installations or portions of electrical installations may not be able to meet Part 2 of this
Standard. However the standard permits use of specific design and installation method for such
installations as detailed below, so that compliance question is taken care of.
• Such designs shall satisfy the fundamental safety principles of Part 1 of this Standard
and
Section 1- Scope, Application and Fundamental Principles 85
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•
Shall result in a degree of safety from physical injury, fire and electric shock not less
than that which, in other circumstances, would be achieved by compliance with the
particular requirements of this Standard; and
Shall satisfy the other requirements of this Standard as detailed in this Clause. The
remaining portions of such installations shall comply with Part 2 of this Standard.
If all or part of the design/construction of the electrical installation is not based on the deemed to
comply methods in Part 2 of this Standard, this choice must be made by the designer prior to final
certification of construction by the person carrying out the construction.
It is expected that any departures from Part 2 of this Standard must be formally acknowledged by
the owner or operator of the installation.
In all above cases, it is essential that the designer shall document the Part 1 design. Such
documentation shall be in the English language and clearly identify the following, including.
• the owner or operator’s acknowledgment as to any departure from Part 2 of the
Standard
• any requirements where the design requires specific installation use by the owner or
operator of the electrical installation and provide a copy of these requirements to the
owner or operator
• why Part 2 of AS/NZS 3000 was not adopted
• the verification requirements that are required to be undertaken to ensure full
compliance with the requirements of the Standard
• how compliance with Part 1 of the Standard is being achieved with alternative
designs/ features
• the verification(s) undertaken to ensure full compliance with AS/NZS 3000, and the
results of such verification
Such documentation shall be retained by the designer in the office and further a full set shall be
kept on-site at the electrical installation in the custody of person having overall responsibility for
the installation.
All parts of an electrical installation that do not comply with Part 2 of this Standard shall be
verified as complying with the specific design and with Part 1 of this Standard prior to being placed
in service.
Persons undertaking designs that depart from Part 2 of AS/NZS 3000 shall be competent.
5.8
Summary
In this chapter we reviewed some of the common terms used/ adopted in the electrical systems that
are also referenced in the AS/NZS 3000 The standard identifies shock currents, excessive
temperatures and explosions as three major risks associated with electricity use and provides
guidelines to be implemented for minimize the hazards due to these risks. Shock currents due to
direct contacts can be prevented by use of proper insulation, by providing suitable barriers, by
providing obstacles or by keeping the equipment out of reach under normal conditions of use.
RCD’s are recommended to protect against shock currents but they are not capable of preventing
shock currents. Hazards associated with high temperature risk can be minimized or prevented by
proper arrangement of equipments with adequate space around them, by providing protective
devices to disconnect the supply/ equipment in quick time in case of high currents and voltage
effects that are common sources of high temperatures. It is also essential that personnel in an
installation shall be protected against injury from moving equipments. The equipments shall also
be protected against external influences like weather, dampness, corrosion, etc.
The equipment design shall duly consider the applicable power supply characteristics taking care of
probable variations. The distribution system design shall adopt a number of circuits for feeding the
86 Practical Electrical Wiring Standards - AS 3000:2018
various equipments and loads so as to avoid inconvenience due to supply disruptions and also to
facilitate safe operation of individual items. While selecting equipments it shall be ensured that
they meet relevant standards of AS/NZS as applicable or equivalent international standards. The
installation shall adopt sound practices and consider important factors like prevention of external
influences/ hazards, adopting manufacturer’s recommendations, ensuring proper polarization of
sockets, etc., so that the system can be safely operated without causing any hazards.
It is also necessary that the completed installation shall be verified for compliance to standards and
tested in line with guidelines given in section-8 of the standard. All part-1 design compliance shall
be properly documented and kept as records in the installation for ready reference. In case of any
deviations to the standard, the reasons for the same shall also be documented with alternative
designs adopted indicating clearly how such alternative designs meet the safety stipulations of the
standard.
6
Section 2 – General Arrangement,
Control and Protection
Section-2 of the AS/NZS 3000 gives the recommendations the control of electrical systems and
equipments with proper switchgear and control gear incorporating isolation switches and
protective devices to ensure safety for people and properties from the probable hazards during the
use of electricity. In this chapter we will discuss the safe practices to be adopted for safe isolation
of electrical circuits in case of abnormal conditions and implementation of circuits to facilitate
these controls. We will also discuss the main protective devices to be incorporated in the low
voltage distribution systems including the need and methods for coordinating these devices for
proper isolation of faulty circuits without affecting the healthy systems. The chapter also covers the
stipulations given in the standard related to safe layout arrangements, design features of internal
components, etc., for electrical switchboards.
Learning objectives
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6.1
Arrangement of Electrical circuits and equipments
Electrical equipment control requirements
Isolation requirements of ac and dc systems
Requirements and arrangement of isolation switches
Emergency switching requirements
Fault protection and protective devices
Discrimination and Coordination of protective devices
Residual Current devices for protection
Protection against abnormal voltages
Switchboards and requirements
Selection and installation of switchgear and controlgear (Clause
2.1)
For proper design, correct construction and safe operation of the electrical installations, it is very
much essential to properly select the switchgear and controlgear which protect and control the
various circuits and equipments in an installation. Following are some of the important
considerations to be taken into account while selecting the Switchgear and controlgear in an
installation.
• The switchgear shall facilitate control or isolation of the total electrical installation in
case of major shutdowns and emergency situations. It shall also be able to achieve the
88 Practical Electrical Wiring Standards - AS 3000:2018
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6.2
isolation of individual circuits or individual items of apparatus for safe maintenance,
testing, fault detection or repair, as and when needed
The switchgear shall be able to monitor and automatically disconnect the main power
supply in the event of an overload, short-circuit or excess earth leakage current in the
electrical installation or individual circuits being protected
It shall also offer protection against failures of equipments and associated systems
from overvoltage or undervoltage conditions
To achieve these functions safely, the switchgear and controlgear shall be grouped
and interconnected as per requirements, enclosed in appropriate manner for
protection against external influences and shall be located in such a way to ensure
safe and easy access at all times
It shall be able to control and protect the circuits and apparatus that are intended to be
protected without affecting the other parts of the installation (selectivity)
Installed in accordance with the requirements of this Section, and the additional
requirements as specified in the manufacturer’s instructions
Arrangement of electrical installation (Clause 2.2)
To ensure safe operation of different apparatus and equipment in an installation, it is necessary that
the electrical distribution system shall be divided into an appropriate number of separate circuits
taking into account the following:
• Some equipment may be operated independently and some may be operating as a
group. For proper segregation of the different circuits, the designer of the system
shall identify the relationship between the various equipments including any specific
requirements of some equipment to be operated as a group as well as the special
needs to be met by the end user of the installation.
• Selection of the rating of the circuit components in relation to the load characteristics
and operating characteristics of the equipment to be connected in that circuit
• The limitations in the system like possibilities of supply failure to critical equipment,
overload conditions due to specific operation needs, etc., and the features that are to
be added to locate a fault consequent to a circuit failure, for corrective actions
• The features to be incorporated in the circuits to help and perform maintenance
works, alterations, additions, etc. for specific parts of the installation without a need
for interrupting or affecting the supply to other parts of the installation that can still
continue to be in service under such works
• To ensure that circuits for safety services are separated from the circuits normally
used to supply the remainder of the electrical installation, so that these safety services
can remain unaffected or controlled independently as appropriate under emergency
situations
It is recommended to group the circuits of an installation and ensure that the respective groups are
separated from each other in such a way that each can be independently controlled with suitable
controlgears matching specific characteristics/ load duty cycles. Typical groups of loads can be
• Lighting
• Socket-outlets
• Heating and/or air conditioning appliances
• Motor-driven equipments
• Auxiliary services, such as indication and control
• Safety services
Appendix B clause B2 provides recommendation for typical radial distribution arrangement
incorporating circuits divided as per the above grouping and each group provided with independent
controls/ isolation arrangements at the respective incoming points to get the following benefits.
• One circuit only will be shut down (by fuses or circuit-breakers) in case of a fault in
that circuit, without disturbing other circuits
• Finding the location of the fault is getting simplified
Section 2 - General Arrangement, Control and Protection 89
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•
Maintenance or extensions to a circuit can be performed by isolating this circuit
keeping the remainder of the electrical installation in service
Conductor sizes can be optimized to suit the decreasing demand towards the final
subcircuits
The appendix recommends due consideration of the following design aspects while concluding the
circuit arrangements for the various services and loads.
• Determine the minimum current-carrying capacity for conductors, dependent upon
the method of installation and the external influences
• Determine voltage drop requirements for the minimum conductor sizes and increase
the same, if conditions demand a lower drop
• Determine the automatic disconnection of supply requirements for the type of loads/
circuits
• Determine overcurrent and short circuit requirements and include necessary devices
coordinated with the chosen conductors
As per appendix C of the Standard, each item of equipment that has a current rating in excess of 20
A per phase should be connected to a separate and distinct circuit. Where more than one item of
equipment (<20A) is to be connected to a circuit, it is recommended for limiting the maximum
number of points in a circuit taking into consideration ALL the following:
• Number and type of each load group (lighting, socket-outlets or appliances, etc.) to be
supplied in combination from a single circuit
• The operating characteristics of the different items of equipment, including periodic
and daily duty cycles
• Circuit current for the operating conditions and the coordination with cable and
protective device ratings to minimize the risk of an overload fault
• The likely effects of an overload fault on the circuit, including loss of supply to
critical equipment needed for a special function, e.g. security, emergency, medical or
critical information and telecommunications purposes
The appendix C in the Standard also includes a table (No. C1) (Table A1.1 in appendix A of this
manual) which gives guidelines for deciding the number of points for lighting, 10A/15A/
20Asockets, etc. that can be connected from a subcircuit based on the conductor size, protective
breaker rating, etc along with the maximum watts to be limited. It can be noted from the table
C1 in the standard that sockets are not permitted in a subcircuit unless the minimum breaker
size is 13 amperes and cable size is 2.5 sqmm, exception being a socket-outlet installed more
than 2.3 m above a floor for the exclusive connection of a luminaire as a lighting point.
While designing the circuits, each single-phase circuit, and each multiphase circuit that requires a
neutral conductor for the operation of connected equipment, shall preferably incorporate a separate
neutral conductor. However a common neutral conductor may be used for two or more circuits
subject to satisfying the following conditions:
• The continuity of the common neutral conductor shall be maintained in such a way
that it does not depend on connections at the terminals of electrical equipment,
including control switches. It means that neutral shall not be connected through
terminals or switches as these are likely to get disturbed and isolate the neutral
inadvertently during normal service
• Final subcircuits shall be controlled and protected by linked circuit breakers or linked
switches that cannot break automatically
• The neutral conductor shall be marked to identify the associated active conductors in
each switchboard
• Single appliances with alternative sources of supply (such as a water heater, space
heater or air conditioner) shall have one common isolating switch controlling all
sources of supply
90 Practical Electrical Wiring Standards - AS 3000:2018
The current-carrying capacity and size of a neutral conductor shall be determined from the currentcarrying capacity of the associated active conductors and shall be installed in accordance with the
provisions of Section 3 of the Standard, which are covered in our next chapter.
Appendix C of AS/NZS 3000 provides guidelines on deciding the operating loads of different
groups, designing the different circuit arrangements and selection of associated protective devices
for safe and reliable operation of an installation. These guidelines are informative in nature for
design purposes and are covered in a separate appendix of this manual.
While planning the system, the maximum demand of power in consumer mains, submains and final
subcircuits shall take into account the intended present usage pattern in the installation and the
likely future growth by adopting any of the following procedures.
• By direct calculations generally considering the recommendations set forth in
appendix-C of the standard (Appendix-1 of this book)
• By making proper assessment of the duty cycles, occupancy conditions, etc., by
competent personnel
• By direct measurement during normal service by means of appropriate demand
meters recording the demand over definite periods of every 15 minutes
• By the current rating / capacity of the breaking device in limiting the demand
It is also necessary that operating characteristics of the distribution system like nominal voltage
rating, current rating, frequency, temperature rise, duty, and fault level shall all be duly considered
while selecting equipments and appliances and putting them into service. In regard to the current
rating, the assessment shall consider the normal continuous current as well as the short term
maximum current likely to flow in the system including the likely duration of such higher currents
for proper operation of the system without interruptions.
6.3
Electrical equipment control requirements (clause 2.3)
Electrical installations shall include necessary devices to ensure prevention and/or removal of
common hazards associated with the electrical installation in addition to ensuring maintenance of
electrically activated equipment. The standard classifies such control devices, according to one or
more of the specific functionalities, as noted below:
• Isolation (to enable isolation of supply to electrical equipment with features to
prevent electrical equipment from being inadvertently energized)
• Emergency (to help removal of an unexpected danger during operation in the shortest
possible time)
• Mechanical maintenance (to enable safe maintenance of the circuits/ equipments
without the possibility of getting electrocuted)
• Functional (to control a circuit for operational control only and not for any safety
reasons.)
All the above devices ensure a deliberate action to be carried out in addition to the normal method
of operation required to energize a circuit. These deliberate actions include one or combination of
the following to prevent inadvertent energization.
• Provision for fitting of a padlock in the device used for isolation
• Putting up warning tags or notices to caution against energizing the equipment.
• Location of such devices within a lockable space or enclosure that cannot be accessed
without key and/or special permissions.
Following paragraphs give further recommendations related to the incorporation of the main
control devices identified above.
6.4
Isolation
Isolation is one of the important operations to isolate the source of hazard/ fault in causing further
damages during fault conditions, fires, etc. In this part of the standard, stipulations are given on the
Section 2 - General Arrangement, Control and Protection 91
practices to be followed for incorporation and proper use of isolation switches in electrical
distribution systems for safety, which are covered in the following paragraphs.
6.4.1 Isolation of ac and dc conductors
A normal ac circuit uses a minimum of two conductors to provide supply to equipment. Though it
may look that breaking any of these conductors could isolate the equipment from being operated,
for safety reasons it is necessary to ensure that isolation switches are properly adopted as noted
below for the ac circuits.
Phase conductors: All active conductors of an ac circuit shall be capable of being isolated by a
device used for isolating its supply.
No switch or circuit-breaker shall be inserted in the neutral conductor functioning as neutral +
protective earth conductor (PEN). Neutral conductor isolation shall be normally avoided in other ac
circuits but may be provided subject to taking into consideration of the following conditions
• A switched neutral pole may be provided subject to the conditions that the neutral
circuit shall not open before opening the corresponding active pole and also that it
shall close before the active pole is closed.
• Where an item of switchgear is required to disconnect all the live conductors of a
circuit, it shall be having provisions to ensure that the neutral conductor cannot be
disconnected or reconnected without the respective active conductors also being
disconnected or reconnected simultaneously.
• A switch controlling a fire pump shall open the neutral conductor also.
Earthing conductor of an ac circuit shall never be isolated or switched and shall remain intact under
all conditions of service, shutdown and maintenance.
It shall also be remembered that fuses shall not be inserted in ac neutral circuits because these fuses
may likely open (blow) under fault conditions defeating all the above considerations related to
neutral isolation.
In regard to the dc circuits it shall be ensured that both the poles of a dc circuit shall be capable of
being isolated by a device for isolation, except in cases where one pole is connected to earth at the
source end.
It shall however be noted that a semiconductor (solid-state) device shall NOT be used for isolation
purposes, in both ac and dc circuits.
6.4.2
Requirements and arrangement of isolation switches
Devices for isolation of a circuit shall be able to satisfy ALL the requirements stipulated below:
• Shall be capable of withstanding an impulse voltage likely to occur at the point of
installation, or shall have an appropriate contact gap to withstand the impulse without
breaking.
• Shall not falsely show that the contacts are open leading to confusions and dangerous
situations.
• Shall clearly and reliably indicate the positions of the device. (Usually it can be with
symbols like ‘O’ for OFF and ‘I’ for ON engraved in the device).
• Shall be designed and installed with suitable features that prevent unintentional
closure due to external forces/ causes like vibration, etc.
• Shall disconnect all active conductors of the relevant supply, when operated.
• Shall be readily available near the equipment to be isolated to enable isolation as
quickly as possible when needed.
In addition, following design requirements shall be met by the isolation switches for specific
applications as appropriate.
92 Practical Electrical Wiring Standards - AS 3000:2018
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Where a device adopted for isolation is not capable of breaking or interrupting
normal load current, suitable measures shall be taken to prevent operating of such a
device while carrying the current i.e. it shall open or close only when there is no
current flowing in the circuit. Suitable measures recommended are means of
interlocking its operation with an associated circuit breaker which shall be operated
before operating this isolation device or ensuring that these devices can be operated
only by authorized persons.
Where a switching device is used for isolation, it shall be capable of being secured in
the open position and it shall not accidentally go back to the close position, when left
in open position.
Where switching is NOT required, the isolation can be achieved by adopting any one of the
following.
• Multi-pole or single-pole disconnectors (off load isolators)
• Plugs and socket-outlets
• Fuses or removable links
• Special terminals that do not require the removal of a conductor.
All devices used for isolation shall be clearly identified to indicate the circuit or equipment that
they isolate, when operated.
6.5
Main isolation switches (Clause 2.3.3)
These are isolation switches that isolate the whole supply of an installation. It is mandatory that the
supply to every electrical installation shall be controlled from a main switchboard by means of one
or more main isolating switches provided therein to control the whole of the electrical installation.
Main switches used for isolation of main supply shall be decided taking into account the following
requirements:
• The number of main switches, their location, arrangement and identification shall be
such to allow for their effective operation (of isolating the supply) in an emergency
situation with minimum efforts.
• Where multiple supplies are provided, each supply shall be controlled by an
independent main switch or switches located on its respective (main) switchboard.
• Each part of an electrical installation supplying an emergency system shall be
controlled by a main switch or switches and the same shall be separate from those
used to control the remainder of the electrical installation.
Following are the items that need NOT be controlled by the main isolation switches as that might
bypass some controls during operation or disturb some other equipment’s function needed for
safety located in the vicinity of the switch.
• Consumer mains.
• Equipment installed for service protection, control or electricity consumption
metering purposes, etc as may be required by an electricity distributor.
• Ancillary equipment, measuring devices and associated wiring that are required to be
connected to the supply side of the main switch or switches generally confined within
the switchboard.
• Equipment such as voltage sensing equipment, connected on the supply side of a
main switch usually for protection.
• Fault-current limiters.
• Surge diverters installed to protect consumers mains or main switchboards.
Main switches shall be accessible when needed and shall be properly identified to facilitate
operation or to avoid accidental operation. The standard specifies that the main switches shall
satisfy the following access and identification requirements.
• These shall be readily accessible without any obstruction and (handles) shall be
located not more than two meters above the ground, floor or a suitable platform to
Section 2 - General Arrangement, Control and Protection 93
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6.6
enable easy operation by a person of normal height standing on ground, floor or
platform without requiring any other tools.
Buildings with multiple occupants are usually provided with metering and
distribution starting at a common point in the premises. These premises shall have
independent main switch(es) accessible to the respective occupier(s) to isolate their
portion of installation when needed by them for attending any works or repairs.
Alternatively each occupier shall have ready access to the main isolation switch of
the total installation.
All such switches shall be legibly marked ‘MAIN SWITCH’ and shall be easily
distinguishable from other switchgear components/ switches by means of grouping,
colouring or any other recognized methods to ensure easy identification and also
enable fast operation under emergency situations.
Where more than one main switch is provided, each of these switches shall be
marked to identify clearly the electrical installation or portion it controls, to avoid
confusions or misjudgment by persons operating the switches.
Switches isolating an alternate supply or transferring an alternate supply when
operated shall indicate the position of the switch at which the alternative supply is
getting controlled.
In installations having more than one supply, a prominent notice shall be provided
indicating the presence of such other supplies and the location of other main
switchboards corresponding to such supplies at each main switchboard, to ensure that
all alternate supplies are also easily and correctly located and isolated under
emergency conditions.
All switches shall be clearly identified to indicate the circuits that they isolate. In case
a circuit cannot be isolated in a distribution board or a switchgear assembly for
functional reasons, a warning shall be affixed to that board or assembly identifying
such circuits and the isolating point.
Other main switches (Clause 2.3.4)
Though a main switch is normally required where the utility company brings in the main supply,
few more additional main switches might be needed in the installation depending on the internal
distribution arrangements. Take the case of an outbuilding, an independent structure of a premise
that is completely separated from the area containing the switchboard through which the premise
receives the supply. The standard recommends that the electrical installation in such an outbuilding
shall be considered as a separate electrical installation and provided with a main switch for its
installation, if it meets the following conditions.
• It has a maximum demand of 100 A or more per phase
• It is provided with a switchboard.
Further, every submain and final subcircuit exceeding 100 A per phase shall be controlled by a
separate isolating switch on the switchboard at which the circuit originates. This requirement may
not be applied in locations where fault-current limiters or fuses protect small submains which are
teed off from larger submains, e.g. Rising sub mains at each floor. The other items with separate
main switches are
• A generating set or UPS system used as the alternative supply in an installation.
• Appliances and accessories, socket outlets, heaters, capacitors, motors, etc. generally
covered in section-4 of the standard.
6.7
Emergency switching (Clause 2.3.5)
There are installations which are prone for emergency conditions requiring immediate
disconnection of its power supply alone, without disturbing the main supply. The standard
recommends adoption of emergency switching for removing an unexpected danger in the following
installations.
• Machinery.
• Conveyors
94 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
•
•
•
•
Groups of machines operating simultaneously.
Pumping facilities for flammable liquids.
Ventilation systems.
Certain large buildings, e.g. department stores.
Electrical testing and research facilities that use different types of high voltage
equipments and devices.
Boiler rooms.
Large kitchens.
Teaching laboratories.
High-voltage discharge lighting, e.g. neon signs.
Means for such emergency switching shall consist of a single switch isolating the main supply to
the equipment or simultaneously operating several items by single action to isolate the particular
hazard. The standard recommends that a device adopted for emergency switching shall satisfy the
following requirements.
• The switches used for systems with motors shall be capable of breaking the full-load
current of the relevant parts of the electrical installation being isolated. Since the
motors draw very high currents when they get stalled due to some problems or
operating conditions, the isolating switch shall be able to break such stalled motor
currents as well, where appropriate.
• The switch shall be operated manually directly interrupting the main circuit, where
feasible. A device operated by remote control such as a circuit-breaker or a contactor
also may be permitted as emergency switch, in which case it shall be capable of
opening on de-energization (fail safe control) of its coil, or another technique of
suitable reliability shall be employed
• It shall have means to be kept in latching or being restrained in the ‘OFF’ or ‘STOP’
position unless the hazard is removed for safe supply restoration.
• It shall not re-energize the relevant part of the electrical installation upon release of
the device i.e. no auto restart feature shall be provided for such switch.
• It shall have a manual resetting feature for re-energizing or closing before the
electrical equipment can be switched ON again.
6.8
Switches for maintenance (Clause 2.3.6)
It is necessary to consider devices for disconnecting the power supply to specific electrically
powered items where mechanical maintenance might involve a risk of physical injury, if supply is
kept ON. Such maintenance switches shall meet the following normal requirements.
• Shall have capability for manual operation;
• Shall clearly and reliably indicate the ‘OFF’ position.
• Shall be designed or installed in such a manner as to prevent unintentional closure.
These switches shall additionally incorporate following facilities to prevent accidental starting of
the electrically powered equipment during mechanical maintenance and/or shut down.
• locking the switch in the open position or
• a means of shutting down the switch in a lockable enclosure or
• facilities for the attachment of a warning notice or notices
Locking provisions may NOT be provided where such shut down is continuously under the control
of the person performing maintenance. Like any other isolation switches, devices used for shutting
down during mechanical maintenance shall be marked and located such that they are readily
identifiable with its corresponding equipment and are convenient for the intended use.
6.9
Functional (control) switching (Clause 2.3.7)
Devices adopted for switching off electrical equipment or part of an electrical installation for
ONLY operational reasons but do not involve safety requirements are called functional switches.
Typical examples could be a switch that switches off a lamp or an appliance where the main circuit
Section 2 - General Arrangement, Control and Protection 95
feeding other loads connected in that circuit may still be operating without getting disturbed. These
may be standard ON/OFF switches, semiconductor (solid state) devices, or contactors.
Disconnectors, fuses or links shall not be used for functional switching purposes. A single
functional switching device may be used to control several items of an apparatus intended to
operate simultaneously. Following are the conditions that generally call for using functional
switches.
• A part of a circuit that may be required to be controlled independent of other parts of
the electrical installation, say a pump motor in a residential unit.
• In general, any current consuming apparatus requiring control shall be provided with
an appropriate functional switching device to control it separately.
As per the standard, the functional switches shall meet the following requirements:
• They shall be suitable for the duties that they might be required to perform for the
particular load or group of loads to be controlled. The type of load (continuous,
intermittent, inrush currents, etc) to be controlled, the frequency of operation of the
switch (number of times per day, per hour, etc) and the anticipated number of
operations (over a normal period) should be taken into account when assessing the
onerous duty to be met by such switches.
• These switches are not required to switch all active conductors of a circuit, like a
lamp switch which isolates only the phase.
• Functional switching devices that are used for controlling loads having a significantly
low power factor shall be subject to an appropriate de-rating factor, unless device is
designed for the same.
• Functional switching devices need NOT be identified to indicate the ‘ON’ or ‘OFF’
position except when these are adopted for common appliances say a water kettle.
6.10 Fault protection and protective devices (Clauses 2.4, 2.5)
It is necessary to provide appropriate protection devices to protect equipments from common faults
expected in the LV distribution systems. This will avoid potential damages to the equipment as
well as the surrounding equipments or properties depending on the severity and duration of such
faults. Following methods of protection are recognized in this Standard to minimize the faults and
its causes, as we have seen in section 5.3.1 of this manual (Protection against shocks due to indirect
contacts).
• Automatic disconnection of supply.
• The use of Class II equipment with double or reinforced insulation.
• Electrical separation of live parts without connecting to earth or any other circuit.
While the first method requires a monitoring device to open a circuit, the other two methods are
construction features of equipments. We will have a brief review on the automatic disconnection
devices which are covered in detail in the standard for selection, coordination, etc in the following
paragraphs.
6.10.1 Automatic disconnection
Automatic disconnection refers to disconnection of equipment or system from source of supply
under fault conditions. The automatic disconnection is essentially intended to limit the prospective
touch voltage arising between simultaneously accessible conductive parts in the event of a fault.
Limiting the touch voltage is generally achieved with following provisions in the LV electrical
installations.
• Connecting exposed conductive parts of a system to a protective earthing conductor.
• Disconnection of supply by a protective device when the continuously monitored
current by this device exceeds its pre-set safety value.
The most common devices that are employed to provide automatic disconnection of mains under
specific fault conditions in low voltage systems are enclosed fuse-links, Miniature circuit-breakers
(MCB’s) and Moulded-case circuit-breakers (MCCB’s) that monitor over currents/ short circuit
96 Practical Electrical Wiring Standards - AS 3000:2018
currents and isolate the circuit when these go beyond acceptable limits. In addition, fixed setting
residual current devices (RCD’s) are used for protection against shock currents which
automatically disconnect supply when earth leakage currents exceed a safe value. These devices are
also referred to as residual current circuit breakers, earth leakage circuit breakers (RCCB, ELCB),
etc in some countries.
While the fuses blow themselves to isolate a circuit, the MCB’s and MCCB’s include thermal and
magnetic devices that monitor and actuate their trip coils to open the breaker contacts and
disconnect the circuit automatically. The modern MCB’s and MCCB’s are usually provided with
electronic protection. While MCB’s and fuses are designed and rated to operate for specific/fixed
currents, MCCB’s are mostly provided with adjustable settings to select the current for automatic
disconnection purpose.
For larger currents in the range of 1000 amperes and above in bigger installations, air circuit
breakers are employed up to around 5000 amperes. These are also nowadays provided with
electronic trip releases with adjustable settings to protect against over current, short circuit and
earth fault having different characteristic curves.
Unlike the fuses and breaker types discussed above, the residual current device (RCD) senses the
unbalance (i.e. residual difference) between the currents in the phase and neutral conductors of a
single phase circuit (or between sum of three phase currents and neutral current in a three phase
circuit). These unbalances are normally caused by currents getting bypassed (leaked) to earth
through persons coming in contact with the circuit or due to any other internal leakages, which are
unacceptable.
Once the leakage current exceeds a preset value typically in the order of few milliamperes, the trip
coil of the device is actuated by the internal circuit to open the main contacts. Optionally these
devices are available with combination of MCB’s for overcurrent and short circuit protections but
such protections are achieved by an independent circuit different from the earth leakage monitoring
circuit.
Figure 6.1 illustrates these different protective devices which are widely used in all parts of the
world.
Section 2 - General Arrangement, Control and Protection 97
Figure 6.1
Typical low voltage protective devices
All these protective devices are connected in series with the circuit being protected and carry the
load current under normal conditions. All these devices except RCD have inverse characteristics of
current Vs operating time, with curve drawn on logarithmic graph. The RCD is an instantaneous
tripping device.
Table 6.1 gives the common ranges of currents for which these devices are usually adopted.
Characteristics of the fuses, MCB’s and MCCB’s are generally as shown in the figures 6.2 and 6.3.
98 Practical Electrical Wiring Standards - AS 3000:2018
Table 6.1
Common protective devices in LV systems for automatic disconnection
Device
Over current
operation
From fraction of
amperes to hundreds
of amperes
Short circuit
current operation
Upto around 100kA
Earth leakage
current operation
Not applicable
MCB’s
From fraction of
amperes to about
100 amperes
Upto around 10kA
Not applicable
MCCB’s
Normally beyond 100
A and generally upto
about 1000 Amperes
Upto around 50kA
Optional
ACB’s
Above 800 amps
Upto around 120kA
Optional
RCD’s
Not applicable
Not applicable
From 30 to 300mA
Time
Fuses
Current
Figure 6.2
Typical fuse characteristics
Long
delay (bimetal)
operation area
100%
Instantaneous
(electromagnet)
operation area
Overcurrent
Thermal/Hydraulic-Magnetic
Operating time
Operating time
Section 2 - General Arrangement, Control and Protection 99
100%
ramp
Overcurrent
Electronic
Figure 6.3
Typical characteristics of MCB/MCCB
It is to be noted that semi-enclosed re-wireable fuses shall not be used as means of disconnection
due to their unreliability and possibilities of safety hazards while inserting/ replacing blown fuses.
6.10.2
Protection against overcurrent
The standard recommends arrangement of overcurrent protection for consumer mains in
accordance with one of the following methods.
• Providing both short-circuit protection and overload protection at the origin of the
consumers mains (at the point of supply)
• Providing short-circuit protection at the origin of the consumers mains and overload
protection at the main switchboard
In addition, an overcurrent protective device or devices ensuring protection against overload
current and short-circuit current shall also be placed at the origin of every circuit and at each point
where a reduction (distribution transit points) occurs in the current carrying capacity of the
conductors.
Devices for protection against overcurrent shall not be provided for circuits where unexpected
opening of the circuit could cause a danger that might be greater than the danger of overcurrent
itself e.g. emergency system supplies, lifting magnets, exciter circuits of machines and the CT
secondary circuits. It is preferable to consider alarm provisions in these circuits to caution such
faults for taking suitable action by competent personnel in place of automatic disconnection.
The overcurrent protection devices have inverse-time characteristics. These shall be rated for
interrupting capacity at least equal to the prospective shortcircuit current at the point of installation.
The equipment or circuit being protected shall be able to carry the overload current for a specified
duration till interrupted by the protective device. Following are the common devices used/
recommended for overload protection in LV distribution systems.
• Circuit-breakers incorporating short-circuit and overload releases.
• Circuit-breaker + fuse combinations.
• Enclosed fuse links and Fuse combination units
6.10.3
Selection of the breaker/ fuse rating
Circuit breakers and fuses are the most common protective devices for overload protection in LV
systems. The current ratings of these protective devices and the equipment/ cable being protected
shall be coordinated to satisfy the following equation.
100 Practical Electrical Wiring Standards - AS 3000:2018
I B ≤ I N ≤ I Z ………………………. 6.1
where,
I B = the maximum current or maximum demand for which the circuit is designed
I N = the nominal current rating of the protective device
I Z = the continuous current-carrying capacity of the conductor
The equation 6.1 is apparent for safe design. AS/NZS 60529 specifies effective currents to flow in
the respective protective devices to make them operate in conventional time, which are based on
the operating characteristics of a typical circuit breaker and fuse manufactured in the country.
For circuit breakers, it shall be 1.45 I N (for effective breaking) and for fuses, it shall be 1.6 I N (for
fusing)
If I 2 is the current of the device to ensure its effective operation, considering equation 6.1, we have,
• I 2 ≤ 1.45 × In for a breaker
I 2 of a fuse shall be ≤ 90% I Z (based on 1.45/1.6 = 0.9)
• I 2 ≤ 0.9 × I Z for fuse
Hence, while choosing the nominal ratings I N of the breaker or fuse, following equations shall be
considered.
• I B ≤ I N ≤ I Z in case of breakers
• I B ≤ I N ≤ 0.9 I Z in case of fuses
Since the breakers are usually provided with adjustable operating currents, it shall be ensured that
the selected breaker can be set for a particular I N applicable for a circuit.
As already seen, the overload protection is to be avoided in case where its operation can cause
more danger than overload itself. In addition, AS/NZS 3000 stipulates that the overload protective
device for equipment or a subcircuit may be omitted in any of the following situations.
• If effective protection is provided by a protective device located on the supply side of
its origin or the point of reduction in current-carrying capacity.
• For a conductor on the load side of a change in current-carrying capacity that is
effectively protected by an overload protective device located on the supply side of
the conductor OR for a conductor supplying electrical equipment that is not capable
of causing an overload current (like an heating appliance) and the conductor has no
other branch circuits or socket-outlets connected between the origin of the conductor
and the electrical equipment
• For installations of telecommunications, control, signaling and the like.
6.10.4
Protection against shortcircuit current
Protective devices shall be provided to interrupt any shortcircuit current flowing in the conductors
before such current causes danger due to thermal and mechanical effects produced in them and
connections. Similar to overload protective devices, short circuit protection device shall be installed
at the origin of every circuit and at each point where a reduction occurs in the current-carrying
capacity of the conductors. These may also be omitted in case of dangers due to interruptions
causing more danger than short circuits itself. These may also be omitted for conductors connecting
generators, transformers, rectifiers or batteries to their associated switchboards subject to wiring
not placed close to flammable materials and remainder of their circuits are provided with short
protection at the switchboards.
The time (t) in which a given shortcircuit current (I) will raise the conductors from the highest
admissible duty temperature to the limit temperature may be approximated by the following
equation.
Section 2 - General Arrangement, Control and Protection 101
t=
K 2 S2
………………………. 6.2
I2
where
t = short circuit duration in seconds
K = factor dependent on the material of the conductor, the insulation and the initial and the final
temperatures that decides the maximum short circuit current the conductor can carry for a
specific duration (e.g. for copper conductors with PVC insulation, K = 111 for 40°C
ambient).
S = cross-sectional area of the conductor in mm2
I = effective short-circuit current in amperes (r.m.s)
AS/NZS 3000 provides illustrative figures showing the connection positions of overload and short
circuit protective devices in typical LV installation circuits. Some of these illustrations are
reproduced in figure 6.4 for better understanding of the stipulations.
Main supply
Main supply
Overload and Short
circuit Protectoion
device
Short circuit
Protectoion device
A
Overload and short
circuit protective
device
Overload and short
circuit protective
device
Sub circuits
No branch
circuits
between A, B
B
Over load
protective
device
Basic Protection in every main circuit and every
subcircuit
A
B
Over load
protective
device
Alternate position for overload protective
devices
Figure 6.4
Typical positions of protective devices (based on AS/NZS 3000)
6.10.5
Protection against Internal arcing Fault currents
Internal arcing of a switchboard is one major hazard that shall be prevented by proper protective
features. The arcing is possible due to arcing fault currents to the earthed parts while the equipment
is in service, or when the device is undergoing maintenance checks during which time the insertion
and removal to source of supply can lead to high arcing currents. Arcs created by a fault do not
remain stationary. The interaction between an arc and the electromagnetic field caused by the fault
current flow will cause the arc to move away from the source point with the arc behaving very
much like a conductor placed in a magnetic field. The arc also causes sudden heating of the air in
its immediate vicinity causing a violent expansion much like an explosion. This can result in the
dislocation of loose components around the fault point and their being thrown like projectiles
outwards from the arc. Following are some important effects of arc flash:
• Electric arcs produce some of the highest temperatures known to occur on earth: up to
35000 degree Fahrenheit. This is four times the temperature of the sun’s surface
102 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
The intense heat from the arc causes a sudden expansion of air. This results in a blast
with very strong air pressure
All known materials are vaporized at this temperature. When materials vaporize they
expand in volume. The air blast can spread molten metal to great distances with force
Energy released is a function of system voltage, fault current magnitude and fault
duration
For low voltage systems, a 3 to 4 inch arc can become stabilized and persist for an extended period
of time. Figure 6.5 gives the temperature characteristics and effects of arcing fault currents, if
allowed to continue for a few cycles without disconnecting the main circuit.
Figure 6.5
Typical consequences of arcing fault currents
AS/NZS 3000 recommends that protection against arcing currents shall be provided for all larger
current (800 A or more per phase) switchboards by automatic disconnection of the system using
suitable sensing devices to limit as far as practicable the harmful effects of an internal arcing fault.
In addition, the following construction/ installation design features are essential to reduce the
probability of initiation of arcing currents.
• Additional insulation
• Separation methods to reduce the prospect of initiation of a switchboard internal
arcing fault between live parts of different phases and components (by having
busbars, control units, main power terminals, etc separated from each other using
proper insulated barriers with sufficient clearances)
• Having proper settings for protection against possible arcing currents by adopting
time setting as per equation 6.3 given below
The arcing fault current between phases, or between phase and earth, is found to be in the
maximum range of around 60% of the prospective shortcircuit current. Refer table 6.2.
Section 2 - General Arrangement, Control and Protection 103
Table 6.2
Typical values of arcing fault currents
Maximum short circuit current
Arcing fault currents
10kA
6.56kA
20kA
11.85kA
30kA
16.76kA
40kA
21.43kA
Hence the protection shall be initiated, i.e. pick up of disconnection device shall be done at a
current preferably at around 30% of the three-phase prospective fault level to ensure effective
protection. Accordingly, to minimize damage to the switchboard, the interrupting time is
recommended not to exceed the value obtained from the following equation.
t=
Ke × Ir
I1f.5
………………………. 6.3
Where,
t = clearing time in seconds
l f = 30% of the prospective fault current
l r = current rating of the switchboard
k e = 250 constant, based on acceptable volume damage
Protective devices, such as arc fault detection devices (AFDDs), may be used to protect against the
effects of arc faults for final subcircuits, including fire hazards. Typical applications include the
following:
(a) In premises with sleeping accommodation
(b) In locations with risks of fire due to the nature of processed or stored materials (e.g. barns,
wood-working shops, stores of combustible materials)
(c) In locations with combustible construction materials (e.g. wooden buildings)
(d) In fire propagating structures
6.11 Discrimination and coordination of protective devices
The protective devices take some minimum time to first effectively detect and then to isolate the
circuits completely. During such time there is a possibility of heavy currents flowing in these
devices till the fault is cleared. The energy that is flowing through these devices and circuits is
called let through energy which is calculated with the formula I2t, where I is the let through current
in amperes and t is the duration of this let through current in seconds.
Since short circuit currents are much more than the normally occurring overload currents, it is very
much essential that the characteristics of the protective devices shall be coordinated in such a way
that the energy let through by the short-circuit protective device does not exceed that which can be
withstood by the overload protective device without getting damaged. Where multiple protective
devices are connected in series, it is possible that the same currents are detected or flowing in all
the protective devices along the path. The characteristic curves (time Vs current) of these protective
devices are typically of inverse time characteristics as already noted. Hence as a standard
requirement to ensure selectivity, discrimination between protective devices shall be ensured by
having time/current curve of downstream device below that of upstream protective device(s) for a
given fault current. During such times it shall also be ensured by proper settings that the other
protective device(s) monitoring the healthy circuits shall not operate/ impact their circuits.
104 Practical Electrical Wiring Standards - AS 3000:2018
Figure 6.6 shows the typical curves of different protective devices 1, 2, 3 all connected in a series
circuit having device 1 closer to the source and device 2 located between 1 and 3, typically drawn
on logarithmic scale.
1
Time
3
2
t1
t2
t3
I
Current
Figure 6.6
Typical time discrimination for fault current protection by multiple devices in a series circuit
It can be noted that the same magnitude of overload current would be carried by all the three
devices for a fault after device 3. Here we expect the device 3 to operate earlier so that devices 1
and 2 do not operate once the fault is cleared. This will also ensure that some other circuits
connected between 1, 2 as well as 2, 3 are not affected for a fault away from these circuits. For
achieving the same, it is necessary that the time of operation of device 1 shall be > device 2 shall be
> device 3 for a fault closest to device 3. This is realized by choosing the curve of a device (for the
rating or selected setting) matching its location in the circuit and by ensuring these curves do not
cross each other in the operating current range of a particular circuit.
Consider an example of a system having breakers C1 and C2 in series, with C1 breaker ahead of
C2 breaker. The discrimination of protection shall be ensured by following criteria:
• For ratings of C2 greater than or equal to 800 A, discrimination shall be provided
between overload curves and up to the instantaneous setting of C1
• For ratings of C2 greater than or equal to 250 A and less than 800 A, discrimination
shall be provided between overload curves and is recommended up to the
instantaneous setting of C1
Discrimination of breakers is deemed to be achieved if;
• the overload setting of C1 ≥1.6 × C2, e.g. 1000 A for C1 with 630 A for C2 and
• the instantaneous setting of C1 ≥1.6 × C2, e.g. C1=5 ×1000 A with C2=5 × 630 A.
• For ratings of C2 less than 250 A, C1 ≥ 2 × C2
In a similar way, discrimination between HRC fuses F1 and F2 (F2 in downstream) in series is
deemed to be achieved if;
• For times >0.01 s when F1 ≥ 1.6 × F2., e.g. 16 A with 10 A
• For times < 0.01 s when F1 ≥ 2 × F2., e.g. 20 A with 10 A (based on the total I2 t of
F2 ≤ pre-arcing I2 t of F1)
Figure 6.7 shows the typical recommendation given in the standard for circuit breakers C1 and C2
connected in series as explained above.
