WM_Spring07_Cover.qxd
26/3/07
8:16 pm
Page 1
WIRING
MATTERS
Spring 07 Issue 22
NEW 17TH EDITION TO
BE LAUNCHED IN 2008
Extract fans in dwellings
Competency for persons involved
in electrical installation work
Equipotential bonding
Installing downlighters safely
Uninterruptible power supplies
WM_Spring07_Cover.qxd
9/3/07
3:02 pm
Page 3
8/3/07
10:03 pm
Page 1
17th EDITON
WM_Spring07.qxd
1
NEW 17TH EDITION TO
BE LAUNCHED IN 2008
A BRIEF OVERVIEW
by Geoff Cronshaw
Introduction
BS 7671: 2008 Requirements for
Electrical Installations, IEE Wiring
Regulations 17th Edition is scheduled
to be issued on January 1st 2008 and is
intended to come into effect 6 months
later. The 17th Edition will be
completely restructured compared to
the present 16th Edition and includes
changes necessary to maintain
technical alignment with CENELEC
harmonisation documents. The new
edition will adopt the IEC numbering
system. In addition, the layout and
parts will be completely revised; for
example, many of the chapters will be
completely rewritten. The current
Part 6 (special locations) will become
Part 7 to align with IEC. The next
edition of BS 7671 will include
additional sections on special
locations not currently included in
BS 7671 and the existing special
locations will be revised to align with
changes in CENELEC harmonisation
documents. This article is based on the
draft for public comment and therefore
the actual requirements of the 17th
Edition may change.
against indirect contact becomes fault
protection. Socket- outlets up to 20A for
use by ordinary persons require 30mA
RCD protection and socket-outlets up
to 32A for mobile equipment for use
outdoors require 30mA RCD
protection. Note that certain
exceptions are permitted – refer to
Regulation 411.3.3.
There are new additional
requirements for the connection of
low voltage generating sets including
SSEGs in Chapter 55.
Section 559 Luminaires and Lighting
Installations is a new series of
Regulations concerning lighting
installations and also includes
highway power supplies and street
furniture previously in Part 6.
Chapter 56 has been expanded and
includes requirements for emergency
escape lighting and fire protection
applications.
There are also changes to inspection
and testing. Changes have been made to
the requirements for insulation
resistance; when testing SELV and
PELV circuits at 250 V, the minimum
insulation resistance is raised to 0.5 M;
for systems up to and including 500 V,
(including FELV), the minimum
insulation resistance is raised to 1.0 M.
What’s new?
Part 1 adds requirements to protect
against voltage disturbances and
implement measures against
electromagnetic influences.
Part 3 adds requirements for safety
services, e.g. emergency escape lighting,
and fire protection applications. Also,
Chapter 36 requires that an assessment
shall be made for each circuit of any
need for continuity of service
considered necessary during the
intended life of the installation.
In the new Chapter 41, the terms
protection against direct contact
becomes basic protection and protection
IEE Wiring Matters | Spring 07 | www.theiet.org
8/3/07
5:18 pm
Page 2
17th EDITON
WM_Spring07.qxd
2
Special installations or locations
The next edition of BS 7671 will include
additional sections on special locations
not currently included as follows:
Marinas (Section 709)
Exhibitions, shows and stands
(Section 711)
Floor and ceiling heating systems
(Section 753)
Mobile or transportable units
(Section 717)
Fairgrounds, amusement parks and
circuses (Section 740)
Photovoltaic power systems
(Section 712).
Special locations are areas of
increased shock risk, for example:
Marinas. There are particular risks
associated with electrical installations
in marinas. The environment of a
marina or yachting harbour is harsh for
electrical equipment. The water, salt
and movement of structures accelerate
deterioration of the installation. The
presence of salt water, dissimilar metals
and a potential for leakage currents
increases the rate of corrosion. There
are also increased electric shock risks
associated with a wet environment by
reduction in body resistance and contact
with earth potential.
Exhibitions. There are particular
risks associated with exhibitions,
shows and stands. These arise from:
1. The temporary nature of the
installation
2. Lack of permanent structures
3. Severe mechanical stresses
4. Access to the general public.
Changes to the Existing Requirements
for Special Locations
The current special locations contained
in the IEE Wiring Regulations will be
revised to align with the latest IEC and
CENELEC standards.
For example, the requirements for
locations containing a bath or a shower
unit will require 30mA RCD protection
on all circuits in a bathroom/shower
room. Zone 3 is no longer defined. Socket
outlets other than SELV and shaver
units are allowed 3 metres horizontally
beyond the boundary of zone 1.
Supplementary equipotential bonding is
no longer required providing main
equipotential bonding is installed in
accordance with Chapter 41.
The requirements for swimming
pools now include fountains and the
zones have changed from A, B, and C
to 0, 1, and 2.
In agricultural and horticultural
premises and construction sites the
reduced disconnection times and 25
volt equation no longer appear. The
UK has retained the use of reduced
low voltage supplies for construction
sites which will continue to be a
requirement in the 17th Edition.
In caravan/camping parks each
socket outlet must now be
individually protected with
overcurrent and RCD protection.
Changes to Appendices
Appropriate changes have been made
to the existing Appendices 1 to 7. In
addition the following new Appendices
are now included:
Appendix 8 Current-carrying capacity
and voltage drop for busbar trunking
and powertrack systems
Appendix 9 Definitions – multiple
source, d.c. and other systems
Appendix 10 Protection of conductors
in parallel against overcurrent
Appendix 11 Effect of harmonic
currents on balanced three-phase
systems
Appendix 12 Voltage drop in
consumers’ installations
Appendix 13 Methods for measuring
the insulation resistance/impedance
of floors and walls to Earth or to the
protective conductor system
Appendix 14 Measurement of fault
loop impedance, consideration of the
increase of the resistance of conductors
with increase of temperature.
Further information
Important: This article is only
intended as a brief overview and only
gives a small number of the changes.
For further information on the 17th
Edition please refer to the IET
website: www.theiet.org/DPC Published by IET Publishing & Information Services Michael Faraday House, Six Hills Way, Stevenage, Herts, SG1 2AY, United Kingdom
Tel: +44 (0)1438 313311 Fax: +44 (0)1438 313465
Sales and Project Coordinator L Hall +44 (0)1438 767351 lhall@theiet.org | Editor G D Cronshaw +44 (0)1438 767384
gcronshaw@theiet.org | Contributing Editors J Ware, M Coles, J Elliott | Design Sable Media Solutions
IEE Wiring Matters is a quarterly publication from the Institution of Engineering and Technology (IET). The IET is not as a body responsible for
the opinions expressed.
©2007: The Institution of Engineering and Technology. All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted in any form or by any means without the permission in writing of the publisher. Copying of articles is not permitted
except for personal and internal use. Multiple copying of the content of this publication without permission is always illegal. Web-offset
printing by Wyndeham Heron, The Bentall Complex, Colchester Road, Heybridge, Maldon, Essex, UK
Co-operating Organisations The Institution of Engineering & Technology acknowledges the contribution made by the following
organisations in the preparation of this publication: British Electrotechnical & Allied Manufacturers Association Ltd – R Lewington,
P D Galbraith, M H Mullins | Department for Communities and Local Government – I Drummond | Electrical Contractors Association – D Locke,
S Burchell | City & Guilds of London Institute – H R Lovegrove | Energy Networks Association – D J Start | Electrical Contractors Association
of Scotland SELECT – D Millar, N McGuiness | Health & Safety Executive – K Morton | Electrical Safety Council | ERA Technology Limited –
M Coates | British Cables Association – C Reed | Scottish Building Standards Agency | DTI – D Tee | CORGI – P Collins | GAMBICA –
K Morris.