Section 2 - General Arrangement, Control and Protection 105
Figure 6.7
Coordination of breaker characteristics in series (Source: AS/NZS 3000, figure 2.13)
Appendix-B, clause B3 of the standard also gives additional details related to the selection criteria
for breakers and fuses and the need to match the ratings of protective devices with the circuit being
protected. Figure 6.8 gives the typical overcurrent characteristic curves of fuses and circuit
breakers which shall be kept below the damage curve of the protected cable, to achieve real
protection.
Figure 6.8
Typical overcurrent protection of cables (Source: AS/NZS 3000, Figure B2)
6.12 Simplified protective device selection (Appendix C, C3)
In the above section, we understood the need for selection of protective devices matching the
current capacity of the conductors. AS/NZS 3008.1 series provide a comprehensive set of tables
and calculation methods taking into account different cable/conductor types, installation methods
and external influences to determine the current carrying capacity of the cables under different
conditions. However for many typical and simple applications, the need to refer AS/NZS 3008.1
may not be warranted and an alternative approach may be considered - that is by limiting the
current that can be provided to the circuit by the selection of appropriately rated protective devices.
Tables C5 and C6 of appendix C provide guidance on the ratings of protective devices for singlephase and three-phase cable applications respectively, for use with cables of cross-sectional area
106 Practical Electrical Wiring Standards - AS 3000:2018
from 1 mm2 to 25 mm2, under a range of installation conditions. Table 6.3 given here is part of
those tables for immediate reference.
Table 6.3
Protective device ratings for different cable sizes (Source: AS/NZS 3000, Table C6)
The settings given in Table 6.3 are for cables used in single phase applications. For settings related
to unenclosed cables used in three phase applications, table C7 of AS/NZS 3000 may be referred. It
is to be noted that the tables are only for immediate cross checks. Exact installation conditions like
grouping, multiple circuits, etc shall be taken into account for further improvements on the
selection, where the installation conditions warrant such finer selections.
On similar lines, appendix I provides guidance to the ratings of circuit-breakers and existing semienclosed rewireable fuses or plug-in circuit-breakers that may be used to provide protection against
overload where alterations, additions or repairs involve the use of existing conductors of an
imperial size. These recommendations are as per table 6.4.
Section 2 - General Arrangement, Control and Protection 107
Table 6.4
Protective device ratings for imperial size cables (Source: AS/NZS 3000, Table I1)
V 75 insulation
Two-core
sheathed
cable of
imperial size
Installation
conditions
1/.044
V 60 insulation
CB
(Amps)
Rewireable
Fuse in
existing CB
or plug
(Amps)
CB
(Amps)
Rewireable
Fuse in
existing CB
or plug
(Amps)
Unenclosed
13
10
10
8
1/.044
Partially surrounded
10
8
8
6
3/.029
Unenclosed
16
12
13
10
3/.029
Partially surrounded
13
8
10
6
3/.036
Unenclosed
20
16
16
10
3/.036
Partially surrounded
16
10
10
8
1/.064
Unenclosed
20
16
16
12
1/.064
Partially surrounded
16
10
13
8
7/.029
Unenclosed
32
20
25
16
7/.029
Partially surrounded
20
16
16
12
7/.036
Unenclosed
40
25
32
20
7/.036
Partially surrounded
25
20
20
16
7/.044
Unenclosed
50
32
40
25
7/.044
Partially surrounded
32
25
25
20
6.13 Residual current devices for protection (Clause 2.6)
Requirements for RCD protected circuits in domestic, non-domestic, non-residential and medical
installations have been added, and RCD requirements for alterations and repairs clarified in the
2018 issue of the Standard.
Figure 6.9 shows the typical principle of an RCD, where an unbalance in the coil of the RCD due
to earth leakages leads to tripping of the main power contacts of the device by internal circuitry.
RCD is usually provided with a test pushbutton to check its healthiness to operate under real
leakages.
108 Practical Electrical Wiring Standards - AS 3000:2018
Figure 6.9
Typical RCD circuit operation with phase and neutral current unbalance
It is to be noted that RCD is always an additional protection over and above the overcurrent and
short circuit protection recommended. The use of fixed setting RCD’s with a rated operating
residual current not exceeding 30mA is recognized as providing additional protection in areas
where excessive earth leakage current in the event of failure of other measures of protection or
carelessness by users could present a significant risk of electric shock. Reasons for 30mA choice
are:
• RCD’s with a sensitivity of 30mA will operate before fibrillation of the heart
• Use of a 10mA RCD may cause unwanted tripping and needs to be considered only if
absolutely needed
The nominal rated current of an RCD installed shall be more than the maximum of the following
two values to ensure that the device is not allowed to carry beyond its normal rated current.
• The maximum demand of the portion of the electrical installation being protected by
the device
• The highest current rating of any overload protective device on the electrical
installation part being protected
RCD’s shall be complying with AS/NZS 3190, AS/NZS 61008.1 or AS/NZS 61009.1. In New
Zealand, an RCD shall be of a type where tripping is ensured for residual alternating current as well
as residual pulsating direct current. In Australia, an RCD shall be of the type that tripping is
ensured when the waveform is sinusoidal and not necessary for residual pulsating d.c.. No earthing
or protective bonding conductor shall pass through the magnetic circuit of an RCD to avoid false
operation and/or nullifying the effects of residual currents.
It is necessary to incorporate RCD protection operating at a maximum residual current of 30mA in
all types of residential installations for all final subcircuits that supply the following:
• one or more socket-outlets
• Lighting points (This term includes combination fan, light and heater units, exhaust
fans and ceiling sweep fans in an installation)
• Directly connected hand-held electrical equipment, e.g. directly connected hair dryers
or tools forming part of domestic/ residential installations
Section 2 - General Arrangement, Control and Protection 109
In case of non-residential installations, protection by RCD with a maximum residual current of
30mA shall be applied in the following circuits.
• Final subcircuits supplying socket-outlets where the rated current of any individual
socket-outlet does not exceed 20 A and
• Final subcircuits supplying lighting where any portion of the circuit has a rated
current not exceeding 20 A and
• Final subcircuits supplying directly connected hand-held electrical equipment, e.g.
hair dryers or tools
Nevertheless, the RCD requirement may be omitted for a socket-outlet or a connecting device
specifically intended for the connection of a fixed or stationary electric cooking appliance like a
range, oven or hotplate unit subject to meeting the following conditions.
• The socket-outlet is located in a position that is not likely to be accessed for general
purposes and
• The socket-outlet is clearly marked to indicate the restricted purpose of the socketoutlet and that RCD protection is not provided
In some of the installations a single RCD may not be sufficient to control multiple loads because of
the nature of loads connected in the system and a single RCD might trip the supply to the whole
installation. Hence to ensure minimum impact with the operation of a RCD, AS/NZS 3000
recommends the restriction of load points in the final subcircuits as below, mainly for residential
installations.
• not more than three final subcircuits shall be protected by a single RCD
• In case of more than one final subcircuit, a minimum of two RCDs shall be installed
Australian installations are normally to be provided with 30mA RCD to ensure human safety
against shock currents. However type ‘S’ RCD with a rated residual current in the range 100mA to
300mA may be used as a main switch in a domestic electrical installation basically as additional
protection against the initiation of fire caused by current leakage across insulation in addition to the
30mA RCD in the subcircuits for protection against shock currents.
Use of RCD with sensitivities BELOW 30mA is not mandatory in Australia. However in New
Zealand, additional protection by RCD with a maximum residual current of 10mA is desired for
final subcircuits supplying socket-outlets in areas accessible by children like
• kindergartens
• day care centres for pre-school children
• Primary schools
In case of additions/ alterations involving complete replacement of circuit protection on a
switchboard, additional protection by RCD shall be provided for the final subcircuits supplied from
that switchboard. Similarly new socket-outlets that are added to an existing circuit shall also be
protected by an RCD.
6.14 Automatic disconnection time (Appendix B)
Effective fault protection by means of automatic disconnection of supply is based on disconnecting
the mains supply to the affected section by means of a protective device. This method of protection
relies on the combination of two conditions:
• Provision of a conducting path for circulating the fault current designated as ‘earth
fault-loop’
• The interruption of the fault current within a maximum time by an appropriate
protective device. This maximum time depends on parameters, such as the highest
touch voltage, the probability of a fault, and the probability of a person touching
equipment during a fault.
110 Practical Electrical Wiring Standards - AS 3000:2018
Acceptable limits of touch voltage and duration under fault conditions are based on knowledge of
the effects of electric current on the human body that we reviewed earlier.
AS/NZS 60479 defines two components that permit the establishment of a relationship between the
prospective touch voltage that does not usually result in harmful physiological effects on any
person subjected to that touch voltage and its acceptable duration. These two components are;
• The effect on the human body of electrical currents of various magnitudes and
durations flowing through the body and
• The electrical impedance of the human body as a function of touch voltage
IEC/TR 61200-413 gives the maximum duration that a person may be in contact with an exposed
live part of a circuit for a range of touch voltages in the form of a curve (touch voltage Vs
acceptable time) under normal conditions that are dry with floor having substantial resistance
(Curve L for dry conditions) giving duration in milliseconds. These recommendations are;
• It may be possible by a person to sustain a touch voltage of 50 V indefinitely
• Same person can not sustain a touch voltage of 100 V and must be disconnected
A study was made of the influence of the variations in the different parameters on the value of the
prospective touch voltage U T and it is noted that the touch voltage in a subcircuit for a nominal
voltage U 0 can be calculated as below.
UT =
c × U0 × m
………………………. 6.4
(1 + m)
The touch voltage in a system is dependent on the two parameters as below
• A parameter ‘c’ that represents the proportion of the supply voltage available at the
reference point during operation of the protective device, that may vary from a value
of 0.6 for a circuit very far from the source to a value of 1.0 for a circuit supplied
directly from the source.
• A parameter ‘m’ which is the ratio of the cross-sectional area of the phase conductor
to the cross-sectional area of the protective earthing conductor in the circuit
considered.
Using a mean value for c=0.8 and a ratio m = 1, that exist in most of the final subcircuits, the
prospective touch voltage U T using equation 6.4 for a 230V circuit can be calculated as equal to 0.8
× 230 × 1/2 = 92 Volts.
According to curve L for dry condition referred above, this corresponds to a disconnection time of
0.4 seconds.
Accordingly it is stipulated that for a 230 V supply, disconnection time for earth faults SHALL
NOT exceed 0.4 second for final subcircuits that supply socket-outlets having rated currents not
exceeding 63 A or hand-held Class I equipment or portable equipment intended for manual
movement during use. The disconnection time shall not exceed 5 seconds for such of those circuits
where it can be proved that people are not exposed to touch voltages that exceed the safe values,
during the fault conditions
6.15 Protection against voltage effects (Clauses 2.7, 2.8)
6.15.1 Overvoltage protection
Following are the causes for overvoltage that can lead to unsafe situations in an installation, which
might be normally difficult to assess quantitatively in advance.
• An insulation fault between the electrical installation and a circuit of higher voltage.
• Switching operations.
• Atmospheric phenomena (lightning).
Section 2 - General Arrangement, Control and Protection 111
• Resonant phenomena.
Hence it is necessary that external sensing devices would be needed to detect such unforeseen
situations to ensure safe/ automatic disconnection. Overvoltage protective devices shall generally
meet the following requirements
• Overvoltage protective devices shall be reliable and shall not operate at voltages less
than or equal to the highest normal operating voltage to avoid unnecessary tripping
leading to disturbances under normal operation/ short time fluctuations within
tolerance limits.
• They shall not cause hazards to persons or livestock while operating.
• Transformer windings that operate at different voltages shall be insulated from one
another by insulation with a specified test voltage or by conductive screen connected
to the protective earthing conductor to ensure automatic disconnection of the supply
in the event of a fault.
AS/NZS 3000 does not cover requirements of installations for protection against overvoltages due
to lightning effects. AS/NZS 1768 shall be referred to decide the need/ type of lightning protection
requirements for a building.
6.15.2 Undervoltage protection
This protection is not normally mandatory but shall be employed where a drop in voltage could
cause damage to an electrical installation or part of an electrical installation or electrical equipment,
unless such damage is considered an acceptable risk.
Generally undervoltage isolation need not be instantaneous. It shall have suitable time delay to take
care of voltage drops during starting of motors and supply voltage fluctuations lasting few cycles.
In addition, further time delay may be incorporated to isolate the equipment after detecting
undervoltage subject to the condition that the electrical equipment being protected can withstand a
brief interruption or loss of voltage without danger.
6.16 Switchboards (Clause 2.9)
A switchboard or switchboards is normally provided in an electrical installation for receiving the
main supply and for the mounting of switchgear protective devices. A main switchboard shall be
provided for each electrical installation system for mounting the main switch or switches when the
multiple earth neutral (MEN) system of earthing is used. Switchboard construction generally
complying with the relevant requirements of the AS 3439 series is considered to meet the safety
requirements of AS/NZS 3000. The standard AS/NZS 3000 stipulates guidelines for the location
and the layout arrangements of switchboards in low voltage installations and these
recommendations are covered in the following paragraphs.
6.16.1 Location Requirements
While locating a switchboard in an installation, it is important to take into account the following
considerations.
• The switchboard shall be installed in well-ventilated places to minimize the high
temperature risks and associated hazards.
• It shall be protected against the effects of moisture to which they may be exposed by
adopting suitable enclosure, roof, etc. Moisture is generally the direct cause for
current leakages, shortcircuits, etc.
• The location shall meet the present and future requirements of the premises/
installation by ensuring sufficient space for the initial installation and future
replacement of individual items of the control/ protective devices as reasonably
anticipated and for undisturbed accessibility to facilitate safe operation, testing,
inspection, maintenance and repair expected during normal service life of the
switchboard and installation.
112 Practical Electrical Wiring Standards - AS 3000:2018
6.16.2
Layout and exit requirements
The layout of switchboards in an installation shall meet the following stipulations of the standard.
• Shall be located such that the switchboard and its access is not obstructed by the
structure or contents of the building or by fittings and fixtures within the building
• Shall be provided with adequate space around the switchboard on all sides where
persons have to pass through for safely and effectively operating and adjusting the
equipment/ devices provided in the switchboard.
• Shall be provided with sufficient exit facilities for the room or enclosed area
accommodating the switchboard to enable a person leaving the vicinity of a
switchboard under emergency conditions in a safe manner within the shortest
possible time.
A switchboard is considered to have sufficient access for regular operation and maintenance when
it is meeting the following conditions.
• A minimum undisturbed space of 600mm around all sides of the switchboard even
with the switchgear doors in fully open position and/or with the large internal drawout type circuit breakers, if any, are racked out.
• Minimum 750mm wide by 1980mm high openings or doorways for the switchboard
room to allow necessary access to the switchboard room or enclosure (approximately
2.5 feet x 6.5 feet).
In addition, the following design features shall be adopted for the access/ exit doors of the
switchboard rooms.
• The doors of switch rooms or rooms dedicated to switchboards shall always open in
the direction of exit. The doors located close to the switchboard shall be able to be
opened from inside of the room without using any key or tool.
• Where more than one door is provided for access to the same switchboard, those
doors should be spaced well apart, possibly one at each end and others, if any, equally
spaced in between.
• Doors of enclosures dedicated to switchboards opening into a passage or narrow
access way, due to space limitations, shall have suitable provisions to keep these
doors properly secured in the open position so that workers are prevented from
inadvertently getting pushed towards the switchboard when they use the access way
during the course of their works.
Figure 6.10 identifies most of these requirements for a typical installation.
Section 2 - General Arrangement, Control and Protection 113
Figure 6.10
Typical clearances to be maintained for switchboards (Source: AS/NZS 3000 figure 2.19)
In regard to safe exit facilities for the rooms with switchboards, it is necessary to provide more than
one alternative emergency exit path when a switchboard is of the type meeting ANY ONE of the
following system design requirements.
• It has a prospective short-circuit current of 15 kA or more
• It is supplied by a circuit with a nominal capacity of not less than 800 A per phase
• It is more than three metres in length.
6.16.3
Exclusion of areas for switchboard locations
The standard stipulates to restrict locating the switchboards in the following areas.
• Within 1.2m above the ground, floor or platform level. This condition is generally
applicable for boards with easily accessible live parts. However this condition may
not be considered, if access to live parts is provided with suitable barriers and/or
accessed by authorized personnel with proper locking arrangements.
• Above open water containers or fixed or stationary cooking appliances unless the
switchboard is provided with a suitable enclosure meeting required classifications or
it is installed in an exclusive cupboard with close-fitting doors, preventing it from
these containers/ appliances
• Within close proximity of a bath or shower or above specific zones of a swimming
pool or spa pool. Exact distances and zones of installing switchboards in such damp
locations are covered in a separate section.
• Within a sauna
• Within a refrigeration room
• In locations adopting sanitization or hosing-down operations, unless the switchboard
provided with a minimum degree of protection of IPX6
• Fire exit and egress path locations
• Near fire-hose reels or within a cupboard containing a fire-hose reel
• Near automatic fire-sprinklers (main switchboards as well as safety services boards)
• In hazardous areas as defined in AS/NZS 2430, unless they are meeting hazardous
area construction/ certification requirements
114 Practical Electrical Wiring Standards - AS 3000:2018
6.16.4
Switchboard construction
Switchboard construction shall meet the following stipulations/ requirements as per the standard.
• Live parts shall have protection against easy access/ arm’s reach by means of suitable
enclosures and/or barriers, though some exceptions may be permitted in nondomestic
installations subject to additional protection and access control to live parts
• The switchboard shall be able to withstand the mechanical, electrical and thermal
stresses that are likely to occur in the service conditions to ensure unimpeded service
• To avoid shortcircuit arcing currents through air between bare live conductors of
different potentials, it is necessary to maintain minimum clearances between such
parts. All bare conductors and bare live parts of a switchboard shall be rigidly fixed
and ensured that a minimum clearance or creepage distance in air is maintained
between such conductors or parts of opposite polarity or phase and between such
conductors or parts and earth in accordance with the AS/NZS 3439 or AS/NZS 61439
series of Standards and these are not disturbed during the service
• Where two or more circuit-breakers are mounted in the same row, the operating
means/ handles of these devices shall be orientated in one common direction that
would cause its circuit to open or close, without mismatch
• Fuse using screw-in carriers shall be connected in such a way that the centre contact
is on the supply side of the fuse base
6.16.5
Bars or links
Bars or links provided in the switchboards shall have facilities for securely terminating the
conductors between metal surfaces. All screws which are in direct contact with conductors on
tunnel type terminals shall be of the type designed not to cut the conductor. It is preferable to
consider two screws to provide enough contact and strength for tunnel type connections. One screw
may be considered adequate if the outside diameter of the screw is not less than 80% of the tunnel
diameter. Alternatively, the connections shall be arranged such that the conductor is clamped by
suitable ferrules or plates in direct contact with the conductor.
Neutral bar/links shall be of adequate current-carrying capacity and shall be located in an
accessible position to allow all conductors to be safely connected without moving other nearby
cables or isolating the supply to the switchboard. These shall be designed such that the incoming
neutral conductor cannot be inadvertently disconnected from the bar or link. Separate neutral
terminals shall be provided for the incoming neutral conductor terminating at the switchboard and
also for all the neutral conductor(s) associated with each outgoing circuit originating at the
switchboard.
6.16.6
Identification
Main switchboard and the access rooms/ doors shall be properly identified with legible writing/
engraving giving the names of the switchboards and cautioning about the rooms for taking suitable
precautions or to be aware of the hazards while entering. Notices indicating the location of the
main switchboard shall be of permanent construction and shall incorporate the term ‘MAIN
SWITCHBOARD’ in contrasting colours.
Terminals of bars, links, circuit-breakers, fuses and other electrical equipment mounted on a
switchboard shall be marked or arranged to identify the corresponding active and neutral
connection for each circuit. The terminals for the connection of the MEN link and for the main
neutral conductor shall be legibly and indelibly marked at the main neutral bar or link. Some times,
the MEN connection may be made at another location such as a substation. In such cases, the
location of the MEN connection shall be clearly identified at the main switchboard.
Section 2 - General Arrangement, Control and Protection 115
6.16.7
Switchboard Wiring
The switchboard wiring shall be designed and installed to withstand any thermal and magnetic
effects on the conductors. In case of switchboards are provided with hinged door or where there are
provisions to remove switchboard panel covers, all conductors connected to the switchboard shall
incorporate the following features.
• Have sufficient free length of free cable(s) to allow the panel to be disturbed or
moved into a position little away for carrying out any future works, when needed
• Have proper arrangements to suitably fix and retain the wires in position without
getting unduly stressed or moved at the terminals of electrical equipment when the
panel is getting moved or being fixed in its location
• Shall be arranged to prevent undue pressure on electrical equipment mounted behind
the panel
• Wiring shall be installed in such a manner that, in the event of fire, the spread of fire
will be kept to a minimum. Where a switchboard is enclosed in a case or surround,
any wiring systems entering the switchboard enclosure should pass through openings
which provide a close fit. In some cases internal sealing should also be considered to
avoid ingress of foreign matters/ dust particles and fire.
6.17 Summary
Section-2 of AS/NZS 3000 defines requirements for safe operation of an installation with proper
control and isolation of power supply feeding the total installation as well as the faulty circuits
depending on the nature of the fault conditions. These are achieved by means of switchgear and
controlgear incorporating the suitable devices to monitor fault currents. To ensure proper
monitoring, the circuitry of the installation shall incorporate proper grouping of different loads to
achieve selectivity in isolation. The switchboard controlling the installation shall withstand all
external influences for uninterrupted service with adequate ventilation and safe access
arrangements.
AS/NZS 3000 recognizes four switching conditions in an installation Viz., isolation, Emergency,
mechanical maintenance and functional switching. These switches shall incorporate provisions for
pad locking them in OFF position to avoid them getting inadvertently closed when unsafe.
Adequately rated isolation switches are required at the main supply receiving point of the
installation. Where multiple supplies are involved, each supply shall be provided with a main
switch. Additional main switches may be called for in an installation when the power demand
extended to an outbuilding is 100A or more per phase.
Emergency switches are adopted for motors, conveyors, etc to isolate the supply to the particular
load to ensure prevention of possible hazards under some unhealthy situations. The maintenance
switches are used to shutdown the power supply to a load to be maintained and to prevent
electrocution during its maintenance. Functional switches are used for just controlling individual
loads of an installation without disturbing power supply to any other circuit or device.
Overcurrent and shortcircuit current protections in LV systems are achieved by using fuses/MCB/
MCCB/ ACB based on the nominal current and/or fault current to be sensed and these devices
directly carry the load current under normal and fault conditions. These devices shall be selected
and characteristics coordinated to ensure discrimination in isolation. Shortcircuit protection is to be
considered at the point of supply while overload devices may be located in the subcircuits.
Protection against arcing current faults is needed when the switchboard rating exceeds 800
amperes. These high current boards require proper insulation and internal live parts segregation to
minimize the possibility of arcing faults. The standard also provides the recommended settings to
be adopted for typical protective devices like circuit breakers and fuses based on the conductor
sizes to limit the currents carried by such conductors.
116 Practical Electrical Wiring Standards - AS 3000:2018
The supply disconnection under earth leakages can be achieved by incorporating 30mA sensitivity
residual current device in all residential circuits controlling socket outlets, lighting and hand held
equipments. It might also be necessary to consider more RCD’s to minimize the disturbances to
the total installation.
Protection against overvoltage and undervoltage, though not mandatory, will ensure protecting the
equipments and installation from the possible influence of temperature effects. It is necessary to
consider these protections with some time delay to take care of short time disturbances in the power
supply while some big equipment are switched.
The rooms enclosing main switchboards shall have exit doors properly arranged based on the size
and importance of the boards. It is necessary to ensure provision of barriers, maintaining proper
clearances and proper connections of bars, links, terminals, wires, etc within the switchboard for
better reliability in its operation.
Section 2 - General Arrangement, Control and Protection 117
7
Section 3 – Selection and Installation of
Wiring Systems
Wiring systems distribute the power for voltage transformation and electricity consumption in
electrical installations. These comprise of bare or insulated conductors made of good conductivity
materials and routed in associated enclosures, trays, supports, etc. Both underground and above
ground wiring system requirements are covered under Section-3 of AS/NZS 3000. In this chapter
we will review the types of wiring systems commonly adopted in the LV systems and the
recommendations given in the standard for the requirements to be complied by the wiring systems.
Our study will also include selection methods of conductor sizes to meet the current capacity and
voltage drop constraints of a system. We will also review the recommended guidelines for the
installation practices to be adopted for these wiring systems.
Learning objectives
•
•
•
•
•
•
•
•
•
•
7.1
Types and selection of wiring systems
External influences
Sizing of conductors based on current capacity
Voltage drop considerations in conductor sizing
Installation requirements of wiring systems
Enclosure of cables
Underground wiring systems
Aerial wiring systems
Cables supported by a catenary
Earth sheath return systems
Types of wiring systems (clause 3.2)
The power supply to an installation from the utility companies and its distribution within the
installation are accomplished by means of bare or insulated conductors made of copper or
aluminium. The term ‘cable’ is normally used to conductors covered with insulation. With the
evolution of distribution systems and advances in technological front, the insulated cables have also
undergone many improvements to achieve higher safety and efficiency in transmitting and
distributing power. The cables can be classified in many ways, notable being voltage class (High
voltage and low voltage cables), End application (Power and control), conductor material (copper,
Aluminium, etc), insulation adopted (PVC, XLPE, etc), additional protection offered (armoured,
unarmoured, screened, lead sheathed, etc), number of conductors (single core, three core, etc) and
so on. Following are the common types of cables used for low voltage applications in Australia
with either copper or aluminium as the main conductors.
118 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
Insulated and sheathed types. The normal insulations adopted for the conductors are
polyvinyl chloride (PVC) and cross linked poly-ethylene (XLPE). These cables may
be unarmoured types or armoured types with or without additional screen (screen
generally adopted for HV cables). The cables are usually coved by an overall outer
sheath made of PVC, black being the common colour for power cables, though other
colours are possible
Mineral insulated, metal sheathed (MIMS) cables adopted in specific applications/
areas prone for high operating temperatures
Earthing cables, either PVC insulated or bare
Figure 7.1 shows typical construction features of common cables of single core and three core
cables used in LV systems illustrating the conductor/ insulation/ armour arrangements.
Figure 7.1
Typical cross section of a power cable with single/multiple conductors
AS/NZS 3000 classifies these wiring systems based on the installation methods adopted in a
location and discusses broadly about the following types of wiring systems for low voltage
applications.
• Open wiring systems (without enclosure)
• Enclosed wiring systems
• Supported on catenaries
• Supported over insulators (called aerial conductors, mostly bare)
Generally catenary type and insulator supported aerial wiring systems are run above the ground.
The term Catenary is generally referred for overhead lines supported in cantilever arrangement
hanging outside the poles. The catenary types referred in the standard are mainly for systems
adopted in LV installations in Australia for indoor as well as outdoor applications while aerial
conductors are invariably limited for outdoor use exposed to weather conditions. The open and
enclosed wiring systems mostly adopted within installations usually comprise of insulated cables
(with exception of earth conductor) and are normally routed as below.
• On a surface directly clamped or on cable tray or ladder or in trunking (duct)
• On a surface partly surrounded by thermal insulation
• On a surface fully surrounded by thermal insulation
• Buried direct in the ground and covered by soil
Section 3 - Selection and Installation of Wiring Systems 119
7.2
Selection criteria (clause 3.1)
The design and installation of electrical wiring systems shall meet the following requirements to
ensure high level of safety:
• Shall incorporate protection against physical contact with live parts of the system.
This can be achieved by covering the wiring systems with durable insulation
materials or by placing the live parts out of reach
• The selection of the conductor sizes shall meet the current-carrying capacity, voltage
drop and other minimum size requirements to provide continuous and reliable service
• Further reliability is to be achieved by providing electrical continuity of all
interconnections, joints and terminations within the system
• The wiring systems shall be adequately supported by properly fabricated supports,
suspensions and fixings along their routes, as appropriate
• The selection of all materials of the wiring system like cables, trays, enclosures, etc
shall meet its intended area of application e.g. fire-resistance, explosion protection,
safety services
• Shall be protected against mechanical damage, environmental and other external
influences by adopting enclosure or other approved means
• Installed in accordance with the requirements of this Section and the additional
requirements as specified in the manufacturer’s instructions
7.3
External influences (Clause 3.3)
Though cables are selected based on application requirements and economic considerations, it is
also necessary to take care that they are not affected by exposure to the customary external
influences expected in an application. Following are the typical factors these are exposed, which
shall be duly considered while selecting and installing the wiring systems for reliable and extended
life in any installation.
• Ambient temperatures (vary from place to place and time/ seasons)
• Heat sources
• Water or high humidity
• Solid bodies
• Corrosion/ Pollution
• Mechanical forces
• Vibration and other kinds of stresses
• Flora/ Fauna
• Solar radiation
• Hazardous areas
• Thermal insulation
The materials adopted in the wiring system shall be capable to take care of the expansion/
contraction due to varying temperatures in the ambience during its normal service without getting
degraded or disturbed in its inherent characteristics. Following are the standard design ambient
temperatures to be considered for the wiring systems while making their selection:
• In case of cables running in air, for all methods of installation, reference ambient
temperature shall be 40°C in Australia and 30°C in New Zealand.
• In case of cables buried direct in the ground or installed in underground enclosures,
the same shall be 25°C for Australia and 15°C for New Zealand.
If materials of these systems in an installation are likely to face temperatures above 60°C or below
0°C, manufacturer’s instructions shall be followed.
Another common issue with the wiring system is the probable presence of heat sources nearby and
the standard recommends considering the following alternatives to overcome the influences of such
heat sources.
• Shielding against the heat source.
120 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
Placing the systems sufficiently away from the source of heat.
Selecting a system duly considering the additional temperature rise which may occur
due to such exposure.
Reducing the current being carried by the conductor ( by choosing little larger size)
Local reinforcement or substitution of insulating material.
A method equivalent to one or more of those listed in Items (a) to (e).
Appendix-H of the standard classifies the types of wiring systems based on their ability to maintain
circuit integrity under fire conditions for specified time and mechanical damage of specified
severity, similar to IP classification of enclosures. The salient requirements and methods of
classification for wiring systems are briefly covered in a separate appendix of this manual.
High humidity and entry of water could lead to insulation failures and fault conditions. Hence when
wiring systems are installed in humid conditions or in close proximity to water substances,
following recommendations shall be considered.
• Selection of suitable wiring system that is not damaged by the presence of high
humidity or entry of water.
• In case water is likely to be collected in the service period including possibilities of
condensation on a wiring system (its enclosure), right and proper drainage points
shall be provided along its route with provisions for harmless escape.
• If a wiring system is prone to face wave action (water currents), additional
mechanical protection would be required to overcome its effects.
Entry of foreign particle present in the vicinity can also affect the performance of the wiring system
if it exceeds permitted limits as accumulation would disturb the free flow of air along the system
ultimately leading to poor dissipation of heat. Hence it is necessary to consider suitable provisions
to keep away or clearing such accumulation by suitable means.
In regard to use of wiring systems in corrosive and polluting areas, the materials chosen shall have
sufficient resistance to survive such atmospheric conditions. It is also possible that when dissimilar
materials are placed in contact or interconnected, galvanic action can take place corroding the
anode part. Similarly there could be some materials which can cause mutual deterioration or
hazardous degradation of items in close proximity. The installation shall ensure such materials are
kept away and are not in contact with wiring systems including their enclosures/ supports. In a
similar way presence of flora and fauna in the surroundings can also affect the performance of the
wiring system which shall be avoided or where unavoidable, materials selected shall be able to
overcome the probable hazards created by such plants and pet animals.
The other possibilities that could affect wiring systems are the mechanical forces during normal
service due to external factors or nature of operation of equipments in the area. The wiring systems
likely to face mechanical forces shall overcome or avoid their effects by duly considering the
following.
• Mechanical characteristics of the wiring system to suit and withstand such forces
intermittently or continuously.
• Selecting the proper location to run the wiring the system which can avoid possibility
of exposure to such mechanical forces.
• Provision of additional local or general mechanical protection to take care of the
possible impacts arising out of such forces.
The following practices are recommended for the cable installations to reduce the effects of
mechanical stresses due to such mechanical impacts.
• Provision of adequate supports either continuously or at appropriate intervals
matching the cable weight.
• Use of suitable fixings to hold the cable in position without getting damaged.
• Ensuring proper connections to match the cable size and its termination ends so that
mechanical strains at joints and terminations are kept to the minimum.
Section 3 - Selection and Installation of Wiring Systems 121
•
•
7.4
Ensuring minimum bending radius limits of cables where applicable.
Additional flexibility provisions to take care of any movement or tension during
normal use.
Sizing of conductors
AS/NZS 3000 recommends sizes of conductors adopted in wiring systems to be decided taking into
account any or all the following:
• Current carrying capacity of the conductor (Normal and short circuit conditions, as
appropriate) in line with applicable standards and manufacturer recommendations
• Minimum size criteria related to specific circuits
• Acceptable voltage drop at the equipment terminals
Following paragraphs cover the salient recommendations on these considerations.
7.4.1 Current carrying capacity (Clause 3.4)
The current carrying capacity of a cable or conductor is decided by its material, cross section, and
the insulation. But in all these cases the basic criteria is the maximum temperature that is allowed
in these cables/ conductors while carrying the specific current. The cable selection in Australia and
New Zealand are to meet the operating temperature limits as given in table 7.1, while carrying the
maximum expected current in the system where these cables are used. These are based on the
specific characteristics of the insulation adopted for different types of cables (manufactured in line
with AS/NZS 5000 series, AS/NZS 3191, AS/NZS 3808 and AS/NZS 60702.1).
Table 7.1
Temperature limits for cables (Source: AS/NZS 3000, Table 3.2)
Operating temperature of conductor O C
Type of cable insulation
Under Normal use
Maximum permissible
Thermoplastic types
V-75
HFI-75-TP, TPE-75
V-90
HFI-90-TP, TP-90
V-90HT
75
75
75
75
75
75
75
75
75
75
Elastomeric types
R-EP-90
R-CPE-90, R-HF-90, R-CSP-90
R-HF-110, R-E-110
R-S-150
90
90
110
150
90
90
110
150
Cross-linked polyethylene
X-90, X-90UV, X-HF-90
X-HF-110
90
110
90
110
MIMS
100
250
Other types PE, LLDPE
70
70
Generally the ratings as per manufacturers’ recommendations shall be considered in case of
variations to the values specified in table 7.1 or in applicable standards. However Polymeric cables
with normal operating temperatures below 75°C shall NOT be adopted in Australian and New
Zealand conditions.
122 Practical Electrical Wiring Standards - AS 3000:2018
Higher continuous operating temperatures are permissible for bare metal sheathed cables subject to
taking into account the following
• Suitability of cable terminations and mountings.
• Locating the cable away from combustible materials.
• Locating the cable away from locations where there are more possibilities for persons
touching the exposed surfaces.
• Protection against other environmental and external influences.
• Minimizing and limiting the temperature rise on the terminals of electrical equipment.
Where a single cable is unable to meet the expected current capacity needed, two or more parallel
conductors may be permitted when unavoidable. In such cases, the cable selection and installation
shall duly consider the following.
• Minimum cross-sectional area of each parallel conductor shall be 4 sqmm.
• Parallel conductors shall be of same material and same cross-sectional area to ensure
equal impedance and current sharing.
• These shall have approximately the same length and shall also substantially follow
the same route.
• Both ends of all the parallel conductors shall be effectively joined by clamping,
soldering or other suitable means.
• The selected current-carrying capacity of the conductors shall take into account the
method of installation and applicable derating factors.
• Continuous current rating of each conductor may be selected to be below the overall
current to be carried together (due to sharing). However each individual parallel
conductor shall be rated to withstand the prospective fault-current available at the
point of installation by choosing appropriate cross section and no reduction due to
sharing is to be applied for the SC rating.
It is also necessary that suitable protective devices coordinated with the continuous currentcarrying capacity of the cables shall be provided upstream of these cables such that the protective
devices operate and trip the circuits below the maximum current rating of the cables. (Refer the
previous chapter).
7.4.2
Minimum size of conductors (Clause 3.5)
AS/NZS 3000 recommends minimum size of conductors to be used for the various low voltage
circuits in an installation. Table 7.2 gives the details of the same as recommended in the standard
(not applicable for switchboard and ELV circuits).
Table 7.2
Minimum size of cable conductors in electrical installation (Source: AS/NZS 3000, Table 3.3)
Conductor
Type of conductors
Purpose
Insulated conductors
Insulated conductors
Insulated conductors
Bare conductors
Insulated flexible
Aerial wiring
Aerial wiring
Socket outlets
Other circuits
Control and signal
Power/ earthing
Power/ earthing
Material
Copper
Copper
Copper
Copper
Copper
Copper
Aluminium
Minimum cross
section mm2
2.5
1.0
0.5
6
0.75
6
16
Following are the recommendations in regard to the minimum size of neutral conductors to be
adopted and as may be noted, these are mostly interdependent on the main phase conductors.
Section 3 - Selection and Installation of Wiring Systems 123
•
•
7.4.3
In case of single-phase two-wire circuits, the neutral conductor or conductors of
consumers main, submain or final subcircuits shall have a current-carrying capacity
not less than the current-carrying capacity of the associated active conductor or the
total current to be carried, in case of multiple active conductors.
In case of multiphase systems, the neutral conductor of multiphase consumers mains,
sub mains or final sub circuits shall have a current capacity of not less than that of the
largest associated active conductor. Also the current-carrying capacity of the neutral
conductor of a multiphase circuit shall take into account the harmonic currents to be
carried (if applicable) which is normally taken as 100% of the maximum harmonics
flowing in the system. Harmonics are generated by the use of computers, fluorescent
lamps, variable speed devices, etc.
Voltage drop considerations (Clause 3.6 and Appendix C)
The cable conductors have inherent resistance leading to a voltage drop from source to the
equipment terminals, while they carry the currents. The manufacturers and applicable standards
normally specify the positive and negative tolerance limits to the operating voltage of equipment
for getting long and satisfactory performance. It is necessary that, under normal service conditions,
the voltage at the terminals of any power consuming electrical equipment shall be not less than the
lower limit specified by the relevant standard and/or manufacturer. Where the electrical equipment
concerned is not covered by a standard, the voltage at the terminals shall be such as not to impair
the safe and reliable functioning of the concerned electrical equipment. Since it is not possible to
limit the current, the only option is to reduce the conductor resistance by increasing its section.
It is stipulated that the cross-sectional area of every current-carrying conductor shall be such that
the voltage drop between the point of supply (mains) of the low voltage electrical installation and
any point in that electrical installation does not exceed 5% of the nominal supply voltage at the
mains. However in case the point of supply is the low voltage terminals of a substation located on
the premises containing the electrical installation and dedicated to the particular installation, the
permissible voltage drop may be increased to 7%.
For calculating the voltage drop in a circuit, the maximum current in the conductor during normal
operations shall be considered as per below recommendations:
• Total of the connected load supplied or Maximum demand of the circuit or Current
rating of the circuit protective device at which the installation would trip under faults.
• For final sub circuits having distributed load (such as socket-outlets or lighting), half
the current rating of the protective device may be used due to diversity factor.
• In case of parallel conductors, the maximum current carried by a single conductor
under normal current sharing conditions may be considered for calculating the drop
Appendix C of AS/NZS 3000, clause C4 provides a table to determine the voltage drop in typical
single and three phase circuits based on the formula given in AS/NZS 3008.1 series. The basic
formula used in the AS/NZS 3008.1 series for voltage drop calculation in a circuit is
Vd =
(L × I × Vc )
…………………………… 7.1
1000
Where,
V d = Voltage drop on circuit in volts (V)
L = Route length of circuit; in metres (m)
I = Circuit current (usually maximum demand) in amperes (A)
V c = Cable voltage drop per ampere-metre length of the circuit; in millivolts per ampere-metre,
(mV/Am)
When the voltage drop is expressed in percentage as % V d in terms of the circuit voltage V o in
volts, the equation 7.1 becomes,
%Vd =
(L × I × Vc ) 100
………………… 7.2
×
1000
Vo
124 Practical Electrical Wiring Standards - AS 3000:2018
Which can be simplified in terms of Am per % V d as below
(L × I ) 10 × V0
=
% …………………….. 7.3
% Vd
VC
Using the above formula Table 7.3 provides the values of right hand side of the terms (10 ×
Vo)/Vc, using the values of Vc from the AS/NZS 3008.1.1 series considering common PVC cable
types operating at 75°C for 230V (single phase) and 400V (three-phase) circuits respectively.
This table of appendix-C can be used to quickly calculate the voltage drop of any circuit and also to
choose a correct cable size based on the permissible drop while adopting these standard cables
when current and length are known. Typical worked out examples using this table are also given
below.
Table 7.3
Reference Table to calculate voltage drop of circuits (Source: AS/NZS 3000, Table C8)
Am per %V d
Cable conductor
size
230V circuits
400V circuits
1 sqmm
45
90
1.5 sqmm
70
140
2.5 sqmm
128
256
4 sqmm
205
412
6 sqmm
306
615
10 sqmm
515
1,034
16 sqmm
818
1,643
25 sqmm
1,289
2,588
35 sqmm
1,773
3,560
50 sqmm
2,377
4,772
70 sqmm
3,342
6,712
95 sqmm
4,445
8,927
Example 1: To decide the minimum size of cable for a permissible voltage drop
To find the minimum size of cable for a current of 60 amperes in 230V circuit with a length of
50metres and to limit the voltage drop at 3%,
Am of the conductor is
= 60 × 50 = 3000 ampere metres
So Am for the required 3% drop = 3000/3 = 1000
Select a value, which is above this figure from the table. We note that the PVC cable size for this
application shall be 25 sqmm minimum corresponding to 1289 Am per %Vd
Section 3 - Selection and Installation of Wiring Systems 125
Example2: To find the likely voltage drop for the selected cable matching minimum current
carrying capacity
Expected voltage drop for a route length of 100metres in a 400V circuit for a current of 40A:
Am of the cable is
= 40 × 100 = 4000 ampere metres
Considering a size of the cable as 10 sqmm or 16 sqmm that can carry 40 amperes, the
corresponding values of amperes per %Vd, for these cables are 1034 and 1643 respectively.
Hence for the expected 4000 ampere-metres, the dropFor 10 sqmm cable
= 4000/1034 = 3.87 %
For 16 sqmm cable
= 4000/1643 = 2.43%
Depending on the permissible voltage drop, the cable size can be selected.
7.5
Installation requirements of wiring systems
The standard provides stipulations for the installation of wiring systems on the following aspects:
• Electrical connections
• Identification of wiring systems
• Cable installation practices
7.5.1 Electrical connections (Clause 3.7)
Connection forms an important factor for effective distribution related to receiving supply at a
point or feeding power to the equipment. These normally involve conductor to terminal
interconnections and the standard recommends following factors to be duly taken care for ensuring
good and reliable connections.
• Material of the conductor (copper/ aluminium) and its insulation type.
• Number and shape of the stranded wires forming the conductor.
• Cross-sectional area of the conductor.
• Number of parallel conductors to be connected together at a point.
• Temperature likely to be attained by the end terminals in normal service such that the
effectiveness of the insulation of the cable conductors is not impaired.
• Suitable prevention methods to avoid entry of moisture and the siphoning of water
through any cable or wiring enclosure.
When connecting aluminium conductors, the following unique issues associated with the use of
aluminium should be considered:
• Removal of the aluminium oxide film getting formed at the conductor termination
ends.
• The relative softness of aluminium, especially at increasing temperatures during
operating conditions.
• The different coefficient of linear expansion of aluminium and other metals.
• Possible galvanic actions due to contacts with dissimilar metals (Normally equipment
terminals may be copper or brass)
The following considerations are necessary when connections are executed.
• Insulation at ends shall not be removed beyond the required length as needed for
termination. When connections are made between insulated conductors, these
connection points shall also be insulated with a degree of insulation not inferior to the
insulations used for the conductors. Damaged insulation during installation shall be
reinstated.
• Connections shall be strong enough to avoid getting loosened due to vibrations, etc
• Use of mechanical connection devices for the connection of conductors shall comply
with standards and these shall be used as per manufacturer’s recommendations.
• The stranded conductors shall be retained together by using crimping tools.
126 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
•
•
7.5.2
The connections shall not impact any undue mechanical stress on the conductors
during installation and service.
When soldered connections are adopted, it shall take into consideration the
temperature rise and stress under fault conditions, as these can easily dislodge these
connections
Joints shall be avoided in flexible cords as they are prone for failures during normal
use. Ensure that these are always used with cable couplers for joining
In case of connections involving Aerial conductors, no soldering connections shall be
adopted and these shall take into consideration exposure to Sun, weather conditions,
etc
All underground cable connections shall be sealed to prevent the entry of moisture at
these points.
Earthing conductor connections shall be made considering the possibilities of failures
and methods implemented to avoid such failures. The improvements can be achieved
by independent retention method in case of soldered connections, use of proper
screws depending on terminal types, etc.
Joints in cables shall be enclosed like using a junction box to provide adequate
protection against possible external influences.
Identification of wiring systems (Clause 3.8)
Installation wiring conductors shall be clearly identified indicating their intended function like
active conductor, neutral conductor, earthing conductor or equipotential bonding conductor. It is
usual to adopt insulation having specific colours to enable easy identification. Table 7.4 gives the
recommended colours of insulation to be used for various services.
Colour identification by sleeving or other means, using colours corresponding to those listed in
Table 7.4 may be adopted as a means of identification mainly at the termination ends at
switchboards, etc., and such sleeving or other means shall be of colour-fast, non-conductive
material compatible with the cable and its location.
Table 7.4
Insulation colour codes in electrical installation (Source: AS/NZS 3000, Table 3.4)
Function of conductors
Recommended Insulation colour
Protective earth
Yellow or Green + Yellow
Equipotential bonding
Yellow or Green + Yellow
Neutral
Black / Light blue
Active
Any colour other than the above
combinations of Green, yellow,
Green/yellow, black or light blue.
Some exceptions for colour identification are permitted for the conductors under the following
circumstances/ usage.
• Protective earthing and equipotential earthing conductors: In case of aerial bare
conductors, screen conductor and limitations in the manufacturing like use of silicon
compounds.
• Active/ neutral conductors: For insulated multi core cables identification may be done
with different means like numbering, lettering, etc and in case of insulated aerial
conductors, identification may be done by ribs.
7.5.3
Cable installation practices (Clause 3.9)
Section 3 - Selection and Installation of Wiring Systems 127
Wiring systems shall be installed in accordance with the generally accepted safe and sound
practices that can protect the electrical installation against mechanical or electrical failure under
normal service, wear and tear, as well as under any abnormal conditions that may reasonably be
anticipated. Following are some of the important provisions to be considered.
• Proper fixing with saddles, clamps, etc at regular intervals to prevent cable sags.
• Abrasion protection like use of bushings when passing near metallic surfaces.
It is necessary to consider protection against mechanical damages for the wiring systems in the
following cases as the systems in these locations are most likely to get disturbed knowingly or
unknowingly.
• Located in or under the floors
• Located above the roofs
• Suspended below the ceilings
• Concealed in the walls
• Below raised floors
At locations where the wiring systems change directions along the route, cables of the wiring
system shall be bent in a manner that their sheathing or insulation shall not get damaged or
stressed. The recommended bending radii for the cables shall be adopted as per manufacturer’s
guidelines. Where such guidelines are readily unavailable, it shall be ensured that adopted bending
radius is minimum 6 times the cable diameter for unarmoured sheathed cables and minimum 12
times the cable diameter for armoured sheathed cables.
In addition to the mechanical protection wiring systems shall be installed in an earthed metallic
armouring, screen, covering/ enclosure or protected with an RCD of 30mA sensitivity in the
following installations
• Systems concealed within 50 mm from the surface of a wall, floor, ceiling or
roof
• Systems located more than 150 mm from internal wall-to-wall or wall-toceiling corners
• Systems fixed in position by either fasteners or passing through an opening in a
structural member.
• Systems passing through a structural member, or fixed in position, within 50
mm from the face of the supporting member to which the lining or roofing
material is attached
It shall be noted that insulated, unsheathed cables enclosed in conductive wiring enclosures shall
not be installed without short-circuit protection.
In case of low voltage track systems following stipulations generally govern.
• The complete track system shall be installed so that it is open to view throughout its
entire length (not necessarily from one position).
• The entry of dust or contamination shall be minimized and the system shall be
supported securely in position without sagging or undue stress.
It is necessary to consider suitable segregation or adopt double insulation when the wiring systems
are run in same enclosure or run very close to other systems/ installations as applicable in the
following typical cases to avoid detrimental effects and consequent failures.
• Systems of different electrical installations.
• Different parts of the same electrical installation.
• Circuits of an electrical installation operating at different voltages, such as extra-low
voltage and low voltage.
• Circuits of an electrical installation supplying different safety services.
• Circuits of safety services and the remainder of the electrical installation
• Between electrical installations and non-electrical installations, such as gas and water
lines.
128 Practical Electrical Wiring Standards - AS 3000:2018
•
Between electrical installations and telecommunications/ data cable installations.
Following recommendations are to be considered in such cases.
• Conductors that form part of different electrical installations shall not be installed
within a common enclosure.
• Cables of high voltage circuits and cables of low or extra-low voltage circuits shall
not be enclosed in the same wiring system.
• Wiring systems shall not be installed in the vicinity of services that produce heat,
smoke or fumes likely to be detrimental to the wiring system.
• Wiring systems shall be suitably protected against the hazards likely to arise from the
presence of other services in normal use.
A typical segregation arrangement commonly applied with electrical communication cables is as
shown in figure 7.2.
Figure 7.2
Segregation of LV and communication cable (Source: AS/NZS 3000, Figure 3.8)
Selection and installation of wiring systems shall be ensured in such a way as to minimize the
spread of fires. Wiring systems, such as conduits, cable ducting, cable trunking, busbars or busbar
trunking systems and flush boxes that penetrate elements of building construction with
requirements for adherence to a specified fire rating shall be internally sealed to the degree of firerating of the respective element before penetration and shall also be externally sealed. The sealing
materials shall be of adequate mechanical stability to withstand the stresses that may arise through
damage to the support of the wiring system because of fire and shall also permit thermal movement
of the wiring system without reduction of the sealing quality.
In a.c. circuits, the eddy current and circulating currents are quite common due to mutual
inductance effects with closed metal parts and other energized systems at varying potential values.
It is necessary to avoid such unnecessary currents to prevent overheating of cables and wiring
systems, associated failures, etc. Following are some of the recommendations to keep off such eddy
currents and circulating currents in the installation.
• Single-core cables in lead or other non-ferrous metal sheathing may be used for a.c.
circuits subject to running them in trefoil formation throughout their entire length
except for less than 2 metres at either end for termination purpose.
• Where trefoil formations are not possible, the cables shall be placed as near as
practicable to each other with the sheathing of the cables bonded at both ends and at
intervals not exceeding 30 metres along the cable run, by a conductor having
conductivity not less than that of the cable sheath.
Section 3 - Selection and Installation of Wiring Systems 129
7.6
Enclosure of cables (Clause 3.10)
Insulated, unsheathed cables shall be enclosed in a wiring enclosure throughout their entire length,
though it is not mandatory to always install sheathed cables in a wiring enclosure. However where
the sheath of a cable is removed for termination, joints, etc., the exposed cores of the cable shall be
adequately enclosed. Following stipulations apply for such enclosed wiring systems similar to open
wiring systems.
• Preventing entry of water
• Proper supports of systems and enclosures at such points
• Ensuring Mechanical and Electrical Continuity
• Adoption of proper Bending radii
• Passage for conductors in conduits with bushings and to avoid sharp angle that could
fail in due course, etc
• Terminations securely anchored to avoid disconnections due to mechanical forces,
vibrations, etc
• Provision for normal expansions due to ambient and service conditions.
• Opening/ access provisions for cable trunking.
• Avoiding spread of fire
Following are some of the common wiring enclosures adopted in the installations. These shall be
chosen to have adequate strength and shall be of adequate size to meet the number, size and weight
of cables to be enclosed within them, including provisions for future additions/ alterations.
• Steel conduits or other metallic tubing or conduits.
• Flexible metallic conduits.
• Rigid and flexible non-metallic conduits.
• Corrugated non-metallic conduits.
• Cable trunking with or without compound filling.
7.7
Cables installed in conduits (Appendix C, clause C6)
In LV systems, installing the wires and the cables in metallic/ PVC conduits is the most common
practice. Appendix C, clause C6 provides guidelines for the number of cables to be installed in
conduits, which is an essential reference while designing the systems. The number of cables that
can be installed in a circular conduit is dependent on the size of the conduit vis-à-vis size of the
cables. It is also necessary to maintain adequate free space inside the conduits for pulling the cables
in them, to take care of the reduction of space available from the circular geometry of the cables
and enclosures, which is termed as space factor. The number of cables to be limited in a conduit is
given by the following relation.
Number of cables per conduit =
Internal cross sec tion area of conduit
× Space factor …. 7.4
Total cross sec tion area of all cables
AS/NZS 3000 recommends the following space factors to be adopted for this purpose.
• For one cable in a conduit: 0.5
• For two cables in a conduit: 0.33
• For three or more cables in a conduit: 0.4
Refer to Tables, C10, C11 and C12 of the standard which provide data on the maximum number of
cables recommended per conduit based on the equation 7.4 for single core, two core and four core
PVC V90 cables used in most of the installations. These tables use manufacturer’s data for cables
with standard conduits of bore dimensions for rigid PVC, corrugated and profile wall smooth bore
types given in AS/NZS 2053 Parts 2, 5 and 6.
The tables are on the assumptions that the conduit is relatively short in length, clear of obstructions
and distortions, and quantity and arrangement of impediments, such as bends, is minimized. Where
this is not the case, the number of cables given in the tables should be reduced suitably to ensure
that the maximum cable pulling tension and bending radius are not exceeded. It is also to be noted
130 Practical Electrical Wiring Standards - AS 3000:2018
that the current carrying capacity of cables in conduits is suitably reduced due to poor heat
dissipation which shall be duly considered while selecting the cable sizes. AS/NZS 3008 provides
de-rating factors for grouping of cables in conduits.
7.8
Underground wiring systems (Clause 3.11)
Where cables are installed underground, they shall meet the following requirements.
• Shall be suitable for the environment in which they are placed
• Shall have proper protection against inadvertent damage likely to be caused by
manual or mechanical excavation works which are the common hazards to such
systems.
• Shall be provided with suitable warning notices, marking or other acceptable means
to minimize the risk of inadvertent damage likely to be caused by manual or
mechanical excavation works.
The underground wiring systems are broadly classified into three main categories in the standard as
noted below.
Category A: Where the wiring system is suitable for installation below ground due to its inbuilt
characteristics without any further need of mechanical protection. Following are the systems that
are recognized to fall under category A.
• Cables enclosed in heavy-duty insulating conduit without further mechanical
protection.
• Cables enclosed in insulating wiring enclosures encased within concrete.
• Sheathed cables enclosed in galvanized steel pipe (not standard metal) without further
mechanical protection.
• Armoured sheathed cables or neutral-screened cables buried direct in the ground
without mechanical protection.
Category B: Where the wiring system is suitable for installation below ground only when provided
with additional mechanical protection for the cable or cable enclosure. Following are sthe ystems
that fall under category B.
• Cables enclosed in medium-duty insulating conduit with additional mechanical
protection.
• Sheathed cables buried direct in the ground with extra mechanical protection.
Category C: Where the wiring system is laid within a channel chased in the surface of rock and
covered with concrete.
Any Category A or Category B wiring system that comprises cables not installed in a wiring
enclosure shall be laid on a bed of not less than 50 mm thick sand or friable soil free of sharp stone
and covered by not less than 50 mm thick of the same material. In case of Category B wiring
system, additional mechanical protection shall meet ALL the following requirements.
• Shall be placed within 75 mm above the wiring system
• Shall be not less than 150 mm wide covering the full system
• Shall overlap the wiring system by at least 40 mm on either side
The category B systems are to be provided with one or combination of the following features
related to such additional protections.
• Precast concrete slabs having a thickness of not less than 40 mm and a classification
of not less than grade 20 in accordance with AS 3600 or NZS 3104.
• Concrete slabs cast on-site having a thickness of not less than 100 mm.
• A continuous concrete pour having a thickness of not less than 75 mm.
• Fibrous cement slabs having a thickness of not less than 12 mm.
• Bricks manufactured specifically for the protection of electric cables.
• Polymeric cable cover strip complying with AS 4702.
Section 3 - Selection and Installation of Wiring Systems 131
Underground wiring systems shall be installed with a minimum depth including cover and
protection as per Table 7.5. The depth shall be considered from the upper surface of the ground or
below any poured concrete laid on that surface to the upper surface of the wiring system of a
Category A and C systems or to the upper surface of the additional mechanical protection of a
Category B system.
Table 7.5
Depth of underground wiring systems (Source: AS/NZS 3000, Table 3.6)
Location
of wiring
system
Within
confines
of a
building
external to
a
building
Covering on the surface of
ground above the wiring
system
Poured concrete of
75 mm minimum
Thickness
No surface covering or
less than 75 mm
thickness of concrete
Poured concrete of
75 mm minimum
Thickness
No surface covering or
less than 75 mm
thickness of concrete
Category
A
Category
B
Category
C
0mm
(just
below)
0mm
(just
below)
0mm
(just
below)
500 mm
500 mm
50 mm
300 mm
300 mm
50 mm
500 mm
500 mm
50 mm
The standard gives typical illustrations highlighting these recommendations. For immediate
understanding some of these illustrations given in the standard are shown in figures 7.3 to 7.5 here.
The note identified in these drawings refers to the sand bedding requirements of the category A and
B systems noted earlier.
Figure 7.3
Category A installation located below 75mm minimum thick concrete (Source: AS/NZS 3000, Figure 3.10)
132 Practical Electrical Wiring Standards - AS 3000:2018
Figure 7.4
Category B installation located below natural ground (Source: AS/NZS 3000, Figure 3.14)
Figure 7.5
Typical Category C installation (Source: AS/NZS 3000, Figure 3.16)
Wiring systems installed underground shall be identified by an orange marker tape complying with
AS/NZS 2648.1. In order to provide early detection of the presence of underground wiring during
excavation work, these marker tapes shall be positioned at approximately 50% of the depth of
cover above the wiring system or any additional mechanical protection provided for that system. In
case of category C type chased in rock, orange marker tape shall be laid directly on top of the
wiring system before the concrete is poured.
All underground wiring systems shall be spaced not less than 100 mm from other underground
services, though relevant authorities may insist for higher distances also. Typical recommendations
of the standard on this requirement are reproduced in table 7.6 here.
Table 7.6
Minimum separation between UG services and UG wiring systems (Source: AS/NZS 3000, Table 3.7)
Minimum separation
to UG LV wiring
system
Minimum separation of
the service line
enclosure to LV system
earth electrode
Water service pipes upto
65mm internal dia
100 mm
500 mm
Water service pipes above
300 mm
500 mm
Type of Service
Section 3 - Selection and Installation of Wiring Systems 133
65mm internal dia
7.9
Saitary drainage
100 mm
Storm water drainage
100 mm
Gas
100 mm
Telecommunication
100 mm
500 mm
600 mm
500 mm
----
Aerial wiring systems (clause 3.12)
Following are the typical features considered for aerial wiring systems running above ground in
outdoor installations, with copper or aluminium as the material of the conductor. Generally aerial
systems may not be allowed by regulatory authorities in areas with dense high bushes due to
possibilities of quick spreading of fire.
• Hard-drawn bare conductors
• polymeric insulated cables
• neutral-screened cables
• Parallel-webbed, twisted, or bundled insulated cables.
It is necessary that the aerial conductors shall be insulated for the following situations:
• For any conductor span that is attached to a building or structure except those
between and supported by two independent poles or similar independent supports.
• For any conductor span running close to a building, building opening or structure
within arms reach.
• Above swimming pools and areas where sailing craft or irrigation pipes are used.
• In areas declared by the responsible Fire Authority as being subject to bushfires,
where required by the regulatory authority or the electricity distributor.
The minimum size of aerial conductors shall be as follows:
• Copper or aluminium conductors installed as aerial conductors shall have not less
than seven strands and shall be not smaller than 6 mm2 for copper or 16 mm2 for
aluminium.
• Aerial steel conductors shall have not less than three strands.
Aerial conductors for low voltage systems shall be installed such that clearances from ground,
buildings and structures other than public roadways are not less than those given in Table 7.7 and
the length of span of aerial conductors shall not exceed the values specified in Table 7.8 for the
appropriate type and size of conductor. For more details refer relevant tables of the Standard.
Table 7.7
Minimum clearances for aerial conductors (Source: AS/NZS 3000, Table 3.8)
Type of
aerial
conductors
Minimum height above buildings, structures,
ground or elevated areas
Over areas Over roofs Over other
Over areas
used for
roofs and
not used
used by
traffic
or
structures
by
vehicles
resort
vehicles
From
buildings:
Horizontal
clearance
from walls,
etc.
Bare live
5.5 m
5.0 m
3.7 m
3.0 m
2.0 m
Insulated
and
4.6 m
3.0 m
3.0 m
2.0 m
1.0 m
134 Practical Electrical Wiring Standards - AS 3000:2018
unsheathed
live
Neutral
screened
cable
4.6 m
3.0 m
2.7 m
0.5 m
1.0 m
Table 7.8
Maximum unsupported spans of aerial conductors (Source: AS/NZS 3000, Table 3.9)
Type of Service
Cross section
Maximum span
Insulated annealed copper
including neutral-screened
≥6 sqmm
60 m
Insulated hard-drawn copper
including two-, three- and fourcore twisted but excluding
neutral-screened
6 sqmm
10 sqmm
≥16 sqmm
40 m
50 m
60 m
Neutral-screened cables with
harddrawn copper conductors
- two conductors
- three conductors
- four conductors
- two, three or four conductors
Upto 10 sqmm
Upto 10 sqmm
Upto 10 sqmm
16 sqmm
40 m
60 m
50 m
60 m
Insulated or bare aluminium
excluding neutral-screened
16 sqmm
≥25 sqmm
50 m
60 m
Aerial bundled cables
(aluminium conductor)
≥25 sqmm
60 m
The aerial conductors are usually supported on pin insulators above the poles. Pin-type insulators
shall not be used for supporting aerial conductors in following situations; instead strain types shall
be adopted.
• Where the strain tends to lift or otherwise separate the conductors from the insulators
• Where the direction of the conductors is changed by more than 30°.
Section 3 - Selection and Installation of Wiring Systems 135
Figure 7.6
Typical aerial conductors on poles with pin and strain insulators
Any hardware or fittings used in association with the aerial line shall be made of corrosion-resistant
material or shall be suitably protected against corrosion, to withstand exposed the weather
conditions for providing safe and longer life.
The aerial conductors of different voltages and different services shall be adequately spaced to
prevent contact with each other under all conditions of sag and sway, quite common in varying
temperature conditions. It shall be ensured that the spacing between conductors at supports,
measured in any direction shall be not less than the values identified in table 7.9.
Table 7.9
Spacing between aerial conductors at supports (Source: AS/NZS 3000, Table 3.10)
≤10 m
Spacing for insulated
conductors
0.2 m
Spacing for bare
conductors
0.4 m
>10 m upto 25 m
0.3 m
0.5 m
>25 m upto 45 m
0.4 m
0.6 m
>45 m upto 60 m
0.5 m
0.7 m
Span
There are many tables available in appendix D of the standard giving minimum sizes of posts,
poles and struts for aerial line sizes up to 2 × 4core 95 sqmm aluminium and 4 × 7/3.5 sqmm
copper conductors with a maximum total weight of 2.7 kg/m. The appendix in the standard covers
details related to the following types of poles and supports.
• Timber posts and poles
• Square timber struts
• Angle iron struts
136 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
Steel poles
Steel square section
Fabricated steel poles.
The sizes of poles and supports are dependent on the normal loading applied by the tension of the
aerial conductors which is a factor of the conductor type, span length between supports and the
acceptable design sag. Appendix D, Tables D3 to D12 of the standard shall be referred to select the
correct type of poles based on the application requirements.
7.10 Cables supported by a catenary (Clause 3.13)
Cables supported by a catenary shall be stranded types with double insulation or the equivalent of
double insulation, ensuring suitability for Sun light when employed in outdoor. Following
guidelines shall be adopted when designing and installing wiring systems on catenary supports.
• The catenary shall provide uniform support
• It shall consist of material resistant to corrosion or deterioration like the main wiring
system
• Shall be adequately and effectively fixed at both ends
• Shall be capable of withstanding all mechanical stresses likely to occur in service
especially effects of wind or ice
• Shall be mounted at a sufficient height above the ground to prevent danger to persons
or livestock, or damage to the cable being supported.
Cables supported by a catenary wire in indoor shall maintain not less than 100 mm from any
moving parts or parts of equipment operating at an elevated temperature.
7.11 Earth sheath return systems (Clause 3.16)
The earth sheath return system is one where the copper sheath of a mineral insulated metal
sheathed (MIMS) cable is used as both a protective earthing (PE) conductor and a neutral (N)
conductor simultaneously as a PEN conductor. As per the standard, copper sheath cables alone may
be used as a combined protective earthing and neutral (PEN) conductor and shall be installed
taking into consideration of the following.
• The sheath shall be of adequate cross-sectional area and conductivity.
• The ESR shall be adopted only in electrical installations where the MEN system of
earthing is used and shall commence at the location where the neutral and earthing
conductors are connected to form the MEN connection.
• Where the ESR system is adopted as a neutral and protective earth to electrical
equipment, then the neutral and protective earth shall not be combined again to form
another ESR system.
• The ESR system shall not be installed in hazardous areas.
• Conductors used in an ESR system shall have a minimum size of 2.5 mm2.
• At every joint in the sheathing and at terminations, the continuity of the earthing
conductor shall be ensured by a bonding conductor in addition to sealing and
clamping of the external conductor
• The resistance of the bonding conductor at joints should not exceed that of the cable
sheath.
• Two conductors, one for earth and one for neutral shall be separately used at each
termination.
• Where several cables are associated, e.g. single-core cables used in a multiphase
circuit, the cables shall be arranged in trefoil or run close to each other as discussed
earlier.
• There shall be identification on the switchboard at which the circuit originates to
indicate that the circuit is using the ESR system.
• No switch shall operate in the neutral conductor of an ESR system.
• Only electrical equipment identified as suitable for use in conjunction with an ESR
system shall be used.
Section 3 - Selection and Installation of Wiring Systems 137
It may be noted that the circuits employing ESR systems are unable to be protected by RCD’s.
7.12 Summary
The wiring systems comprise of bare/ insulated cable conductors supported and/or enclosed in
suitable covers like trunkings, trays, pipes, etc. Section-3 of AS/NZS 3000 covers wiring systems
that may be open type, closed type or supported on catenaries/ insulators. In wiring systems, these
types shall be selected and installed to take care of external influences like ambient temperature,
heat sources, water, humidity, mechanical forces, corrosion, etc. The ambient temperature for these
systems is generally taken as 40 deg C in Australian conditions and 30 deg C in New Zealand
conditions. To take care of external forces, the systems shall be properly supported at regular
intervals ensuring proper connections, bending radii, etc.
The sizing of the conductors in wiring systems shall be based on the current capacity requirements
of the circuit and acceptable voltage drop within the installation. In regard to current carrying
capacity the maximum operating load of the circuit shall be considered with suitable load factors.
It is also necessary to ensure that the minimum size adopted in power circuits is 2.5 sqmm copper
for insulated cables, 6 sqmm for aerial copper conductors, 16 sqmm for aerial aluminium
conductors, etc. In regard to voltage drop considerations a 5% drop is considered nominal, though
in specific cases where the substation is internal to the installation, a 7% drop is accepted.
The standard also specifies guidelines to be adopted for electrical connections, proper identification
of cables and terminating ends including best practices to be adopted in the installation practices. In
case of aluminium connections, it is necessary to take care of the flexibility of aluminium at higher
temperature and formation of aluminium oxide. The insulation of the earth cables are generally
identified with yellow, Green or their combination. For neutral cables, the standard recommends
black or light blue colour insulation to be adopted while for active conductors these shall preferably
be brown for single phase and red, white, dark blue for the three phase circuits.
Additional mechanical protection and abrasion protection shall be considered depending on cables
laid in walls, near metal surfaces, etc. The cable enclosures shall also be installed taking care of
bending radii, supports, mechanical protection, etc like bare cables.
The Standard defines three classifications of wiring systems for underground applications.
Category A is without additional mechanical protection like cables in heavy conduits, armoured
cables, etc. whereas category B adopts additional mechanical protection e.g. cables in medium duty
conduits, direct buried sheathed cables, etc. Category C type is chased in rock with concrete poured
as its covering. In all these underground cable categories, their locations shall be identified with
orange tapes above the cables along their paths to caution the presence of cables during excavation.
Systems of different installations and different services shall be segregated generally maintaining a
minimum clearance of 100mm between electrical and non electrical pipes.
The aerial conductors shall be supported with poles and insulators at specified spans based on
conductor cross section. These are generally avoided in bushy areas to avoid fire spreads. These are
to be maintained at a minimum height of about 2 to 6 meters from ground level depending on the
areas above which these are routed. The conductors shall also be spaced with a minimum of
200mm to 700mm between them depending on the type (bare or insulated) and the span adopted.
Earth sheath return systems are generally adopted in MIMS cables where the neutral and earth are
combined as a single PEN conductor and recommended in MEN systems. These systems shall not
adopt switched neutral and can not be protected by RCD’s for leakages.
138 Practical Electrical Wiring Standards - AS 3000:2018
8
Section 4 – Selection and Installation of
Electrical Equipment
The reliability of electrical systems also depends upon correct selection and installation of
equipment and appliances to provide satisfactory and safe service during its life period, without
creating problems to the source as well. In section 4 of AS/NZS 3000, recommendations are
provided for correct selection and installation of common appliances used in electrical
installations. The section briefs recommendations on the socket outlets, lighting systems, heating
systems, etc., commonly adopted in low voltage systems. We will also review the recommendations
related to selection and installation of major electrical equipments like motors, transformers, etc.
Learning objectives
•
•
•
•
•
•
•
8.1
Selection and installation criteria
Protection against thermal effects
Connection of Electrical equipment
Requirements for Socket-outlet connections
Requirements for Lighting systems
Requirements of Cooking appliances and electrical heating systems
Basic requirements for major equipments like Electricity converters, Motors,
Transformers, and Capacitors
Selection and installation criteria (Clause 4.1)
In our earlier chapters we reviewed the requirements related to design, construction features and
safe practices to be followed for the electrical installations. The electrical equipment operating in
such installations shall be selected and installed with features as may be needed to take care of the
following to serve their purpose.
• Shall be able to function properly under all external influences to which they are
likely to be exposed during service life
• They shall prevent any adverse effects on the electrical installation during their
operation and use
• Shall operate safely when properly assembled, installed and connected to supply as
per approved specifications, standards and good manufacturing practices
• It shall be ensured that there is no danger from electric shock, fire, high temperature
or physical injury in the event of reasonably expected conditions of overload,
abnormal operation, fault or any external influences
140 Practical Electrical Wiring Standards - AS 3000:2018
While equipments are expected to satisfactorily give the intended service based on the supply
characteristics, they shall also be able to function properly under one or more of the following
external influences that might be expected in a particular installation. Accordingly the equipments
shall have suitable design aspects by incorporating additional means of protection features to
effectively protect themselves and the installation against the presence and extent of such external
influences.
• Mechanical forces, vibrations, etc.
• Exposure to adverse weather including rain, snow, ice and ultra violet rays from Sun
as applicable when installed outdoor
• Presence of water and possibilities of splashing, spraying, submersion or high
humidity in the areas of operation
• Flora, including vines, weeds, flowers and plants of all types
• Fauna, including cats, dogs, horses, cattle, etc.
• Excessive dampness and poorly sealed underground cellars, etc.
• Corrosive fumes, liquids or polluting substances, particularly those used in a
sanitization process associated with the food industry
• Galvanic action with dissimilar metals
• Accumulation of dust or solid foreign bodies
• Steam
• Oil
• High and low temperatures
• Explosive atmospheres (often referred to as ‘hazardous areas’)
• Seismic activity (earthquakes and tremors)
Following electrical disturbances that might be expected out of the equipment operation shall be
assessed and equipments and installation shall be provided with specific protection features to
avoid these effects impacting the supply characteristics to other equipments and other installations.
• Power factor – High or low
• Excessive voltage fluctuations due to abnormal duty cycles and internal
characteristics
• Severe distortion of current waveforms, like effects of harmonics
• Electromagnetic interferences
8.2
Protection against thermal effects (Clause 4.2)
High temperature of the installation and equipments can lead to burns, fire accidents, etc., and shall
be avoided for ensuring safety and protection to people and properties. Hence following
recommendations shall be followed to effectively protect the installation and systems against high
temperature effects:
• Selection and installation such that temperature characteristics of the installed
equipment do not adversely affect itself or the electrical installation feeding it or any
other installation in close proximity, whether electrical or not
• Provision of adequate ventilation where more heat is expected to be generated in
normal operation so that the operating temperatures are maintained well below the
rated or specified limits applicable for the installation
One of the major dangers associated with the high temperatures is fire. Hence following
precautions shall be taken while installing equipments to prevent possibilities of spread of fire in
case of abnormal faulty conditions:
• Avoid installation in a position where it might cause a fire hazard
• Plan the storage of flammable materials suitably
• If electrical equipment is expected to attain surface temperatures that would cause a
fire hazard to adjacent materials, or expected to cause arcs and sparks, it shall be
installed in an enclosure of applicable temperature or with low thermal conductance.
Alternatively they shall be screened suitably and kept away at sufficient distance to
dissipate the heat.
Section 4 - Selection and Installation of Electrical Equipment 141
•
•
•
•
Use of flammable or combustible material for enclosures shall be totally avoided.
Also ensure that in case of enclosures made of special materials, they meet the
service temperature requirements.
Electrical equipment, including switches, socket-outlets and other accessories shall be
with a minimum horizontal separation of 300 mm and vertical separation of 600mm
from any opening of a fire rated wall, ceiling or floor
The openings made for electrical equipment installed in fire-rated barriers shall not
penetrate beyond 50% of the thickness of the barrier
The temperatures of accessible parts of electrical equipment within arm’s reach shall
be limited to a maximum temperature as stipulated in table 8.1. Where part of the
electrical installation is likely to attain a temperature exceeding these limits, under
normal load conditions, shall be guarded so as to prevent accidental contact even if it
is for a short period as these could cause burns.
Table 8.1
Limits of temperature rise for parts within arm’s reach (Source: AS/NZS 3000, Table 4.1)
8.3
Accessible part
Metallic parts
Non Metallic parts
Operation with hand held
55 deg C
65 deg C
Parts intended to be touched but
not hand held
70 deg C
80 deg C
Parts that need not be touched
during normal operation
80 deg C
90 deg C
Connection of electrical equipment (Clause 4.3)
Following are the recommended or standard types of connections adopted for connection of
electrical equipments to source of power supply. In all these cases the source and equipment shall
be as close as possible with proper mechanical protection for the connecting cable/ wires;
• Direct connection with or without installation coupler
• By socket-outlet through a plug usually with flexible cord attached to the equipment
• By using connecting device like junction box, ceiling rose or permanent connecting
device
The equipment wiring forming part of equipment and interconnecting the equipment to the source
of supply shall meet the following stipulations:
• It shall be as short as practicable. A maximum flexible cord or cable length of 2.5 m
is recommended
• It shall have a current-carrying capacity not less than the maximum load of the
connected appliance or luminaire. The minimum cross-sectional area of 0.75 mm2 for
flexible cords is recommended, other than those specified for portable or hand-held
appliances and luminaires
• These shall be protected against short-circuit with fuses, etc
• In case of metallic or conductive enclosures, these shall include a protective earthing
conductor of suitable cross-sectional area that will ensure operation of the circuit
protective device, in the event of a fault to earth, without damage to the protective
earthing conductor
142 Practical Electrical Wiring Standards - AS 3000:2018
8.4
Socket-outlets (Clause 4.4)
Socket outlets are common power distribution devices in any installation. As you might have come
across, the socket outlet construction and voltage ratings differ in various countries as per local
practices and distribution voltages. Figure 8.1 shows some of the different models of socket outlets
used in many countries including Australia and New Zealand.
Figure 8.1
Typical socket outlets and matching plugs in LV systems
Socket outlets require matching plugs to ensure safe and reliable connection. The Standard requires
the following minimum features to be ensured for these outlets:
• Shall have their voltage conspicuously marked. This is normally engraved during
manufacture
• The socket outlets shall adopt different construction features to ensure plugs or
equipments of different voltages are not connected to a particular voltage rated
sockets. The construction shall prevent insertion of an extra-low voltage plug into a
socket-outlet connected to a circuit of greater than extra-low voltage. AS/NZS 3112
contains a specific plug and socket-outlet arrangement recommended for ELV
applications.
• The socket and plug connections shall incorporate protection against contact with live
pins of plugs, when inserted or during equipment operation
• Insertion and removal of plugs shall not cause damage to the equipment cord due to
improper matching
• The plug insertions shall exactly fit into the outlets and the connections shall ensure
additional protection against touching/ coming in contact with the pins of plugs by
that might not be fully inserted into the socket outlets (e.g. in areas frequented by
children)
• All sockets shall be provided with an earthing contact
As an additional safety, it is preferable to consider the following for the socket installation to avoid
accidental contacts and shocks.
• Using recessed type socket-outlets
• Using plugs with insulated pins
• Placing the socket-outlets out of reach
Section 4 - Selection and Installation of Electrical Equipment 143
The standard recommends following to be taken care in specific conditions of installation of
outlets:
• Shall be protected from entry and accumulation of dust and water where these outlets
are provided on floors or horizontal planes
• Where outlets are installed within 75 mm of a floor, the corresponding plug shall be
withdrawn in the horizontal plane
• The plugs shall not become loose or malfunction due to gravity, vibration or the
weight of the flexible cord or cable
• Where the outlets are provided for the connection of a fixed or stationary appliance or
a luminaire that is not readily accessible, the socket-outlet shall be securely fixed to a
structure or support to ensure that no mechanical strain is placed on the installation
wiring connections when inserting or removing the plug to the outlet
• The requirements related to socket outlets in damp and wet areas will be covered in a
subsequent chapter
• Where socket-outlets are installed in building surfaces that are required to provide
fire-resistance or acoustic properties, measures shall be taken to ensure that these
properties are maintained
Each socket-outlet shall be individually controlled by a separate switch that complies with either
AS/NZS 3133, AS/NZS 60669.1 or AS 60947.3 and shall switch ON/OFF all active conductors.
The switch shall have a current rating, at its operating voltage, not less than the current rating of the
socket-outlet it controls. A maximum of two outlets are permitted to be controlled by a switch
subject to the outlets being adjacent to each other. In such cases, the current rating of the switch
shall be not less than the total current rating of the two socket-outlets or the current rating of the
overcurrent protective device on the circuit; whichever is less.
Each switch operating a socket-outlet shall be as close as practicable to the socket-outlet and shall
be clearly marked to indicate the socket-outlet(s) or the connected electrical equipment that the
switch controls. In specific cases, where the switch is located little away from the socket-outlet it
shall be installed in a convenient and readily accessible position as close as practicable to the
socket-outlet; and both the switch and the socket-outlet shall be provided with legible, indelible and
uniform labels indicating their relationship.
Such markings are not mandatory where the socket-outlet is located more than 2.5 metres above the
ground, floor or platform level provided for the connection of a specific lamp, luminaire or
appliance and which is not accessible for general use.
Polarization of sockets is important for safe use with due consideration for the following:
• Where socket-outlets of the same type form part of an electrical installation the order
of connection of the socket-outlets shall be the same
• All socket-outlets for three-pin-flat pin plugs shall have earth, active and neutral in a
clockwise direction, when viewed from the front of the outlet
8.5
Lighting equipment and accessories (Clause 4.5)
This part of the section covers specific requirements related to lamp holders which are normally
used for incandescent lamps (now increasingly fitted with compact fluorescent lamps) and
luminaires with specific types of lamps. Requirements for the safe installation of recessed
luminaires have been enhanced, and an updated list of luminaire classifications added in the 2018
issue of the Standard.
The lamp holders shall be located to minimize the risk of direct contact with live parts of a lampholder when the lamp is removed or replaced and also to avoid mechanical damage to the lamp or
lamp-holder. Lamps, luminaires and their associated ancillary gear shall be so installed as not to
cause undue temperature rise, ignition or deterioration of the materials on which they are mounted.
144 Practical Electrical Wiring Standards - AS 3000:2018
The luminaires shall be made of materials that are rated with the temperatures expected in the areas
of operation.
Following additional protections are recommended where lamps are operating close to flammable
materials:
• The lamps shall be suitably shielded by a shade, reflector, guard or enclosure to
prevent contact with the hazardous material
• Some lamps such as spotlights generate heat in the illuminated surface due to their
basic principle of operation. These kinds of lamps shall be separated by such a
distance that the material will not attain excessive temperature. Table 8.1 gives the
minimum distances to be maintained in such cases where information is not provided
by the manufacturer.
Table 8.2
Minimum distances of lamps to flammable materials (Source: AS/NZS 3000, Table 4.2)
Lamp Rating
≤ 100 Watts
Minimum distance to
flammable material
0.6 m
> 100 and upto 300 Watts
1.0 m
> 300 watts
1.8 m
Recessed type luminaires are increasingly used in air conditioned modern installations. In case of
adopting such luminaires, the temperature rise at the rear of a recessed luminaire shall be limited to
prevent damage to adjacent materials and to minimize risks of fire. These shall be ensured by
employing luminaires certified for such applications or by providing fire resistant enclosures.
It is also necessary that such recessed luminaires and their auxiliary equipments are installed to
facilitate sufficient cooling air movement through or around the luminaire so as not to impair the
thermal insulation or other material, common in such installations. Where thermal insulation is of a
type that is not fixed in position, e.g. loose fill, a barrier or guard constructed of fire-resistant
material, these luminaires shall be provided and secured in position to maintain the necessary
clearance. Figure 8.2 gives minimum clearances to be maintained for recessed type luminaires
based on the type of lamps used.
Section 4 - Selection and Installation of Electrical Equipment 145
Figure 8.2
Minimum clearances for recessed luminaires (Source: AS/NZS 3000, Figure 4.9)
Recessed luminaires are classified as follows by AS/NZS 60598.2.2:2016, Appendix ZZ:
(a) Non-IC luminaire A recessed luminaire
(b) Do-not-cover luminaire
(c) CA90 luminaire
(d) CA135 luminaire (New Zealand only)
(e) IC luminaire
IC-4 luminaire
146 Practical Electrical Wiring Standards - AS 3000:2018
Cooking appliances and heating systems
Electricity is used for generating heat for the functioning of cooking appliances, water heaters,
room heaters, etc. Following paragraphs highlight some of the important stipulations covering these
appliances and heaters.
8.6.1 Cooking appliances (Clause 4.7)
This is generally a fixed or stationary cooking appliance having an open cooking surface
incorporating electric heating elements like cooktop, deep fat fryer, barbecue griddle, etc. These
appliances shall be provided with a switch operating in all active conductors of the power supply to
appliance. The switch shall be mounted close to the appliance in a visible accessible position. The
switch should be mounted within 2 metres of the cooking appliance but not on the cooking
appliance. This requirement does not apply to enclosed cooking appliances such as wall ovens,
microwave ovens and grills.
In New Zealand, a cooking appliance shall be connected to the electrical installation wiring by a
socket-outlet or by a coupler having suitable current rating.
Requirements for cooking appliance switching devices are clarified in issue 2018 of the Standard
for improved safety outcomes.
8.6.2 Hot water and steam appliances (Clause 4.8)
All appliances producing hot water or steam normally operate with thermostat controlling their
functions. These appliances shall be additionally protected against overheating by means of an
appropriate non-self resetting device, functioning independently of the thermostat.
Every unvented water heater shall be installed in such a direction that the pressure relief devices
and the protective device terminals are readily accessible for operation, inspection and adjustment,
as may be needed.