ISSN 1749-978-X
IEE Wiring Matters | Spring 07 | www.theiet.org
8/3/07
5:18 pm
Page 4
EXTRACT FANS
WM_Spring07.qxd
4
EXTRACT FANS IN DWELLINGS
by John Ware
Kitchens and bathrooms now are often fitted with
extract fans either to meet the requirements of the
Building Regulations or at the owner’s request. Such
fans will need to be replaced or repaired during the
lifetime of the installation. The IET Helpline often
receives enquiries relating to two aspects of these
fans; the isolation and switching requirements of
BS 7671 and the ventilation requirements of the
Building Regulations.
In this article we will explain briefly the concepts of
isolation and switching and then discuss these
functions in relation to extract fans and in an
upcoming article we will cover the ventilation
requirements placed by Part F of the Building
Regulations.
Isolation and switching.
The term isolation and switching, as used in BS 7671:
2001 Requirements for Electrical Installations (The
IEE Wiring Regulations) refers to four distinct
functions:
Isolation,
Switching off for mechanical maintenance,
Emergency switching, and
Functional switching.
Isolation and switching: The four functions
Isolation:
Purpose:
For:
Switching off for mechanical maintenance:
Purpose:
To enable non-electrical work to be carried out safely on the switched
circuit or equipment
For:
Non electrically-skilled persons
Functional switching:
Purpose:
To enable proper functioning and control of electrical equipment
For:
The user of the installation
Emergency switching:
Purpose:
To cut off rapidly electrical energy to remove an unexpected hazard
For:
Anyone
Table 1: Summary of the four functions of Isolation and Switching
Summary of requirement
The concepts of isolation and switching are
summarized in Table 1.
Isolation
The definition of isolation, in BS 7671 (The IEE
Wiring Regulations) 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 or section from every source of electrical
energy’. Isolation is provided to permit an electricallycompetent person to work safely on all or part of an
electrical installation. Once electrical equipment has
been securely isolated from the source of energy and
any electrical energy has been discharged, a skilled or
instructed person should be able to safely access parts
that are normally live, or may become live, without
the risk of danger from electric shock, electric arcing
or explosion or from electrically-powered equipment
and machines.
The corresponding definition in the Electricity at
Work Regulations (EWR) adds the specific
requirement that the isolation is to be secure. This
IEE Wiring Matters | Spring 07 | www.theiet.org
To enable electrical work to be carried out safely on the isolated circuit or
equipment
An electrically-skilled or supervised person.
Regulation(s) in BS 7671
and the EWR which refer
Every circuit to be capable of being isolated
461-01-01
Neutral conductor to be capable of being isolated
(with certain permitted exceptions1)
461-01-01
460-01-04
Group of circuits may be isolated by a common means
461-01-01
A means of isolation to be provided as close to the
origin of an installation as possible
460-01-02
476-01-01
Means of isolation to be non-automatic
460-01-01
Means of isolation to be suitably positioned or durably marked
461-01-05
Means of isolation not to be inadvertently or unintentionally energized
461-01-02
Means of isolation may need to be secured in the open position
476-02-02
Requirements where there is more than one device to be operated
461-01-03
514-11-01
Circuits containing stored energy may need to be discharged2
461-01-04
Means of isolation to be secure
EWR (Regulation 12)
Table 2: Principal requirements in BS 7671 and the EWR applicable
to isolation in dwellings
1
A neutral conductor falls within the definition of a live conductor. With permitted exceptions for TN-S and
TN-C-S systems, the neutral conductor is required to be able to be isolated from the corresponding
supply conductor. A means of isolation of the neutral conductor in a TN-S or TN-C-S system is not
generally required where that conductor can reliably be regarded as being at Earth potential. The neutral
conductor (PEN or N) for supplies provided in accordance with the ESQCR 2002 is considered to be
connected with Earth by a suitably low resistance. For a TT system, the neutral conductor as well as the
phase conductor are required to be provided with a means of isolation.
2
Note that motor circuits may include a capacitor such as a start capacitor which would store energy. The
stored energy has to be discharged before work can be undertaken on the circuit.
means the isolated equipment cannot be re-energised accidentally
or inadvertently.
The procedure of ‘Isolation’ includes (a) correct identification
of the equipment and circuit to be worked on, (b) disconnection,
(c) securing the means of disconnection, (d) posting notices and
8/3/07
10:13 pm
Page 5
EXTRACT FANS
WM_Spring07.qxd
5
8
9
*…>Ãi
ˆ}…Ì
Ã܈ÌV
>˜
/ˆ“iÀ
՘ˆÌ
ˆ}…Ì
>˜Ê>ÃÃi“LÞ
iÕÌÀ>
8
9
"/\Ê
ˆÀVՈÌÊ«ÀœÌiV̈ÛiÊVœ˜`ÕV̜ÀÃʘœÌÊŜܘ
Figure 1: Insertion of a local isolator in the lighting and extract fan circuit for a windowless
bathroom in dwellings
Figure 2: Three-pole isolator which can be
locked off. Courtesy of MK Limited
(e) proving dead. In some cases
additional precautions will also be
needed. Please refer to the HSE
publication: Electricity at Work: Safe
Working Practices.
Requirements applicable to isolation
are given in BS 7671: 2001
Requirements for Electrical
Installations and Regulation 12 of the
EWR and these requirements are
summarized in Table 2.
Isolation of an extract fan
Let us take the case of an extract fan in
a dwelling and assume the installation
is part of a single-phase TT system. The
issue of isolation will arise during the
lifetime of the installation when an
electrician is called in to repair or
replace the extract fan. The electrician
will need to isolate the fan from the
source of energy in order to work safely
on it. As the installation is part of a TT
system, both the phase and neutral
conductors will need to be isolated.
The main switch in the consumer unit
In many dwellings the main switch of
the consumer unit can form a perfectly
adequate means of isolation. In such a
case, the electrician will switch off the
main switch, take precautions that it
will not be inadvertently or
unintentionally switched back on, and
then having verified all the conductors
Local isolator
Often a local isolator, such as a pull
cord switch, a switched fused
connection unit or a switched device
installed outside the bathroom but
next to the door leading in to the
bathroom is provided. Providing the
switch meets the requirements of
isolation, the advantages of using such
a switch are:
of the fan circuit are dead, be able to
work in safety on the fan. The
difficulty that can arise when using
the main switch as the means of
isolation is that it is often remote from
the extract fan and there is a very real
risk of someone inadvertently
switching the supply back on. The
electrician is therefore required to
take adequate precautions against this
happening by means such as securing
the main switch with a padlock or
locking the cupboard or door where
the consumer unit is located and
putting notices.
A circuit-breaker in the consumer unit
In the case of an installation forming
part of a TN system where the
neutral conductor can be regarded as
being reliably connected with Earth,
it may not be necessary to disconnect
both conductors and safe isolation
may be able to be achieved by
isolating the phase conductor by
means such as switching off and
locking off the appropriate circuitbreaker providing the device is
suitable for such use. Note that
putting a bit of insulation tape over a
switched-off circuit-breaker is not
sufficient. Once again, part of the
isolation procedure is to verify that
all conductors are indeed ‘dead’.