The over temperature protection fitted to unvented water heaters shall operate directly in the circuit
wiring to the heater elements and shall NOT be arranged for control through relays or contactors.
These shall also be provided with an independent isolating switch in addition to any automatic
switch incorporated in the heater structure and the isolating switch shall be installed on or adjacent
to the water heater or on the switchboard at which the water heater final subcircuit originates.
8.6.3 Room heaters (Clause 4.9)
These are normally installed permanently as one or multiple units in applicable rooms. All room
heaters shall have the following provisions to ensure safe operation.
• An individual isolating switch is a must for each room heater or for controlling each
group of room heaters
• The isolating switch shall be installed in a readily accessible position in the same
room or just adjacent to its room entrance or on the switchboard at which the room
heater final subcircuit originates
8.6.4 Heating cable systems (Clause 4.10)
Cables for electric heating systems may be laid in floors and ceilings. These cables shall be of the
appropriate type meeting such heating applications. While installing electrical heating cables,
following requirements shall be met.
• Shall not be in contact with flammable materials
• Shall be completely and adequately embedded in the substance they are intended to
heat
• Shall not suffer any detrimental effect due to flexing or movement of the substance in
which they are embedded
Section 4 - Selection and Installation of Electrical Equipment 147
The heating cables shall also be provided with separate functional & isolation switches similar to
the room heaters. All heating cables shall be provided with following additional safety protection
features.
• RCD with a fixed rated residual current not exceeding 30 mA
• Earthing of all conductive covers or where the floor heaters are without a conductive
covering, an earthed metallic grid with a spacing not exceeding 30 mm shall be
provided above the under-floor heating cable
• Adequate mechanical protection to prevent damage
• Suitable signs and identification cautioning their existence
8.1
Electricity converters (Clause 4.12)
These are basically electrical devices that convert the a.c. to d.c. or some stabilized form using
electronic circuits or rotary equipments. The converters covered by this standard are:
• Uninterruptible power systems (UPS)
• Semiconductor power converters (and inverters)
• Voltage stabilizers
• Motor-generator sets
• Rotary converters
Figure 8.3
Static UPS-General configuration
Figure 8.4
Rotary converter system
148 Practical Electrical Wiring Standards - AS 3000:2018
It is recommended to refer relevant standards applicable for selection and installation of specific
converters like AS/NZS 62040 series for UPS, AS 3011 for batteries, etc. The Wiring rules specify
following as the minimum safety requirements to be complied in installations using such
converters.
• The converted supply shall be controlled by a manually operated isolating switch, or
switches, at the output of the converter, or at the switchboard to which the output is
connected, and shall be located adjacent to the converter equipment
• Each electricity converter shall be controlled by switches or devices suitable for
starting and stopping the converter. Where there is more than one switch or device for
this purpose, they shall be grouped together and shall be clearly identified.
• An electricity converter shall be so arranged that it cannot supply energy upstream of
the point of connection to the installation (reverse feeding) either directly or
indirectly. This may be achieved with suitable components or interlocking
arrangements.
All converters shall be provided with overcurrent protective devices which shall be located as close
as practicable to the output terminals of the electricity converter. The unprotected interconnecting
conductors shall never exceed 15 metres in length. Such interconnecting conductors shall be
covered with metal or non flammable material.
Electricity converters, particularly static converters, such as UPS, shall be arranged to ensure that
the continuity of the neutral conductor to the load is not interrupted during bypass or maintenance
switching. The output of an electricity converter shall be provided with the same type of earthing
system used for the associated electrical installation.
Harmonic currents are common in installations adopting converters. Where an electricity converter
is intended to operate in parallel with a network or other source, circulating harmonic currents shall
be limited so that the current-carrying capacity of the conductors is not exceeded. Such harmonic
currents are normally limited by use of filters or suitable impedance in the converter circuits.
The battery installations can produce harmful acids that could corrode the floors and nearby
materials and it is necessary to ensure proper precautions to avoid spillages and to provide acid
resistant tiles where needed (These requirements are getting reduced with sealed types increasingly
used in modern installations). Similarly these are also susceptible to release gases and suitable
protections shall be needed to ensure safety to people and properties in such battery installations.
8.2
Motors (Clause 4.13)
Every motor
functions.
•
•
•
in an installation shall be provided with a switching device to give the following
To start and stop the motor
Emergency stopping
Isolating the motor for mechanical maintenance
It is generally possible to achieve all the above functions with one switch. In certain installations,
one switch may be controlling a group of motors connected within a system like a split air
conditioner unit. It is not mandatory to have such switches where the motors are part of the
appliances or rated below 150VA or when connected through socket outlets having their own
isolating switch.
Since motors draw large currents during starting and also while getting stuck up without rotation
due to unknown reasons ((locked rotor and stall currents), it is necessary to consider switches
adequately rated to meet such high currents, without getting impaired. These currents may be
considered four times the motor rated full load current in case of d.c. motors and eight times the
rated currents for a.c. motors, when manufacturer’s data is unavailable.
Section 4 - Selection and Installation of Electrical Equipment 149
Where unexpected restarting of a motor might cause danger, such motor shall be provided with
means to prevent automatic restarting after stopping. Similarly where reversal in direction of motor
during such restarting could pose danger, suitable protections shall be provided to avoid reverse
direction of rotation.
Motors shall have protective devices to protect against overload and this is mandatory for all
motors rated above 0.37 kW. These devices shall be properly coordinated with the motor
characteristics and other system protection devices.
Over temperature of motor windings shall be avoided by proper selection of the motor ratings in
line with the load to be operated. In addition proper over temperature protection methods shall be
adopted for unattended motors as noted below
• Motors having a voltage rating > 480V a.c. for shaded-pole type motors
• Motors rated for > 240V a.c. for other motors
• Other types of motors that are rated above 2250 watts
Figure 8.5
Typical three phase motor circuit with basic protections
The over temperature protection is generally achieved by proving thermal overload devices in the
motor circuits or by having built-in thermal devices. The over temperature may also be due to
intermittent over load currents for short durations, motor duty cycles with frequent starts/ stops, etc.
The over temperature protective device shall isolate the motor supply directly once such high
temperatures are detected. Such isolation shall be atleast in one pole of a d.c. or an a.c. motor
supplied with earthed systems. In case of unearthed systems, the isolation shall be in all poles of a
d.c. motor and atleast in 2 poles of an a.c. motor.
150 Practical Electrical Wiring Standards - AS 3000:2018
Figure 8.6
Typical temperature rise of a motor winding with varying duty cycles.
The over temperature device is not mandatory for motors associated with fire protection device or
where such opening of motor circuit could create a hazard and also for unattended submersible
pump motors immersed in water that have a rating not greater than 2250 W.
It shall not be out of place to mention that the motors covered by this standard are mostly motors
forming part of appliances and normally encountered in day to day use of residential and
commercial installations. Depending on the importance and size of motors, many more protection
devices are available and are selected/ incorporated for the safety of the installation and economical
reasons.
8.3
Transformers (Clauses 4.14, 4.16)
Similar to the motors, transformers is a separate subject not possible to be covered in this text. The
standard is not intended to detail requirements for all types of transformers and is basically limited
to the installation requirements related to the low voltage side of a transformer used in low voltage
installations. This standard does not cover requirements of instrument transformers used in
metering and protection circuits, luminous discharge types and transformers used in motors/ similar
equipments.
The Standard stipulates that conductors connected to the secondary windings of a transformer shall
be controlled with isolation switches and protected against over current and fault conditions like
short circuit by use of correctly matched devices.
The autotransformer shall not be used to supply equipments having a voltage rating of less than its
highest input or output voltage. For step up transformers no connection shall be made between the
primary and secondary windings, other than by a protective earth conductor.
The transformers when installed inside an enclosed area shall be provided with suitable ventilation.
Oil filled transformers with a liquid dielectric having a flashpoint <250°C and with total capacity
more than 50 litres shall be provided with an oil collection pit to drain the oil in the event of fires.
In addition installing such transformers in a chamber of adequate fire resistance and/or provision of
sills or other means shall be considered.
Section 4 - Selection and Installation of Electrical Equipment 151
Figure 8.7
Typical indoor transformer installation with oil pit
8.4
Capacitors (Clause 4.15)
Capacitors are devices normally used for improving the power factor of an a.c. system. These are
also used in applications such as motor starting, harmonic filters, etc. The capacitors in a circuit can
cause high inrush currents and hence the switching devices and connected distribution systems
used for the capacitors shall be designed to match these high currents. In addition to selecting the
capacitors for the highest voltages and currents and temperatures they are likely to be operating, the
associated accessories shall also be chosen to match the high switching currents of these capacitor
units. The Standard recommends considering the following design requirements in a capacitor
installation.
• Switches shall be provided for capacitor units like any standard electrical equipment.
The operating duty for the circuit breakers, contactors and switches used for
switching the capacitors shall be AC 6b rating as per AS 60947.
• The conductors connected to the capacitor shall have a current-carrying capacity not
less than 135% of the rated current of the capacitor; or the setting of the circuitbreaker; whichever is the greater
• In case of capacitors permanently connected to a motor circuit, Conductor current
rating shall be not less than one-third of the rating of the motor circuit conductors or
135% of the rated current of the capacitor; whichever is the greater
• Capacitors with values greater than 0.5 μF shall be provided with a discharge path
The capacitors retain the voltage at their terminals for some time duration even after its main power
supply is disconnected. Hence discharging the voltage at the capacitor terminals is an important
requirement to ensure safety when maintenance or repair works are taken up on systems with
capacitors. Hence as a safety, voltage between the capacitor terminals shall be reduced to less than
or equal to 50 Volts within one minute after disconnecting the power supply for capacitors rated
upto 650V and within five minutes of disconnecting supply for capacitors rated above 650 V,
before any work is taken up in the capacitor circuits (even in case of discharges it is recommended
to physically measure the voltage before any work is taken up).
152 Practical Electrical Wiring Standards - AS 3000:2018
For capacitors connected in parallel with appliances terminals permanently, (capacitors controlled
by the controlgear of the appliance itself), the windings of the appliance shall be connected to form
a permanent discharge path for the capacitor. No switch or fuse shall be inserted between the
capacitor and the appliance unless the capacitor itself incorporates a permanently connected
discharge device.
In case of independent capacitors used in a system, the capacitor shall be controlled by a circuitbreaker fitted with an overcurrent release. In such cases, provision shall be made for the discharge
of the capacitor by the use of auxiliary contacts of the circuit-breaker which automatically connect
a discharge device to the capacitor immediately it is disconnected from the supply or by
incorporating a permanently connected discharge device in parallel with the capacitor. This is
generally applicable for individual shunt capacitors rated not more than 100 kvar operating
independently or in parallel with groups of shunt capacitors. Like appliance capacitors, it is
essential that no fuse or switch shall be connected between the auxiliary contacts of the circuitbreaker and the discharge device.
In circuits with automatic operation, a capacitor may be controlled by a quick make-and-break
switch or contactor and protected by fuses. In such cases, the switching shall incorporate some sort
of manual operation to cater for the individual isolation of each capacitor or for the capacitor bank
as a whole.
Like transformers, capacitors also use dielectric oil internally, though the quantity per capacitor
may be much less. Nevertheless, necessary precautions shall be taken to prevent burning of this
liquid dielectric oil and in the unlikely event of fires, the products of combustion of the liquid
(flame, smoke, toxic gases) shall be prevented spreading to other parts of the premises by suitable
installation and segregation practices.
8.5
Airconditioning and heat pump systems
Requirements for airconditioners and heat pumps where the internal unit (or units) are supplied
from a switchboard or circuit separate to that of the compressor, and new exceptions have been
added in 2018 issue of the Standard.
Airconditioning and heat pump systems incorporating a compressor shall be provided with an
isolating switch (lockable) in accordance with Clause 2.3.2.2, installed adjacent to but not on the
unit, which isolates all parts of the system, including ancillary equipment, such as head units, from
the same location.
For split system airconditioning units, where the manufacturer requires the airconditioning system
to be connected to the electricity supply by means of a plug and socket at the internal unit, the
isolating switch installed at the external unit shall control the socket-outlet located at the internal
unit.
8.6
Lifts
Requirements for lifts installed for general use and that are not emergency lifts (safety services)
have been added.
8.7
Summary
The selection and installation of electrical equipment and appliances shall take care of the external
influences they are likely to face during the service without affecting the electrical installation and
ensuring basic safety features like prevention of shock, high thermal effects, etc. The selection and
Section 4 - Selection and Installation of Electrical Equipment 153
installation shall incorporate means to avoid disturbances to the supply such as power factor,
voltage fluctuations, harmonics and electro magnetic interferences.
The equipments shall be installed to ensure that they do not become source of fire when located
close to flammable substances. This is to be accomplished by adopting proper metal enclosure,
segregation, use of certified equipments, etc. The standard specifies maximum limiting
temperatures for accessible metallic and non metallic parts of the equipment and its enclosures,
which shall be ensured in an applicable installation.
The connection of an equipment or appliance may be direct or through a socket and/or with an
installation coupler or through a junction box, ceiling rose, etc. It shall be ensured that the wires
from such connecting devices to the equipment shall be as short as practicable with ratings
matching the equipment/ appliance being connected. These shall be protected with fuses and earth
continuity.
Each socket outlet shall be installed with an isolating switch matching the socket rating, unless the
installation permits it without switch. The matching plug shall exactly fit into its socket and the
installation shall ensure avoidance of accidental contacts of open pins of the plug during insertion
or removal. Specific guidelines are to be followed when these sockets are mounted on horizontal
plane or within 75mm from a floor level.
Lighting equipments may be simple lamp holders with matching lamps or luminaires designed for
specific lamps. These lighting accessories shall incorporate basic safety features to enable safe
access and safe lamp replacement as may be needed. The recessed luminaires in air conditioned
areas shall be installed ensuring minimum distances from the insulation materials and also with
enough clearances all around to facilitate adequate air circulation.
The standard also covers requirements related to cooking appliances, heating systems, etc. These
shall all be provided with proper thermal protective devices and isolation switches for safe
functioning. The heating cables in floor shall ensure proper grounding practices to avoid accidental
contacts.
In regard to electricity converters like UPS, stabilizers, etc, it is necessary to provide individual
isolation switches with provisions to avoid back feeing to the supply system, where such
possibilities exist. The harmonic currents shall be taken care while adopting electronic converter
systems in an installation by means of properly sized conductors and by providing harmonic filters.
Battery installations of converters like UPS would require special precautions to avoid accidental
contacts to their terminals and possible acid spillages.
Motors shall be provided with isolating switch for emergency stopping as near as possible. Motors
rated above 0.37kW shall be provided with thermal protection to avoid high winding temperatures
and consequent failures. A separate study on motor requirements shall be needed to decide on the
other protections to be adopted for a motor.
Transformer requirements related to low voltage connections are covered in this standard. When
total oil in transformer installation is beyond 50 litres, it is necessary to provide separate pits for oil
drainage during fires and oil leakages.
In regard to the low voltage capacitors it is necessary that the switching devices and conductors
associated in their circuits are designed to match the high switching currents of the capacitors.
Another important requirement for capacitors is provision of discharge path for charged capacitors,
when disconnected from the mains before it could be accessed for any maintenance work.
154 Practical Electrical Wiring Standards - AS 3000:2018
9
Section 5 - Earthing Arrangements and
Earthing Conductors
In an earlier section we reviewed about the importance of the earthing in electrical systems. In this
chapter we will review the AS/NZS 3000 requirements related to design and installation of earthing
in low voltage systems with specific stipulations on the MEN system commonly adopted for LV
distribution systems in Australia. We will also discuss the earthing conductor size selection
procedures and recommended arrangements for equipment earthing. A brief review will be made
on the equipotential bonding and earthing requirements for non electrical services. The chapter
will also cover the earth fault loop impedance and the recommendations on the earth loop
impedance value to be maintained in the LV systems.
Learning objectives
•
•
•
•
•
•
•
•
9.1
Requirements of Earthing system
Multiple Earthed Neutral (MEN) system
Earthing conductors
Equipment earthing
Earthing arrangements
Equipotential bonding
Earth fault loop impedance and need for limiting circuit lengths
Earthing requirements for other (non-electrical) systems
Requirements of earthing system (Clauses 5.1, 5.2)
The earthing arrangements shall be selected and installed with necessary features to perform some
important functions expected from them for safe operation of the electrical installations. The
following paragraphs give the various features that are commonly adopted in a system to ensure its
safe operation:
• The supply to the protected part or a circuit shall be automatically disconnected in the
event of a shortcircuit of live conductors to earth or excessive earth leakage current to
avoid consequent hazards like fire, shocks, etc. This is achieved by means of
protective earthing arrangements to enable proper sensing of fault conditions for
immediate disconnection.
• There are some equipment/ devices that require an earth reference to function
correctly rather than for safety purposes. This is achieved through functional
earthing (FE) arrangements, e.g. Cathodic protection, radio interference signal
suppression, clean earth in instrumentation systems, etc
154 Practical Electrical Wiring Standards - AS 3000:2018
•
•
It is necessary that the system shall be able to mitigate voltages appearing on exposed
conductive parts of equipment and extraneous conductive parts of the installation due
to various reasons like mutual inductance, etc that would make these parts getting
charged and becoming unsafe during normal service. This is achieved through
equipotential bonding arrangements and by having features to provide connection of
exposed conductive parts and extraneous conductive parts to earth.
Until the isolation of a fault by automatic disconnection, the system fault path shall
be capable of carrying sufficient quantity of earth fault and earth leakage currents
without danger or failure from thermal, electromechanical, mechanical, and
environmental and other external influences. This is achieved by providing effective
and reliable low fault loop impedance path in the earth circuit so that the fault
conditions can be monitored and sensed correctly to ensure disconnection at
appropriate times
Three phase LV systems usually have the source neutral earthed solidly. Connecting an impedance
between neutral and earth is not a recommended practice for consumer LV installations. Majority
of the installations under the purview of AS/NZS 3000 are provided with solid earthing for the LV
neutral. However where impedance is added in the neutral, like in the case of some special
installations and in some diesel generator installation, etc., it shall be ensured to meet the following
requirements.
• The value of earthing resistance shall be in accordance with the protective and
functional requirements of the electrical installation and shall be continuously
effective
• earth-fault currents and earth-leakage currents shall be carried by the impedance
without danger, particularly ensuring that associated thermal, thermo/
electromechanical stresses do not affect the system/ safety requirements
• Shall be adequately robust and if needed, shall have additional mechanical protection
appropriate to meet the external influences expected in the system/ environments
9.2
Multiple Earthed Neutral system (Clauses 5.3)
Multiple Earthed System (MEN) described in Australia and New Zealand standards is similar to
TN-C-S system defined in IEC 60364 and IEE wiring regulations which we briefly covered in an
earlier chapter with an appendix giving more details on the same. The letters in TN-C-S refer to the
following:
• T - the distribution system is directly connected to earth—at the neutral point of the
supply transformer
• N - The exposed conductive parts are connected to the earthed point of the
distribution system. It is neutral or if neutral is unavailable it could be one of the
phases
• C - the neutral and protective conductor functions are combined in a single conductor
(the PEN conductor of the source)
• S - the protective conductor function is separated from the neutral—separate neutral
and earth conductors within the installation
The 2018 issue of the Standard defines the MEN system for further clarity. MEN connection
requirements have been added regarding location in an accessible position.
In every electrical installation there shall be an MEN connection (also known as the MEN link) at
the main switchboard that receives the mains supply from the distributor. The function of the MEN
connection is to enable the earthing of the supply system neutral conductor within the electrical
installation. This is done by means of a connection from the main earthing terminal/connection or
bar of the installation to the earthing terminal provided on the MEN link. Where the MEN
connection is done using insulated conductors, the insulation shall be colored green or in a
combination of green and yellow. An MEN System in an installation generally comprises of all the
following components:
Section 5 - Earthing Arrangements and Earthing Conductors 155
•
•
•
•
•
•
Protective earthing conductors connecting all exposed conductive parts within the
plant as required
Main earthing conductor (Copper or aluminium) used for interconnecting all these
protective earth conductors to the earth
Main earthing terminal, connection or bar of the plant used for terminating one end of
the main earthing conductor with its other end connected to earth electrode
MEN link/connection from the above earthing terminal/connection/bar to the mains
supply neutral bar
Earth electrode within the plant connecting all the above parts to the earth through the
main earth conductor
Equipotential bonding conductors of extraneous conductive and other parts as
applicable/ available in the system
Typical MEN systems showing connections to earth at consumer installation and substations are
shown in figures 9.1 and 9.2.
Figure 9.1
General arrangement of MEN connections (Source: AS/NZS 3000, Figure 5.1)
156 Practical Electrical Wiring Standards - AS 3000:2018
Figure 9.2
Alternative arrangement of MEN connections (Source: AS/NZS 3000, Figure 5.2)
9.3
Selection of earthing conductor (Clauses 5.3.2, 5.3.3)
Copper is the most common and recommended conductor to be adopted for earthing connections.
Copper conductors used for earthing shall be of high conductivity type and may be in any one of
the following forms:
• Multi-strand conductors
• Circular braided conductors
• Solid conductors subject to a minimum cross-sectional area of 10 mm2 and a
minimum thickness of 1.5 mm except where a cable standard permits a smaller
conductor size
The cross-sectional area of a copper main earthing conductor with multiple strands shall be not less
than 4 mm2 and need not be greater than 120 mm2.
The next alternative is using aluminium earthing conductors which shall meet the following
requirements:
• Solid conductors of 10 mm2 or less (without strands)
• Minimum 16 mm2 conductors for main earthing conductors
• The installation shall provide satisfactory terminations with features to prevent
corrosion
• Shall not be installed underground or in other damp situations unless it is designed for
these conditions
Materials other than copper or aluminium also may be used as an earthing conductor. In such cases
the resistance of the alternative material shall be not greater than the resistance of the equivalent
copper earthing conductor size determined in accordance with appropriate methods. Also its degree
of corrosion resistance shall not be inferior to copper/ aluminium.
Protective earthing conductors may include the following:
(a) Earthing conductors that comply with Clause 5.3.2.1, separately installed
(b) Earthing conductors that comply with Clause 5.3.2.1, in a common enclosure with live
conductors.
Section 5 - Earthing Arrangements and Earthing Conductors 157
(c) Earthing conductors in multi-core cables
(d) Busbars
Under special conditions following may be accepted as earthing conductors:
• Conductive conduit, tube, pipe, trunking and similar wiring enclosures subject to
enclosing live conductors within
• Conductive sheaths, armours and screens of cables that are parts of the live conductor
cable
• General conductive framework subject to ensuring proper connections for earth
continuity
• Catenary wires not less than seven strands and with a minimum cross-sectional area
of 8.5mm2, constructed of hard-drawn copper or galvanized low carbon (mild) steel
Earthing conductors shall be provided with insulation except for the following options:
• Aerial conductors
• Flat braided conductors
• Busbars
• Sheaths of MIMS cable
• Catenary supports
• Metallic wiring enclosures
• Copper earthing conductors buried direct in the ground
9.3.1
Minimum size of earthing conductors
Sizing of a copper earthing conductor can be generally chosen from Table 9.1 given in this section
and the size is normally based on the cross-sectional area of the largest active conductor supplying
the portion of the electrical installation being protected. In case of using earth conductors of
different material, the considerations given in earlier section shall be taken into account, subject to
acceptance of such alternative materials by competent personnel and relevant authorities.
For installations using a number of parallel connected active conductors in the mains, the
equivalent active conductor size shall be determined by summing up the cross-sectional areas of the
individual largest active conductors of the system. Then the minimum earth conductor size shall be
chosen from the table in relation to the equivalent size so arrived or where the equivalent size is not
appearing in the table in relation to the next larger size of the equivalent size.
In some cases the size of the main earthing conductor need not be determined in relation to the size
of the largest main active conductor size. Following are the typical cases, where sizes different
from the main active conductor are accepted and it may be necessary to cross check those sizes
based on calculations method in larger systems:
• Where double insulation is maintained between the point of supply and the load
terminals of the protective devices for the submains and final subcircuits from the
main switchboard, the minimum size of the main earthing conductor may be
determined in relation to the cross-sectional area of the largest active conductor of the
outgoing submain or final subcircuit.
• Where the cross-sectional area of the consumers’ mains is chosen to be larger than
that required to carry the maximum demand of the installation because of voltagedrop limitations, the minimum size of the main earthing conductor may be chosen in
relation to the minimum cross-sectional area of the cable size required to carry the
maximum demand.
• Copper or aluminium conductors installed as aerial earthing conductors shall have not
less than seven strands and shall be not smaller than 6 sqmm for copper or 16 sqmm
for aluminium.
Sizes marked with # in table 9.1 may have to be increased in case of higher fault currents, mainly
to lower the earth fault loop impedance.
158 Practical Electrical Wiring Standards - AS 3000:2018
Table 9.1
Minimum earth conductor sizes (Source: AS/NZS 3000, Table 5.1)
Minimum earth conductor sizes for smaller systems and loads shall be decided depending on the
application and insulation types as noted below:
• Minimum size of copper earthing conductor in the form of single-core insulated cable
or flexible cord shall be 2.5 sqmm
• Minimum size of an earthing conductor incorporated with associated live conductors
in sheathed multicore cable shall be 1 sqmm
Section 5 - Earthing Arrangements and Earthing Conductors 159
•
9.3.2
Minimum size of earthing conductor in the sheath of a multicore flexible cord, shall
be not less than the cross-sectional area of the largest active conductor in the flexible
cord provided largest active conductor in the flexible cord is in between 0.5 and 2.5
sqmm, mainly for such of those flexible cords used to supply a hand-held or portable
appliance
Size of earthing conductors by calculations
The minimum cross-sectional area can also be determined by calculation using the following
equation, based on fault current to be carried and its duration. This equation is to be used only for
systems having disconnection times not less than 0.1 second but not exceeding 5 seconds.
S=
I2 × t
K
………………………. 9.1
where
S = cross-sectional area of protective earthing conductor, in sqmm
I = RMS value of the fault current in amperes that would flow through the overcurrent
protective device of the circuit concerned in the event of a short-circuit of negligible
impedance
t = the disconnecting time of the overcurrent protective device in seconds, corresponding to
the value of fault current I
K = factor dependent on the material of the protective earthing conductor, the insulation and
other parts and the initial and the final temperatures (e.g. for copper conductors with PVC
insulation not laid up with other conductors, K = 136 and for bare copper K=170)
If a calculation results in non-standard size, conductors of the nearest higher standard crosssectional area available in the market shall be used.
Functional earthing conductors that are solely for correct operation of electrical equipment, or to
permit reliable and proper functioning of electrical installations need not comply with requirements
for main and protective earthing conductors. The sizes are usually chosen low but with adequate
strands/ protection to ensure reliable connection. Where earthing combines both protective and
functional purposes, the requirements for protective conductors shall be adopted for sizing.
9.4
Earthing connections (Clauses 5.3.4, 5.3.5, 5.3.6)
The main switchboard of every electrical installation shall be provided with a main earthing
terminal/connection or bar at an accessible and relatively undisturbed position, as we noted in an
earlier chapter. The following conductors of the installation shall be connected to this main earth
connection in the switchboard, either directly or indirectly, to form an equipotential bonding
network.
• Protective earthing conductors.
• Main earthing conductor.
• MEN connection.
• Equipotential bonding conductors.
• Functional earthing conductors, if required.
It is recommended that the functional earth connection is independent to the protective earth, as
otherwise it may be conflicting with the manufacturers requirements. Thus functional earth
connection shall preferably be limited to the main earth connection point only.
9.4.1 MEN Connection
In every electrical installation there shall be an MEN connection (also known as the MEN link) at
the main switchboard.
160 Practical Electrical Wiring Standards - AS 3000:2018
NOTE: The function of the MEN connection is to connect the earthing system within the electrical
installation to the supply neutral conductor by means of a connection from the main earthing
terminal/connection or bar to the earthing terminal on the main neutral bar.
Exceptions:
1) The NE connection may be made at an earth bar within an owner or user operated supply
substation (refer to Figure 5.2)
2) The NE connection may be made at an electricity distributor neutral bar within the electrical
installation, e.g. at the supply substation or meter panel, if so required by the distributor
3) The NE connection may be made through an earthing conductor or terminal, provided by the
electricity distributor
NOTE: An earthing conductor or terminal provided by the electricity distributor may include a
special earthing conductor, the conductive sheath of a supply cable, or a neutral bar at a substation.
The MEN or NE connection shall be located in an accessible position for disconnection and
testing purposes.
The minimum size of this MEN connection conductor shall be not less than the current carrying
capacity of the main supply neutral conductor. However in some cases depending on non provision
of the shortcircuit protection arrangement or non provision of double insulation at supply end may
demand a higher MEN conductor size to be used.
Where the MEN connection is insulated, the insulation shall be colored green or in a combination
of green and yellow in accordance with Clause 3.8 of the Standard.
9.4.2
Earth electrodes
The connection of the electrical installation earthing system to the general mass of earth shall be
achieved by means of one or more earth electrodes of acceptable types. The materials and
dimensions of the earth electrode shall be chosen to withstand corrosion and shall have adequate
mechanical strength based on the nature of the soil and environmental conditions. Table 9.2 gives
typical types and installation of earth electrodes recommended in the standard.
Table 9.2
Acceptable types of earth electrodes (Source: AS/NZS 3000, Table 5.2)
Section 5 - Earthing Arrangements and Earthing Conductors 161
Based on the soil conditions commonly prevalent, it is usual to consider the vertical earth
electrodes to be driven to a minimum depth of: 1.2 metres in Australia and 1.8 metres in New
Zealand. In case of using strip-type earth electrodes buried in a horizontal trench, these shall be laid
at a depth having not less than 0.5 metre from soil surface and with a minimum horizontal length of
3 metres in Australia and 7.5 metres In New Zealand.
Following requirements shall be met for the earth electrodes and their installation methods to
ensure the desired protection and safety:
• Shall have effective contact with moist soil that shall not be subject to excessive
drying out. This may have to be done either by keeping them exposed to outdoor
conditions where cold conditions prevail or covering the electrodes suitably to avoid
loss of moisture in case of hot conditions.
• Shall be separated from conductive enclosures of other buried services such as water,
gas, telecommunications and flammable liquid, to reduce possible electrolytic/
galvanic corrosion
• The main earthing connection to an earth electrode shall be accessible for the
purposes of inspection, testing or modifications, as may be needed at any time
9.5
Equipment earthing (Clause 5.4)
As we reviewed earlier, it is necessary to ensure all equipment are provided with protective earthing
connected to earth to keep the touch potential of contact surfaces to zero or within acceptable limits.
The equipment shall be connected to earth by means of conductors in the form of cables, cords,
busbars or similar forms of current-carrying material or through another earthing medium such as
conductive parts of cables, wiring enclosures, switchboard framework, etc available in a system.
It is necessary to ensure that all the following are kept connected to earth under all conditions:
• Earthing contact of every socket-outlet
• Every lighting point
• Transformers supplying ELV lighting systems
• Exposed conductive parts of luminaires
• Conductive poles, posts, struts, brackets, stay wires and other conductive supports of
low voltage aerial conductors
• Parts of structural metalwork, including conductive building materials
• Exposed conductive parts of a submersible pump
Following items need not be earthed:
• Electrical equipment complying with AS/NZS 3100 for double insulation
• Cables affording double insulation normally connecting to such equipment
• Exposed conductive parts supplied by an SELV or a PELV system
Structural metalwork including conductive building materials
Structural metalwork forming the frame of a structure containing an electrical installation or part
thereof, including sheds or similar structures that are permanently connected to the electrical
installation wiring, shall be earthed. The size of the earthing conductor used for earthing the frames
shall be determined from Clause 5.3.3 in relation to the cross-sectional area of the largest active
conductor that is contained within the framework of that electrical installation.
For combined outbuildings, each outbuilding shall contain its own individual bonding connection
to the conductive frames within that outbuilding.
All other conductive building materials shall be earthed where—
a) the risk of contact with live parts of electrical equipment or insulated, unsheathed cables exists; or
b) double insulation of cables in contact with conductive building materials is not permanently
and effectively maintained.
162 Practical Electrical Wiring Standards - AS 3000:2018
9.6
Earthing arrangements (Clause 5.5)
A main earthing conductor shall be taken from the main earthing terminal/connection or bar
provided at the main switchboard to an earth electrode located as close as possible and in as direct a
manner as possible. It shall not be connected to the terminal of any accessory, luminaire or
appliance.
The connection of the main earthing conductor to the earth electrode shall meet the following
requirements:
• be accessible for visual inspection and for the purposes of testing
• be made by means of a suitable device having good conductivity and in accordance
with the manufacturer’s recommendations
• be protected against mechanical damage and corrosion
• be prominently labeled indicating WARNING: ‘MAIN ELECTRICAL EARTHING
CONDUCTOR—DO NOT DISCONNECT’ to caution against accidental
disconnection.
The resistance of the main earthing conductor, measured between the main earthing
terminal/connection/bar end and the earth electrode including the connection to the earth electrode,
shall be not more than 0.5 Ω.
Protective earthing connections of equipments shall be directly connected to the main earthing
conductor or to another point on an earthing system that is subsequently connected to the main
earthing conductor. These may be done through one or a combination of the following:
• To an earthing terminal/connection or bar at the main switchboard provided
specifically for the connection of earthing conductors and which is directly connected
to the main earthing conductor
• To any point on the main earthing conductor
• To an earthing terminal/connection or bar at a distribution board provided specifically
for the connection of protective earthing conductors
• Any point on a protective earthing conductor providing facilities for earthing at a
distribution board
It shall however be noted that a protective earthing conductor of a distribution board shall not be
used for the connection of earthing facilities for another distribution board or for connection of an
equipment fed by another switchboard.
Continuity of earthing connections shall be ensured by adopting proper connection practices. Star
or cutting washers or similar devices that effectively cut through paint or similar coatings are
acceptable across bolted or clamped joints. Where electrical equipment is connected in the form of
a plug and socket-outlet, appliance plug or similar connecting device, connection of exposed
conductive parts to earth shall be made automatically, before the live connections are made while
inserting the plug. In a similar way, it shall be ensured that when any plug portion is withdrawn
from the corresponding socket-outlet the earth connections are NOT broken before the live
connections are broken.
9.6.1
Outbuildings
All parts of an electrical installation in or on an outbuilding that are required to be earthed in
accordance with Clause 5.4 shall be earthed by one of the following methods:
a) Individual outbuildings The earthing system in an individual outbuilding shall be either—
i) connected to a protective earthing conductor connected in accordance with Clause 5.5.2.1;
or
ii) connected as a separate MEN installation in accordance with Clauses 5.5.3.1(c) and 5.5.3.2.
Section 5 - Earthing Arrangements and Earthing Conductors 163
b) Combined outbuildings The earthing system in a combined outbuilding shall be connected to a
protective earthing conductor, connected in accordance with Clause 5.5.2.1, and shall not be
connected as a separate MEN installation.
Separate MEN installation The earthing system in a separate MEN installation shall be connected to
the submain neutral conductor supplying the outbuilding. In this case, the submain neutral
conductor supplying the outbuilding is a combined protective earthing and neutral (PEN) conductor.
The electrical installation in the outbuilding shall be regarded as a separate electrical installation,
and shall be earthed in accordance with other relevant Clauses of this Standard..
9.6.2
Wiring systems
Exposed conductive parts of wiring enclosures shall be earthed at the end adjacent to the
switchboard or accessory at which the wiring enclosure originates. Similar procedure shall be
followed for earthing connections of the conductive sheathing, armouring or screening of cables or
cords.
9.6.3 Unprotected consumers mains
Exposed conductive parts associated with consumers mains that are not protected with short circuit
protection on the supply side shall be earthed by a conductor with a current-carrying capacity not
less than that of the main neutral conductor. This conductor shall be connected to the main neutral
conductor or bar or the main earthing terminal/connection or bar. The cross-sectional area of the
MEN connection shall be not less than that of the main neutral conductor.
9.6.4 Protection of earthing connections
The earthing connections and conductors shall be suitably protected against mechanical damages
by ensuring that the clamps, clips, etc that are holding the cables do not pass through the conductor
strands and guarding these with metallic barriers or other suitable means. Similarly these shall be
protected from chemical deterioration due to corrosive effects by using corrosion resistant materials
and by ensuring prevention of moisture by sealing as well as prevention of contacts between
dissimilar metals, etc. Using the available wiring enclosures to include earthing conductors shall
also be considered, wherever possible.
9.7
Equipotential bonding (Clause 5.6)
Equipotential bonding requirements have been expanded and clarified through enhanced
requirements for showers, bathrooms, pools and spas in 2018 issue of the Standard.
Equipotential bonding arrangements shall be provided to avoid any potential differences that may
occur between electrical equipment connected to the electrical installation earthing system and any
conductive piping (including taps, etc) that may independently be in contact with the mass of earth.
It is necessary to adopt equipotential bonding for the fixed extraneous conductive parts generally
for all the following applications.
• Conductive water piping. It shall be bonded to earth as close as practicable to the
entry of the conductive water piping to the building
• Other metallic piping systems. These may not be bonded in case they are effectively
earthed by connection to an associated item of electrical equipment e.g. water pipes
connecting the electric hot water systems.
• Metal cable sheaths and metallic wiring enclosures
• Any conductive reinforcing within a concrete floor or wall of a room containing a
shower or bath.
• The exposed conductive part of any electrical equipment in the swimming and spa
pools within the classified zones and any exposed conductive parts of electrical
164 Practical Electrical Wiring Standards - AS 3000:2018
•
equipment that are not separated from live parts by double insulation and are in
contact with the pool water, including water in the circulation or filtering system.
Telephone and telecommunication earthing systems at an enclosed terminal provided
for the purpose or directly to the earth electrode by an independent connecting device
and shall be clearly identified.
Additional equipotential bonding requirements apply for:
(a) Patient areas of hospitals, medical and dental practices and dialyzing locations, in accordance
with AS/NZS 3003
(b) Explosive atmospheres, in accordance with Clause 7.7
(c) Telecommunications installations, in accordance with AS/NZS 3015
(d) Film, video and television sites, in accordance with AS/NZS 4249
(e) Photovoltaic arrays, in accordance with AS/NZS 5033
(f) Grid connected inverters, in accordance with AS/NZS 4777.1
(g) Generating systems, in accordance with Clause 7.3
(h) Separated circuits, in accordance with Clause 7.4
Conductive water piping
Conductive water piping that is both—
(a) installed and accessible within the building containing the electrical installation; and
(b) continuously conductive from inside the building to a point of contact with the ground,
shall be bonded to the earthing system of the electrical installation
Bonding conductor size shall generally take into consideration the following recommendations as
per the standard.
• Metallic piping, cable sheaths and wiring enclosures shall have a cross-sectional area
of not less than 4 sqmm.
• Metallic parts of wet areas like showers, pools, etc shall have a cross-sectional area of
not less than 4 sqmm.
• Telephone and telecommunication earthing system shall have a cross-sectional area
of not less than 6 sqmm.
• Refer to AS/NZS 2381.1 and the AS/NZS 61241 series of Standards for minimum
sizes of equipotential bonding of items in hazardous areas.
It may be noted that bonding of extraneous conductive parts and their connection to the earthing
system also help to get reduction in earth fault-loop impedance that is necessary to keep the
disconnection time within a short time, as applicable.
Section 5 - Earthing Arrangements and Earthing Conductors 165
9.8
Earth fault loop impedance (Clause 5.7 and Appendix B)
The path available or used for the circulation of fault current in a system is called the earth fault
loop. In an MEN system, the earth fault loop comprises of the following parts.
• The active conductor as far as the point of the fault, including supply mains, service
line, consumers mains, submains (if any) and the final subcircuit
• The protective earthing conductor (PE), including the main earthing
terminal/connection/ bar and MEN link connection
• The neutral-return path, consisting of the neutral conductor (N) between the main
neutral terminal/bar and the neutral point at the transformer, including supply mains,
service line and consumer mains
• The path through the neutral point of the transformer and the transformer winding
The impedance of the earthing system shall be limited to that which will generate sufficient current
in the protective device to cause operation of that device within the required time.
The following equation fulfils this requirement.
ZS ≤
U0
………………………. ………………………9.3
Ia
Where
Zs = the impedance of the earth fault-loop comprising the source, the active conductor up to the
point of the fault and the return conductor between the point of the fault and the source
I a = the current required to cause the automatic operation of the disconnecting protective device
within the required disconnection time
U o = the nominal a.c. r.m.s. voltage to earth (230 V)
Figure 9.3 shows the impedance path of an earth loop in common LV systems covering the supplier
and customer installations together. At the instant of the fault, current will flow through the earth
fault-loop and its magnitude is limited only by the total system impedance Zs that is equal to the
sum of all the individual impedances in the loop as below.
Zs = Z AB + Z BC + Z CD + Z DE + Z EF + Z FG + Z GH + Z HA …………9.4
As noted from figure, impedances Z AB , Z BC , Z FG , Z GH and Z HA are all upstream of the protective
device within the electrical installation and are regarded as being external to the reference point;
accordingly they may be collectively referred to as Z ext . The remainder that are downstream (or
‘internal’ within the installation) may be referred to as Zint (Z CD , Z EF ), therefore, Zs = Zext + Zint.
166 Practical Electrical Wiring Standards - AS 3000:2018
Figure 9.3
Simplified MEN system identified with earth fault loop (Source: AS/NZS 3000, Figure B5)
When an electrical installation is being designed, Zext may or may not be available (it will depend
on the electricity distributor’s transformer and supply cables). If it is not available Zint may be
determined by either of the following two methods:
Method 1: When the length and cross-sectional area of conductors are known, we know from
above that
Zint = Z CD + Z EF ……………………………………………...9.5
Reactance may generally be ignored for conductors of 35 mm2 or less where the active and earthing
conductors are in close proximity to one another. Thus, for such circuits, the current I a may be
calculated using only conductor resistance as per the following equation.
Ia =
U0
………………………. ………………………9.6
R PE + R L
Where
U O = the nominal a.c. r.m.s. voltage to earth
R PE = the resistance of the protective earthing conductor from the reference point to the exposed
conductive part (See figure 9.3)
R L = the resistance of the phase (active) conductor from the reference point to the exposed
conductive part.
In case of finer values using impedance values for conductors, these are available in the AS/NZS
3008.1 series and referring to the tables in the standard for the known conductor sizes, Z int can be
arrived at.
Section 5 - Earthing Arrangements and Earthing Conductors 167
Method 2: When the length and cross-sectional area of the supply conductors are not known, it
may be assumed that there will always be 80% or more of the nominal phase voltage available at
the position of the circuit protective device. Therefore, Zint should be not greater than 0.8 Zs. This
may be expressed as follows:
Zint = 0.8 U o /I a …………………………………………..9.7
Above equation may be expressed in terms of circuit length by considering (active and earth)
conductor sizes and protective device tripping current. This gives rise to the following equation.
L max =
0.8 U 0 S ph S pe
I a ρ (S ph + S pe )
…………………………………9.8
Where,
L max
= Maximum route length of the conductor in metres
Uo
= nominal phase volts (230 V)
ρ
= resistivity at normal working temperature in Ω-mm2/m
= 22.5 × 10−3 for copper
= 36 × 10−3 for aluminium
Ia
= Trip current setting for the instantaneous operation of a circuit-breaker; or the current
that assures operation of the protective fuse concerned, in the specified time
S ph
= cross-sectional area of the active conductor of the circuit concerned in mm2
S pe
= cross-sectional area of the protective earthing conductor concerned in mm2
It is not desirable to have an impedance value that can affect the operation of a protective device,
when provided resulting in the fault to continue without disconnection. Hence it is always
important to crosscheck the loop impedance for effective and correct operation for the expected
fault currents. The impedance values are basically functions of route lengths and conductor sizes.
The formula given in equation 9.8 can be used to calculate the maximum route lengths beyond
which the impedance of a particular size of conductor would limit the magnitude of the shortcircuit current below that is required to operate the protective device.
Appendix B, Table B1 can be used as a ready reference to find typical maximum conductor lengths
of a conductor size for safe operation of a protective device of particular rating. Once the length is
exceeded in an installation, alternate conductor (higher cross section) shall have to be chosen to
reduce the overall loop impedance for effective operation of the device. Table 9.3 here provides a
glimpse of some part of the Table B1 for immediate understanding of typical maximum circuit
lengths for a few conductor sizes commonly used in low voltage installations. .
168 Practical Electrical Wiring Standards - AS 3000:2018
Table 9.3
Maximum circuit lengths of conductors for protective device ratings (Source: AS/NZS 3000, Table B1)
Minimum current Ia for circuit-breakers considered for the purpose of the values arrived in Table
9.3 is the mean tripping current as per the applicable standard AS/NZS 60898.
Type B breaker = 4 × rated current
Type C breaker = 7.5 × rated current
Type D breaker = 12.5 × rated current
Ia for fuses are approximate mean values taken from AS 60269.1.
From the table 9.3, it may be noted that for say a conductor size of 25 sqmm, if the protective
device rating chosen is 100 amperes, the cable length using type B breaker shall be just below
100metres to limit the overall loop impedance. Where the actual circuit length is more than
100metres, there is no alternative other than to go for the next conductor size with a different
device rating. During actual operation changing the device rating may be possible but it is
impossible to modify the conductor sizes without associated implications of cost, matching devices,
etc. Hence the conductor size selection at the stage of design is very much important as otherwise,
it could create serious implications at a later date.
The maximum route lengths given in the table for the phase conductor are from the point of
connection to the point of use and are related to a disconnection time of 0.4 seconds for the device
under use. When the nominal phase voltage of the electrical installation is not 230 V, the maximum
Section 5 - Earthing Arrangements and Earthing Conductors 169
length may be determined by multiplying the lengths in this table by a factor of Uo/230 e.g. for a
nominal phase voltage of 240 V, the factor ~1.04 may be used.
For other cable sizes/ protective device ratings, reference shall be made to table B2 in AS/NZS
3000 appendix B.
9.9
Earthing requirements for other systems (Clause 5.8)
In general, the earthing arrangements of the following systems should be independent systems;
however, if they are connected to the electrical installation main earthing system they should not
reduce the integrity of the electrical installation earthing system:
• Lightning protection.
• Static electricity protection.
• Radio frequency interference (RFI)-screened installations.
• Information technology installations.
• Explosion protection systems.
• Cathodic protection systems.
9.10 Summary
The earthing system in an installation plays an important role to ensure that the touch voltage of all
contact surfaces are kept at earth potential for safety of persons in case of short circuit currents
between live parts to the earth. The earthing connections adopted for achieving this objective is
called protective earthing. This is different from functional earthing which is basically intended
some specific operations purpose like cathodic protection, etc and not for safety purpose. The value
of earth resistance determines the system’s ability to minimize the touch potential and the earth
fault loop impedance determines the minimum time taken by protective device for operation under
the specific fault current of the system, both of which shall be as low as possible.
It is necessary to adopt correct sizes of earthing conductors to carry the expected fault currents in a
system. The earth conductor is usually made of copper and AS/NZS 3000 recommends minimum
size of main earth conductor to be considered for the system which is generally based on the active
conductor sizes of the main supply. It is also necessary to use the calculations method to arrive at
the earth conductor sizes depending on the likely short circuit currents and the short circuit current
protection adopted in the system.
In Australia the LV installations are usually adopted with multiple earthed neutral systems which
are equivalent of TN-C-S system specified in IEC. The MEN system consists of a common neutral
and protective earth conductor from the main power supply distribution company which is
terminated usually on the main switchboard of the installation. It is necessary that the protective
earthing connections from various equipments of the consumer installation are connected to the
MEN point at the main switchboard, directly or indirectly. The MEN point at the switchboard is
connected to the earth through an earth electrode of atleast 1.2 m depth made of acceptable material
(copper or steel rod/ strip) of sufficient size as per the recommendations of the standard.
For reliable earthing system it is necessary to ensure that the earth connections of all equipments
and extraneous conductive parts are securely done by using corrosion resistant material with
suitable protections adopted to safeguard against mechanical forces and corrosion effects.
It is necessary that all conductive pipes, reinforcing parts in wet areas, etc are also connected to the
earth by means equipotential bonding with minimum 4 sqmm copper conductors. The earth fault
loop impedance shall be effectively controlled within a reasonable value to ensure safe
disconnection time of 0.4 seconds is achieved by the protective device adopted for short circuit
protection. Guidelines are available to choose proper conductor sizes and protective device ratings,
which shall be duly considered during design stages.
170 Practical Electrical Wiring Standards - AS 3000:2018
10
Section 6 – Damp Situations
Moisture and humidity are the major concerns for maintaining the electrical insulation
characteristic which is very vital for the reliable operation of the electrical systems and to avoid
flash over, shock currents, etc. Hence it is very important that the electrical systems operated in
areas that are constantly exposed to water shall be selected and installed with utmost care to
maintain insulation properties and safety. Locations from bath/ shower/ water containers used by
common man in every residential installation to comparatively commercialized areas like
swimming pools, spa pools, sauna heaters, etc frequented by many people always use water; hence
these areas demand special attention for selecting and installing proper electrical equipments.
Section 6 of AS/NZS 3000 stipulates the rules to be followed for the electrical systems installed in.
such damp locations. In defining these rules, AS/NZS 3000 divides such areas into multiple zones
depending on the quantum of water exposure likely to be faced by these installations at the water
storage/consuming points as well as at the locations surrounding such areas. Based on these
defined zones, the standard recommends the types and characteristics of systems to be adopted in
each damp area to minimize the possibility of common hazards expected.
Learning objectives
•
•
•
•
•
•
•
Basic requirements to be met in damp areas
Requirements in bath, showers or other fixed water container areas
Requirements in swimming pools, paddling pools and spa pools or tubs
Requirements in fountains and water features
Requirements in locations containing sauna heaters
Requirements in Refrigeration rooms
Requirements in General hosing down areas
10.1 Basic requirements (Clause 6.1)
Generally electrical equipments in damp situations shall meet all the basic safety requirements
related to construction, protection systems, earthing, etc as stipulated in Part-1, Section-1 and Part2, sections-2 to 5 we reviewed till now for protection of personnel and properties from the hazards
of electricity use. The installations in damp situation have no exception for these stipulations.
Nevertheless, section-6 of the standard stipulates some of the important and specific requirements
to be considered additionally for electrical installations in damp situations over and above the rules
covered in the earlier sections. These additional requirements are basically aimed for meeting the
following objectives and functions in damp areas.
• An increased risk of electric shock is always possible for electrical installations
operating in damp areas due to more likelihood of contacts by the body with earth
potential through constantly available water or moisture in these areas which are good
170 Practical Electrical Wiring Standards - AS 3000:2018
•
conductors of electricity. There are also more chances for the reduction in body
resistance of the persons in contact with water in these areas that can easily enhance
the flow of dangerous shock currents at lower contact potential values when
compared to such possibilities in general areas. Hence the systems installed in these
damp areas shall be able to minimize the additional risks of increased direct contacts
through water and moisture as well as dangerous leakage currents at very low touch
potential on contact surfaces, by adopting adequate and foolproof safety measures.
The enclosures of electrical equipment are usually specified with IPXX category of
protection in which the second X defines how much resistance the enclosure can offer
against entry of water. In damp areas, there are always more chances for water
entering into the enclosures of electrical items/ appliances in different directions with
different forces due to varying patterns and constant use of water and also more
chances of high humidity affecting the insulation characteristics. These can lead to
short circuit currents and associated hazards. Hence the equipment enclosures
selected and installed in damp areas shall be able to offer adequate protection against
water and humidity to minimize insulation failures for ultimately safeguarding the
personnel and properties from such hazards and possible damages.
The standard stipulates specific additional requirements to be considered to meet the above two
important objectives of the installations in the following common damp locations, which are
covered in the subsequent paragraphs:
• Baths, showers and other fixed water containers
• Swimming pools, paddling pools and spa pools or tubs
• Fountains and water features
• Saunas
• Refrigeration rooms
• Sanitization and general hosing-down operations
10.2 Locations of baths, showers or fixed water containers(Clause 6.2)
The requirements specified in this part of the standard apply to electrical installations in locations
containing a bath, shower or other fixed water containers (those designed to contain water for
normal use but exclude fortuitous containers or areas not intended to contain water for normal
operations) and also in locations just adjacent to these areas classified into specific zones,
depending on the degree of possibility of water or moisture entering and impacting safety in those
zones.
10.2.1 Baths and showers
Areas containing baths, showers, etc including surrounding areas are divided into four zones Viz.,
0, 1, 2 and 3 and the standard defines the type and specific requirements to be adopted for the
electrical installations in each of these four zones. It is to be noted that there are some minor
variations to the distances defining the upper limit of boundaries for some of the zones in Australia
and New Zealand.
Zone 0 shall be the complete interior part of a bath or a shower base which could be either a raised
hob or a depression in the floor irrespective of whether the water stagnates for some time or gets
drained fast in this area.
Zone 1 for a bath (but without shower) shall be the area covered between the following boundary
planes:
• Zone 0 boundary plane at the bottom, which is generally the top of the curbed bath
area
• The complete vertical projection planes taken along the sides of the internal rim of
the bathing location all around
Section 6 - Damp situations 171
•
Horizontal plane lying at top above the zone 0 top which is defined at a maximum
height of 2.5 metre above zone 0 boundary plane for Australian Installations and at a
maximum height of 2.25m above zone 0 boundary plane for New Zealand
installations, assuming there is no shower fixed in the bath area.
When the bath contains a shower, the vertical projection of zone-1 boundary considered just along
the sides of rim in the above case shall be shifted outwards and taken at 1.2 metre radial distance
from fixed plumbing connection of the shower, subject to having the plumbing connection of the
shower at a maximum height of 2.4metres in Australia and 2.25 metres in New Zealand.
Where the height of plumbing connection of shower exceeds 2.4 m in Australia, or 2.25 m in New
Zealand, the horizontal plane boundary at top of Zone-1 boundary shall also be extended upwards
to the actual height of the plumbing connection, with the vertical boundary on the sides remaining
at the same 1.2m.
Zone 2 of the bath/ shower area shall be the area contained within the following planes:
• The floor plane just outside zone 1
• Area covered between the end vertical projection plane limiting Zone 1 (that might
vary depending on presence of shower) and the parallel vertical plane located further
outwards at 0.6 m from this Zone 1 vertical projection
• The horizontal plane 2.25 m above the floor at the top
Zone 3 shall be the area limited by the following planes.
• The floor plane outside zone 2
• The vertical plane between Zone 2 end defined above and the parallel vertical plane
2.4 m further external to Zone 2 boundary along the sides
• The horizontal plane above Zone 2 top at 2.5 m above the floor in Australia and
horizontal plane above Zone 2 top at 2.25 m in New Zealand (depending on shower
height) at top
• In Australia, Zone 3 includes the area above Zone 2 up to 2.5 m above the floor at top
of the bath, whereas it starts from 2.25m in New Zealand.
Showers Zone 1 has been clarified for different shower head configurations in 2018 issue of the
Standard.
Figure 10.1 illustrates these four zones and their limits for a typical bath without a shower in plan
view (all dimensions are in meters).
172 Practical Electrical Wiring Standards - AS 3000:2018
Figure 10.1
Plan view defining the classified zone limits of bath without shower and barriers (Source: AS/NZS 3000, Figure 6.1)
Figure 10.2 shows the elevation views defining these zone classifications of the bath locations
without shower as well as with shower where the plumbing connection is located below a height of
2.25m from the floor level. It can be noted that zone 1 boundary is getting extended upto 1.2m
from plumbing connection when provided with shower instead of bath rim end when it is without
shower. These figures also depict the 2.5m and 2.25m rules to be applied in Australia and New
Zealand.
Section 6 - Damp situations 173
Figure 10.2
Elevation views showing classified zone limits of bath/ shower without barriers (Source: AS/NZS 3000, Figures 6.2, 6.4)
Though the bath with shower in the figure 10.2 shows an overall dimension of 1.2+0.6+2.4= 4.2m
as the end of zone 3, all bath rooms may not extend upto this length. Hence a wall of a room
located within this distance from bath area (in any direction) will effectively become the end of
zone 3 in that direction. Similarly ceilings, walls with or without windows, doors and floors that
are limiting the distances of the surrounding walls in a bath room also will limit the associated
zones, as applicable. Further it is possible to reduce or prevent the force of water coming out of the
shower by providing some intentional barriers, such as screens, doors, curtains and fixed partitions
along the rim or just after the rim. Such barriers which effectively offer adequate and constant
protection against spraying water from the shower may also be considered to limit the extent of
174 Practical Electrical Wiring Standards - AS 3000:2018
zone 1 classified in the earlier cases, subject to keeping such barriers upto a height of minimum
1.8metres from the floor level of the bath. Figure 10.3 defines the changes applicable to the limits
of zones having a 1.8m high barrier which can be located at ≤ 1.2 meters and accordingly zone 1
end also will get shifted towards the bath location effectively reducing the other zone limits as well,
compared to areas having no such barrier.
Figure 10.3
Elevation view showing classified zone limits of bath shower with vertical barrier (Source: AS/NZS 3000, Figure 6.11)
10.2.2
Fixed water containers
These are permanently installed water containers installed or located above ground to store water
for general residential uses made of concrete or any other material. In these installations, only two
zones (Zones 0 and 2) are classified in the standard, which is described below. Here it shall be
noted that the limiting boundary distances of zone 2 from zone 0 end will vary depending on the
capacity of water stored in the container.
Zone 0 shall be the area of the interior of the water container
Zone 2 for an individual water container with a capacity not exceeding 40 litres and having fixed
water outlets shall be the area contained within the boundary planes as described below:
• Zone 0 boundary plane at bottom
• The vertical plane located 0.15 m away all around the internal rim of the water
container in the sides
• The horizontal plane 0.4 m from the water container top
Zone 2 for water containers having either a capacity exceeding 40 litres or a water outlet through a
flexible hose shall be the area contained within the following limits:
• Zone 0 boundary at bottom
• The vertical plane located 0.5 m away from the internal rim of the water container on
the sides
• The horizontal plane 1.0 m from the water container top, at the top
Figure 10.4 gives the typical limits of the zones for the second case (>40L) defined above.
Section 6 - Damp situations 175
Figure 10.4
Classified zone limits for water containers> 40L or with flexible outlets (Source: AS/NZS 3000, Figure 6.11)
10.2.3
Equipment features and protection requirements
In the bath, shower and water container areas, the following measures of protection against electric
shocks observed in earlier chapters shall NOT be considered and are strictly prohibited, because
these measures can not control the water though these may control people.
• Protection by means of obstacles to prevent access to live parts
• Protection by placing equipments out of reach of the people
Hence it is necessary to achieve the required safety requirements by adopting equipments having
suitable construction meeting the characteristics needed or avoiding the use/ installation of the
items in case the required characteristics can not be met at these locations. It is also to be noted that
in these locations as well as in the areas discussed subsequently, an equipment satisfying
requirements of one particular zone will satisfy requirements of other lower zones (Note: safety
requirements for Zone0> Zone 1> Zone 2> Zone 3) and not vice-versa. Accordingly equipment
satisfying zone 0 requirements can be used in all zones, equipment satisfying Zone 2 requirements
shall be used only in Zone 2 and 3, and so on. It is necessary to check the suitability of zones for
equipment as certified by manufacturers or certifying authorities to take a decision of installing the
same at other zones, when such options are explored.
Following are some of the rules that shall be considered/ assessed before taking a decision to install
a particular electrical item in the shower, bath and water container areas.
Except for luminaires, no other electrical devices like sockets, switches and switchboards are
permitted in Zone 0 of these areas. Following are the MINIMUM degree of IP classification
protection to be offered by the equipments located at respective zones in these areas (Note that only
the second numeral of IP classification is important in these areas).
• In Zone 0: IPX7
• In Zones 1 and 2: IPX5 in communal baths/ showers; IPX4 in other locations
• In Zone 3: IPX5 in communal baths/ showers; No specific limitation in other
locations
176 Practical Electrical Wiring Standards - AS 3000:2018
It is to be noted that the degree of protection required in communal areas used by many people
exceed the requirements defined for residential areas normally used by limited number of persons.
Socket-outlets shall not be installed in Zone 0 or 1. Also socket-outlets shall not be installed within
0.3 metre of the floor of a bathroom, laundry or other similar location where the floor is likely to
get drenched.
Similarly no socket outlets are permitted in zone 2 and 3 if they fall within 0.3 m of zone 1
boundary. However socket-outlets may be installed within Zones 2 and 3 beyond this 0.3 m limit,
in case these socket outlets are protected by an RCD with a fixed rated residual current not
exceeding 30mA subject to enclosing the outlets in a cupboard that maintains the minimum
enclosure classification applicable for the zone.
As a special concession, socket outlets installed in Zone-2 may be without RCD if the appliance
used is of the automatic switching type like a shaver supply unit complying with AS/NZS 3194.
Similarly socket-outlets installed in Zone 3 supplied as an SELV or a PELV system can also be
without RCD protection.
Switches and accessories are not to be installed in Zone 0 and also within 0.3 metres of Zones 1, 2
and 3. Beyond 0.3 m, these may be permitted subject to satisfying the minimum enclosure IP
category noted above.
Switchboards are not permitted in all the zones of these installations. Table 10.1 in a nutshell
summarizes the stipulations for areas containing bath, shower and water containers.
Table 10.1
Electrical items requirements in classified zones of bath, shower and water container
(Source: AS/NZS 3000, Table 6.1)
Section 6 - Damp situations 177
10.3 Swimming pools, paddling pools and spa pools/tubs (Clause 6.3)
The requirements for spa pools/ tubs are divided into three categories as noted below.
• Spa pools/ tubs having maximum capacity of 680 litres are considered to be same as
the bath shower we reviewed just above. Accordingly spa pools and tubs upto 680
litres capacity shall follow zone classifications and installation requirements as
defined under section 10.2 above.
• Spa pools and tubs having capacities beyond 5000 litres are considered to have same
characteristics like swimming pools and paddling wheels. The requirements for
installations at these locations are given in clause 10.3.1 below along with swimming
pools and paddling wheels.
• Separate zone classifications are applicable for spa pools/ tubs having 680 to 5000
litres capacity and these are defined in the subsequent section 10.3.2.
It may be noted that similar to the bath areas, ceilings, walls with or without windows, doors and
floors, barriers and fixed partitions, e.g. a 1.8 m solid fence, that limit the extent of a room or area
containing a swimming pool, paddling pool, spa pool or tub and fixed partitions that provide
effective protection against spraying or splashing water, will limit the associated zones defined for
these areas.
10.3.1 Swimming pools, paddling pools and spa pools/tubs > 5000 litres
Three zones zone 0, 1 and 2 are applicable in these areas, as defined below:
Zone 0 shall be the area of the interior of the water containing part of the swimming pool or
paddling pool upto its brim level.
Zone 1 shall be the area limited between;
• Zone 0 limit and surface expected to be occupied by persons on the sides of the pools/
tubs at the bottom
• a vertical plane 2.0 m from the internal rim of the water container along the sides
• The horizontal plane upto 2.50 m above the floor or the surface at the top of the pool/
tub
178 Practical Electrical Wiring Standards - AS 3000:2018
In addition following considerations shall be applied for zone 1, when the pool contains a diving
board, springboard, starting block or a slide:
• vertical plane situated 1.50 m all around the diving board, springboard, starting block
or slide
• Horizontal plane 2.50 m above the highest surface expected to be occupied by
persons at the top
Zone 2 shall be bounded by the planes as noted below:
• Area between the vertical plane limiting Zone 1 and the parallel vertical plane 1.50 m
external to Zone 1
• the floor, or surface expected to be occupied by persons at the bottom
• The horizontal plane 2.50 m above such floor or surface at the top
10.3.2 Spa Pools or Tubs between 500 to 5000 litres capacity
For capacities from 500 to 5000L, following defines the zone limits.
Zone 0 shall be the area of the interior of the spa pool or tub.
Zone 1 shall be the area limited by:
• Zone 0 at the bottom
• the vertical plane 1.25 m from the internal rim of the pool or tub and the floor, or the
surface expected to be occupied by persons along the sides and
• The horizontal plane 2.50 m above the floor or surface at the top
Figures 10.5 and 10.6 give some typical illustrations reproduced from AS/NZS 3000 showing the
above definitions.
Figure 10.5
Classified zone limits for in-ground swimming pools (Source: AS/NZS 3000, Figure 6.15)
Section 6 - Damp situations 179
Figure 10.6
Classified zone limits for spa pools / tubs not exceeding 5000 litres capacity (Source: AS/NZS 3000, Figure 6.18)
10.3.3
Electrical installation requirements
Following requirements shall be considered for the electrical items to be installed within the
various zones of swimming pools, paddling wheels and spa pools/ tubs discussed above.
As like bath areas, protections by means of obstacles or by placing items out of reach are NOT
permitted in these areas.
Where electrical equipment is in contact with pool water, failure of insulation may result in a
hazardous voltage appearing across or through the pool water. A very low voltage is sufficient to
present a hazard to persons immersed in the water. Protective measures to be considered are;
• locating the electrical equipments at a considerable distance away from these
locations with all plumbing connections made of non-conductive material
• Metal grids or barriers inserted in any plumbing connections between the electrical
equipment and pool and connected to the equipotential bonding system
• Use of an RCD with a fixed rated residual current not exceeding 30 mA to protect
circuits supplying Class I (earthed conductive parts) equipments installed in these
areas
Following are the IP classifications permitted in the respective zones for the enclosures of the
electrical live items to be installed in these areas;
• Zone 0: IPX8
• Zone 1: IPX5
• Zone 2: IPX4
The wiring systems installed in these areas shall be able to prevent entry of moisture and water.
Accordingly bare aerial conductors shall not be installed above Zones 0, 1 or 2. Other types of
aerial (insulated) cables may be installed subject to meeting the safety clearances defined in the
earlier chapter (Section 3, Clause 3.12 of AS/NZS 3000).
180 Practical Electrical Wiring Standards - AS 3000:2018
Socket-outlets shall not be installed in Zone 0. However these may be considered in zone 1 area
subject to meeting ALL the following conditions:
• Enclosure having the required ingress protection
• Outlets located at a height of not less than 0.45metres above the ground level and at a
horizontal distance beyond 1.25 metres from the internal rim of the water container
• Supplied from an independent circuit or operation at SELV/PELV or controlled by
RCD having less than 30mA sensitivity
For zone 2, the first and third of the above conditions shall apply and there is no restriction of
distances for locating the devices from zone 1 boundary limit.
The luminaires, appliances and other standard electrical equipment are permitted in all zones of
these areas subject to meeting enclosure IP classification and designed/ constructed for use at
swimming pools, etc. Further the following additional requirements shall be met.
Zone 0 luminaires, appliances, etc shall be rated and supplied as below:
• Shall be rated below 12V AC/ 30V ripple free d.c.
• Shall be operating as SELV/ PELV system
• Shall be supplied from a source located outside zone 0
Zone 2 luminaires, etc shall meet ANY ONE of the following requirements:
• Shall be supplied as an SELV/ PELV system
• Shall be of class II (double insulation) construction
• Shall be class I type with earthing and RCD backup protection against current
leakages
Table 10.2 summarizes the stipulations to be considered in swimming pools, paddling wheels and
spa pools/ tubs for ready reference.
Table 10.2
Electrical items requirements in classified zones of swimming pools, paddling wheels or spa pools/ tubs
(Source: AS/NZS 3000, Table 6.2)
Equipment
Zone 0
Zone 1
Zone 2
In the following table, all multiple conditions numbered with i), ii), etc shall be considered together
for a zone, where given.
Socket –
Outlets
Not permitted
Switches/
accessories
Not permitted
Not permitted for general use.
(i) IPX4 AND
(ii) (a) separated supply
For pools, equipment satisfying OR (b) SELV/PELV
all i, ii and iii below are
supply
acceptable
OR (c) RCD protection
(i) IPX5 AND
(ii) (a) ≥0.45 m high and
≥1.25m from internal
rim OR
(b) under and ≥0.5 m from
edge of fixed continuous
horizontal barrier ≥1.25m
wide
(iii) (a) separated supply
OR (b) SELV or PELV supply
OR (c) RCD protection
IPX5
IPX4
Section 6 - Damp situations 181
Appliances,
Luminaires
and other
equipment
(i) IPX8 AND
(i) IPX5 AND
(ii) cerified for use
(ii) (a) SELV or PELV supply
AND
OR
(iii)12V a.c./30 V d.c.
(b) Class II construction
SELV or PELV
OR
(c) Class I construction
supply from source
fixed in position and
outside zone AND
RCD protection
(iv) No earth
(i) IPX4 AND
(ii) (a) SELV/ PELV
supply OR
(b) Separated supply
OR
(c) Class II
construction OR
(d) Class I
construction and
RCD protection
Heating cable
systems
Not applicable
Permitted where embedded in
the floor area under the Zone
and protected with earthing
(Refer previous chapter)
As for Zone 1
Switchboards
Not permitted
Not permitted
Not permitted
Electricity
generation
Systems
Not permitted
Not permitted
Not permitted
Electricity
distributor’s
electrical
equipment
Not permitted
Not permitted
Not permitted
182 Practical Electrical Wiring Standards - AS 3000:2018
10.4 Fountains and water features (Clause 6.4)
In addition to taking care of reduced body resistance and increased chances of contacts with live
parts, the requirements under this section are also intended to protect electrical material and
electrical equipment from the corrosive effects of chemicals used in the treatment of water in
fountains and water features. However it is to be noted that the stipulations given under this section
need not be applied for fountains or water features which comply with any of the following:
• the depth of water is within 300mm or
• Suitable means are provided in these areas to restrict entry of persons to the water
Similar to the other areas, ceilings, walls with or without windows, doors and floors that limit the
extent of a room or area containing a fountain or water feature and fixed partitions that provide
effective protection against spraying or splashing water, limit the associated zones defined below.
10.4.1 Zone definitions
Two zones (Zones 0 and 1) are classified for fountains and water features:
Zone 0 shall be the area of the interior of the water storing part including any recesses in their walls
or floors or the interior of water jets or waterfalls.
Zone 1 shall be the area limited by:
• Zone 0 limit at the bottom
• the parallel vertical plane 2.0 m from the internal rim of the water containers on the
sides
• The floor or surface expected to be occupied by persons in the immediate
surroundings and the horizontal plane 2.50 m above the zone 0 boundary plane
When the fountain or water feature contains sculptures and decorative water containers, following
areas shall be suitably added/ extended for zone 1:
• Parallel vertical plane situated 1.50 m around the sculptures and decorative water
containers at the sides
• Horizontal plane 2.50 m above the sculptures and decorative water containers at the
top
Figures 10.7 and 10.8 give typical zones for a fountain and water feature discussed above.
Section 6 - Damp situations 183
Figure 10.7
Zone boundary limits for fountains and water features – Plan view (Source: AS/NZS 3000, Figure 6.20)
Figure 10.8
Zone boundary limits for fountains and water features – Elevation view (Source: AS/NZS 3000, Figure 6.21)
184 Practical Electrical Wiring Standards - AS 3000:2018
10.4.2 Electrical equipment features/ installation
As like other areas, protections by means of obstacles or by placing items out of reach are NOT
permitted in these areas.
Protective measures to be considered are:
• earthed system with RCD protection having a fixed rated residual current < 30 mA or
• supplied at either extra-low voltage or low voltage through an isolating transformer
complying with AS/NZS 61558, but not earthed
Degree of protection of enclosures in these zones shall meet the following minimum
classifications:
• In Zone 0: IPX8
• In Zone 1: IPX5
Wiring systems for a fountain or water feature shall be installed so as to prevent entry of moisture
to any connection and water siphoning through any wiring enclosure or cable. The types
recommended are elastomer or thermoplastic insulated and sheathed copper cables or flexible cords
suitable for immersion in the type of water being used. Where these wiring systems are likely to
face mechanical damages, installing these in a wiring enclosure is recommended.
Socket-outlets shall not be permitted in Zone 0 or Zone 1.Switches and other accessories shall not
be installed in Zone 0. However switches and other accessories may be installed in Zone 1 subject
to providing the required IP enclosure.
Luminaires, appliances and other electrical equipment shall be permitted where designed and
constructed specifically for use in a fountain or water feature and provided with the required degree
of protection. In addition the following conditions shall apply:
• Zone 0 luminaires, appliances, etc shall be operated as SELV/ PELV system and
supplied from a source located outside zone 0 with a nominal rating of below 12V
a.c./ 30V ripple free d.c. These shall not be provided with earthing in accordance with
the ELV supplied luminaire requirements we studied in the chapter on earthing
recommendations (Section 5 of the standard)
• Zone 2 luminaires, etc shall be supplied as an SELV/ PELV system or shall be of
class II (double insulation) construction or shall be of class I construction with
earthing and 30mA RCD backup protection against current leakages.
NO switchboard shall be installed within the classified zones similar to the other areas discussed
earlier.
10.5 Locations containing sauna heaters (Clause 6.5)
There is no zone 0 applicable for rooms or enclosures containing heating equipment used
exclusively for sauna heating. Three zones 1, 2 and 3 are defined for these applications.
Zone 1 shall be limited by;
• Parallel vertical plane 0.5 meter from the external edge of the sauna heater on the
sides
• floor, or surface expected to be occupied by persons at the bottom
• The cold side of the thermal insulation of the ceiling at the top
Zone 2 shall be the area limited by;
• vertical plane between Zone 1 and the cold side of the thermal insulation of the walls
of the sauna room or enclosure at the sides
• floor, or surface expected to be occupied by persons at the bottom
• The horizontal plane 1.0 m above the floor at the top
Section 6 - Damp situations 185
Zone 3 shall be the area limited by the;
• Same as zone 2 limit on the sides
• Location between horizontal plane 1.0 m above the floor and the cold side of the
thermal insulation of the ceiling at the top
Figure 10.9 shows typical sauna heater area defining the zones indicated above in the elevation
view.
Figure 10.9
Zone boundary limits for sauna heaters – Elevation view (Source: AS/NZS 3000, Figure 6.22)
As like other areas, protections by means of obstacles or by placing electrical items out of reach are
NOT permitted in these areas.
Installation of electrical equipment and wiring in classified zones shall consider the following
stipulations:
• Only electrical equipment belonging to the sauna heater shall be permitted in zone 1
• Zone 3 Electrical equipment shall be suitable to withstand a minimum temperature of
125°C and the insulation of conductors shall be suitable to withstand a minimum
temperature of 170°C
• There are no special requirements concerning heat resistance of electrical equipment
in zone 2, except for meeting enclosure IP classification
Any equipment in these areas, other than sauna heater, shall be protected by an RCD with a fixed
rated residual current < 30 mA.
Degree of protection required for electrical enclosures shall be at least IPX4B or IP24 for all
permitted items within the sauna room.
Wiring systems should be installed outside the zones, on the cold side of the thermal insulation. If
the wiring system is installed in Zone 1 or 3 on the warm side of the thermal insulation, the system
shall be heat-resistant type meeting the maximum temperatures indicated above. Metallic sheaths
186 Practical Electrical Wiring Standards - AS 3000:2018
and metallic conduits shall not be accessible in normal use. Suitable types of wiring systems
recommended are insulated, unsheathed cables in non-metallic enclosures or sheathed cables.
Socket-outlets, switches and other accessories shall not be installed within a sauna room or
enclosure except those forming part of the sauna heater.
A switchboard shall not be installed within any classified zone of a sauna heater.
10.6 Refrigeration rooms (Clause 6.6)
These are increasingly used as freezers or cold rooms to refrigerate and preserve food articles and
some other items. These are rooms bounded by walls with insulations and temperatures maintained
at same value in all parts of the room. Hence in these rooms only one zone (no need of numbering)
is applicable, which comprises of all of the area within the refrigeration room.
As like other areas, protections by means of obstacles or by placing items out of reach are NOT
permitted in these areas.
Electrical equipment within the room shall have protection classification of at least IPX4B or IP24
Wiring in these areas shall be of a type that will not be affected by the operating temperature of the
room and that will not provide pockets or channels in which moisture might accumulate, or allow
to get passed into electrical equipment. PVC cables are not to be considered in these areas due to
sub zero temperature which are susceptible to failures due to possibilities of bending, flexing or
vibration at temperatures below approximately 0°C. Following wirings are permitted for use in the
refrigeration rooms:
• Unenclosed sheathed cables including MIMS cables
• Insulated or sheathed cables enclosed in a wiring enclosure that has adequate draining
facilities
Sealing shall be done with a compound that does not set hard for each wiring enclosure at points
passing from refrigerated to non-refrigerated space and at cable entry points into motors,
luminaires, switches, etc. Socket-outlets, switches/ accessories and controlgear having the required
degree of protection and that are permanently sealed shall alone be used in these areas.
Lamp holders shall be the all-insulated type or any other suitable type that precludes the possibility
of any external metal portion becoming live. Lamp holders shall not be suspended within 2.50 m of
the floor or ground with a flexible pendant.
Fixed appliances shall be fitted with internal heaters or in enclosures that would prevent the
retention of moisture in addition to meeting the IP classification, as specified.
A switchboard shall not be installed in a refrigeration room.
10.7 Sanitization and general hosing down areas (Clause 6.7)
Some of the well known areas where sanitization and hosing down operations are frequently
carried out are as below:
• Areas keeping livestock
• Areas producing feed, fertilizers, vegetable or animal products or where these items
are stored, prepared or processed
• Areas growing plants, such as greenhouses or hydroponic installations
• Areas where agricultural or horticultural products are produced, prepared or
processed, e.g. dairies, drying, stewing, pressing out, fermenting, butchering, meat
processing, etc
• Car wash bays, etc.
Section 6 - Damp situations 187
Here also just one zone is classified which comprises of areas that are normally affected by the
sanitization and hosing down operations and shall generally be bounded by the following:
• Location between the floor or the base of a recess in the floor and a horizontal plane
2.0 m above this floor top
• Side walls enclosing the area of these operations
• Location on a ceiling within 1.0 m of walls surrounding the area
As like other areas, protections by means of obstacles or by placing items out of reach are NOT
permitted in these areas.
Degree of protection for enclosures shall be minimum IPX5 where low or medium pressure hosingdown is used and shall be minimum IPX6 in areas where high pressure hosing is adopted. Though
switchboards are allowed only in these damp areas, it is subject to maintaining the minimum degree
of protection of IPX6. The switchboards and wiring enclosures shall ensure adequate protection
against water and moisture entry into the electrical items by proper sealing.
10.8 Summary
Section-6 of AS/NZS 3000 specifies the rules applicable for electrical systems and installations in
the damp areas like bath, shower, swimming pools, fountains, etc, where water presence exists all
the time or most of the time. Such damp areas affect the moisture content and humidity in the
surroundings also which will directly impact the insulation of electrical systems. There are also
more chances for getting direct contacts to live conductors through water used in these areas.
Further, persons exposed to damp areas have their body resistance considerably reduced which
increases the chances of shock currents due to low touch potential possible in these areas. Hence
the electrical installations shall be able to offer adequate safety protection against possibilities of
shock currents and water/ moisture issues by choosing proper enclosures for electrical items and
taking other safety precautions in these areas.
The standard divides each of these damp areas into different zones suffixed with numerals 0, 1, 2,
etc indicating the severity of water exposure chances in a particular zone for a particular
application. The lower the numeral, higher are the chances for electrocution demanding additional
safety precautions in such zones compared to zones suffixed with a higher numeral.
Zone 0 in case of baths, fountains, water containers etc are the parts carrying water and no
electrical items are permitted in zone 0 except for luminaires subject to limiting their operating
voltages and using certified items, etc. Zone 1 is usually defined upto some distance on the sides
and upto some height from zone 0 boundary top with distances in the order of about 1.5metres to
2.5meters. Areas beyond zone 2 are classified as zone 3 upto some distances/ height away from
zone 2 limits. A barrier with a minimum height of 1.8 metre at boundary of zone 0 as well as walls,
ceilings, windows, etc around the damp areas normally limit the extents / dimensions of these
zones. This barrier could be simple screens in bath/ shower areas and shall be solid walls in most of
the other areas like swimming pools, etc.
No switchboards are permitted in any of these installations, except in areas where hosing down
operations are carried out. It is necessary to ensure all enclosures are adequately protected with
proper IP category usually having its second digit in the order of 7 or 8 for items located in zones 0
and 1 of specific installations; with second digit of 4 or higher for items located in zone 2 and 3 in
most of the installations. It is also necessary to consider RCD protection or SELV/PELV systems in
most of the areas, mainly in classified zones 0 and 1 to minimize the possibilities of shock currents
and the associated hazards.
188 Practical Electrical Wiring Standards - AS 3000:2018
11
Section 7 - Special Electrical
Installations
Section 7 of AS/NZS 3000 defines requirements to be met by specific installations that necessitate
special attention due to their functions or unique characteristics and the same are covered in this
chapter. We will discuss the requirements for safety services that come into prominence during
emergency situations like fire, etc. Our study will also cover the specific requirements for captive
electricity generating systems used in electrical installations as alternative supply. The chapter
also includes the basic requirements of separated supply systems and the features of SELV and
PELV systems that are extensions to these separated supply systems. We will also go through the
stipulations given in the standard regarding HV installations, hazardous area requirements and
temporary installations like demolition sites, shows and carnivals, etc.
Learning objectives
•
•
•
•
•
•
•
Requirements for safety services
Requirements for Electricity generation systems
Protection by electrical separation
Requirements for Extra-low voltage electrical installations
Requirements for High voltage electrical installations
Requirements for explosive hazard areas
Requirements for other installations like demolition sites, shows and carnivals, etc.
11.1 Applicable areas (clause 7.1)
This section covers the minimum requirements to be adopted in the design, selection and
installation of electrical systems for the following services/ installations:
• Electrical systems feeding safety services
• Electricity generation systems
• Systems supplied by electrical separation
• Extra-low voltage electrical installations
• High voltage electrical installations
• Installations in areas where an explosive hazard may arise
• Electrical installations of construction and demolition sites, electro medical treatment
areas; relocatable installations and the sites, marinas and recreational boats, shows
and carnivals, shows and carnivals, cold cathode illumination systems,
telecommunication networks power supplies, cranes and hoists, lifts, generating sets,
188 Practical Electrical Wiring Standards - AS 3000:2018
outdoor sites under heavy conditions, electric fences; and film, video and television
sites.
11.2 Safety services (Clause 7.2)
The term ‘safety services’ refers to parts of the electrical installation in building areas that are
essential for the safe operation of the important services like fire detection, fire alarm and
extinguishing systems, smoke control systems, evacuation systems such as lifts, etc., The power
supply to these services cannot be bypassed or kept on hold under any circumstances as otherwise
it could compromise the protection to people and properties within the building. The rules for
safety services are intended to ensure that electricity supply is not inadvertently disconnected for
electrical equipment that are required to be operated during emergency conditions, especially
where there is no alternative supply to feed such demanding services. These were named as
“emergency services” till the previous edition of this standard. The supplies associated with these
services are generally prefixed with “emergency” and/or “essential” in the building codes of
Australia and New Zealand.
Fire and fire control equipment in major buildings comprise generally all the following parts:
• Fire hydrant pumps to pump water in fire water lines for different locations to control
the fire
• Pumps to feed for automatic sprinkler systems, water spray or deluge systems and
similar fire-extinguishing systems
• Pumps feeding through fire-hose reels, where such hose reels could be the only
means of fire protection to spread water, i.e. in locations where fire hydrants and
automatic fire-sprinkler systems are NOT installed
• Fire detection and alarm systems to initiate prompt actions at the correct times
• Air-handling systems that are required to exhaust fumes and gases and to control the
spread of fire and smoke during fire accidents
The expression ‘safety services’ generally excludes items which do not affect the operation of the
safety equipment like the following:
• Escalators or moving walkways (travelators).
• A lift in a single private residence that is installed in accordance with AS/NZS
1735.18 need not comply with the requirements of this Clause (Clause 7.2)
• Lifts that are not defined as emergency lifts in the National Construction Code (NCC)
or New Zealand Building Code (NZBC)
• Pumps for ‘jacking’ or water pressure maintenance, the failure of which does not
deprive the fire hydrant or sprinkler pump of adequate water supply.
• Fire detection, alarm and intercom systems with battery backup complying with AS
1670 or NZS 4512
• Smoke alarms installed in single private residences (see Clause 4.6 for information
relating to smoke alarms).