1. The electrician may decide it is safe
to isolate the extract fan at this switch
leaving the main switch at the
consumer unit on thereby permitting
operation of other electrical
equipment in the dwelling, for
example, the lighting circuits and the
socket-outlet circuits.
2. Providing the isolator is local to the
extract fan the electrician may decide
there is minimal risk of it being
inadvertently or unintentionally
switched back on while he is working
on the extract fan because he is next to
it and can stop anyone interfering with
it. Securing the means of isolation is
thereby simplified.
Once again, the conductors that are
to be worked on have to be proved
to be dead.
IEE Wiring Matters | Spring 07 | www.theiet.org
8/3/07
10:05 pm
Page 6
EXTRACT FANS
WM_Spring07.qxd
6
Mechanical maintenance
An extract fan should be maintained
periodically and such maintenance
normally includes cleaning as the fan
will almost certainly get dirty. In
order for such maintenance to be
performed safely, a means of
switching off for mechanical
maintenance is to be provided
(Regulation 462-01-01 refers). The
means of switching off is not
necessarily intended to provide
protection against electric shock but
has to reliably stop the fan turning
(Regulation 462-01-03 refers).
Switching off for mechanical
maintenance, in this case, is to enable
non-electrical maintenance to be
performed safely without the risk of
injury from mechanical movement.
The demands of safety are such that
the same means of preventing
unintentional or inadvertent reclosure
of the switch must be provided as for
the function of isolation. The means
provided for the function of isolation
will, in almost all cases, be sufficient
to meet the requirements for
switching off for mechanical
maintenance.
Functional switching
Functional switching is an operation
intended to switch ‘on’ or ‘off’ or vary the
supply of electrical energy to all or part
of an installation for normal operating
purposes. The purpose of functional
switching is to enable current-using
equipment, such as an extract fan, to be
controlled for normal operating
purposes. The control may be manual,
such as a simple fan on/off switch or
automatic such as a timer circuit or
variable such as a speed control.
Emergency switching
The provision of emergency switching
requires the identification and
assessment of reasonably foreseeable
dangers. Regulation 463-01-01 refers. An
extract fan in a bathroom or kitchen is
unlikely to cause danger such that an
emergency switch will be required.
IEE Wiring Matters | Spring 07 | www.theiet.org
A mains voltage extract fan may be installed in zone 1, 2 or 3 providing
the additional requirements listed below are met:
Requirement
Zone 11
IP rating
At least IPX4 (Regulation 601-06-01)
RCD protection
30 mA RCD
protection is
required2.
(Regulation
601-09-02)
Zone 2
Zone 3
The fan has to be suitable for the
conditions (Regulation 512-06-01)
Note that Manufacturer’s
instructions for mains voltage fans
may include the recommendation
that 30 mA protection be provided.
Such instructions should be followed.
A SELV extract fan may be installed in zone 1, 2 or 3 providing the requirements
listed below are met:
Source
The safety source, such as the transformer, may have to be installed outside of the zones
(Regulation 601-08)
IP rating
At least IPX4 (Regulation 601-06-01)
The fan has to be suitable for the
conditions (Regulation 512-06-01)
Table 3: Requirements applicable to a fixed extract fan installed in a location containing a bath tub or
shower basin
1
Fixed current-using equipment such as an extract fan may be installed in zone 1 if (i) it is suitable for the conditions of that zone,
(ii) can reasonably only be located in that zone and (iii) RCD protection is provided (See Regulation 601-09-02(iii)).
2
The Residual Current Device used is required to have a rated residual operating current not exceeding 30 mA in accordance with
Regulation 412-06.
Requirement
Zone 1
Zone 2
Zone 3
Outside the
Zones
Outside the
bathroom
Local isolator
permitted?
Isolator operating at mains
voltage not permitted. Only SELV
switches are permitted
(Regulation 601-08-01)
A local isolator may be installed but it
has to be suitable for the conditions
(Regulation 512-06-01)
IP rating
At least IPX4
(Regulation 601-06-01)
Isolator has to be suitable for the
conditions (Regulation 512-06-01)
Table 4: Requirements applicable to a local isolator1 for an extract fan installed in or adjacent to a
location containing a bath tub or shower basin
1
Note that plate-type switches are unlikely to have a suitable IP rating and will need to be installed in zone 3 or outside the zones.
Often such switches are installed on the outside of the bathroom but next to the door leading into the bathroom. Pull cord switches are
unlikely to have a suitable IP rating and while the pull cord itself may enter zone 1 or 2, providing it is an insulating pull cord, the body of
the switch will have to be installed in zone 3 or outside the zones. Hence, in a bathroom with a ceiling height greater than 2.25 m or 3 m
the body of the pull cord switch may be installed in zone 3 or outside the zones (But see Regulation 512-06-01) and the insulating pull
cord allowed to enter zone 1 or zone 2 (See Regulation 601-02-01 and Figures 601A and 601B in BS 7671).
Windowless bathrooms.
Windowless bathrooms and toilets have
to be fitted with a fan that often includes
a timer circuit to ensure the fan
continues to run for a period after the
light has been switched off. Figure 1
illustrates a typical configuration for a
circuit. The timer unit is often part of
the fan assembly. A two pole isolator
inserted at X-X in the circuit would
effectively isolate the fan and timer unit
but would also isolate the bathroom light
which would result in inconvenience
for the electrician who would then need
to provide a light source.
A three-pole isolator inserted at Y-Y in
the circuit would be preferable as it
would permit the light to be left
energized whilst the fan or timer unit
was being worked on.
The electrican called upon to work on
the timer unit or extract fan has to verify
that the correct circuit has been isolated
and all the conductors are indeed dead
before he starts work as there is a real
risk of incorrect wiring in such a circuit.
WM_Spring07.qxd
8/3/07
5:21 pm
Page 7
THING E
Y
R
E
EV
TRAD ™
e
h
t
for T DAY !
– NEX
˜‡ˆ˜iÊiÝÌÀ>VÌÊv>˜
ÕV̈˜}
>̅Àœœ“
Figure 3: Inline extract fan and ducting
Additional requirements applicable to an extract fan
installed in a location containing a bath tub or shower
basin in a dwelling
A fixed extract fan can be installed in a location containing a
bath tub or shower basin providing the additional
requirements listed in Table 3 are met.
The requirements given in Table 4 are required to be met for
a local isolator installed in or adjacent to a location containing
a bath tub or shower basin.
Supplementary bonding
Where a mains voltage fan or its local isolator, if any, is
installed in zones 1, 2 or 3, local supplementary bonding has to
be provided connecting together the protective conductor of
the fan circuit and extraneous-conductive-parts in these zones
(Regulations 601-04-01 and 601-04-02 refer).