Evacuation equipment coming under safety services include intercom/speaker systems used under
emergency conditions, emergency lighting systems in egress paths, lifts used for carrying people
(excluding lifts in residential houses), etc.
Following are the essential requirements to be met by the supply systems feeding safety services:
• Separation of supplies feeding safety services from supplies that are feeding rest of
the installation/services
• Incorporation of properly discriminated protective devices
• Providing alternate sources of power supply
The following paragraphs define the minimum regulations for implementing the above features in
the electrical systems controlling safety services.
Section 7 - Special Electrical Installations 189
11.2.1 Separation of supply
Separation refers to provision of isolation switches to separate or independently control supply to
an area without disturbing other areas, which is a fundamental safety concept we discussed earlier.
In regard to power supply for safety services areas, it is stipulated that each part of an electrical
installation supplying a safety service shall be controlled by a main switch or switches that shall be
independent of other switches used to control the remainder of the electrical installation. It is
necessary that the control circuits of the safety services are also made independent similar to the
power circuits. The switches controlling various services in an installation are to be typically
segregated / arranged as per figure 11.1
Figure shows the main isolation switch, and the separation switches controlling the safety services,
doing the separation function, we are discussing here.
Figure 11.1
Typical supply separation for safety services (Source: AS/NZS 3000, Figure 7.2A)
The safety services in a building structure that is fire-separated portion of a main building (usually
separated by 120 minutes fire rated walls) or structure that is regarded as a separate building (out
building) shall also be provided with independent switches for controlling the internal safety
services. These switches in such independent buildings shall be separated from the switches
controlling other parts as well as other types of safety services in the main premises. Similarly the
lifts intended for safety services for emergency fighting and emergency evacuation shall be
provided with switch controls independent of switches controlling supplies to the other common
lifts within the same building.
All these main switches of the safety services and their connections shall ensure the following:
• They shall not be controlled by any main switch/ switches of the general electrical
installation
• Accordingly, these shall be connected on the supply side of applicable general
installation main switches so that these services continue to receive power supply
even when the upstream main switch is operated for isolation. This condition may not
be applicable for safety services provided with alternative supplies, high voltage main
switch, etc.
• They shall control only electrical equipment that is regarded as safety services,
exception being the lighting and socket-outlet circuits in rooms housing fire hydrant
190 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
•
or sprinkler pumps subject to incorporating properly rated over current and RCD
protection in these circuits
No switch shall be introduced between the main switch of safety services and
switchboards from which safety services supply is derived
These switches shall be adequately protected against mechanical damage in case their
location or conditions of use warranting the same
All these switches shall be kept closed (ON) under normal conditions. These might be
kept OFF only by authorized personnel or under maintenance after ensuring alternate
safety system/ necessary precautions.
Identification and labeling shall be legible and easily locatable using contrasting
colour to help for easy operation under emergency conditions. These switches shall
have caution notice marked “IN THE EVENT OF FIRE, DO NOT SWITCH OFF’.
The wiring systems used in power supply and control of safety services equipment
shall be capable of maintaining an adequate supply to such equipment when exposed
to fire. It is preferable to consider wiring systems of specific WS classification in line
with AS/NZS 3013
It shall be ensured that the conductors of safety services are not enclosed with
different safety services or with conductors of any other system. Adopting a
segregation of atleast 50mm or providing effective barrier from the other service
conductors is recommended when the safety service wiring systems are to be run
without enclosures.
The fire water pump motors may generally be designed for automatic operation upon detection of
fire with their main power contactors controlled from remote. It is necessary to provide a manually
operated isolating switch on the supply side of such automatically controlled pump motor
controller which shall remain ON under all normal conditions. This switch shall be installed
adjacent to or on the pump motor controller panel and shall be provided with a device for locking
the switch in the closed (ON) position to avoid inadvertent opening.
11.2.2
Protective devices and discrimination
It is necessary that protective devices against fault currents shall be avoided in safety systems
where the operation of the protective devices may pose more danger than the fault currents
themselves. However where overcurrent protective devices are provided, these shall be selected
such that a fault on one safety service will not result in loss of supply to other safety services and a
fault on the general electrical installation will not result in loss of supply to safety services.
To ensure proper discrimination with circuit breaker and high starting currents of the large size
motors used in such systems, the characteristics of overcurrent protective devices provided on
circuits supplying fire-pump motors shall meet ALL the following requirements:
• Inverse time characteristic
• Rated to carry 125% of the full-load motor current continuously or shall have to
match 125% of the circuit-breaker setting adopted
• Shall open the circuit in not less than 20 seconds at 600% of the full-load motor
current after taking care of currents during start up
• Where a single device protects multiple motors in a group, it shall be selected with
rating equal to 125% of the sum of the all motor rated currents and 600% rated
current of the largest motor in the group
It is also necessary to AVOID over temperature protection for fire-pump motors as the operation of
these devices might reduce the operating time of these pumps during emergencies.
The control and interlocking circuit for automatic operation of fire-pump motor shall meet the
following stipulations:
• It shall be directly fed from the active and neutral conductor of the pump circuit
• Shall be arranged to ensure that the active conductor of the control circuit is directly
connected to the coil of the operating device within the starter
Section 7 - Special Electrical Installations 191
•
It shall not be provided with additional overload protective devices other than those
provided for the pump-motor main circuit
11.2.3 Alternative supply systems
Alternative supply systems in low voltage installations are usually diesel generating sets or similar
ones used to generate power supply at the same nominal voltage and frequency as the mains
supply. These are brought into operation to maintain continuous supply to safety services in case of
failures or maintenance of the main power supply feeding the building. In such installations, a
changeover switch is adopted to feed the safety services to choose the power source. This
changeover switch shall be installed on the main switch board to enable connecting the safety
services from the alternative source, when it is needed.
It shall be necessary to have a reasonably accurate assessment on operating loads of different safety
services to ensure that the generating sets and other independent sources of supply are selected with
adequate capacity to supply all the safety services simultaneously. While doing such sizing,
allowance would also be required for motor start-up by plant sequencing and for motor starting
currents, as applicable.
11.3 Electricity Generation Systems (clause 7.3)
The requirements stipulated under this part of the standard either supplement or amend the
requirements of Sections 2 to 6 of AS/NZS 3000 covered so far. These stipulations are applicable
for all types of electricity generation systems that supply power directly to an electrical installation
or a part of it, either continuously or whenever demanded, whether standalone set or operated in
parallel with grid. The rules are applicable for the following alternative sources normally adopted
in low voltage installations:
(a) Alternative and supplementary supply: A generator set, typically combustion engine-driven,
that—
(i) provides an alternative or stand-by ac electricity supply in the event of failure of the normal
power supply to the installation; or
(ii) is used as the primary power supply to an electrical installation;
or
(iii) is used as part of a stand-alone power system.
(b) Stand-alone power system: A system that is not connected to the power distribution system of a
network provider. Stand-alone systems may be supplied with power from one or more of the
following:
(i) Photovoltaic array
(ii) Wind turbine or mini-hydro turbine
(iii) Engine driven generator set in the form of an ac supply or a dc supply.
(c) Inverter system: An inverter system that provides an ac power supply from an interactive
inverter using a renewable energy source, such as photovoltaic, wind turbine or mini-hydro
turbine. In the event of the renewable energy output available exceeding the electrical installation
load, subject to formal approval of the electricity distributor, any surplus energy available is
exported into the distribution system.
If the output available from the renewable energy sources is insufficient for the installation
loading, the shortfall in energy required is imported from the network. The interactive inverter of
the system also provides control of the exporting and importing of energy from the system and
network.
(d) Battery system A battery system that provides supply from an alternative energy source, such
as a generator set, photovoltaic array, wind turbine or mini-hydro turbine, to charge a battery bank
and provide a d.c. supply to an electrical installation.
Individual AS/NZS publications are available covering each of the above systems and it is
necessary to refer those applicable standards for proper selection and features to be applied in these
systems. The following paragraphs give the minimum standards to be considered for these systems.
192 Practical Electrical Wiring Standards - AS 3000:2018
11.3.1 Control and protection requirements
The supply output from an alternative generation source shall be controlled by an isolating switch
or switches located just at its terminal ends or at the installation switchboard from which the
alternative source feeds supply to the installation. In addition suitable control switches or devices
shall be provided for starting and stopping electricity generation system. In case multiple switch or
devices are used for this purpose they shall be grouped together and clearly identified.
The electrical system of the generation unit shall be so arranged that it shall not be able to backfeed
its generated power upstream to the grid from the point of its connection in the installation, by
adopting suitable interlocks. However in some installations the backfeeding of excess power to the
grid might be allowed, like installations having cogeneration/ captive power generating units
operating in parallel with the supply grid. This is allowed subject to incorporation of proper
protective devices and after entering into an agreement with utility company.
The isolating switch(es) used to control the supply output from a generating unit shall be installed
adjacent to or on the electricity generation system so that a person operating the switch has a clear
view of any person working on the electricity generation system. The switch may be combined
with overcurrent protection (like MCCB/ACB). These shall have manual operating provision and
the manual operation shall NOT be overpowered through intentional/ accidental bypassing or by
internal controls of the unit like a programmable control systems, etc. Where batteries form part of
the generating system, these shall also be controlled by an independent switch located just adjacent
to the batteries or controlled together with the main power isolating switch.
Generating systems shall be provided with the following minimum protections generally
complying with the rules discussed in the earlier chapters.
• Overcurrent protection
• Short circuit and earth fault protection
• Protective earthing
Overcurrent/ overload protective devices of power generation systems shall be located as close as
practicable to the output terminals of the electricity generation system. It shall be ensured that the
unprotected interconnecting conductors to an electrical installation are as short as practicable and
shall never be more than 15 metres in length.
It is necessary to assess the short circuit ratings of such systems and to incorporate devices having
adequate capacity to withstand the effects of such fault currents.
Where these generating units operate in parallel with grid the possible circulating harmonic
currents in such systems shall be limited by using compensating windings in the generator or by
implementing suitable filters, impedance, etc.
Where changeover switches are employed for feeding the generated alternative power, the
changeover device shall open all active conductors of the normal supply when the alternate supply
is getting connected to the installation. Figure 11.2 shows the changeover arrangement involving 4
poles of main source and generator. Where three pole switches are employed, the neutral of the
generator will be directly connected to the earth without the fourth pole.
Section 7 - Special Electrical Installations 193
Figure 11.2
Alternative supply connection to a switchboard with 4 pole changeover switch and a local MEN connection (Source:
AS/NZS 3000, Figure 7.3)
The method of connection of a grid-connected inverter system shall be in accordance with the AS
4777 series of Standards and shall also meet any stipulations by the electricity distributor. Figure
11.3 shows typical grid connected system and the change over arrangement normally agreed by
supply distributors.
Figure 11.3
Typical grid connection of alternative supply with inverter (Source: AS/NZS 3000, Figure 7.7)
In stand alone generator system (generator not operating in parallel with the grid), the consumer
installation mains requiring alternate source of supply shall be connected directly from the output
of the electricity generating system, typically as shown in figure 11.4. This shall be provided with a
local MEN connection like in the utility supply.
194 Practical Electrical Wiring Standards - AS 3000:2018
Figure 11.4
Typical connection of standalone generator with local MEN connection (Source: AS/NZS 3000, Figure 7.4)
11.3.2 Neutral and earthing connections
The generation system shall incorporate protective earthing in line with AS/NZS 3000 section 5
stipulations and MEN connections, exception being generators rated below 25kVA but only when
used as separated supply. The neutral to earth connection (MEN connection) of the generation
system shall also be made within the installation at the switchboard to which the electricity
generation system is connected.
The incoming neutral of a generator to its main MEN switchboard shall be directly connected
without any switch. However, where multiple engine-driven generating sets are operated in
parallel, the connection to the neutral point of the windings of each generating set to earth shall be
controlled through independent isolating switches. This is to ensure that only one neutral point of
any one of the generators is connected to earth at any time of parallel operation to avoid circulating
currents through these sets. It shall also be necessary that neutral and earth conductors of a set are
not operated in parallel, except when the conductors are adequately rated to carry the maximum
rated fault current of the generator distribution system.
11.4 Electrical Separation (Clause 7.4)
Electrical separation refers to isolated supply, where the supply to a load is isolated from the main
source by adopting isolation transformers of suitable ratings with ratio of 1:1 or downwards, being
common in LV systems. Protection by electrical separation is one of the recognized methods of
safety in the standard and is recommended to prevent shock current due to contact with exposed
conductive parts that get energized during internal faults.
11.4.1
Typical arrangement
A separated supply could be achieved by any one of the following alternatives
• Using an isolating transformer with its output supply separated from the mains input
with use of double insulation or equivalent
• Connecting a generator output having its output separated from the frame of the
generator
• An isolated inverter similar to the isolating transformer
The separated supply shall meet all the following requirements:
Section 7 - Special Electrical Installations 195
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Separated circuit voltage shall not exceed 500 Volts
All live parts of a separated circuit shall be reliably and effectively electrically
separated from all other circuits, including other separated circuits and earth within
the same installation
The earthing connections are prohibited in these systems. Exposed conductive parts
of electrical equipment supplied by a separated circuit shall NOT be connected to the
protective earthing conductor or to the exposed conductive parts of the source of
supply.
Cables and supply flexible cords to electrical equipment from separated supply shall
be protected against mechanical damages or otherwise arranged so as to ensure that
any damage that might occur is readily visible to ensure easy detection and to
minimize faults
Figure 11.5 gives different types of separated supplies and their connection possibilities as
recognized in the standard.
196 Practical Electrical Wiring Standards - AS 3000:2018
Notes referred in the figure:
1 If Class II, omit earth connection from final subcircuit.
2 Transformer or generator output protections may adopt a double-pole circuit breaker or HRC fuse.
3 Equipotential bonding conductors of sockets must be insulated and NOT connected to earth. In case of multiple socket
outlets all earth pins shall be interconnected but isolated from earth.
4 An RCD cannot serve the purpose when the equipotential bond is connected to the frame of the generator
Figure 11.5
Typical separated supply sources and their connections (Source: AS/NZS 3000, Figure 7.8)
It might be quite possible that a separated circuit feeds supply to multiple equipments in an
installation. In such cases, the exposed conductive parts of all those equipments shall be connected
together by an insulated equipotential bonding conductor. It shall also be ensured that this bonding
conductor is separated and is not connected to earth or a protective earthing conductor or exposed
conductive parts of another circuit or another separated circuit or any extraneous conductive parts
in the installation. In multiple equipment systems it is also necessary to take care that all the
following are connected to the equipotential bonding conductor
• Earthing contact of all socket-outlets
• Earthing conductors in any supply cable or flexible cord installed on the separated
circuit.
The multiple equipment system connected from separated supply requires the following
stipulations also to be met.
• Exposed conductive parts of the source of supply that are earthed, shall not be
simultaneously accessible with any exposed conductive part of the separated circuit.
• A protective device shall operate to disconnect the separated circuit automatically in
the event of two faults resulting in exposed conductive parts being connected to live
parts of different polarity.
• If the protective device is a circuit-breaker, the protective device shall open in all
unearthed conductors substantially together.
11.4.2
Verification
The separation adopted in each of the separated circuits shall be checked physically and verified by
proper tests in addition to the other verification guidelines applicable for electrical installations as
per section 8 of the standard, covered in the next chapter.
The measurements shall be made using a 500V dc insulation resistance tester to find values of the
insulation resistance across the following:
• Separated circuit and the transformer primary winding, if a transformer is the source
of the separated supply
• Separated circuit and any other wiring
• One separated circuit and any other separated circuit
• Separated circuit and earth
• Exposed conductive parts and earth
• Exposed conductive parts and earth contact of a socket-outlet in single equipment
systems
In case of multiple equipments connected from separate supply, the insulation resistance
measurements would be needed across the following:
• The separated circuit and the equipotential bonding conductor
• Equipotential bonding conductor and earth
• Equipotential bonding conductor and any equipotential bonding conductor of another
separated circuit in the installation
Insulation resistance obtained by all such measurements shall be a minimum of 1 MΩ.
Section 7 - Special Electrical Installations 197
In addition, the continuity of bonding conductors shall be checked by measuring resistance of the
equipotential interconnections in the separated circuit. The resistance values so measured shall not
exceed 0.5Ω
11.5 Extra-low voltage electrical installations (clause 7.5)
As we have seen, the voltage value is one of the main components deciding the shock current
values and separation is able to achieve safety from the intricacies of the main supply. The extra
voltage installations covered in this section try to achieve both these benefits by having separated
supplies with lower operating voltages not exceeding 50V ac or 120V ripple-free dc. Following are
the recognized extra low voltage systems.
• Separated extra-low voltage (SELV) where no protective earthing is adopted.
• Protected extra-low voltage (PELV), where one conductor of the output circuit is
earthed
Both PELV and SELV systems shall meet the following basic requirements:
• The source of supply shall be so arranged that nominal operating voltage shall not
have the possibility of exceeding the limits of 50 V ac or 120 V ripple-free dc
• Live parts of the circuits shall be electrically segregated from each other and from
circuits at higher voltages
An electrical installation operating at extra-low voltage but not complying with the above
requirements shall be deemed to be operating at low voltage and shall be subject to the relevant
requirements of other sections of the wiring rules.
Following are the recognized ways of arranging SELV and PELV systems.
• A safety isolating transformer complying with AS/NZS 61558 to step down to the
required range of ELV values
• A source of current that is independent of voltages. It can be an engine-driven
generator or an electrochemical source such as a battery.
• A source of current separated from higher voltage electrical installations, such as a
motor-generator set, with electrically separate windings having a degree of electrical
separation equivalent to that of an isolation transformer
• Electronic devices where the voltage at the output terminals cannot exceed extra-low
voltage even in the case of an internal fault. In these cases, the output terminals may
be allowed to have momentary voltages exceeding ELV limitations under transient
conditions but it is subject to the condition that the voltage at the output terminals
automatically and immediately gets reduced to extra-low voltage as soon as contact is
made with live parts under direct or indirect contacts.
Live parts of these systems shall be arranged in such a way that short-circuits or arcing between
live parts or between live parts and other conductive materials will not take place under the
reasonably expected service and environmental conditions.
Other specific requirements to be met by SELV/PELV systems are as below:
• The drop in voltage at any point in an extra-low voltage electrical installation shall be
limited to be within 10% of the nominal value when all live conductors are carrying
the circuit operating current, unless the equipment is capable of operating at still
lower values as per manufacturer’s guarantees
• The supply to an extra-low voltage electrical installation shall be controlled by a main
switch or switches operating in all unearthed conductors, provided that the operation
of the main switch for the high voltage part of the electrical installation does not
cause disconnection of the main supply (possible when ELV supply is derived from
main supply)
• Every extra-low voltage circuit shall be individually protected at its origin against
overload and short-circuit by adopting a fuse or a circuit-breaker
198 Practical Electrical Wiring Standards - AS 3000:2018
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11.5.1
Plugs in ELV circuits shall not be able to enter sockets of other voltage systems and
similarly the sockets shall not accept plugs of other voltage systems
ELV Sockets shall not have a contact for a protective earthing conductor
Conductors and insulation of cables shall be suitable for the intended purpose and
need not be further protected unless installation conditions so demand
The separation of live parts from those of other circuits and from earth shall be
confirmed by a measurement of the insulation resistance. The insulation resistance
value shall be not less than 0.5 MΩ when tested with a 250 V dc insulation resistance
tester.
SELV system additional requirements
In line with the requirements of separated supply systems, SELV circuits shall not be connected to
other circuits or earth or earthing conductors/ exposed conductive parts of another system as well
as extraneous conductive parts in the installation. However connection to extraneous conductive
parts may be made where electrical equipment is inherently required to be so connected but subject
to complying with the condition that the extraneous conductive parts shall not attain a voltage
exceeding that of the SELV circuit.
In dry indoor conditions, where the nominal voltage of SELV exceeds 25 V ac or 60 V ripple-free
dc, protection against electric shock due to direct contact shall be provided using barriers/
enclosures or alternatively by adopting proper insulation. In wet and outdoor conditions these
protection barriers/ insulation might be warranted for all voltages.
Where barriers/ enclosure are provided they shall afford a minimum protection category of IPXXB
or IP2X or when provided with insulation, it shall be capable of withstanding a test voltage of 500
V ac for 1 minute.
A switch used for isolation in SELV circuit may operate in one less conductor than the number of
conductors used in the circuit.
11.5.2 PELV system additional requirements
PELV circuits differ from SELV in the way that one conductor of the output circuit is earthed by an
appropriate connection to earth normally at the source of the equipment where from the SELV
originates.
The direct contact shall be prevented in PELV circuits from 6 V ac or 15 V ripple-free dc operating
voltage onwards. However, in dry indoor conditions, live parts of PELV circuits can be considered
for protection from direct contact only for circuits exceeding 25 V ac or 60 V ripple-free dc, subject
to ensuring that a large contact area (more than 8000 sqmm) with the human body is not expected
in such dry installations. The prevention of direct contact to live conductors is achieved by use of
barriers/ enclosures or proper insulation. When such protection is provided with barriers or
enclosures they shall afford a minimum degree of protection equal to IPXXB or IP2X.
Alternatively, when insulation is adopted it shall be capable of withstanding a test voltage of 500V
ac for 1 minute.
A switch used for isolation in PELV circuit shall operate in all unearthed conductors.
11.6 High voltage electrical installations (clause 7.6)
For protection and earthing purposes, this clause applies to all the electrical equipment up to and
including any low voltage cables as well as low voltage switchgear associated with high voltage
transformers. The standard recommends that high voltage parts of these installations shall follow
the requirements as stipulated under AS2067 in Australia and follow the requirements as stipulated
under New Zealand Electrical Regulations in New Zealand.
Section 7 - Special Electrical Installations 199
The below mentioned installations incorporating high voltages in their internal systems and
operation are NOT covered under this category.
• Electric discharge (neon or equivalent type) illumination systems
• X-ray equipment
• High frequency equipment
• High voltage wiring and electrical equipment enclosed within self contained electrical
equipment where appropriate precautions have been taken to prevent contact with
high voltage conductors for supplying low voltage in the installation
The standard specifies that the low voltage parts of HV installations shall generally follow all the
rules and regulations specified from sections 1 to 6. In regard to the HV parts of these installations,
the standard recommends to duly take care of the following features in design, selection and testing
phases.
• Insulation levels to withstand highest voltage and/or impulse withstand voltages
• Minimum clearances to live parts taking into account electrode configurations and
impulse withstand voltages
• Minimum clearances under special conditions
• The application of various devices connected to the system
• The methods of installation of equipment, cables and accessories
• General requirements of installations regarding choice of circuit arrangement,
documentation, transport routes, lighting, operational safety and labelling
• Special requirements with respect to buildings
• Protection measures with respect to access
• Protection measures with respect to fire
• Provision of earthing such that the system operates under all conditions and ensures
safety of human life where there is legitimate access
• Testing
The details related to the above topics are quite voluminous and cannot be covered in this manual
except that we need to understand that all the basic requirements related to insulation, access,
isolation, protective earthing, protective devices, enclosures, minimum safety clearances, testing,
etc required for LV systems as covered in this manual are equally important for high voltage
installations also. The main difference being the amount of voltages and power controlled in HV
installations which are comparatively much higher than LV systems and hence the HV installations
require additional features to achieve the same kind of safety expected in low voltage systems.
11.7 Hazardous areas (clause 7.7)
The subject on Hazardous area and its definitions/ requirements is specialized and is relevant not
only for electrical equipments but also for process and instrumentation applications. The hazardous
areas recognized in Australian Standards follow the guidelines as per IEC standards that are
somehow different from the guidelines followed in USA. The basic awareness needed in these
areas is that the electrical devices are always prone to create sparks due to movement of energized
contacts which can easily become the main source of fire in areas that accommodate or process
explosive chemicals, flammable liquids and the like. Hence it is necessary that electrical
installations in hazardous areas shall be able to prevent the sources of fire (sparks) from the
electrical parts coming in contact with hazardous substances present in such explosive/ flammable
areas. These are usually achieved by accommodating live parts in special enclosures or by adopting
special design features to minimize the possibility of sparks coming in contact with external
hazardous gases or substances prevailing in the surroundings.
The hazardous materials that are of concern are:
• Flammable Gases/ Vapour
• Flammable liquids having partial pressure to liberate flammable vapour at normal
conditions
200 Practical Electrical Wiring Standards - AS 3000:2018
•
Combustible dust / Flakes or fibres that burn when mixed with air or in layer form
that could burn when ignited
In AS/NZS 3000 following two general hazardous areas are recognized
• Gas hazardous areas - in which flammable gases or vapors may be present in the area
in sufficient quantities to produce an explosive gas atmosphere. Three Zones (Zone 0,
Zone 1 and Zone 2) are recognized in these areas and defined in AS 2430.1.
• Dust hazardous areas - which are hazardous because of the presence of combustible
dust, fibers or flyings. Three Zones (Zone 20, Zone 21, Zone 22) are recognized in
these areas and defined in AS/NZS 61241.3
The higher the zone number, the smaller is the risk of an explosion in that zone. Table 11.1
identifies the typical zone classifications specified in the IEC/AS for flammable gases and
flammable liquids producing flammable vapour. It may be recalled that the classified zones in
damp areas also try to follow a similar concept.
Section 7 - Special Electrical Installations 201
Table 11.1
IEC/AS Hazardous area classifications
Zone
Condition of the area
0
An area in which an Explosive gas/air mixture is
continually present or present for long periods
1
An area in which a gas/air mixture is likely to occur in
normal operation
2
An area in which a gas/air mixture is not likely to
occur in normal operation, and if it occurs, it will exist
only for a short time
Table 11.2 gives typical zones applied for areas frequented with combustible dust materials.
Generally it is considered that 1 mm or less thickness of dust is not likely to result in formation of
explosive atmosphere and may not require special precautions.
Table 11.2
IEC/AS Hazardous area classifications for combustible dust
Zone
Condition of the area
20
An area in which combustible dust, as a cloud, is present
continuously or frequently, during normal operation, in sufficient
quantity to be capable of producing an explosive concentration
of combustible dust in mixture with air, and / or where layers of
dust of uncontrollable and excessive thickness can be formed.
21
An area in which combustible dust, as a cloud, is likely to occur
during normal operation, in sufficient quantity to be capable of
producing an explosive concentration of combustible dust in
mixture with air.
22
An area in which combustible dust, as a cloud, can occur only
infrequently and persist only for a short period, or in which
accumulation of dust layer can give rise to an explosive
concentration of combustible dust in mixture with air.
Selection and installation of electrical equipment in all these hazardous areas shall comply with the
appropriate requirements as specified in AS/NZS 2381.1 or AS/NZS 61241.14 and normal
equipments shall not be used.
Additional competencies, inspection techniques and maintenance / repair methods in these
hazardous areas shall be based on the requirements given in the applicable parts of the AS/NZS
2381 and AS/NZS 61241 series
11.8 Other specific installations (clause 7.8)
Following special installations are recognized in this standard for complying with safety norms
highlighted and it is recommended to refer the respective standard indicated against each in regard
to the regulations to be followed in such installations.
• Construction and demolition sites - AS/NZS 3012
• Electromedical treatment areas - AS/NZS 3003
202 Practical Electrical Wiring Standards - AS 3000:2018
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Transportable structures and vehicles including their site supplies - AS/NZS 3001
Marinas and recreational boats - AS/NZS 3004
Shows and carnivals shall comply with AS/NZS 3002
Cold-cathode illumination systems (electric discharge illumination systems) AS/NZS 3832
Extra-low voltage (d.c.) power supply installations within public telecommunications
networks - AS/NZS 3015
Generating sets for the supply of electricity at voltages normally exceeding 50 V a.c.
or 120 V d.c. - AS/NZS 3010
Electric fences - AS/NZS 3014 and AS/NZS 3016
Electrical equipment and temporary electrical installations on film, video and
television sites - AS/NZS 4249
Any other LV installations not covered in these standards shall follow the requirements as per
AS/NZS 3000, as appropriate to the type and nature of the installation.
11.9 Supplies for electric vehicles (NZ only)
The particular requirements of this Clause (Clause 7.9) supplement or amend the requirements of
Sections 2 to 7 of this Standard for parts of electrical installations intended for the charging of
electric vehicles for New Zealand only.
NOTES:
1 Appendix P contains information on the modes of charging used for charging systems used for
electric vehicles.
2 Appendix C contains guidance on assessing the contribution of electric vehicle charging to
maximum demand of an installation
11.10 Summary
Section 7 of AS/NZS 3000 specifies requirements to be complied in special installations such as
safety services, alternative supply systems, separated supply installations, etc including
construction/ demolition sites, shows and carnivals, etc. The term safety services refers to
emergency systems that are to be operated in emergency situations to safeguard personnel and
properties and includes fire and smoke detection/ control systems, evacuation systems like lifts, etc.
The installations shall essentially consider separation of power supplies feeding equipments of
safety services so that these services can be independently controlled in all conditions including
emergency situations with normal or alternative supply arrangements. These services shall
incorporate properly rated and discriminated protective devices and be ensured for uninterrupted
availability to take care of any kind of emergency situation at any time.
The captive power generation systems can be any of the sources like engine-generator set, inverters
with batteries, etc. These systems shall also include basic protection features such as overcurrent
protection, shortcircuit protection and protective earthing like the other main installations generally
in line with the stipulations covered in this standard. Normally these may be operated as stand
alone system or in parallel with grid with single or multiple sets. 4pole/ 3pole changeover switches
are adopted to feed areas requiring alternative supply. The neutral of a generator shall be connected
to the main MEN connection without any isolating device. However where multiple generating sets
are operated in parallel it is necessary to connect one generator neutral alone to the earth with the
others kept open to prevent circulating currents. To achieve this suitable isolation switches shall be
incorporated in each generator neutral circuit.
Electrical separation is normally used by having an isolation transformer and avoiding all earthing
connections for the conductive and extraneous parts in an installation. In these systems, it is
recommended to interconnect all earthing contacts of socket outlets, cables, etc to an equipotential
bonding conductor that is insulated from earth for easy verification/ testing. It is important to
Section 7 - Special Electrical Installations 203
verify the conformity of separated supply systems mainly in regard to separation of earth
connections, equipotential connection, etc. The separated extra low voltages and protected extra
low voltage (SELV and PELV) systems are sources rated below 50V a.c. or 120V d.c. SELV
systems do not adopt protective earthing while PELV systems have one conductor of output
circuits connected to earth. These ELV systems can be combination of isolation transformers
and/or electronic devices rated to supply the required currents without exceeding the specified
limits. It shall be ensured that the conductors of all separated systems are isolated from those of the
other systems. It is also essential to adopt special types of socket-outlets and plugs so as to avoid
interchange with high voltage supplies/ appliances. In dry conditions these ELV systems need not
have additional protection for shock hazards upto 25V a.c. or 60V d.c. However additional
protection by using suitable barrier or adopting suitable insulation shall be provided beyond these
ratings.
In regard to HV installations, hazardous areas and other specific installations, the standard
recommends referring the other applicable AS/NZS/IEC standards for specific requirements but it
is necessary that these systems shall also ensure all basic safety requirements expected as per
AS/NZS 3000 with some special construction to take of the hazards and characteristics expected in
these systems.
204 Practical Electrical Wiring Standards - AS 3000:2018
12
Section 8 – Verification
Section 8 of AS/NZS 3000 provides guidelines for verification of various parts of an electrical
installation to check for their compliance with the stipulations given in this standard before the
systems are put into service. In this chapter we will review the importance and requirements of
these verifications along with the brief verification checklists given in the standard for major parts
of a typical LV installation. We will also go through the tests to be conducted on the systems before
and after energisation along with the stipulated guideline values to be considered for verifying
such test results. The tests include insulation resistance tests, fault loop impedance measurements,
RCD testing, etc and are mandatory for all LV installations.
Learning objectives
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Basic requirements
Checklist and documentation
Mandatory tests
Testing sequence
Insulation and polarity tests
Measurement of earth fault loop impedance
RCD testing
12.1 Basic objectives and requirements (clause 8.1)
AS/NZS 3000 stipulations so far covered are all with the main objective of ensuring safety to
personnel, properties and livestock. There is no doubt the objective can be achieved but it is
important that the installation follows all the key recommendations of the standard. As we have
seen in the initial chapter, the equipment and installation need to successfully pass the tests to
prove that they will provide the safe and expected performance during actual service.
Section-8 of the standard is aimed to achieve the above goal by providing guidelines and specific
instructions covering the following.
• To verify that the requirements of this Standard have been met by the new electrical
installations as well by any alterations, additions or repairs to an existing electrical
installation
• All the new and altered installations after completion are inspected as far as is
practicable to ensure for correct ratings and for correct installation as per established
standards. In regard to the modifications, alterations, additions or the repairs to an
existing installation it is compulsory to ensure that these do not impair the safety of
the existing electrical installation.
206 Practical Electrical Wiring Standards - AS 3000:2018
•
After the above inspection, all the new and altered systems are tested for checking
their correct design and ratings by means of established procedures using reasonably
simple instruments to ascertain that the equipment and installation will provide safe
and satisfactory performance under the specific supply and environmental conditions.
Though most of the tests and inspections are done before energisation, certain types of tests may
have to be made after the electrical installation has been placed in service, to check integrity with
the supplier systems as well and for checking proper operation of some specific and important
devices like RCDs.
Section-8 of the standard recommends the following steps to complete the verification and testing
of the installations:
• Visual Inspection to verify that all external provisions, connections and safety
clearances as per the norms are provided and available
• Mandatory tests shall be conducted after completion of inspection for checking
compliance of the installation and equipment to the basic design and safe
performance needs
• Establishment of system conditions based on tests covering different parts of the
installation by measuring insulation resistance, loop impedance, etc including checks
on circuit connections, polarity, etc and ensure that they all meet basic acceptable
requirements for successful operation without impairing safety to the installation.
• Tests on RCDs.
It is to be noted that there might be requirements to refer other standards for some of the tests on
specific installations such as separated supplies, SELV and PELV installations, electromedical
installations, transportable structures and vehicles and marinas, etc for which the standards
identified in appendix-A shall be referred.
It is also essential to have records of all the tests with the date of initial energisation of an
installation on-site.
12.2 Safety precautions and testing devices
The main objective of AS/NZS 3000 is to ensure safety of persons and avoid damage to property
and electrical installation equipment and there is no exception for these objectives during
inspection and testing as well. Hence it is necessary to ensure that all the inspections and tests are
carried out applying well defined safety practices, so that there are no untoward incidents or causes
that could affect the people, properties and the installation during these verifications and tests.
Following are the minimum precautions needed as per AS/NZS 3017. This standard also provides
illustrations on the test setups to be adopted for the various tests recommended in AS/NZS 3000.
• Treat all equipment, such as cables and terminations, etc as being energized until
proven otherwise before getting in contact with them
• Follow safe working practices as per AS/NZS 4836 like use of proper PPE, etc
• Understand the correct use of the equipment to be used and its rating before applying
a test voltage or passing a test current
• Ensure that the equipment being used, including any test leads, probes or clips are
suitable for the test voltages, are in good working order and have no damaged or
loose parts that might lead to direct contacts with test voltages
Following are some of the main instruments and devices needed to complete most of the tests
recommended in AS/NZS 3000.
• Insulation resistance tester
• Ohmmeter
• Voltage indicator (e.g. lamp, neon, LED device or meter)
• Suitable probes
• Trailing leads
• A range of resistors of known values
Section 8 – Verification 207
•
•
A suitable instrument for measuring fault-loop impedance and
A suitable instrument or device for checking the operation of a RCD
Where appropriate, the instruments used for tests should have valid calibration from approved
laboratories for their correct measurements.
12.3 Visual Inspection (Clause 8.2)
It is necessary that the visual inspection shall be carried out when the installation works have been
completed and before the relevant part of the electrical installation is to be placed in service to use
normal supply. It might also be possible that the visual inspection of some part of the electrical
installation may not be accessible on completion of works due to the type of enclosure in the
building structure, etc. In such cases, it is recommended to complete the inspection of that part
during the course of the installation itself and making proper records before covering up.
Basically the visual inspection is aimed to check and ensure compliance of all the following.
• Availability of protection features to safeguard against direct contact with live parts,
e.g. availability of insulation and enclosure for the electrical equipment.
• Availability of protection features to defend against indirect contact in equipments
having exposed conductive parts based on the type and nature of the installation e.g.
provision of double insulation or isolating transformers, in specific areas.
• Protection methods adopted against the common hazardous parts and conditions, e.g.
enclosure door conditions, guarding or screening of flammable materials, protection
from hot surfaces and parts that may cause physical injury when operating, etc.
• Protections available to prevent spreading of fire during fire accidents, e.g. sealing
provisions for wiring systems while penetrating fire barriers.
• General conditions of various parts of an electrical equipment which are likely to
create hazards in due course of time because of the nature of the surroundings and
other conditions of the installation e.g. signs of damage on external and internal parts
of an equipment that could impair safe operation and/ or its disconnection.
Following paragraphs highlight some of the common checks to be made on different parts of a
typical low voltage installation. These may not be exhaustive but cover most of the salient points
that are important in the respective parts.
12.3.1 Checks on consumer mains
Verify the compliance of consumer mains for the following:
• Current carrying capacity of the system based on the demand expected
• Voltage drop is likely to be within limits, e.g. minimum size of conductors for the
current expected
• Conditions of underground installation e.g. enclosure conditions/ material, depth of
burial, mechanical protection provided, etc.
• Aerial connection practices adopted, e.g. maintenance of safety clearance distances.
• Proper connection of wiring at the mains
• Space availability for maintenance checks/future modifications
• Protection against external influences, mostly weather conditions likely to be faced
while in service
12.3.2 Verifying switchboards
The switchboards of the installation shall be visually inspected for the following requirements.
• Location, e.g. unobstructed access, exit doors and egress path sizes
• Protective devices installed, e.g. rating of overload and residual current protective
devices, expected normal and fault current ratings and compliance of component
ratings to the same
• Provision of isolating devices and their features, e.g. main isolation switches, locking
features, etc.
208 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
12.3.3
Connecting devices and their arrangements, e.g. neutral/earth bars and active links
Connection and fixing methods adopted for wiring and switchgear
Identification and labelling provisions on the switchboard
Protection against external influences like IP classification, etc to meet the conditions
of a location
Ventilation conditions
Verifying wiring systems
Following checks are recommended on the wiring systems of the installation.
• Conductor size – Current carrying capacity and voltage drop based on main circuit
and subcircuit ratings
• Identification of cable cores with colour, sleeving, etc as per acceptable norms
• Adequate support and fixing for all the wires and associated parts like enclosures, etc.
• Conditions of connections like bending radii, adequate supports, etc.
• Conditions of enclosures like material, sealing, etc.
• Compliance to particular installation conditions, e.g. underground with proper
identifications/ protections, aerial with proper spans/ supports, emergency systems
arrangements, etc.
• Segregation of electrical installations from other services, as needed, like barriers,
distances, etc.
• Protection against external influences, e.g. grinding of or protection against sharp
corners, enclosure IP classification, armouring, etc.
12.3.4
Checks on Electrical equipment
Availability of following provisions and features shall be checked
• Provision of isolation and switching devices for protection against injury from
mechanical movement devices and motors
• Provision of isolation and switching devices for protection against high temperature
effects e.g. motors, room heaters, water heaters
• Locking devices and caution labels
• Provision of individual switching devices for particular electrical equipment/
appliances e.g. socket outlets, cooking appliances, etc
• Particular installation conditions, e.g. locations affected by water, explosive
atmospheres, extra-low voltage, high voltage and provisions made to take care of
such conditions
• Compliance with applicable Standard of an equipment or device, as applicable
• Quality and conditions of the connection, support and fixing arrangements provided
for the switches, socket outlets, distribution systems, etc.
• Protection against external influences as needed like barriers, covers, etc.
12.3.5
Checks on earthing
The earthing system plays an important role in the safety condition of an installation and hence it is
important that all earthing provisions as per the standard requirements are complying to ensure
safety. Following are the typical checks needed.
• Proper MEN connection at the main switchboard or any other recognized location
• Earth electrode provision, type, location and material as per acceptable practice
• Earthing conductors, e.g. material, conductor size, identification with colour codes
for insulation, etc.
• Equipotential bonding conductor provisions, as needed, including their size,
identification, etc.
• Proper connections, joints and terminations of earthing conductors at all equipments,
links, busbars, etc.
• Protection against external influences with conduits, etc as may be needed
• Earthing arrangements for other non electrical systems
Section 8 – Verification 209
•
Creation of earthed situation that will ensure earthing of additional electrical
equipment
12.3.6 Fixed wire appliances
Ensure following checks on the appliances with fixed wiring
• They are correctly positioned and are suitable for the environment in which they are
located
• Connections of conductors to electrical equipment are correct. When this connection
is via flexible cord the cord shall be anchored at both the electrical appliance and the
supply fitting
• Electrical appliances are correctly mounted and protected against mechanical damage
• Covers are in place preventing access to live parts or basic insulation
After completing all the checks for the various items of an installation, the checklist forms shall be
duly filled to confirm compliance to the requirements and retained for future records.
12.4 Mandatory tests (clause 8.3)
Following are the mandatory tests to be carried out on an installation before energisation to verify
its compliance with the standard.
• Condition and continuity of the earthing system (by means of resistance measurement
for main earthing, protective earthing and bonding conductors)
• Insulation resistance of various switchboards and circuits
• Polarity of the connections
• Correct circuit connections
• Measurement of earth fault loop impedance to confirm automatic disconnection of
supply within acceptable time, when needed
• Operation of RCDs
12.4.1 Sequence of tests
It is recommended to adopt the sequence as per figure 12.1 for the various mandatory tests to be
done on an installation.
Figure 12.1
Sequence of mandatory tests (Source: AS/NZS 3000, Figure 8.1)
210 Practical Electrical Wiring Standards - AS 3000:2018
If the electrical installation fails a test, it could be due to some unexpected or unknown fault
conditions prevailing in the equipment or in its connections. Such faulty conditions shall be
located, analyzed and rectified, after which that test and any preceding tests that might have been
influenced due to such fault condition shall be repeated once again to prove compliance. AS/NZS
3017 shall be referred which provides detailed procedures and setups to be followed while carrying
out the various mandatory tests highlighted in figure 12.1.