Inline extract fans
Inline extract fans providing air extraction through ducting
are often installed in the loft above a bathroom to provide
bathroom ventilation as illustrated in Figure 3. An extract fan
mounted in the loft above the bathroom is not ‘in the
bathroom’ and hence is not subject to the supplementary
requirements placed by Section 601 of BS 7671. Note that the
fan is, of course, subject to the general requirements contained
in the other Parts of BS 7671. S
W
E
R
SC INGS
FIX OLS
O
T
&
ders,
l
i
u
b
r
ge fo ans
n
a
r
ssive electrici ices.
a
m
r
A
,
nters at trade p
e
p
r
a
c
s
mber for your
u
l
p
&
e NOW
Phon atalogue.
c
FREE
S
PRICE
E
D
A
S
TR
DUCT
O
R
P
0+
14,00
IVEReYtails
L
E
D
AY ewfix.com for d
D
T
X
NE ns. See scr
ptio
ery o
deliv
y
a
d
Next
OVER
950
PAGES
our
y
t
e
G
Ee
E
R
F talogu
a
c
W!!
OW
N
NO
1
4
1
0
6
9
0
m
o
0
c
0
08 ewfix. nters
scr rade Cou
T
ons
locati
rs for
te
n
u
co
307
/trade
IRING
.com
te: W
ewfix
e quo
r
s
c
a
s
le
P
Go to
IEE Wiring Matters | Spring 07 | www.theiet.org
8/3/07
11:12 pm
Page 8
COMPETENCY
WM_Spring07.qxd
8
COMPETENCY FOR
PERSONS INVOLVED
IN ELECTRICAL
INSTALLATION WORK
by Jon Elliott
Introduction
The IET technical advice line often
receives calls relating to the
competency of persons carrying out
electrical installation, maintenance,
inspection & testing and similar
activities and what qualifications are
required to be classed as an
“electrician”. The helpline often takes
enquiries from persons currently
working as electricians who have no
formal qualifications who now wish to
find an appropriate qualification, and
from those who wish to enter the
electrical industry from a wide range
of backgrounds.
A number of typical questions
relating to competency, qualifications
and training for electricians are given
below with appropriate answers.
What is an electrician?
The term electrician is generic, has no
legal status and is in no way protected.
Anyone may refer to themselves as
IEE Wiring Matters | Spring 07 | www.theiet.org
being an electrician. As such, the term
is no indicator of a person’s level of
training, technical qualifications
achieved, or the extent of relevant work
experience accumulated - all factors
which will affect a person’s ability to do
electrical work properly. The ability to
perform a particular task properly is
defined as competency. Generally
speaking, a suitably qualified and
competent electrician should be able to
perform a wide range of installation
activities in domestic, industrial and
commercial installations. Persons
wishing to employ an electrician
should ask for some evidence of their
competency. This might include
production of certificates obtained
from successful completion of
recognised training at a technical or
further education college, a Joint
Industry Board Electrotechnical
Certification Scheme card, or proof of
membership/affiliation with a
recognised industry body.
What are the legal requirements?
The Electricity at Work Regulations
1989 imposes duties on persons
involved in electrical work
commercially whether employers, the
self employed or employees, including
most trainees.
Regulation 16 (Persons to be
competent to prevent danger and
injury) states:
“No person shall be engaged in any
work activity where technical
knowledge or experience is necessary to
prevent danger or, where appropriate,
injury, unless he possesses such
knowledge or experience, or is under
such degree of supervision as may be
appropriate having regard to the
nature of the work.”
It is stated in the Memorandum of
guidance on the Electricity at Work
Regulations 1989 (HSE: 1989) that “the
object of the regulation is to ensure
that persons are not placed at risk due
to a lack of skills on the part of
themselves or others in dealing with
electrical equipment”.
It continues: “the scope of ‘technical
knowledge or experience’ may include:
(a) adequate knowledge of electricity;
(b) adequate experience of electrical
work;
(c) adequate understanding of the
system to be worked on and practical
experience of that class of system;
(d) understanding of the hazards
which may arise during the work and
the precautions which need to be
taken;
(e) ability to recognise at all times
whether it is safe for work to
continue.”
What qualifications are needed to
become an electrician?
People have come into the electrical
installation industry from a number of
routes and may have a number of
different electrically-biased
qualifications.
8/3/07
11:14 pm
Page 9
COMPETENCY
WM_Spring07.qxd
9
Concentrating on electrical
installation work, typically persons
wanting to become electricians
complete a City & Guilds qualification.
Over the years, City & Guilds has
offered a number of qualifications
which provided the knowledge
evidence requirements for electrical
trainees:
- 236 part “A” and “B” certificates (not
available since the late seventies)
- 2360 part 1 (not available since
October 2006) and part 2 (no longer
available from October 2008)
- 2330 part 1 and 2 certificates (both
available from September 2004)
All of the above could, and in the
case of the 2330 can, be studied by
persons not directly employed in the
electrical installation industry.
What subject updating and
further development qualifications
are available?
For persons who have been employed
in the electrical industry for some
time and who have no electrical
qualifications, or who qualified prior
to the advent of the 16th Edition, the
most appropriate subject updating or
“refresher” course is probably the City
& Guilds 2381 “16th Edition”
certificate. Those wishing to refresh
and/or improve their skills in relation
to the inspection, testing and
certification of an installation may
choose to take the City & Guilds 2391
Inspection & testing certificate, which
may also be beneficial to those
wanting to be graded as an Approved
Electrician on the Joint Industry
Board (JIB) Electrotechnical
Certification Scheme, or who wish to
become a Qualified Supervisor for an
NICEIC Approved Contractor or ECA
registered company.
What about persons wishing to
become electricians having
qualifications that are not electrical
installation work specific?
Many persons have entered the
industry having successfully
completed electrically biased BTEC
(and later Edexcel) National Certificate
or diploma courses or other
electrically related qualifications. In
the case of qualifications other than
those provided by City & Guilds listed
above an assessment will have to be
made of their suitability in terms of
providing the necessary knowledge
base for someone involved in electrical
contracting if they wish to obtain a
JIB grading or become affiliated with
other electrical organisations.
However, qualifications primarily
relating to electronics and/or
computing even if at a notionally
higher level in the National
Qualifications Framework may not be
suitable without the addition of some
electrical installation specific
training/experience.
What is the National Qualifications
Framework?
The National Qualifications
Framework categorises all accredited
qualifications on a scale ranging from
entry level through to level 8. In the
field of electrical installation work
levels 2, 3 and 4 are of relevance.
Broadly speaking, level 2 is “first
certificate” level, will cover a narrow
range of work activities and in many
cases may be awarded as an interim
stage on the way to becoming fully
qualified in a particular occupation.
Completion of part 1 of the City &
Guilds 2360 or 2330 qualifications will
result in the award of a level 2
certificate. Level 3 is “craft” level, or
the level required to be competent in a
particular occupation. Completion of
part 2 of the City & Guilds 2360 or 2330
qualifications will result in the award
of a level 3 certificate. Level 4 would
equate to technician level and as such
falls beyond the remit of this article.
Companies or persons carrying out
electrical work in domestic premises
were required to either notify the
relevant Building Control department
for the area where notifiable work was
being carried out prior to starting
work or to become registered as
domestic installers. A qualified
electrician generally met the
requirements to become a registered
domestic installer. However, many
persons carrying out work activities in
domestic premises requiring a degree
of work on the electrical installation,
such as central heating installers and
kitchen fitters did not. As such there
was a need for a recognised
qualification to be developed that
would provide “non electricians” with
the necessary knowledge and skills to
be considered competent for such
work. The examination board EMTA
Awards Ltd (EAL) developed a level 2
qualification for domestic electrical
installers in conjunction with a
What is a domestic installer?