12.4.2
Continuity tests of the earthing system
The continuity of earthing system is most important to serve its purpose of safety and hence the
continuity of the earthing system is to be checked for ensuring proper protection under energized
conditions.
• The circuit protective devices shall be able to detect if there is a fault between live
parts to the earth due to either direct or indirect contacts and will operate without fail
• The electrical equipment parts that are earthed have sufficiently low earth resistance
values and these parts will not reach dangerous voltages when faults occur
Following verifications and tests are recommended to check conformity of main earthing and
equipotential bonding systems:
• Check and ensure continuity of the main earthing conductor between the main
switchboard and the earth electrode
• Measure and ensure that the resistance of the main earthing conductor including
earth rod is within 0.5Ω
• Check continuity of connections between any point on the installation required to be
earthed and the switchboard earth bar or terminal, as applicable
• Ensure that the protective earthing conductor sizes provided are meeting the
installation fault conditions and active conductor sizes as per the standard
• Ensure that all fixed wired appliances that require earthing (Class I equipments) are
connected to earth, using recommended conductor sizes
• Ensure that the connection between any point on the installation required to be
equipotentially bonded and the switchboard earth bar is continuous
• Measure and ensure that the resistance of each equipotential bonding conductor does
not exceed 0.5 Ω
12.4.3
Insulation Resistance tests
Insulation resistance tests are intended to verify the adequacy of the insulation between live
conductors and earth and ensure its integrity while energizing or operating with the normal
operating voltages of the system or equipment. This test is essential to prevent the following
hazards common with insulation failures.
• Shock hazards from inadvertent contacts
• Fire hazards from short-circuits
• Equipment damage during energisation or after some time
Insulation resistance test is done using 500V DC insulation resistance test instrument, usually
termed as megger instrument. The resistance is measured between the active conductors, active
conductor to neutral and active conductors to earth, etc by maintaining 500V DC between the
terminals to be tested for one minute. It shall be ensured that the insulation resistance value so
measured between various isolated/ separated parts is minimum 1 MΩ on the 500 V DC
instrument.
Where consumers mains or submains are not of significant length, e.g. with say 50 m of polymeric
cables, the insulation resistance values in excess of 50 MΩ can be expected. Similarly a lower
value in the order of about 10 kilo ohms (0.01 MΩ) may be acceptable for sheathed heating
elements of appliances or some other higher or lower value as permitted in the applicable standard
for the item under test at the discretion of authorized testing personnel. Figure 12.2 shows typical
arrangement for measuring insulation resistance between active conductor to other conductors and
earth using the instrument, with all untested parts grouped and connected to earth.
Section 8 – Verification 211
Figure 12.2
Typical arrangements for Insulation resistance test on cables
12.4.4
Polarity and correct circuit connection checks
Polarity checks are related to verifying the arrangement of active, neutral and earth conductors to
prevent the following problems:
• Shock hazards that can happen due to transposition of active and neutral conductors
of the consumers mains or sub mains (with MEN connection at outbuilding or
detached portion) resulting in the earthing system to carry a substantial voltage
• Combinations of incorrect connections among active, neutral and earthing conductors
resulting in the exposed conductive parts of the electrical installation becoming alive
when supply is given
• The wrong connection of switches in neutral conductors instead of active conductors,
impacting the parts of appliances, such as appliances and lamp holders to remain
energized even when the switches are in the ‘OFF’ position, that can easily lead to
shocks and faults
The verification of correct circuit connections is intended to ensure the following:
• Protective earthing conductors do not carry current under normal operating
conditions
• No short circuit is prevalent in the system that can otherwise lead to short-circuit
current between live conductors and through part of the earthing system upon
energisation causing considerable fire damage or personal injury, particularly in high
current locations
• Interconnection of different circuits for safe functioning and operation of the various
equipments of the installation, as applicable
Most of the above checks are done by visual inspections, resistance tests, voltage measurements,
test lamps, etc.
12.4.5
Verification of resistance and impedance
Though the standard specifies provision of RCD’s for socket outlets, there are some exceptions
where an installation includes circuits containing socket-outlets that are not protected by an RCD.
212 Practical Electrical Wiring Standards - AS 3000:2018
In such cases, it is necessary to verify that the impedance for detection of small currents is within
acceptable values and automatic disconnection of supply during earth faults is possible under
leakage conditions. These tests shall be done as below:
• Verification of the total resistance (Rphe) of the active and protective earthing
conductors, without energizing the system
• Verification of the earth fault-loop impedance, after connecting the supply
The second test is performed to verify the integrity of the MEN connections and the test covers all
the components of the earth fault-loop including the upstream neutral (PEN) conductors, which will
be effective with the mains supply.
These tests will prove that the protective device will operate to disconnect an earth fault current
within the stipulated operating time without touch voltage getting exceeded as per the standard.
The standard includes two tables to cross check the correct values to be achieved for Rph and earth
fault loop impedance for different conductor sizes and protective device ratings.
The loop impedance measurements should be carried out using a measuring instrument capable of
reading low resistance values keeping MEN connection intact and ensuring safety for the
instrument operator. Where supply is not available, the resistance of the conductors of each
individual circuit may be measured by connecting an ohmmeter with the following arrangement
illustrated in figure 12.3.
• One lead on the active conductor and protective earthing conductor connected
together at the origin of the particular circuit, where the protective device is normally
fitted
• With the above arrangement, connecting one lead of the ohmmeter to the active
conductor and the other lead to the associated protective earthing conductor at a
considerable distance away from the origin
The value of resistance (Rphe) so obtained shall not exceed the value in table 8.2 given in AS/NZS
3000 (Table 3.2 in AS/NZS 3017) for the appropriate conductor size and type of protective device.
Section 8 – Verification 213
Figure 12.3
Typical setup for individual circuit earth fault loop impedance measurement (Source: AS/NZS 3017, Figure 3.22 )
Maximum values of earth fault-loop impedance (Zs at 230 V) to be obtained in such measurements
are given in table 12.1 for different types of circuit breakers and fuses commonly adopted in the LV
installations.
214 Practical Electrical Wiring Standards - AS 3000:2018
Table 12.1
Maximum allowable earth fault loop impedance for 230V circuits (Source: AS/NZS 3000, Table 8.1)
Table 12.1 does not include 5 seconds disconnection time circuit-breakers as these are intended to
operate in the instantaneous tripping zone for earth faults. The values of Zs in the Table are based
on the equation, Zs = Uo/Ia, we reviewed earlier while discussing on earth fault loop impedance.
Where
Z s = earth fault-loop impedance of the circuit
U o = nominal phase voltage (230 V) of the system
I a = current causing automatic operation of the protective device.
For arriving at the maximum permissible impedance for the required disconnection time, I a for
circuit-breakers for the different types are chosen as the mean tripping current in line with AS/NZS
60898 as noted below.
Type B = 4 × rated current
Type C = 7.5 × rated current
Type D = 12.5 × rated current
I a for fuses are approximate mean values considered from AS 60269.1
Section 8 – Verification 215
12.4.6
RCD testing
Though RCD’s are provided their operation can not be taken for granted. These shall be tested for
proper operation under normal supply conditions, immediately after energisation for making any
rectifications, if needed. It may be recalled that the RCD’s are to satisfactorily perform with pure
sinusoidal voltages in Australia, while in New Zealand these shall be suitable for operation during
leakages in pulsating dc circuits also. The tests to check proper operation of each RCD in each final
subcircuit protected by an RCD shall be done as below.
• Using an integral testing device which causes the device to operate under rated
residual alternating current and (in New Zealand) residual pulsating direct current
conditions. If the RCD being tested is marked with the symbol confirming that it is a
type A RCD, the residual direct pulsating current testing may be omitted (in New
Zealand).
• By using a test plug with a resistor between the active and earth pins and checking
the operation of the device when energized with the supply.
Though use of integral test device may establish the RCD is functioning correctly and prove the
integrity of the electrical and mechanical elements within the tripping device, it does not prove the
continuity of the main earthing conductor or the associated circuit protective earthing conductors or
any earth electrode or other means of earthing or any other part of the associated electrical earthing
in the installation.
The test is considered successful if the RCD trips instantaneously to isolate the main supply.
12.5 Summary
Section-8 of AS/NZS 3000 details the guidelines to be followed for inspection and testing on new
installation as well as for alterations/ modifications to an existing installation for making sure that
the installation complies with the stipulations of the standard and hence is safe to operate. It is also
necessary all applicable safety norms are strictly in place while the tests are conducted. Most of the
tests are to be conducted using proper test instruments without energizing the main supply. Some
tests shall be done after installation is put into service.
Section-8 also provides checklists for completing visual inspections on various parts of an
installation covering consumer mains, switchboards, equipments, earthing systems, etc. After
visual inspection, the mandatory tests shall be conducted in the sequence recommended i.e. earth
resistance test, insulation resistance test, polarity and connection tests, resistance measurement,
earth fault loop impedance measurement and finally the tests on RCD’s provided in subcircuits.
The earth resistance values of main earthing conductor shall not exceed 0.5 ohms for an installation
including earth rod and MEN connection and also for bonding connections. The insulation
resistance tests shall give a value of not less than 1 Mega ohms with 500V DC across the insulated
parts. Some exceptions to this value may be considered based on the types of materials and circuits
adopted. Polarity and circuit connection tests are essential to verify there are no exchange in active,
neutral and earth conductors while completing terminations at various parts of the installation as
these could cause short circuits, fire hazards, etc upon energisation.
The combined resistance of the active and protective earthing conductors without connecting mains
supply and the earth fault loop impedance after getting mains supply shall be measured, to check
their impact on the disconnection times of protective devices. The resistance and earth fault loop
impedance values shall be within the maximum values specified in the tables given in the standard
based on the conductor sizes (for resistance), protective device rating and disconnection time
needed for various types of circuit breakers and fuses commonly adopted in LV installations.
Tests on RCD’s shall be verified by using integral test device or any suitable device for their
correct tripping at or just below the rated residual current.
216 Practical Electrical Wiring Standards - AS 3000:2018
Section-8 of AS/NZS 3000 shall be referred along with AS/NZS 3017 to understand the
instruments needed and the test setups to be adopted for completing the recommended tests. All test
results shall be properly recorded along with energisation date and kept for access at site.
Section 8 – Verification 217
Appendix 1
Determination of Maximum Demand for
Electrical Installations
A1.1 Introduction
Appendix C of AS/NZS 3000 provides guidelines that are useful in determining the power demand
of an installation that is ultimately critical in deciding the conductor sizes, protective device
settings, reliable and safe operation of a system, etc during the design and selection of electrical
distribution systems. This part of our manual briefly covers the salient recommendations given in
the standard for the calculation of maximum demand in common LV installation, which is referred
to as “after diversity maximum demand”
While individual load control device and current are based on the nominal current rating of the
load, all the loads in most of the plants do not operate simultaneously. Some devices (like
refrigerators, heating loads, etc) include automatic ON/OFF controls due to which the currents
drawn by such loads do vary under normal use. Similarly all sockets in an installation may not be
serving at all the times. Hence to decide the maximum load of a system it is not necessary to just
sum of all the nominal amperes of different loads but instead you need to apply some factors (Less
than 1, referred to as diversity factor) to decide the actual current to be considered during the
design and selection stages of an installation.
AS/NZS 3000 divides the LV installations into two major types as below:
• Domestic installations basically serving as residences of families in single and multi
apartment buildings
• Non domestic installations where people gather for pleasure, business, education, etc
like motels, schools, churches, etc including factories and business houses
The standard provides tables associated with each of the above type of installation for the diversity
factors to be applied for various electrical loads so as to assess the “after diversity maximum
demand” for the purpose of to be met by the main supply.
The tables given in the standard may also be used to come to a conclusion on the power demand to
be met by captive units that might be planned to supply a part or full installation, with due
consideration on manufacturer’s recommendations and inrush currents for large inductive/
capacitive loads.
218 Practical Electrical Wiring Standards - AS 3000:2018
A1.2 Domestic installations
These are divided into two types Viz., single residence and multiple living blocks with further
division of four categories as below for applying the diversity factors to the loads:
• Single living unit
• 2 to 5 living units
• 6 to 20 living units
• 21 and above units
A1.2.1 Load grouping for domestic installations
The loads in domestic installations are usually divided into separate and multiple circuits as we
reviewed in the manual. The standard groups the common domestic loads using the alphabets A, B,
etc as given below. It also includes some more sub-grouping as given in brackets for each type of
load:
• A. Lighting points (Indoor and outdoor)
• B. Socket outlet points (10A, 15A and 20A- Refer I,J also)
• C. Home appliances (like ranges, cooking appliances, washing machines, etc or
exclusive sockets provided for such appliances)
• D. Heating and Air conditioning (or sockets provided for such items)
• E. Instantaneous water heaters
• F. Storage water heaters
• G. Spa and swimming pool heaters
• H. Communal lighting (applicable for multiple units only)
• I. Socket outlets rated below 10 amperes but not covered by any other group
including B (normally considered for multiple units only)
• J. Appliances rated above 10 amperes in three categories Viz., dryers and water
heaters, space heating and air conditioning, swimming pools and spa heaters
(applicable for multiple units only)
• K. Lifts (Not to be taken as part of domestic load for demand assessment but shall be
considered as non domestic for the purpose of subcircuit design)
• L. Motors (Same consideration like lifts, to be considered as non domestic)
• M. Any other loads not covered above
Table C1 in the standard gives the recommendations on currents and/or diversity factors to be
considered for the different groups identified above. Most of these recommendations are included
in Tables A1.1 and A1.2 attached at the end of this appendix. While referring to the tables, it must
be remembered that for such of the loads indicated as ‘not applicable’, if provided in an
installation, their contributions shall be their respective nominal current ratings and shall be added
to the current contribution from other load groups to arrive at a conservative maximum demand.
A1.2.2 Demand calculations
Following are the important points to be considered while making the demand calculations.
• In case of Multiphase systems - Divide the total points of lighting by the number of
phases (3) and round the result to the next integer for consideration in each phase
• 2 points per every meter run shall be considered for the lighting tracks, taking 0.5A/m
per phase as the connected load
• Fluorescent lamps and discharge lamps shall include the wattages of the ballasts/
capacitors, etc. for their normal current before applying the recommended diversity
• Socket outlets installed above 2.3 meters used for luminaires or for connecting
appliances rated below 150 watts may be considered as one lighting point – Group A
• Permanently connected load rated below 10 Amperes may be included as part of
socket outlet – Group B category (i)
Determination of Maximum Demand for Electrical Installations 219
A1.3 Non domestic installations
These installations are divided into two categories as below for the purpose of load assessment:
• Residential institutions, hotels, boarding houses, hospitals and motels
• Factories, shops, stores, schools, churches and business premises
A1.3.1 Load grouping for non-domestic installations
The loads in non domestic premises are also divided in line with the domestic loads and identified
with alphabets as given below with further break-up in brackets against respective loads:
• A. Lighting points in places other than those in fuel dispensing units
• B. Socket outlets (10A and above 10A. Here 10A sockets shall be further divided as
free outlets or with permanently installed loads)
• C. Appliances for cooking, heating and cooling, including instantaneous water
heaters
• D. Motors excluding those in lifts and fuel dispensing units (includes motors used for
domestic use)
• E. Lifts (includes lifts used in domestic buildings)
• F. Fuel dispensing units (Both lighting and motors together)
• G. Swimming pools, spa, saunas
• H. Welding machines
• J. X ray equipment
• K. Other loads not covered above
Table A1.2 gives part of table C2 in regard to the diversity factors to be applied for the various load
groups given above.
A1.4 Additional guidelines
Table A1.4 provides maximum current values that are to be considered for a final subcircuit
feeding an individual appliance of specific power ratings. Same current values may be applied for
multiple appliances used in a system having combined total power equal to the values given in this
table e.g. appliance consisting of oven and hotplates.
Table A1.4
Current Values to be considered for appliance wattages
Appliance full-load rating
per phase
Upto 5000 W
Maximum demand to be
considered
16 A
Exceeding 5000 W but within 8000 W
20 A
Exceeding 8000 W but within 10000 W
25 A
Exceeding 10000 W but within 13000 W
32 A
Exceeding 13000 W
40 A
In the case of welding machines, total demand needs to be decided using a diversity factor that
varies with the number of machines connected in an installation. Following factors are
recommended for arriving at maximum currents/ demand when an installation is likely to be
operated with two or more arc welding machines:
• Upto two largest welding machines: 100% of each rated primary current plus
• Next largest welding machine: 85% of the rated primary current; plus
• Next largest welding machine: 70% of the rated primary current; plus
220 Practical Electrical Wiring Standards - AS 3000:2018
•
All other welding machines: 60% of the rated primary current
It means that an installation using six arc welding machines each rated for x amperes is expected to
have a demand contribution of (2x+0.85x+0.7x+1.2x) amperes from these six welding machines.
For resistance welding machines, the maximum current that may be considered is 70% of the rated
primary current shall be considered for seam and automatically fed machine and 50% of the rated
primary current for manually operated, non-automatic machine. The additional diversity factors
based on duty cycles may be applied over these currents as per applicable standard of the machine.
A1.5 Steps for demand calculations
The standard recommends adopting the following steps in assessing the maximum demand of an
installation:
• Decide whether the installation is domestic or non domestic or mixed to conclude the
table to be used for deciding the diversity factors applicable
• Group the loads as A, B, C, etc.
• In case of multiphase systems, divide the loads phase wise and decide the number of
lighting points, sockets, etc group wise in each phase
• List out the maximum rated amperes against each load group
• Multiply these group wise currents with the applicable diversity factor as per table
(depending on quantity of each group loads) to decide the maximum currents of each
group
• Sum up the maximum currents in amperes so arrived for each load group, which is
the maximum demand expected for the installation (phase wise, if applicable)
A1.6 Sample calculations
Reference may be made to appendix-C of AS/NZS 3000 that provides various worked out
examples considering different load group combinations for both domestic and non domestic
installations. Example calculations using the relevant tables are provided in the exercises forming
part of this manual.
Determination of Maximum Demand for Electrical Installations 221
Table A1.1
Current and diversity factors for multiple domestic installations (Source: AS/NZS 3000, Table C1)
222 Practical Electrical Wiring Standards - AS 3000:2018
Determination of Maximum Demand for Electrical Installations 223
Table A1.3
Current Ratings and diversity factors for non-domestic installations
224 Practical Electrical Wiring Standards - AS 3000:2018
Determination of Maximum Demand for Electrical Installations 225
Appendix 2
Wiring Systems Classification
A2.1 Basic criteria
The common causes of damages on the wiring systems are accidental fires and
mechanical impacts likely to occur during their service. A wiring system can be provided
with protection against these causes in the following ways to minimize the damages
• Unenclosed or enclosed wiring systems routed suitably by using fireresisting building structural elements with suitable supports and fixings to
get the desired protection and NOT using any fire rated elements
• By using fire-rated elements of building construction
The degree of protection normally varies for the different methods and hence it is
important to differentiate the kind of protections offered by each system. The end user
might be at a loss to understand the actual protection achievable by a particular system
unless some simple method is available to weigh the different systems on a common
ground. AS/NZS 3013 provides guidelines for classifying these systems for this purpose.
The wiring system classification helps the end users in identifying a wiring system’s
capability to maintain circuit integrity under fire conditions for a definite time period and
against mechanical damage of a specific severity.
This classification is analogous to the IP classification (defining the degree of protection
against entry of dust/tools and water inside enclosures) and is recognized by using letters
‘WS’ (to mean wiring system) followed by two characters using X, 1, 2, 3, 4 or 5 to
define the degree of protection offered against fire and mechanical forces. “X” normally
means that a required protection is inapplicable for a specific system. Table A2.1
indicates the explanations to the characters used for identifying a wiring system’s
capability. A third letter W after the two characters means that the system had been type
tested to prove its integrity for protection against fire for the specified duration and then
hosed with water.
226 Practical Electrical Wiring Standards - AS 3000:2018
Wiring system suppliers shall provide installers with complete details of fixing and
support methods to be followed for the wiring system to achieve the desired protections
against fire and mechanical forces including instructions related to the correct orientation
of the wiring system. Installers shall install wiring systems strictly in accordance with the
supplier’s instructions to ensure getting the desired protection.
Table A2.1
Classification adopted in wiring systems
First character: Protection
against Exposure to fire
Character
used in
first and
Minimum time for
second
maintaining circuit integrity
place
Second character: Protection
against mechanical damage
Type of mechanical force
capable to withstand
Not applicable
X
Not applicable
15 minutes
1
Light impact only
30 minutes
2
Moderate impact only
60 minutes
3
Heavy impact
90 minutes
4
Very heavy impact
120 minutes
5
Extremely heavy impact
A2.2 Specific wiring systems to be adopted
Table A2.2 shows types of recommended systems for selected applications along with the
minimum fire protection obtained from these systems.
Table A2.2
Specific recommendations on wiring systems for common applications
Equipment/ applications
Wiring systems recommended
Pumpset systems associated with the
operation of residential sprinkler pumps
WS2X. Minimum 30 minutes fire
exposure shall be possible
Pumpsets for other purposes
WS5X – Minimum 120 minutes fire
exposure shall be possible
Smoke-venting equipment and Electrical
operated Passenger and goods lifts
Not below WS52 for circuits
supplying these
Central emergency lighting circuits
WS4X for submains and certain final
subcircuits
Substation or main switchboard
WS53W in locations likely to get
damage by motor vehicles;
elsewhere WS52W or a minimum
fire-resistance of two hours
Wiring systems Classification 227
A2.3 Wiring systems with or without enclosures
Some wiring systems need not depend on fire-rated elements of building construction for
thermal protection. The required protection can be achieved either by having unenclosed
systems with some sort of integral protection as part of material characteristic during
manufacture or by having an enclosed system with a protective cover outside the wiring
to take care of the impact and cutting energies to meet the desired classification
requirement levels per AS/NZS 3013. The protections with enclosures may be metallic or
non metallic. Non-metallic types are normally expected to provide mechanical protection
between −15 to +60°C. In case of any extended ranges or reduced ranges these shall be
certified by manufacturer by means of external marking over the non metallic enclosures.
Table A2.3 provides some suggestions on the wiring systems to be typically considered
for common applications, but remember that some external influences might call for
higher categories, as well.
Table A2.3
Wiring system types for typical applications
System
type
General application areas
WSX1
In internal domestic or office situations where some damage is
common.
WSX2
In passageways in domestic office and commercial locations
where impact by hand trucks and barrows is most likely.
WSX3
In car parks and driveways where occasional impact of cars or
light vehicles is quite possible.
WSX4
In areas where impact by vehicles not exceeding two tonnes but
with solid frames is anticipated.
WSX5
In areas where impact by laden trucks exceeding two tonnes is
likely.
Where wiring systems are enclosed, the protection must be able to resist the impact loads.
This is usually achieved by having a complete enclosure, such as a conduit, pipe, trunking
or other housing; or by adopting a barrier that is interposed between the wiring system
and the possible source of impact. These enclosures shall preferably have undergone type
tests to confirm their compliance to the expected impact energy and ambient temperature
range of the installation.
228 Practical Electrical Wiring Standards - AS 3000:2018
Table A2.4 gives some common means that are adopted to get the expected protection
from a system.
Table A2.4
Wiring system types/ materials recommended for mechanical damage protections
System
type
Protection
expected
Recommended types of cables/ enclosures
to get the desired protection
WSX1
Light protection
against casual
damage by
pedestrians
- One sheathed cable just adjacent to a
projecting timber batten or similar corner to
ensure the projection is not less than twice the
dimension of the cable in that direction
covering the cable for casual damages.
- Cabling in light or medium-duty conduits
- Small sheet-metal ducts with clip-on covers.
- PVC duct with clip-on covers
WSX2
Against damage
from hand-powered
wheeled devices,
e.g. hand trucks
and wheelbarrows
and wheelbarrows
- Use of Heavy-duty conduits.
- Using Armored cables
- Steel duct with a minimum metal thickness of
1.6 mm, a screw fixed lid and unsupported
width not exceeding 100 mm.
- Fabricated steel cover with metal not less
than 1.6 mm thick, and unreinforced
unsupported width not exceeding 100 mm.
- Any WSX1 system with a 10 mm cover of
plaster or concrete.
- using MIMS cable
WSX3
Medium-duty
protection against
damage from
vehicles, such as
cars and light
commercial
vehicles
- Any wiring system with additional 2.0 mm
sheet steel coverage with a maximum
unsupported width of 100 mm.
- Any WSX2 systems with an additional 1.6 mm
sheet steel coverage and unsupported width
not exceeding 100 mm.
- Galvanized medium tube
- Very heavy-duty conduits.
Wiring systems Classification 229
Appendix 3
IEC Classification of Supply Systems
Based on Earthing Practices
A3.1 IEC definitions on supply systems
IEC 60364 discusses the types of earthing adopted in low voltage installations for protection
against direct and indirect contacts and provides a method of classifying the LV supply systems
based on the type of earthing adopted as well as the method used to extend the system earth to
consumer installations. We already reviewed the meanings of specific alphabets like T, N, etc used
to define these systems. IEC standard also discusses the comparative merits of the different types of
systems for specific applications. The salient features of these systems are briefed in this appendix.
A3.1.1 TN system
TN system refers to a system having one or more points of the source directly earthed with the
exposed metal parts using protective conductors. It is further subdivided into the following types
depending on the neutral-earth connection configuration.
TN-C system
This is a system in which the same conductor functions as the neutral and protective conductor
throughout the supply and consumer installation as per figure A3.1.
230 Practical Electrical Wiring Standards - AS 3000:2018
Figure A3.1
Schematic of a TN-C system
TN-S system
This system incorporates separate conductors for neutral and protective earth functions throughout
the system. In this type, the utility company provides a separate earth conductor back to the
substation as per figure A3.2.
Figure A3.2
Schematic of a TN-S system
The required connection back to the substation is most commonly adopted by means of an earth
clamping to the sheath of the incoming supply cable which facilitates connections to both supply
side earth conductor and the consumer installation earthing terminal.
TN-C-S system
In this system the neutral and protective functions are done by a single conductor for part of a
system. Usually the supply neutral and earth are combined at electricity distributor’s side with a
conductor named as PEN (protective Earth and neutral) conductor but they are separated within the
installation as shown in figure A3.3. This is also called as Protective Multiple Earthing (PME for
short). The Earthing terminal of the consumer installation is connected to the supplier's neutral.
IEC classification for Supply Systems based on Earthing Practices 231
Any breakage of the common neutral cum earth wire of the PEN conductor can result in the
enclosures of electrical equipment inside the premises assuming line voltage when there in
insulation failure. It is therefore essential to maintain the connection integrity of this common
neutral-cum-earth conductor.
Figure A3.3
Schematic of a TN-C-S system
A3.1.2
TT and IT systems
In a TT system, no earth is provided by the supplier and installation requires own earth rod (usually
common with overhead supply lines) and is typically adopted as per figure A3.4.
Figure A3.4
Schematic of a TT system
IT Supply is, for example, portable generator with no earth connection or with impedance earth and
installation using its independent earth rod as per figure A3.5.
232 Practical Electrical Wiring Standards - AS 3000:2018
Figure A3.5
Schematic of an IT system
An installation connected to a protective multiple earth supply is subject to special requirements
concerning the size of earthing and bonding leads, which are generally larger in cross-section than
those for installations fed by supplies with other types of earthing. Full discussions with the
Electricity Supply Company are necessary before commencing such an installation to ensure that
their needs will be satisfied. The cross-sectional area of the equipotential bonding conductor is
related to that of the neutral conductor as shown in table A3.1
Table A3.1
Minimum cross-sectional area of main equipotential bonding conductor for PME systems
Neutral conductor section
(mm²)
Main equipotential bonding
conductor section (mm²)
35 or less
10
Over 35 and up to 50
16
Over 50 and up to 95
25
A3.2 Installations where use of TN-C-S system is prohibited
Danger can arise when the non-current carrying metalwork of an installation is connected to the
neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the
neutral, and can thus share the normal neutral current under certain conditions. This current will not
only be that of the installation itself, but also be part of the neutral current of neighboring
installations. It therefore follows that the earthing wires of an installation may carry a significant
current even when the main supply to that installation is switched off. This could clearly cause a
hazard if in a potentially explosive part of an installation, such as a petrol storage tank, the earth
wire were to carry part of the neutral current of a number of installations. For this reason, the use of
PME supply system is prohibited in the case of petrol filling stations. Such installations must be fed
from TN-S supply systems.
IEC classification for Supply Systems based on Earthing Practices 233
The difficulty of ensuring bonding requirements on construction sites means that PME supplies
must not be used for temporary supplies. Electricity Supply Regulations also do not permit the use
of TN-C-S supplies to feed some sites described briefly below.
A3.2.1
Construction sites
The objective of electrical installation on a construction site is to provide lighting and power to
enable the work to proceed. By the very nature of the installation, it will be subjected to the kind of
rough treatment, unlike in most fixed installations. Those working on the site may sometimes be
ankle deep in mud and thus are particularly susceptible to an electric shock. They may be using
portable tools in situations where danger is more likely than in most factory situations. Hence
special regulations are applicable to construction power installations to minimize the danger to
working personnel. Apart from the use of TN-S system certain additional requirements are also
applicable, as given below:
• Distribution and supply switches and isolators must be protected to minimum IP44 to
protect from objects more than 1 mm thick and from splashing water. The main
isolator must be capable of being locked or otherwise secured in the 'off' position.
Emergency switches should disconnect all live conductors including the neutral.
• Sockets on a construction site must be separated extra-low voltage (SELV) or
protected by a 30mA RCD or must be electrically separate from the rest of the
supply, each socket being fed by its own individual transformer. Most sockets may
have to be fed at 110 V from center-tapped transformers to comply with this
requirement as 12V hand tools would draw heavy currents.
• Cables and their connections must not be subjected to strain, and cables must not be
run across roads or walkways without mechanical protection. Circuits supplying
equipment must be fed from a distribution assembly including over current
protection, a local RCD if necessary. Socket outlets must be enclosed in distribution
assemblies, fixed to the outside of the assembly enclosure, or fixed to a vertical wall.
Sockets must not be left unattached, as is often the case on construction sites.
• As the construction proceeds, the installations will be moved and frequently altered.
It is usual for such installations to be subjected to thorough inspection and testing at
intervals, which shall never exceed three months.
• The equipment used must be suitable for the particular supply to which it is
connected, and for the duty expected on site. Where more than one voltage is in use,
plugs and sockets must be non-interchangeable to prevent misconnection.
The requirements for construction sites also apply to sites where repairs, alterations or additions are
carried out, Demolition of buildings and Civil engineering operations, such as road building,
coastal protection, etc.
The special requirements for construction sites do not apply to temporary buildings erected for the
use of the construction workers, such as offices, toilets, cloakrooms, dormitories, canteens, meeting
rooms, etc. These areas/buildings are not subject to changes as construction work progresses, and
are thus exempt from these requirements.
A3.2.2
Marina supply systems
A marina is a location, often a harbor, for leisure craft to berth. Like residential caravans, such craft
require external power supplies and shall take care of some special conditions given below. The
marina often includes shore-based facilities such as offices, workshops, toilets, leisure
accommodation and so on which are exempt from such special requirements.
The electrical installation of a marina is subject to hazards not usually encountered elsewhere.
These include the continuous presence of water and salt, the movement of the craft, increased
corrosion due to the presence of salt water, galvanic action due to dissimilar metals and the
possibility of equipment being submerged due to unusual wave activity in bad weather.
234 Practical Electrical Wiring Standards - AS 3000:2018
Hence the neutral of a TN-C-S system must not be connected to the earthed system of a boat so that
the hazards, which follow the loss of continuity in the supply PEN conductor, are avoided. This
condition rules out the use of PME supplies for marinas. Where TN-C-S supply is provided, it must
be converted to a TT system at the main distribution board by provision of a separate earth
electrode system of driven rods or buried mats with no overlap of resistance area with any earth
associated with the TN-C-S supply. If the marina is large enough, it may be that the supply
company will provide a separate transformer and a TN-S system.
A common installation method is to provide a feed from the shore to a floating pontoon via a
bridge or ramp, and then to equip the pontoon with socket outlets to feed the craft moored to it.
Socket outlets may be single or three-phase types. Where multiple single-phase sockets are
installed on the same pontoon, they must all be connected to the same phase of the supply unless
fed through isolating transformers. Socket outlets should be positioned as close as possible to the
berth of the vessel they feed, with a minimum of one socket per berth, although up to six sockets
may be provided in a single enclosure. Each socket outlet must be provided with a means of
isolation which breaks all poles on TT systems, and must be protected by an over current device
such as a fuse or a circuit breaker. Groups of socket outlets must be RCD protected. Each socket or
group of sockets must be provided with a durable and legible notice giving instructions for the
electricity supply.
A3.2.3
Caravan power supply
A caravan is a leisure accommodation vehicle, which reaches its site by being towed using a
vehicle. A motor caravan is used for the same purpose, but has an engine, which allows it to be
driven; the accommodation module on a motor caravan may sometimes be removed from the
chassis. Caravans will often contain a bath or a shower, and in these cases the special requirements
for such installations will apply. Railway rolling stock is not included in the definition as a caravan.
All dangers associated with fixed electrical installations are also present in and around caravans.
Added to these are the problems of moving the caravan, including connection and disconnection to
and from the supply, often by unskilled people. Earthing is of prime importance because the
dangers of shock are greater. For example, the loss of the main protective conductor and a fault to
the metalwork in the caravan is likely to go unnoticed until someone makes contact with the
caravan whilst standing outside it. The requirements of the Electricity Supply Regulations do not
allow the supply neutral to be connected to any metalwork in a caravan, which means that PME
supplies must not be used to supply them.
The power supply to a caravan must be made using approved type of coupler at a height of not
more than 1.8 m from earth. The coupler socket (fixed to the caravan body) must have a springloaded lid, which will protect the socket when caravan is traveling, with a clear notice near the
socket indicating voltage, current and frequency of the supply required by the caravan. This socket
must be connected to a main isolator with 30mA RCD protection, which on operation will
disconnect all live conductors. All metal parts of the caravan, with the exception of metal sheets
forming part of the structure, must be bonded together and to a circuit protective conductor, which
must not be smaller than 4 sq. mm except where it forms part of a sheathed cable or is enclosed in
conduit.
A3.2.4
Other locations
These are locations such as swimming pools, saunas, and hospitals etc., where danger to human
beings is more probable with the supply. The reason for this is that in some of the locations, the
human beings are partly unclothed, without shoes and are in contact with water or their body is
wet. The effect of any contact with live electrical parts could therefore be particularly dangerous.
A hospital where open-heart surgery is performed poses even greater danger because of the effect
of stray electric currents finding their way to the human heart through medical appliances used in
the treatment. Also explosive hazards may exist in certain hospital locations handling anesthetic
gases. The use of electrical systems in these cases will depend on the zone of use. These zones are
IEC classification for Supply Systems based on Earthing Practices 235
defined in each case depending on the presence of vulnerable human body in the vicinity. Use of
lower voltage isolated systems, earth fault alarms and RCD are advised depending on the
application.
236 Practical Electrical Wiring Standards - AS 3000:2018
Appendix 4
Earthing Regulations and Practices
from Other National Codes
A4.1 USA- National Electrical Code (NEC) Regulations
The design, installation and maintenance of earthing connections in electrical systems have been
exhaustively studied and documented. Mandatory regulations have been established and are in use
by different countries (example: NEC/NFPA codes of USA. Article 250 of NEC code of USA is
dedicated to the subject of earthing). These regulations usually stipulate the minimum requirements
to ensure safety of equipment and personnel but system designers are free to improve on them on
the conservative side.
NEC defines service equipment as switching and protective equipment installed at the point of
entry of power from the electric power utility to the consumer premises. The provisions of NEC are
meant to ensure that the electrical installation on the downstream side of the service equipment is
free from defects that can cause fire, explosion or electric shock hazards under normal or fault
conditions of the system. NEC recommends that all earthing electrodes including cold water
piping, metallic building frames etc. used as earthing electrodes are bonded to the equipment
earthing system at the service equipment earth.
As discussed earlier, bonding of all building earthing in this manner ensures that no dangerous
potential develops between the equipment earthing conductors, metal raceways, building structures,
cold water mains etc. even though their potential with reference to the mean earth potential may be
substantially higher. Any voltage difference between these points will be due to abnormal
conditions such as an earth fault. But even so, the potential difference will not reach the Touch
Potential limits thus ensuring that the system is free of shock hazards.
Section 250-118 of NEC permits the use of various earthing conductors including tubular conduits
used as cable raceway, cable armor and cable trays. Where earthing conductors are employed, it
must be ensured that they are sized to withstand earth fault currents of value and duration
appropriate to the circuit under consideration.
If the conductor sizing is inadequate, the following may result:
• Damage to the insulation of the earth conductor. In case the conductor is bare,
insulation damage may occur in the phase conductor with which it is in contact.
• Fusing of the earth conductor in extreme cases. Though this may result in the
interruption of the fault current, it will cause the potential of the enclosure of the
faulted equipment to rise beyond acceptable limits.
238 Practical Electrical Wiring Standards - AS 3000:2018
A4.2 Earthing practices as per South Africa Bureau of standards
(SABS)
A4.2.1 Industrial power transformer earthing
Industrial power systems in South Africa typically employ the scheme shown in Fig A4.1 for
earthing the MV/LV transformers.
Figure A4.1
Typical industrial transformer earthing practice in South Africa
It may be noted that the transformers are cable fed and they do not incorporate surge arrestors. The
transformer LV neutral is earthed directly to an earth electrode and at the same time connected to
the plant earthing network. The tank is connected to the earth network using at least two earth
leads. The LV system neutral and the earth network of the plant to which the safety earth of all
equipment is connected are thus bonded all through with metallic connections and earth fault
currents return to the neutral via the metallic path.
A4.2.2 Remote installations fed from pole mounted transformers
The earthing requirements associated with remote MV/LV transformer fed installations are
somewhat different from the approach adopted in industrial installations. These are installations
with pole mounted transformer remotely installed and fed by an MV overhead line which in turn
feeds isolated LV consumers through overhead lines. Moreover, the earth electrode resistance
cannot be expected to be too low either.
The arrangement suggested in SABS standards for this type of distribution is shown in Figure
A4.2.
Earthing Regulations and Practices 239
Figure A4.2
Earthing of remote distribution transformers/LV system
It is noteworthy that the installation recommends the LV neutral to be earthed at a point well away
from the transformer (usually at the first LV pole). The tank earthing is mainly to take care of faults
between the MV system and the tank. The MV windings are protected by surge arrestors mounted
on the body of the transformer and the LV neutral is connected to the transformer tank through a
neutral surge arrestor. Though the MV surges will be transmitted into the LV system by the
coupling effect, the high surge impedance of the LV lines will prevent the surges being propagated
into the LV system.
Also the surges being of very short duration will not pose a safety hazard. This design takes into
account the following factors:
• Need to prevent the LV neutral from assuming dangerous touch potential when an
MV to tank fault occurs.
• Need for limiting the voltage across the neutral arrestor.
• Need for detecting MV to tank faults by MV earth fault protection
A4.3 Earth Electrodes
LV electrode resistance should be normally expected to permit sufficient fault currents for
detection. With the LV line to neutral voltage of 240 V the resistance limit works out to 2.4 ohms,
if a earth fault current of 100 amps is to be obtained. However, with the TN-C-S type of system, all
equipment enclosures are directly connected to the neutral at the service inlet itself and thus the
current flow does not involve the earth path at all.
240 Practical Electrical Wiring Standards - AS 3000:2018
So, the limit of LV earthing resistance is decided by the criteria of obtaining sufficient fault current
when there is a MV to LV fault without involving the tank or core (refer to Figure A.4.3)
Figure A4.3
Equivalent Circuit for Combined MV/LV earthing
Assuming an MV Earth fault protection setting of 40 A the earth loop resistance can be arrived as
318 ohms (12700/40) for 22 kV system. The permissible earth electrode resistance works out to
273 ohms (after taking off the values of NGR and substation earth resistance). If we consider a
safety factor of 400%, the maximum value of LV earth resistance can be taken as 68 ohms. The
safety factor will ensure that the seasonal changes of soil resistivity will have no adverse effect on
protection operation. Standard configurations are available in the code for 70 ohm electrodes and
can be used in the design.
A4.4 Earth electrode design factors
The construction of earthing electrodes depends on the local codes applicable. The purpose
however is common. It is to establish a low resistance (and preferably low impedance) path to the
soil mass. It can be done using conductors that are exclusively meant for this function or by
structures/ conductors used for other functions but which are essentially in contact with soil.
However, while using the latter category, it must be ensured that the earth connection is not
inadvertently lost during repair works or for any other reason.
Factors contributing to earth electrode resistance are:
• The resistance of electrode material
• Contact resistance of the electrode with soil
• Resistance of the soil itself
The values of the first two are quite low compared to the last and can be neglected. Soil resistivity
for a given type of soil may vary widely depending on:
• The presence of conducting salts
• Moisture content
• Temperature
• Level of compaction
Earthing Regulations and Practices 241
IEEE 142 gives several useful tables which enable us to determine the soil resistivity for commonly
encountered soils under various conditions which can serve as a guideline for designers of earthing
systems. These are reproduced in tables A4.1 and A4.2.
Table A4.1
Effect of temperature on soil resistivity
Temperature 0C
Resistivity Ohm-m
-5
700
0
300
10
80
20
70
30
60
40
50
50
40
Table A4.2
Effect of moisture content on soil resistivity
Moisture
content %
Resistivity in Ohm-m
Top soil
Sandy loam
Red Clay
2
****
1850
****
4
****
600
****
6
1350
380
****
8
900
280
****
10
600
220
****
12
360
170
1800
14
250
140
550
16
200
120
200
18
150
100
140
20
120
90
100
22
100
80
90
24
100
70
80
242 Practical Electrical Wiring Standards - AS 3000:2018
When it is not possible to obtain the minimum resistance stipulations or if the earth fault current
cannot be dissipated to the soil with a single electrode, use of multiple earth rods in parallel
configuration can be resorted to. The soil layers immediately surrounding the electrode contribute
substantially to the electrode resistance. More than 98% of the resistance is due to a soil cylinderhemisphere of 1.1 times the electrode length. This is called the 'critical cylinder'. Placing electrodes
close to each other thus interferes with the conduction of current from each electrode and lowers
the effectiveness. Hence earth rods are generally arranged in a straight line or in the form of a
hollow rectangle or circle with the separation between the rods not lower than the length of one
rod.