In 2005 the Building Regulations as
applied to England and Wales were
modified to include requirements for
domestic electrical installations.
IET Wiring Matters | Spring 07 | www.theiet.org
8/3/07
9:17 pm
Page 10
COMPETENCY
WM_Spring07.qxd
10
number of interested parties from the
electrical industry. It should be
remembered that due to the limited
course content of this qualification,
successful completion does not meet
the requirements of any electrical
organisation for grading as an
electrician.
What is an Electrical Apprenticeship?
When school leavers wish to become
electricians the best route into the
industry is via an apprenticeship. This
will normally take 3 to 4 years to
complete. The apprentice will receive
practical “on the job” work experience
and training throughout this time and
will be given opportunities to be more
involved in work as time progresses
and their skills and abilities develop.
During the first three years, they
attend a further education college on a
day-release basis (that is, attending
one day per week during college term
time) where the knowledge obtained in
the workplace is reinforced by
instruction and training. They will
also receive key skills training for
communication, application of
number, information technology,
working with others, improving
learning and performance and
problem solving. On completion of
their studies at college they receive a
technical certificate and take the
Achievement Measurement 2 (AM2)
timed practical assessment and with
the assistance of their employer
compile a site based logbook in order
to obtain their level 3 National
Vocational Qualification (NVQ) or
Scottish Vocational Qualification
(SVQ) as appropriate.
What are the JIB Grading requirements?
Electrician
In order to be registered as an
electrician with the Joint Industry
Board for the Electrical Contracting
Industry (JIB) a person must:
have been a registered apprentice or
undergone some equivalent method of
training and have had practical
IET Wiring Matters | Spring 07 | www.theiet.org
training in electrical installation
work, and
have obtained an NVQ / SVQ Level 3
in electrical installation work (or
approved equivalent such as successful
completion of the City & Guilds 2360
part 2 certificate and have passed the
Achievement Measurement 2 (AM 2),
or be able, with the application for
Grading and any other relevant
supporting evidence (i.e. the City &
Guilds Electricians' Certificate) which
may be required, to satisfy the
Grading Committee of his experience
and suitability), and
be 21 years of age (this requirement
may be waived if the applicant has
obtained a pass in the City & Guilds
2360 Electrical Installation Theory
Part 2 Course or approved equivalent),
in addition to the above
Electricians are expected to be able
to carry out electrical installation
work efficiently in accordance with
the National Working Rules for the
Electrical Contracting Industry, the
current IEE Regulations for Electrical
Installations, and the Construction
Industry Safety Regulations.
Approved Electrician
In order to be registered as an
Approved Electrician with the JIB a
person must have met the
requirements to be graded as an
electrician above and must
additionally:
have had two years experience as an
electrician subsequent to the
satisfactory completion of training
and immediately prior to the
application for the Approved
Electrician grade, or be 22 years of
age, whichever is the sooner, and
have demonstrated competence and
obtained a suitable qualification (such
as the City & Guilds 2391 qualification)
in the inspection, testing and
commissioning of installations.
Approved Electricians are expected:
to possess the practical, productive
and electrical engineering skills with
adequate technical supervisory
knowledge so as to be able to work on
their own proficiently and carry out
electrical installation work without
immediate supervision in the most
efficient and economical manner
be able to set out jobs from
drawings and specifications and
requisition the necessary installation
materials
be able to accept responsibility for
the proper completion of jobs and,
if required, supervise other
operatives.
References
Electricity at Work Regulations 1989
(HMSO: 1989)
Memorandum of guidance on the
Electricity at Work Regulations 1989
(HSE: 1989)
Requirements for Grading of Electrical
Operatives. (Joint Industry Board:
2006-2007)
Further Information
Qualifications
In general
- Qualifications and Curriculum
Authority: www.qca.org.uk
Electrical installation work
- City and Guilds: www.city-andguilds.co.uk
Domestic electrical installer
- EMTA Awards Ltd: www.eal.org.uk
Apprenticeships
In England and Wales
- JTL: www.jtlimited.co.uk
- Modern Apprenticeships:
www.apprenticeships.org.uk
In Northern Ireland
- The Electrical Training Trust:
www.ett-ni.org
In Scotland
- The Scottish Electrical Charitable
Training Trust: www.sectt.org.uk
- The Scottish Enterprise:
www.scottish-enterprise.com/
modernapprenticeships
Electrotechnical Certification Scheme
- Joint Industry Board: www.jib.org.uk 8/3/07
11:28 pm
Page 12
BONDING
WM_Spring07.qxd
12
equipment exposed-conductive-part
and the simultaneously accessible
extraneous-conductive-part is:
Uf = If R2
Where:
If is the fault current
R2 is the resistance of the circuit
protective conductor.
(Ignoring any reactance of the circuit
protective conductor, and any small
effect of current flowing in the main
equipotential bonding conductor)
The effect of connecting the main
equipotential bonding conductor to the
extraneous-conductive-part is to
minimise Uf . Without this conductor,
the potential difference would
approximate to the voltage drop
produced by If along the full length of
the earth return path, and this could
be significantly greater than (If R2).
Therefore, failure to install all
necessary main equipotential bonding
conductors within an installation will
certainly increase the shock risk
associated with indirect contact.
Equipotential Bonding
by Geoff Cronshaw
Introduction
BS 7671: 2001 (incorporating
Amendments No 1: 2002 and No 2: 2004)
has requirements for protection
against electric shock, and lists a
choice of five basic measures which
shall be used to protect against
indirect contact. Protection by earthed
equipotential bonding and automatic
disconnection of supply is the most
common measure. Its purpose is that
under earth fault conditions, voltages
between simultaneously accessible
parts are not of such magnitude and
duration as to be dangerous.
IET Wiring Matters | Spring 07 | www.theiet.org
Main equipotential bonding
Regulation 413-02-02 requires main
equipotential bonding to be carried
out. Its importance is often
underestimated (see Figure 1). An
earth fault in the current-using
equipment produces a fault current
(If) which flows along the circuit
protective conductor and back to the
source. A small proportion of the
current may flow through the main
equipotential bonding conductor
directly to earth, and then back to
the source.
The potential difference between the
Installation of main equipotential
bonding conductors
IEE Guidance Note 5 recommends
that main equipotential bonding
conductors should be kept as short as
practicable and be routed to minimise
the likelihood of damage or
disturbance to them. The connections
to gas, water and other services
entering the premises must be made
as near as practicable to the point of
entry of each service, on the
consumer’s side of any insulating
section or insert at that point or any
meter. Any substantial extraneousconductive-part which enters the
premises at a point remote from
the main earthing terminal or bar
must also be bonded to this terminal
or bar.
Extraneous-conductive-parts should
preferably be bonded using individual
main equipotential bonding
8/3/07
9:31 pm
Page 13
BONDING
WM_Spring07.qxd
13
conductors. Alternatively, two or more
such parts may share a main
equipotential bonding conductor, but
where this arrangement is employed
the conductor should be continuous,
i.e. disconnection of the conductor
from one extraneous-conductive-part
must not interfere with or endanger
the security of the bonding of the
other part(s).
Regulation 547-02-01 and Table 54H of
BS 7671 gives sizing requirements for
main equipotential bonding
conductors. However, it is
recommended that the electricity
distributor or supplier should be asked
to confirm their agreement to the
proposed size(s) it is intended to
install.