A4.5 Earth electrode types from some national codes
Examples are given from the National Electric Code (USA) and the South African Standards below
to illustrate different approaches taken for the design of earth electrodes
A4.5.1 Recommendations of NEC
NEC Section 250 covers the different types of electrodes. Those in the first type are metal
underground water pipes, metal frame of building etc. that are not specifically installed for the
purpose of being used as earth electrodes. The other type is known as ‘made electrodes’ whose
specific function is for use as earth electrodes. Examples of these are metal rods/pipes driven into
earth, concrete encased electrodes, buried earth ring etc. It is to be noted that NEC does not require
any special earth electrodes in an installation where some form of buried underground metalwork is
already present and can be used for this purpose (this aspect of NEC is totally at variance with the
prescription of the Australian Standard)
Metal underground water mains used as earth electrodes should be in direct contact with earth for a
minimum distance of 10 ft. and made continuous by bonding around insulating joints. Also,
continuity should not depend on water meters, filters or any other removable devices. The earthing
rods can also be one of the following types:
• An effectively earthed metal frame or structure of a building
• An electrode encased in two inches of concrete located within a concrete foundation
footing at the bottom part, in direct contact with the earth
• A buried earth ring around a building made of at least 20 ft. of copper conductor at a
depth of over 2.5 ft
Underground metal gas piping system shall not be used as an electrode. Also NEC code does not
permit the use of Aluminum as suitable material for earth electrodes.
Where electrodes of above types are not readily available, electrodes specifically made for use as
earth electrodes will have to be deployed. These electrodes shall be driven into the soil in such a
way that a substantial part of their length is below the permanent moisture level of the earth in the
area under consideration. When more than one electrode is used they shall be separated by more
than 6 ft. and preferably by one-rod length and shall be effectively bonded so as to result in a single
composite electrode system. The length of electrodes shall not be lower than 8 ft. and in the case of
pipes/conduits the diameter shall not be less than ¼” trade size. Rods shall not be lower than 5/8”
diameter for use as made-electrodes. The electrodes shall be galvanized or otherwise metal-coated
to minimize corrosive effects of being buried in the earth. Plate electrodes of 2 sq. ft. surface area
and ¼” thick made of iron/steel plates and buried in soil at a depth of 2.5 ft. can also be used as
electrodes.
All non-conductive coatings such as paint/enamel and rust shall be scraped of the surface of the
electrodes to ensure proper contact with soil.
Resistance of made-electrodes shall not be lower than 25 ohms. In case this is not so, multiple
electrodes bonded together shall be used to bring down the resistance value.
Earthing Regulations and Practices 243
244 Practical Electrical Wiring Standards - AS 3000:2018
A4.5.2
Recommendation of South African Standard SCSASAAL9
This standard recommends earth electrodes using buried horizontal conductors, vertically driven
electrodes or a combination of both. Buried horizontal conductors are easier to install and result in
less steeper potential gradients during an earth discharge. This type of electrode (called as trench
electrode) must be installed as deep as practicable but not less than 500 mm below ground level.
The reasons for this are:
• Less prone to mechanical damage
• Better current dissipation using the layer of soil above the conductor
• Lower voltage gradient during discharge
Vertically installed electrodes may be extended to substantial lengths (15 to 90 m depth) and are
able to make contact with low resistivity soils. At these depths the soil resistivity is same
throughout the year and less prone to seasonal variations that are found on the soil layers near the
surface. Vertical rods are also found to have superior surge performance and are useful for
electrodes meant for lightning protection of structures.
Electrodes constructed from a combination of driven rods and radial array of buried conductors
ensure a stable resistance value and good performance for power frequency and high frequency
earth discharges. They minimize the chance of high soil potential gradients and avoid failure of
earth connection due to a single electrode conductor breakage.
This standard specifies the following type of electrodes:
• Non extendable earth rods in a multi electrode (preferably 3-point star) configuration
• Linear trench electrode
• Extendable earth rods
• Vertically Installed conductor
Linear trench electrodes are recommended where vertical rods are not convenient to use.
This standard defines the minimum resistance of the earth electrode in LV installations neutral as
68 Ohms and that of the transformer tank earth electrode (in MV installations of 11 kV or 22 kV)
as 30 Ohms. The reasoning behind this stipulation was explained under section A4.3 of this
appendix.
Standard SCSASAAL9 defines various electrode configurations having resistance of 30, 70 and
150 Ohm in different soils in the form of tables for easy selection. Figure A4.4 identifies some of
these recommended arrangements.
Appendix - 5
Practical Exercises
Introductory chapters to AS/NZS 3000:2007 (Chapters -1, 2, 3 and 4)
S. No
Question
1.1
Indicate the voltage ranges of a.c. and
d.c. installations for which AS/NZS
3000 is applied
1.2
Indicate two main advantages of
regulations for electrical systems
2.1
Name two reasons on which a.c.
systems over score d.c. systems for
power generation, transmission and
distribution.
2.2
Vector 1 lags Vector 2 by 60degrees
and leads vector 3 by 90 degrees.
Indicate these by a vector diagram
2.3
Earth fault can not be detected in
unearthed system. Is it true or false?
2.4
A delta connection in a three phase
system does not have _________
2.5
What is the main feature of functional
earthing?
3.1
Indicate three likely major hazards
due to shock currents
3.2
Indicate two ways of avoiding shocks
or its hazards?
3.3
Name the common insulation and the
temperature rise limit adopted in
electrical machinery
Answer
248 Practical Electrical Wiring Standards - AS 3000:2018
S. No
Question
3.4
Maximum numeral used for IP
classification against water entry and
its meaning are
3.5
An arm’s reach distance can be
reduced by use of ___________
4.1
Main disadvantage of solid earthing is
__________________
4.2
High resistance earthing is generally
limited to _______________
applications
4.3
A non-let go threshold due to shock
currents is possible when the shock
current value is around _________
4.4
Most common reasons for shock
currents are _________________
with live parts
4.5
Protective earthing mainly limits the
________________ of the electrical
equipment enclosures.
4.6
The common LV consumer supply
connection as per IEC grouping based
on earthing is ________ type.
4.7
Protective earthing for non electrical
metallic parts are usually done by
____________
4.8
To sense the fault currents in a
system correctly, it is necessary to
limit the value of ______________
4.9
Indicate two factors that decide the
choice of earth electrode quantity and
its material in a location.
4.10
Indicate the main difference between
core balance CT and other CT’s used
for earth fault detection.
Answer
Practical Exercises 249
Where direct answers cannot be given, it is advised to refer AS/NZS 3000 and indicate the
applicable clause.
Chapter 5 - Section-1 General Principles
S. No.
5.1
Question
Mark the zones of arm’s reach for
a person standing on a surface
shown in plan view below.
Surface “S”
5.2
Indicate, as piece of electrical
equipment, what is the class of the
cellular phone.
5.3
Indicate the limit of the number of
cores allowed in flexible cord.
5.4
A sub DB is provided with
phase/neutral conductors and the
system is MEN. (Neutral earthed
at mains). Indicate whether neutral
conductor in the DB is classified
as live part or earthed part.
5.5
What are the types of ELV
systems? How they differ from
each?
5.6
Indicate the main reason that can
affect the touch voltage of different
people at an enclosure with faulty
circuit.
5.7
What are the limiting values for
ELV?
5.8
TRUE/FALSE – A protection
offered by having removable
barriers shall be provided with
protective interlocks also.
5.9
What is the maximum time limit for
disconnecting the mains in case of
indirect contact while using hand
held class I equipment?
AS/NZS
3000
clause
Answer
250 Practical Electrical Wiring Standards - AS 3000:2018
S. No.
Question
5.10
TRUE/FALSE – RCDs can
prevent shocks due to indirect
contacts.
5.11
What is the maximum permissible
voltage allowed without barriers in
dry areas? Are there any specific
conditions for this?
5.12
Indicate the types of protection
recommended for live parts
supplied at different voltages
within a system.
AS/NZS
3000
clause
Answer
Practical Exercises 251
Chapter 6 - Section 2 Selection of Equipment
S. No.
Question
6.1
A common neutral conductor may
be used for two or more circuits
subject to satisfying certain
conditions. Indicate one such
condition.
6.2
Name common methods adopted
to decide the maximum demand
of an installation?
6.3
List the main locations where
isolation of neutral in an a.c.
system should be avoided.
6.4
What are the conditions where
isolation of single pole is
acceptable in a d.c. circuit?
6.5
When is the electrical installation
in an outbuilding is to be
considered as a separate
electrical installation?
6.6
Which is the odd one out?
a) Fuse b) MCCB c) ACB d) RCD
6.7
TRUE/FALSE - The breaking
capacity of a device protecting a
feeder can be lower than the
breaking capacity of the upstream
protective device.
6.8
What is the accepted sensitivity of
RCDs for human protection
6.9
To avoid unwanted tripping while
using RCDs, ensure that the sum
of the normal leakage currents of
electrical equipment on the load
side of an RCD is less than
______ of its rated residual
current.
6.10
What is the main characteristic to
be taken care while using under
voltage devices in motor circuits?
6.11
_______ space around a
switchboard is considered
essential to facilitate emergency
exit. How is this space measured?
AS/NZS
3000
clause
Answer
252 Practical Electrical Wiring Standards - AS 3000:2018
S. No.
Question
6.12
In general, a switchboard shall not
be located within ____ of the
ground, floor or platform.
6.13
Name a couple of places where
locating a switchboard is
prohibited.
AS/NZS
3000
clause
Answer
6.14. The maximum permitted current capacity of a conductor in an installation is 34 amperes.
Calculate the operating current for the over current protective device in case of a) MCB b) fuse.
(Refer clause 2.5.3 of AS/NZS 3000) with nominal ratings close to the maximum permitted
current,
6.15. Calculate the minimum cross section of PVC insulated copper conductor feeding a circuit that
can develop a fault current of 10kA. Also calculate the recommended size if the protective device
can isolate this circuit in 0.5 seconds in case of short circuits.
Practical Exercises 253
Chapter 7 - Section 3 Wiring Systems
S. No.
Question
7.1
What are the ambient
temperatures applicable for cables
in Australia and New Zealand a) in
air and b) buried below ground.
7.2
For cables exposed to direct
sunlight, ambient temperature
correction factor shall be
considered for a temperature
equal to ________
7.3
The minimum cross section area
of parallel multiple conductors
used in an installation shall be
_______
7.4
The current rating of HRC fuses
should not be greater than
_____% of cable current-carrying
capacity of circuit protected by it.
7.5
What are the minimum copper and
aluminium conductor sizes for
aerial applications?
7.6
Joints in flexible cords may be
acceptable if done with ______
7.7
Indicate the preferred and
alternative colours of cores for the
following uses:
Neutral
Active line
7.8
What are the recommended
bending radii for unarmoured and
armoured cables in the absence of
manufacturer recommendations?
7.9
The support interval for a LV
suspended track system shall be
not more than __________
7.10
What are the conditions under
which use of different voltage level
conductors are permitted without
segregation in LV systems?
AS/NZS
3000
clause
Answer
254 Practical Electrical Wiring Standards - AS 3000:2018
S. No.
Question
7.11
What precaution is needed for
single core cables used in
Alternating Current systems?
What are the reasons for this?
7.12
Indicate a couple of features
recommended for the cable
trunking applications.
7.13
The spacing between aerial
conductors shall be increased by
a suitable margin to take care of
_________________
7.14
In dicate the factors affecting the
clearances to be maintained
between aerial wiring systems at
the supports operating at same
voltages.
7.15
In indoor applications, cables
supported by a catenary wire shall
be maintained not less than
_______ from any moving parts of
equipment operating at an
elevated temperature.
AS/NZS
3000
clause
Answer
Practical Exercises 255
Chapter 8 - Section 4 Appliances and accessories
S. No.
Question
8.1
What are the permitted surface
temperatures for hand held
electrical equipment?
8.2
A maximum flexible cord or cable
length of ______ is recommended
for equipment wiring.
8.3
Three-pin/flat-pin plugs shall be
connected so that, when viewed
from the front of the socket-outlet,
the order of connection of the
three points (Active, neutral and
earth) shall be in the order of
___________ in a clockwise
direction.
8.4
Lampholders without suitable
guards shall be mounted at a
minimum height of _____ above
the floor.
8.5
100W lamp shall be located away
at a mnimum distance of _____
from flammable materials.
8.6
Minimum clearances for recessed
Halogen Luminaire lighting from
the roof structure shall be
_____________.
8.7
All motors rated ______kW and
above shall be protected by
overload protective devices.
8.8
Current rating of conductors used
for capacitors connected
permanently in motor circuits shall
exceed ____________ of the
capacitor current or ________ of
the motor conductor.
8.9
400V Capacitors shall be
discharged to a minimum voltage
of _____ within ____ minutes of
disconnection.
AS/NZS
3000
clause
Answer
256 Practical Electrical Wiring Standards - AS 3000:2018
S. No.
8.10
Question
A drainage pit/ chamber is needed
while having equipments with
dielectric liquid of capacity above
___________.
AS/NZS
3000
clause
Answer
Practical Exercises 257
Chapter 9 - Section 5 Earthing Systems
S. No.
Question
9.1
The MEN system is equivalent to
__________ system as per IEC
regulations.
9.2
The minimum cross section for
solid copper earth conductor shall
be ___________
9.3
Solid earthing conductors of
aluminium shall adopt a minimum
size of __________.
9.4
The size of a bare earthing
conductor in the same system
may be ________ (smaller or
larger) than an insulated earthing
conductor.
9.5
The maximum size of copper
conductors used for earthing
connection to an electrode is
usually ____________sqmm
9.6
The minimum diameter of a
copper clad steel circular earth rod
shall be ____mm
9.7
Name a couple of applications,
where earthing of exposed parts
of enclosures is avoided in an
installation.
9.8
What are the minimum depths to
which vertical-type earth
electrodes shall be
driven________________
9.9
Indicate two conditions, where
earthing of lighting points can be
avoided.
9.10
The resistance between the earth
bar and any structural part of an
installation required to be earthed
shall not exceed __________
9.11
What is the minimum size of bare
copper and aluminium earthing
conductors directly buried below
ground?
AS/NZS
3000
clause
Answer
258 Practical Electrical Wiring Standards - AS 3000:2018
9.12. Calculate the minimum cross section of bare copper earthing conductor for a system that can
develop a fault current of 25 kA and incorporated with a protective device that can isolate the
faulty circuit in 0.4 seconds
9.13 A 240V circuit incorporates active conductor of 35sqmm and earthing conductor of 16sqmm
both copper. Calculate the maximum circuit lengths permitted while using type C breaker rated 100
amperes. Compare it with the values as per table B1 in the standard
Practical Exercises 259
Chapter 10 – Section 6 Damp situations
S. No.
Question
10.1
Indicate two main conecrns
applicable in damp situation in
regard to human safety.
10.2
A barrier upto a height of ______
or upto _______ can reduce the
limits for classified zones in bath
area
10.3
_______ and _______ can also
limit the zone distances similar to
barriers
10.4
Indicate the common shock
protection methods in general
areas that are not accepted in
damp areas
10.5
A zone 2 cerified item can be used
in _______ but not in ________
10.6
________ are generally allowed in
zone 0 areas subject to
considering suitable certification.
10.7
What are the important conditions
for the source of the above items
permitted in zone 0?
10.8
___________ are not permitted
genarally in all damp areas except
for _____________areas
10.9
Generally IP rating is not critical in
Zone ____ of residential bath
areas
10.10
Unlike most of the other areas,
refrigeration areas differ in terms
of zone classification by having
____________
AS/NZS
3000
clause
Answer
260 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 11 – Section 7 Specific installations
S. No.
Question
11.1
Indicate specific considrations for
fire pump controller control supply
arrangement.
11.2
__________ is one of the main
design consderations to be
ensured for alternative power
supply when used with grid supply
.
11.3
Multiple generators operating in
parallel shall be provided with
__________ in the neutral circuits.
11.4
Electrical separation of a.c.
supplies are achieved by use of
____________
11.5
Recommended insulation
resistance for separate supply
circuits shall be minimum _______
with 500V DC
11.6
Voltage drop in an extra low
voltage circuit is allowed upto
________ compared to ________
in normal voltage circuit as % of
nominal operating voltage.
11.7
Aerial conductors of bare and
insulated conductors can be
maintained with same spacing for
________ systems
AS/NZS
3000
clause
Answer
Practical Exercises 261
Chapter 12 – Section 8 Verification
S. No.
Question
12.1
_________ test shall be
conducted as the first test in an
installation as part of verification.
12.2
Resistance of main earthing
consudtor shall be limited to a
valie of ___________
12.3
Earth fault loop impedance
measurement shall be done after
energisation to ensure
___________________
AS/NZS
3000
clause
12.4 Cross check the fault loop impedance values tabulated in Table 8.1 for 32 amps breaker with a
disconnection time of 0.4 sec for Type B and Type C breakers, taking into account the AS/NZS
mean tripping currents of 4 times and 7.5 times the nominal rating for these breaker types
respectively.
262 Practical Electrical Wiring Standards - AS 3000:2018
Appendix 1 – Load demand
A1.1. Determine the maximum demand of a single domestic electrical installation supplied at 240V
single-phase with the following loads:
• 36 lighting points
• 20 m of lighting track
• 12nos 10 A single socket-outlets
• 6nos 10 A double socket-outlets
• 1no 7.5kW range
• 1no 3.6kW water heater
A1.2. Determine the maximum demand of the heaviest loaded phase of a motel complex supplied
by three-phase 415V with the following loads:
• 300nos 60 W lighting points
• 60nos 100 W 1-phase exhaust fans (permanently connected)
• 70nos 10 A 1-phase single socket-outlets (permanently heated or cooled area)
• 6nos 15 A 1-phase socket-outlets
• 1no 12 kW 3-phase electric range
• 2nos 1.5kW 3-phase borewell pump motor
The loads are distributed over the three phases as below
Red
110 lights
18 exhaust fans
20x10A socket outlets
2 x 15 A socket-outlet
3.2 kW oven
0.75 kW pump
White
90 lights
20 exhaust fans
25x10A
2 x 15 A socket-outlets
4.4 kW hotplates
Blue
100 lights
22 exhaust fans
25x10A
2 x 15 A socket-outlet
4.4 kW hotplates
0.75 kW pump
Practical Exercises 263
Appendix 6
S. No
Answer
1.1
1.2
1000V a.c. and 1500V d.c.
• Safety
• Uniformity
• Easy selection of equipment
• Good Workmanship
2.1
•
•
•
2.2
voltage transformation
Three phase and single phase options
Avoidance of commutators in generators
2
1
3
2.3
False. Second earth fault can be detected. It is true for first fault only.
2.4
Neutral
2.5
It is not for safety purposes.
3.1
•
•
•
•
fatality
Falls and injury
Burns
Organ damages
3.2
•
•
•
Limiting touch voltage
Disconnection within shortest time
Avoid probability of contact
3.3
Class F with class B rise
3.4
8 – Continuous immersion (with water pressure)
3.5
Hand tools or ladders
4.1
Higher earth fault currents.
4.2
Large power generator
4.3
10-25 milliamperes.
4.4
Direct and indirect contacts
4.5
Touch voltages
4.6
TN-C-S
4.7
Equipotential bonding
266 Practical Electrical Wiring Standards - AS 3000:2018
S. No
Answer
4.8
Earth fault loop impedance
4.9
•
•
•
Overall earth resistance needed
Soil resistivity
Corrosive nature of soil
4.10
•
•
Not connected in series with main power circuit
Primary current rating
Answers for Practical Exercises 267
Chapter 5 - Section-1 General Principles
S. No.
AS/NZS
3000
clause
5.1
1.4.12
Answer
1.25 m
5.2
1.4.29
Class III (only SELV is used).
5.3
1.4.36
Five
5.4
1.4.63
Live part
5.5
1.4.83 and
1.4.76
5.6
1.4.95
Body resistance.
5.7
1.4.98
50V a.c. or 120V ripple free d.c.
5.8
1.5.4.4
True
5.9
1.5.5.3
0.4 second
5.10
1.5.5.2
False. They only protect from effects of shock.
5.11
1.5.7
5.12
1.5.11.2
PELV (protected ELV) and SELV (separated ELV) are
the types based on whether it is electrically separated
from earth or not.
25V a.c. or 60V ripple-free d.c. where a large contact
area with the human body is not expected and
otherwise the limits are 6V and 15V respectively.
Segregation or over voltage protection
268 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 6 - Section 2 Selection of Equipment
Answer
S. No.
AS/NZS
3000
clause
6.1
2.2.1.2
•
•
By ensuring continuity
By using links at final subcircuits
6.2
2.2.2
•
•
•
Calculation
Measurement with meter
By protective device rating
6.3
2.3.2.1.1
•
•
consumers mains or
where neutral conductor is used as a PEN
conductor
6.4
2.3.2.1.2
•
•
One pole earthed or connected to PE conductor
ELV d.c. circuit
6.5
2.3.4.1
6.6
When its demand equals or exceeds 100 amps per
phase and is provided with a switchboard.
RCD – Doesnot protect against a short circuit
True. But subject to limiting I2 t of conductor up to the
feeder within the expected capacity.
6.7
2.5.4.5
6.8
2.6.1
6.9
2.6.2.1
6.10
2.8.1
6.11
2.9.2.2
•
•
6.12
2.9.2.5
1.2 metres
6.13
2.9.2.5
•
•
•
30mA maximum
One-third
Starting current and its duration.
600 mm
with doors open and breakers racked out.
Near baths/ showers
Near egress paths
Near fire exits
6.14. The maximum permitted current capacity of a conductor in an installation is 34 amperes.
Calculate the operating current for the over current protective device in case of a) MCB b) fuse.
(Refer clause 2.5.3 of AS/NZS 3000) with nominal ratings close to the maximum permitted
current,
Solution:
The nominal current I N shall be less than the allowable current I Z (which is 34 amps)
Nearest MCB nominal current is 32 amps.
The operating current for 32A MCB is 1.45 × 32 = 46.4 amps
Answers for Practical Exercises 269
For fuse, I N shall be less than 90% of 34 amps (30.6 maps). Hence fuse I N rating (closest) may be
30 amperes.
The operating current will be 30 × 1.6 = 48 amperes for the fuse.
NOTE: As indicated in the clause, the above are based on the assumption that the conductor is not
continuously loaded to the full load current.
6.15. Calculate the minimum cross section of PVC insulated copper conductor feeding a circuit that
can develop a fault current of 10kA. Also calculate the recommended size if the protective device
can isolate this circuit in 0.5 seconds in case of short circuits.
Solution:
This is based on equation 2.4 of AS/NZS 3000.
K2 S2
2
t= I
K=111 for PVC insulated copper conductor.
Same clause approves that the conductor can safely withstand the temperature rise caused by short
circuit up to a maximum of 5 seconds.
Substituting the values
10000× 5
111
S=
= 201 sqmm
Nearest conductor size can be 240 sqmm if there is no proper protective device.
With a SC protective device operating in 0.5 seconds, it is possible to use a lower size conductor.
Substitute t with 0.5 in the above equation,
10000× 0.5
111
S=
= 63 sqmm.
Nearest conductor standard size of 70 sqmm may be adopted with SC Protection.
NOTE: K = 111 for 40°C ambient (For Australian ambient conditions).
270 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 7 - Section 3 Wiring Systems
S. No.
AS/NZS 3000
clause
Answer
7.1
3.3.2.1
•
•
7.2
3.3.2.11
20°C + ambient air temperature
7.3
3.4.3
4 sqmm
7.4
3.4.4
90%
7.5
Table 3.3
7.6
3.7.2.8
7.7
Table 3.4
7.8
3.9.6
7.9
3.9.7.5
1.5 metre
7.10
3.9.8.3
•
•
•
•
40 and 30 deg C
25 and 15 deg C
Copper: 6 sqmm
Aluminium: 16 sqmm
Suitable cable couplers
•
•
Black, alternatively light blue.
Red , alternatively colours not associated with
earth and neutral cores
6 times and 12 times of the conductor radius
respectively.
•
cables with double insulation or
cables insulated for the highest voltage they
operate or
cables separated by continuous barriers.
7.11
3.9.10.2
No steel wire or steel tape armour to avoid eddy
current flow and consequent temperature rise in
the cores.
7.12
3.10.3.9
•
•
•
7.13
3.12.3.1
Sags resulting while operating at higher
temperatures (upto 1150 C)
7.14
Table 3.10
7.15
3.13.3
•
•
Opening provisions for covers
Continuous through walls, floors, etc
Accessibility
Span
whether conductors are bare or inulated.
100mm
Answers for Practical Exercises 271
Chapter 8 - Section 4 Appliances and accessories
S. No.
AS/NZS 3000
clause
Answer
8.1
Table 4.1
8.2
4.3.5
2.5m
8.3
4.4.5
Earth, active, neutral
8.4
4.5.1.1
8.5
Table 4.2
600mm
8.6
Figure 4.7
200mm
8.7
4.13.2
0.37kW
8.8
4.15.2.3
135% of capacitor or one-third of motor (whichever
is greater)
8.9
4.15.3.1
50V, 1 minute
8.10
4.16
Metallic: 55 deg C
Non metallic: 65 deg C
1.8m
50 litres
272 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 9 - Section 5 Earthing Systems
S. No.
AS/NZS 3000
clause
Answer
9.1
5.1.3
9.2
5.3.2.1.1
10 sqmm
9.3
5.3.2.1.2
16 sqmm
9.4
5.3.3.1.3
Smaller.
Refer K factors as per standard.
9.5
5.3.3.2
9.6
Table 5.2
9.7
5.4.1.1
•
•
•
9.8
5.3.6.3
In Australia, 1.2 m and
in New Zealand, 1.8 m
9.9
5.4.3
9.10
5.4.6.3
0.5 ohm
9.11
5.5.5.5
Copper – 35 sqmm
Aluminium – Not permitted
TN-C-S
120 sqmm
12mm
•
•
•
Double insulation items
SELV cicruits
PELV circuits
Lamps in all insulated lamp holders
Timber pole mounted
ELV lamps
9.12. Calculate the minimum cross section of bare copper earthing conductor for a system that can
develop a fault current of 25 kA and incorporated with a protective device that can isolate the
faulty circuit in 0.4 seconds
Solution:
This is same as the earlier problem under (Chapter 6) section 2 based on equation 2.4 of the
standard which appears as equation 5.1 in section 5 (5.3.3.1.3) of the standard except for the
insulation
t=
K2 S2
I2
K=170 for bare copper conductor.
Substituting the values
S=
25000× 0.4
= 93sqmm
170
Answers for Practical Exercises 273
Nearest conductor size that can be used is 95sqmm.
9.13 A 240V circuit incorporates active conductor of 35sqmm and earthing conductor of 16sqmm
both copper. Calculate the maximum circuit lengths permitted while using type C breaker rated 100
amperes. Compare it with the values as per table B1 in the standard
Solution:
This is given by equation B7 reproduced below:
L MAX =
0.8U 0 S ph S pe
I a ρ ( S ph + S pe )
For type C breaker, tripping current is 7.5 times 100, i.e. 750 amps
Substituting the values as outline in clause B5.2.2 of the standard
L MAX =
0.8 × 240 × 35 × 16
= 125 m
750 × 0.0225 × (35 + 16)
Table B1 indicates permitted circuit lengths with 35 sqmm active and 10 sqmm earth conductors
with 100 amps breaker, as 85m for type C at 230V.
Corresponding L MAX at 240V is 1.04 × 85 = 88.4 m (Correction factor per note 4), which means
another 35mtrs may be permitted by having increased earth conductor size..
(NOTE: The maximum length obtained only satisfies the fault protection requirements of Clause
1.5.5.3 i.e. automatic disconnection. The tripping time, overload, short-circuit and voltage drop
requirements shall be considered independently for a final decision)
274 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 10 – Section 6 Damp situations
S. No.
AS/NZS 3000
clause
•
•
•
10.1
10.2
6.2.2
10.3
10.4
Answer
Reduced body resistance
Likleyhood of more contacts
Mosture and water affecting insulation
1.8 metres or shower height, whichever is higher.
Ceilings and Walls
6.2.3 etc
•
•
Prortection by obstacles
Keeping live parts out of reach
10.5
Zone 3, Zone 1/ zone 0
10.6
Lamps
10.7
•
•
•
10.8
Switchboards, Hosing down
10.9
Zone 3
10.10
Entire room in one zone
Source located outside zone 0
12V a.c. or 30V d.c. source
SELV/PELV
Answers for Practical Exercises 275
Chapter 11 – Section 7 Specific installations
S. No.
AS/NZS 3000
clause
Answer
11.1
7.2.9.4
Shall be direct tapped from main supply without control
transformer.
11.2
7.3.3
Avoid backfeeding except when governed by written
agreement.
11.3
7.3.6
Isolation switches
11.4
7.4.2
Isolating transformers
11.5
7.4.7.1
1 mega ohm
11.6
7.5.7
10% and 5%
11.7
7.5.11.2
ELV systems
276 Practical Electrical Wiring Standards - AS 3000:2018
Chapter 12 – Section 8 Verification
S. No.
AS/NZS 3000
clause
12.1
Figure 8.1
12.2
8.3.5
12.3
8.3.9.3
Answer
Earth resistance test
0.5 ohm
.
Inclusion of upstream system
12.4 Cross check the fault loop impedance values tabulated in Table 8.1 for 32 amps breaker with a
disconnection time of 0.4 sec for Type B and Type C breakers, taking into account the AS/NZS
mean tripping currents of 4 times and 7.5 times the nominal rating for these breaker types
respectively.
Solution:
Nominal Phase voltage U 0
Nominal current rating of CB
IN
= 230V
= 32 amps
I a for Type B breaker
= 4times 32 amps = 128 amps
Hence maximum fault loop impedance
while using type B breaker
= 230/128= 1.796 ohm
I a for Type C breaker
= 7.5 times 32 amps = 240 amps
Hence maximum fault loop impedance while
using type C breaker
= 230/240= 0.958 ohm
A similar calculation is adopted for all values in Table 8.1 of the standard.
Answers for Practical Exercises 277
Appendix 1 – Load demand
A1.1. Determine the maximum demand of a single domestic electrical installation supplied at 240V
single-phase with the following loads:
• 36 lighting points
• 20 m of lighting track
• 12nos 10 A single socket-outlets
• 6nos 10 A double socket-outlets
• 1no 7.5kW range
• 1no 3.6kW water heater
Solution:
Refer Table C1 under column 2 for single domestic installation.
Following come under Load Group A(i)
36 lighting points plus 20 m of lighting track
=3+4= 7A
Load Group B(i):
12 x 10 A single socket-outlets + 6 x 10 A double socket-outlets = 24 nos 10 A outlets
=10+5= 15A
Load Group C
7500 W range will draw a full load current = 7500/240 = 31.25A
Its demand
=31.25 x 0.5 = 15.6A
Load Group F:
Water heater 3600 W = 3600/240= 15 A
Hence Total demand = 7+15+15.6 +15 = 52.6A
A1.2. Determine the maximum demand of the heaviest loaded phase of a motel complex supplied
by three-phase 415V with the following loads:
• 300nos 60 W lighting points
• 60nos 100 W 1-phase exhaust fans (permanently connected)
• 70nos 10 A 1-phase single socket-outlets (permanently heated or cooled area)
• 6nos 15 A 1-phase socket-outlets
• 1no 12 kW 3-phase electric range
• 2nos 1.5kW 3-phase borewell pump motor
The loads are distributed over the three phases as below
Red
110 lights
18 exhaust fans
20x10A socket outlets
2 x 15 A socket-outlet
3.2 kW oven
0.75 kW pump
White
90 lights
20 exhaust fans
25x10A
2 x 15 A socket-outlets
4.4 kW hotplates
Blue
100 lights
22 exhaust fans
25x10A
2 x 15 A socket-outlet
4.4 kW hotplates
0.75 kW pump
Solution:
This is a motel complex and hence table C2, column 2 will be applicable with 240V phase voltage.
Current = Watts/ Voltage
278 Practical Electrical Wiring Standards - AS 3000:2018
Load
60W
Lights
100W
exhaust
fans
10A
sockets
15A
sockets
Oven/
hotplates
Pump
motor
Total
amps
Group
A
A
B(ii)
B(iii)
C
D
RED
Load
110 points
0.25A/point
at 240V
110×0.25×0.75
18 fans
0.417A per
fan
18×0.417×0.75
20 nos
2900W total
2 nos
15 + (0.5× 15)
3200W
Full amps
750W
Full amps
WHITE
BLUE
Amps
Load
Amps
20.625A
90 points
90×0.25×0.75
16.99A
5.63A
20 fans
6.255
20×0.417×0.75
12.08A
22.5A
13.33A
25 nos
3400W
2 nos
15 + (0.5× 15)
4400W
Full amps
14.17A
22.5A
18.33A
3.125A
77.29A
78.24A
Load
Amps
100 points
18.75A
100×0.25×0.75
22 fans
22×0.417×0.7
5
25 nos
3400W
2 nos
15 + (0.5× 15)
4400W
Full amps
750W
Full amps
6.88A
14.17A
22.5A
18.33A
3.125A
83.75A
Answers for Practical Exercises 279
Appendix 7
Reducing the Impact of Power Supply
Outages
Appendix M of the Standard provides guidance on the mitigation of adverse effects that disruption to power
supply may cause in living and homecare medical situations. Where the owner or operator of an installation
or part of an installation has identified it beneficial to reduce the impact of power supply outages and
provide continuity of supply, the Appendix gives guidance so that the electrical installation will function
correctly for the intended purpose, and take into account the mitigation of foreseeable adverse effects that
disruption to power supply may cause.
Active assisted living
Active assisted living (AAL) systems and services enable independent living through the use of information
and communications technology (ICT) by ensuring usability, accessibility, interoperability, security and
safety for all users.
Homecare medical
Home-based medical procedures such as dialysis, respiratory support and cardiac care and tele-monitoring,
which is the remote collection and transmission of data for ongoing patient management belong to this type
of field.
The following steps are provided to supplement the requirements of the Standard for active assisted living
and homecare medical applications:
(a) Enroll the electrical installation for the priority restoration in the event of supply failure with the
electricity retailer and/or electricity distributor providing power supply to the electrical installation
(b) Ensure that all trees have sufficient clearance from the aerial conductors to prevent damage, or
interruption of electrical supply, if it is aerial power supply
Arrangements
Following steps are provided to supplement the requirements of the Standard for the above said
applications:
(a) Enroll with electricity retailer and/or electricity distributor who provides power supply to the electrical
installation, for priority restoration in the event of supply failure
Circuit Design for Low Voltage Installations 287
(b) Ensure that all trees have sufficient clearance from aerial conductors, if the supply to the electrical
installation is by aerial conductors. Inspections should also be carried out at appropriate intervals and
corrective action taken, if required.
(c) Provide protection discrimination throughout the electrical installation
(d) Ensure that the capacity of the mains, switchboards and final subcircuit wiring is able to supply the
expected maximum loading of the electrical installation
(e) Use miniature circuit breakers (MCBs) or residual current breaker with overload (RCBO) for
overcurrent protection in domestic installations
(f) Use a portable Type 1, 10 mA RCD for each item of medical equipment in home care areas of a
domestic electrical installation
(g) Ensure constant charging of batteries to maintain full charge where batteries are used as energy storage
for power supply equipment and that regular maintenance of batteries is carried out according to battery
manufacturers’ instructions
(h) Provide for easy connection of a portable generating set, by use of a dedicated extension cord socketoutlet wiring in the electrical installation or changeover switch
NOTES:
1. A grid connected inverter is not able to use the output from a PV array when the normal mains supply
to the electrical installation has failed
2. An inverter/ UPS powered from a large battery may be able to provide an alternative power supply
cover for a short term supply failure
288 Practical Electrical Wiring Standards - AS 3000:2018
..
Appendix 8
Electrical Conduits
There are two series of Standards for electrical conduits that run in parallel within Australia and New
Zealand.
The first is the AS/NZS 2053 series and the second is the AS/NZS 61386 series, which is based on but not
equivalent to the IEC 61386 series.
The relevant Standards are as follows:
• AS/NZS
• 2053 Conduits and fittings for electrical installations
• 2053.1 Part 1: General requirements
• 2053.2 Part 2: Rigid plain conduits and fittings of insulating material
• 2053.4 Part 4: Flexible plain conduits and fittings of insulating material
• 2053.5 Part 5: Corrugated conduits and fittings of insulating material
• 2053.6 Part 6: Profile wall, smooth bore conduits and fittings of insulating material
• 61386 Conduit systems for cable management
• 61386.1 Part 1: General requirements
• 61386.21 Part 21: Particular requirements—Rigid conduit systems
• 61386.22 Part 22: Particular requirements—Pliable conduit systems
• 61386.23 Part 23: Particular requirements—Flexible conduit systems
The AS/NZS 2053 series and the second is the AS/NZS 61386 series provide the marking methods for the
conduits. The AS/NZS 61386 series gives the option of using the first four numerals in the classification, as
a minimum, to mark conduits and fittings.
First and second numerals in comparison with duty ratings in AS/NZS 203 series and AS/NZS 61386 series
are shown in the below Tables.
290 Practical Electrical Wiring Standards - AS 3000:2018
Circuit Design for Low Voltage Installations 291
..
Appendix 9
Installation of Arc Fault Detection
Devices (AFDDs)
Appendix O of the Standard provides guidance on the selection and installation of AFDDs to mitigate the
risk of igniting an electrical fire in final sub-circuits downstream of the arc fault detection device (AFDD).
Fire ignition by arc faults is normally due to one or more of the following:
(a) Insulation defects between live conductors leading to fault currents
(b) Broken or damaged (reduced cross-section) conductors under load current conditions
(c) Terminal connections with high resistance
Arc fault detection devices installed in final sub-circuits are capable of detecting fault conditions that result
within the installation.
There are also other types of arcing fault devices used to mitigate the risk of igniting an electrical fire and or
causing an electrical explosion in highcurrent circuits, high-current LV switchboards and HV switchboards,
and these are not the AFDDs described in Appendix O.
There are two types of arc faults:
(i) Series arcing faults - Electrical arc within a single active or neutral conductor, in series with the
connected load
(ii) Parallel arcing faults - Electrical arc between an active and a neutral conductor, or between two active
conductors of different phases, or between a live conductor and the protective conductor, in parallel
with the connected load
Circuit Design for Low Voltage Installations 293
Types of Arc Fault
Miniature overcurrent circuit breakers (MCBs), fuses, and residual current devices (RCDs) are not capable
of reliably protecting against the effects of arcing, and their response times at the level of current associated
with electrical arcing.
AFDDs that do not incorporate integral overcurrent or residual current protection do not provide protection
against sustained thermal overloads, short-circuit currents, or residual currents at power frequency.
NOTE: Installations in which the use of AFDDs may be appropriate include the following:
(a) Premises with sleeping accommodation
(b) Premises and locations for children, handicapped or elderly people
(c) Premises for gathering of people
(d) Locations with risks of fire due to the nature of processed or stored materials
(e) Locations constructed with combustible materials
(f) Fire propagating structures
(g) Locations where irreplaceable goods are stored or displayed and may be endangered
294 Practical Electrical Wiring Standards - AS 3000:2018
..
Appendix 10
Guidance for Installation of Electrical
Vehicle Socket – Outlets and Charging
Stations
Appendix P of the Standard provides guidance for:
(a) Circuits intended to supply energy to electric vehicles
(b) Circuits intended for feeding back electricity from electric vehicles into the supply system
This applies to off-board equipment including the vehicle connector for charging electric road vehicles, with
a rated supply voltage up to 1000 V ac or up to 1500 V dc and a rated output voltage up to 1000 V ac or up
to 1500 V dc.
Electric road vehicles (EV) includes plug-in hybrid road vehicles (PHEV), that derive all or part of their
energy from on-board rechargeable energy storage systems, (RESS), including traction batteries.
Applications to which the Appendix provides guidance are as follows:
(a) The characteristics and operating conditions of off-board charging equipment
(b) The connection between off-board charging equipment and electric vehicle
(c) The required level of electrical safety for off-board charging equipment
Requirements and tests referenced in this Standard can be found in the following Standards:
(a) AS/NZS 61439 series for tests and related requirements for low-voltage switchgear and controlgear
assemblies
(b) IEC 62752 for the in-cable control protection device for Mode 2 charging of electric road vehicles
(IC-CPD) as a part of the complete system
(c) IEC 62196 series for vehicle coupler, plug and socket-outlet
Exclusions
The Appendix does not provide guidance on:
(a) Safety aspects related to maintenance
(b) Charging of trolley buses, rail vehicles, industrial trucks and vehicles designed primarily for use offroad
The Appendix also provided information on the following:
296 Practical Electrical Wiring Standards - AS 3000:2018
•
•
•
•
•
Definitions
Maximum demand
Installation
Devices for protection against indirect contact by automatic disconnection of supply
Other equipment such as socket-outlets and vehicle connectors, ratings of vehicle couplers, socket-outlets
and plugs, permitted socket-outlets or vehicle connectors, types of connection, charging stations and cables,
periodic verification
Circuit Design for Low Voltage Installations 297
Appendix 11
Dc Circuit Protection Application Guide
Appendix Q of the Standard provides provides guidance for the selection of circuit protection and switching
devices operated on a dc supply that would be deemed to meet the design, equipment selection and
installation criteria of this Standard.
Where a single contact is used to interrupt the current flow of a dc circuit, consideration should be given to
the size of the contact, the air gap, arc suppression and the use of multiple contacts.
The Appendix provides information on the following:
• The phenomenon of arc suppression
• Switchgear types
• Dc ratings
• Provision of isolation and overcurrent protection
• Switchboard locations
• Final subcircuit wirings and fittings
• Inverters
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Practical Electrical Wiring Standards- AS/NZ 3000:2018
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Technical Questions
1.
What is the maximum permitted voltage drop at any point of the distribution system?
2.
What is the accepted sensitivity of RCD’s for human protection?
3.
What are the conditions under which use of different voltage level conductors are permitted without
segregation?
4.
What is the permitted surface temperature rise for hand held electrical equipments
5.
Indicate two conditions where earthing of electrical system can be avoided