Regulation 514-13-01(ii) requires a
permanent label to be fixed at or near
the point of connection of every main
equipotential bonding conductor to an
extraneous-conductive-part.
Supplementary equipotential bonding
BS 7671 also has requirements for
supplementary equipotential bonding,
which includes installations and
locations of increased shock risk such
as rooms containing a bath or shower,
as shown in Figure 3.
Where supplementary equipotential
bonding is applied in a particular
location within an installation, e.g. a
bathroom, it has the effect of reestablishing the equipotential
reference at that location for all the
exposed-conductive-parts and
extraneous-conductive-parts which
are bonded together locally. This
further reduces any potential
differences that may arise between
any of these parts during an earth
fault.
current-using
equipment
origin
of
installation
L
<2 m
Uf
N
main earthing
terminal
~
– If
R2
If
circuit
protective
conductor (cpc)
E
extraneous-conductive-part
main equipotential
bonding conductor
earth
Figure 1: Illustration of main equipotential bonding
Figure 2: PME supply (TN-C-S system) Schematic of earthing and main equipotential
bonding arrangements. Based on 25 mm2 tails and selection from Table 54G.
Note: An isolator is not always installed by the electricity distributor.
Ceiling
metal
pipe
luminaire
Pull cord switch
Outside Zones
Zone 2
Zone 3
shower
Switch
for fire
Radiant fire
Cord
3.0 m
Zone 1
Zone 2
Shaver
unit
Zone 3
2.25 m
Zone 0
Outside Zones
metal pipes
*
metal waste
Further information.
For more information on earthing
and bonding refer to IEE Guidance
Note 5. Also a new IEE Guidance Note
8 specifically on earthing and
bonding is due to be published shortly
by the IET. exposed-conductive-part
0. 6 m
2. 4 m
* Zone 1 if the space is accessible without the use of a tool.
Spaces under the bath, accessible only with the use of a tool, are outside the zones.
Figure 3: Supplementary bonding in a bathroom - metal pipe installation with soldered joints
providing reliable electrical continuity
IET Wiring Matters | Spring 07 | www.theiet.org
9/3/07
12:31 am
Page 14
DOWNLIGHTERS
WM_Spring07.qxd
14
Installing Downlighters Safely
Following these steps should ensure a
downlighter will not pose a risk of fire
due to overheating
1. Only use downlighters that conform
to BS EN 60598, the British Standard
for Luminaires and ensure the
requirements of BS 7671 are met
2. Follow the Manufacturer’s
instructions
3. Ensure the requirements of the
Building Regulations (England and
Wales) are met
4. Provide space around the
downlighter
5. Fit the correct lamp
1. EN 60598. International Standard
EN 60598 specifies general
requirements for luminaires
incorporating electric light sources for
operation from supply voltages up to
1 000 V. The requirements and related
tests of this standard cover all aspects
of safety including electrical, thermal
and mechanical in the areas of
classification, marking, mechanical
IET Wiring Matters | Spring 07 | www.theiet.org
construction and electrical
construction.
BS 7671 Requirements for Electrical
Installations requires, in Regulation 511,
that electrical equipment, which includes
luminaires, conforms to an applicable
Standard. Section 422 of
BS 7671 gives requirements for protection
against fire and harmful thermal effects
and Regulation 422-01-02 applies to fixed
electrical equipment such as a
downlighter that, in normal operation,
has a surface temperature sufficient to
cause a risk of fire or harmful effects to
adjacent materials. The Regulation gives
three methods of preventing danger
which are (i) mounting within a suitable
enclosure, (ii) screening or (iii) provision
of sufficient distance from adjacent
material. Refer to the Regulation for full
details.
2. Manufacturer’s instructions. The
manufacturer’s instructions supplied
with the downlighter must be followed.
The Manufacturer may require a
by John Ware
certain amount of space be left around
the back of the downlighter or that the
downlighter must not be covered with
loft insulation, or only lamps of a
particular type and maximum wattage
be fitted or that a fire hood or
intumescent hood be installed.
3. Building Regulations (England and
Wales). The installer must be aware of
the requirements of the Building
Regulations in England and Wales
before installing a downlighter. For
example, before cutting a hole in the
fabric of the building the installer
must ensure that the structural
integrity, fire resistance or other
aspects of the structure are not
compromised. Approved Document B
gives guidance on the precautions to
be taken to inhibit the spread of fire
within a building. Approved Document
A deals with structure and the basic
requirement is that persons installing
electrical equipment must not cut,
drill, chase, penetrate or in any way
interfere with the structure so as to
8/3/07
5:26 pm
Page 15
DOWNLIGHTERS
WM_Spring07.qxd
15
vœœÀ
LœÝÊvˆÌÌi`ʈvʈ˜ÃՏ>̈œ˜
ˆ˜ÃÌ>i`ʈ˜Êۜˆ`
Viˆˆ˜}
Õ“ˆ˜>ˆÀi
Figure 1: Installing a downlighter
F
in the void between a ceiling and an upstairs floor
Lœ>À`Ê>``i`ÊLiÌÜii˜
Viˆˆ˜}ʍœˆÃÌÃ
œvÌʈ˜ÃՏ>̈œ˜
Viˆˆ˜}
Õ“ˆ˜>ˆÀi
Figure 2: Installing a downlighter
F
in a ceiling with a loft space above
cause significant reduction in its load
bearing capacity.
Regulation 4(2) states that, on
completion of electrical installation
work, the building (and parts of the
electrical installations in the building
that were not the subject of work)
should be no worse in terms of the
level of compliance with the other
applicable Parts of Schedule 1 to the
Building Regulations than before the
work was undertaken.
For example, one or more
perforations of a ceiling lining
beneath a floor – made to
accommodate recessed lighting or
similar fittings – may have an adverse
effect on that floor’s performance in
terms of its resistance to fire and
sound penetration. Due regard should
therefore be paid to the guidance in
Approved Documents B and E on
the performance of compartment
floors.
Regulation 4(2) also means that,
when extending or altering an
installation, only the new work must
meet current requirements and there
is no obligation to upgrade the
existing installation unless the new
work would adversely affect the safety
of the existing installation, or the state
of the existing installation was such
that the new work could not be
operated safely, or where there is a
requirement to upgrade imposed by
the energy efficiency requirements of
the Building Regulations.
4. Provide space around the
downlighter. A downlighter can
develop significant heat and sufficient
space must be provided around it.
When installing the downlighter in
the void between the ground floor
ceiling and the upstairs floor, there
should be sufficient space around the
downlighter as illustrated in Figure 1.
The downlighter used must be marked
with symbol: F . Building debris and
other flammable material must be
removed from the void. Cables must be
IET Wiring Matters | Spring 07 | www.theiet.org
8/3/07
5:27 pm
Page 16
DOWNLIGHTERS
WM_Spring07.qxd
16
Dichroic:
Light away from luminaire
Heat back to luminaire
GZ10 holder
Aluminised:
Light and heat away
from luminaire
GU10 holder
GZ10 holder:
Accepts GZ10 & GU10 lamps
GU10 holder:
Only accepts GU10 lamps
Vœœ
cool
Li>“
beam
Figure 4: Cool beam or dichroic lamps forbidden
dichroic
`ˆ … ˆ
reflector
aluminium
 ˆ ˆ
reflector
Figure 3: GU10 and GZ10 bases
secured such that they do not come
into contact with the hot surfaces of
the downlighter.
When installing a downlighter in a
ceiling with a loft space above,
precautions must be taken to ensure
that loft insulation or other material
does not surround or come into
contact with the downlighter.
Installing a board between two joists
as shown in Figure 2 will, and running
the loft insulation over the top of the
board will, in most cases, ensure
sufficient air space around the
downlighter. Once again the
downlighter used must be marked
with the symbol: F .
5. Fit the right lamp. Many
downlighters are designed either for
use with 230 V dichroic lamps fitted
with GZ10 caps or aluminised lamps
fitted with GU10 caps. (See above).
As can be seen in Figure 3 a GZ10
holder will accept lamps having a GZ10
cap and lamps having GU10 cap. A
GU10 holder will only accept lamps
with a GU10 cap due to the chamfer.
A luminaire employing a dichroic
lamp will run hotter than an
equivalent luminaire fitted with an
aluminized lamp.
However, lamps that can be purchased
are GZ10 - dichroic and aluminised and
GU10 - dichroic and aluminised. Use of
dichroic lamps in a luminaire designed
for use with aluminised lamps could
create excessive heat within the
luminaire leading to an unsafe situation
and risk of fire.
The European standard EN 60598
presently caters for this situation by
application, on the luminaire, of a
symbol warning against the use of cool
beam lamps (dichroic) (see Figure 4).
It is recognized that many people
will not know what the above symbol
means nor will they know the
difference between dichroic lamps and
aluminised lamps. To avoid the
occurrence of unsafe situations the
Lighting Association advises its
members to supply only luminaires
suitable for both applications i.e. even
if fitted with a GU10 holder the
luminaire design should accommodate
the additional heat produced by the
possible use of a dichroic lamp. A serious fire occurred in a listed building when a new lamp was fitted in a
downlighter that had not been working for years.
In the attic above, an old oily coat had been thrown down and was partially
covering the non-working downlighter. The heat generated by the new lamp set fire
to the coat and destroyed the upstairs and roof of the property.
IET Wiring Matters | Spring 07 | www.theiet.org
8/3/07
5:28 pm
Page 21
UPS
WM_Spring07.qxd
21
UNINTERRUPTIBLE
POWER SUPPLIES
by Mark Coles
The aim of this article is to give an overview of uninterruptible
power supplies and how to meet the requirements of BS 7671.
What is an uninterruptible power
supply (UPS)?
Fundamentally, an uninterruptible
power supply, or UPS, is a unit which
maintains the electrical supply to a
piece of equipment, or load, following
the failure of the primary source of
supply. The UPS is, therefore, installed
between the source of the electrical
supply and the load.
BS EN 62040-1-1:2003 defines a UPS
as a combination of converters,
switches and energy storage devices
(for example, batteries), constituting a
power system for maintaining
continuity of load power in case of
input power failure.
Static Static UPS systems deliver the
output voltage derived from a stored
source, e.g. a series of batteries
through an inverter.
With a static UPS there will be “no
visible” loss of supply to the load
when the mains supply is lost; the
following, figure 1, shows an example
of the layout of a static UPS system
Rotary Rotary UPS systems consist of
one, or more, electrical rotating
machines to provide the output
voltage, e.g. a generator or multiplesynchronised generators.
Bypass
switch
Electrical
mains supply
UPS
unit
AC output
(Load)
Inverter
Rectifier
Are there different types of UPS?
Fundamentally, there are two
categories of UPS – rotary systems
and static systems.
Under normal circumstances, the
electrical supply can be routed directly
through to the load whilst the rectifier
“rectifies” the a.c. supply to d.c. to
charge the storage batteries.
In the event of loss of the electrical
supply, d.c. from the batteries is
inverted back to a.c. and will supply
the load; the bypass switch opens and
stops the inverted UPS supply from
being routed back to the origin of the
installation. This is known as a passive
standby system.
The bypass switch can be used for
another function. Should the electrical
supply to the installation, or load, be
non-sinusoidal, e.g. the harmonic
content is such that the supply
waveform is no longer considered to be
sinusoidal, the UPS unit may be used as
a “smoothing” device and clean up the
supply for use on sensitive or vulnerable
equipment and critical loads. In reality,
the batteries will be charging whilst
supplying the load. This is known as an
active standby system. Operation in this
mode will also compensate for dips or
surges in the supply.
Static UPS systems are available in
many different sizes, ranging from very
small and simple to very large and
complex. Small and autonomous
systems are available providing circa
1kVA; large UPS units can be paralleled
to provide, in excess of, 1MVA.
Storage
Batteries
Figure 1: Example of a static UPS system
IET Wiring Matters | Spring 07 | www.theiet.org
8/3/07
5:29 pm
Page 22
UPS
WM_Spring07.qxd
22
Bypass
switch
Electrical
mains supply
Diesel
engine
Point of isolation for both supplies
(see Regulation 460-01-02)
AC output
(Load)
Generator
Figure 2: Example of a rotary UPS system
The rotary UPS system generally
sits dormant until it is required.
Control equipment will sense the loss
of mains supply and switch the
installation over to be supplied by the
generator. Usually, there will be a
period of time when the load is
without a supply; this could be a
period of seconds, even minutes,
whilst the prime-mover starts and the
generator attains full speed. This
known as the automatic load transfer
time. Figure 2, shows the layout of an
installation with a back-up generator
or rotary UPS.
The requirements of BS 7671
Isolation and switching
A UPS is a source of energy and, to
comply with Regulation 460-01-01, a
non-automatic means of isolation and
switching should be installed to
disconnect the source from the load.
BS 7671 lists four types of switching –
Isolation, Switching off for
mechanical maintenance, Emergency
switching and Functional switching.
The concepts of isolation & switching
are examined here:
The definition of isolation is:
Isolation
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 or section from every
source of electrical energy.
The definition of a switch is:
IET Wiring Matters | Spring 07 | www.theiet.org
Switch
A mechanical device capable of
making, carrying and breaking
current under normal circuit
conditions, which may include
specified operating overload
conditions, and also of carrying for a
specified time currents under specified
abnormal circuit conditions such as
those of short-circuit. It may also be
capable of making, but not breaking,
short-circuit currents.
Regulation 460-01-02 requires that where
an installation is supplied from more
than one source, a main switch shall be
provided for each source of supply and
a durable warning notice shall be
permanently fixed in such a position
that any person seeking to operate any
of these main switches will be warned
of the need to operate all such switches
to achieve isolation of the installation.
Alternatively, a suitable interlock
system shall be provided.
Characteristics of supply
As with any installation, it is a
requirement of BS 7671 that the nature
of the supply parameters are assessed,
e.g. Ze (Ω) and Ipf (A); UPS systems,
which are a source of supply, are no
exception.
Further, Regulation 551-02-02
requires that the prospective shortcircuit current and prospective earth
fault current shall be assessed for each
source of supply or combination of
sources which can operate
independently of other sources or
combinations.
Protection against electric shock
An important aspect of providing
protection against indirect contact
which can be readily overlooked by the
designer is the need to ensure
satisfactory operation of the relevant
protective device(s) when the
installation, or part thereof, is
energised from a UPS. To be certain
that the requirements of BS 7671 for
protection against electric shock (and
short-circuit) will still be satisfied, the
designer must obtain full information
for the alternative supply and make
the necessary checks of the design,
which will have been based upon the
characteristics of the normal supply
source.
Regulation 551-04-04 requires that
where the conditions for automatic
disconnection of Regulation 413-02
cannot be achieved for parts of the
installation on the load side of the
static inverter, supplementary
equipotential bonding shall be
provided on that side in accordance
with Regulations 413-02-27 and 413-0228. The resistance (R) of the
supplementary equipotential bonding
conductor between simultaneously
accessible exposed-conductive-parts
and extraneous-conductive-parts shall
fulfil the following condition:
R≤
50
I
where: I is the maximum fault current
which can be supplied by the static
inverter alone for a period of up to 5 s.
Further, Regulation 551-04-05 states
that precautions shall be taken or
8/3/07
5:30 pm
Page 23
UPS
WM_Spring07.qxd
23
equipment shall be selected so that the
correct operation of protective devices
is not impaired by direct current
generated by a static inverter or by the
presence of filters.
Protection against overcurrent
Regulation 551-05-01 requires that
where means of detecting overcurrent
of the generating set is provided, this
shall be located as near as practicable
to the generator terminals. A
generator control panel or UPS
equipment may include selfprotection, a feature of which is the
rapid collapse of output voltage to the
load. This will inhibit the operation of
any fault protective device situated
beyond the equipment terminals and
the feature cannot be assumed to
provide a fail-safe operational
arrangement for the user. Safety of
the system as a whole must be
ensured by, if necessary, involving the
equipment supplier.
Earth electrode
Regulation 551-04-03 requires that
protection by automatic disconnection
of supply shall not rely upon the
connection to the earthed point of the
distributor’s network when the
generator is operating as a switched
alternative to a TN system. A suitable
earth electrode shall be provided.
Clause 18.2.1 of BS 7430 states
generator earthing calls for the
provision of an independent earth
electrode. It is necessary that the earth
loop impedance at any point of the
installation is low enough to ensure
operation of the earth fault protection,
and this should be taken into account
when the earth electrode forms part of
the earth fault loop. For independent
earth electrodes associated with the
local earthing of the star point of
generating plant, it is recommended
that the earth resistance should not
exceed 20 Ω.
Supplies for safety services
Safety services, such as fire alarm
systems, sprinkler systems, etc., are
often supplied by UPS systems as loss
of supply to such equipment could
result in loss of life. BS 7671 defines a
safety service as an electrical system
for electrical equipment provided to
protect or warn persons in the
event of a hazard, or essential to their
evacuation from a location.
BS 7671 recognises that UPS
systems may operate in a parallel
configuration. Regulation 566-01-01
requires that protection against shortcircuit and against electric shock
shall be provided whether the
installation is supplied by either of
the two sources or by both in parallel.
Further, Regulation 566-01-02 requires
that precautions are taken to limit
circulation currents, particularly that
of third harmonics or multiples
thereof, in the connection
between the neutral points of
sources.
Harmonic distortion
Static UPS systems may create
harmonics on the sinusoidal
waveform. Other than selecting the
use of low harmonic-producing
equipment, there are two recognised
methods of reducing harmonic
content; install harmonic filters
which are suited to the load of the
UPS or increase the size of the
neutral conductor. Regulation 524-0202 requires that the neutral conductor
is adequately sized to carry the
maximum current likely to flow in it
under normal operating conditions.
IET Wiring Matters | Spring 07 | www.theiet.org
8/3/07
10:19 pm
Page 24
UPS
WM_Spring07.qxd
24
Small systems
Some small UPS systems, circa 1kVA,
can be unearthed and effectively
operate as an electrically separated
system. Note that certain items of
equipment require a reliable
connection to the means of earthing to
operate, i.e. filters within the switchmode power-supplies of personal
computers. Prior to connecting
equipment to a UPS, it must be
ensured that the equipment is suitable
for operation in such circumstances.
Regulation 413-06-03 requires that
where only a single item of equipment
is supplied in this manner, there
should be no connection between the
separated circuit and any other
circuit, or to Earth. The flexible
cable/cord supplying the load, which
is liable to mechanical damage, should
be visible throughout its length. It is
preferred that a separate wiring
system should be used for the
separated circuit (although multicore
cables without magnetic sheath or
insulated conductors in an insulated
enclosure are permitted if the rated
voltage of the cables is not less than
the highest voltage likely to occur and
each circuit is protected against
overcurrent). Every live part of each
separate circuit shall be electrically
separated from all other circuits to a
standard not less than that provided
between input and output windings of
an isolating transformer to BS 3535.
Regulation 413-06-04 requires that no
exposed-conductive-part of the
separated circuit shall be connected to
either the protective conductor of the
source circuit, or to any exposedconductive-parts of any other circuit.
Other considerations
Prolonged loss of supply
In the UK, some areas are more
susceptible to power cuts than others,
particularly rural areas. Should
inclement weather bring down
overhead power lines, for example, the
mains supply could be interrupted for
IET Wiring Matters | Spring 07 | www.theiet.org
quite some time, perhaps days. A static
UPS would not have the capacity to
supply the load for a period of days
but it would, however, provide enough
time to allow back-up of information
and data during the enforced power
outage. This is known as the autonomy
time. If the installation is located in
such an area, a static UPS system
could be used for short term power
loss with a rotary UPS installed to
provide an alternative long-term backup source.
Storage batteries
Static UPS systems are usually
equipped with storage batteries to
meet the power requirements of the
connected load; large loads require
large battery banks. Small UPS
systems may have maintenance free
batteries but large banks will consist
of one of two types of rechargeable
battery, namely, lead-acid or alkaline.
Lead-acid batteries are the most
commonly used rechargeable battery,
they are found in such applications as
cars, motorcycles and electric vehicles.
Note that the correct battery must be
chosen for the particular application.
Alkaline rechargeable batteries,
such as nickel-cadmium, nickel-metal
hydride and lithium ion, are widely
used in small items such as laptop
computers. Large capacity versions of
these cells are now used in transport
and UPS applications.
There are two different types of
lead/acid and alkaline rechargeable
batteries: valve-regulated
(‘maintenance-free’) and vented. In
valve-regulated batteries, any
hydrogen and oxygen produced during
charging does not escape but is
converted back into water. Water
cannot be added to these batteries as
they do not need topping up. In
contrast, vented batteries allow any
hydrogen and oxygen produced to
escape into the surrounding
atmosphere and they require regular
topping up with water. However,
installation, commissioning and
maintenance should only be carried
out by a competent person trained in
this line of work and experienced with
the particular equipment. Sources of further information
1) BS 7671: 2001 (2004) Requirements
for electrical installations
2) BS 7430: 1998 Code of practice for
earthing.
3) The Selection and operation of
uninterruptible power supplies, HES
107/1996. http://www.hse.gov.uk/
research/crr_pdf/1996/CRR96107.pdf
4) Using electric storage batteries safely.
http://www.hse.gov.uk/pubns/
indg139.pdf
5) BS EN 88528-11:2004 Reciprocating
internal combustion engine driven
alternating current generating sets Part 11: Rotary uninterruptible
power systems - Performance
requirements and test methods
6) BS EN 62040-3:2001 Uninterruptible
power systems (UPS) - Part 3:
Method of specifying the
performance and test requirements
Thanks to Uninterruptible Power
Supplies Ltd. for the images used
http://www.upspower.co.uk