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DSS/ 007 / 010 – Code of Practice for the
Protection and Control of HV Circuits
1
Purpose
This code of practice details the network protection philosophy covering the High Voltage (HV)
networks. There is a legal obligation to provide protection to meet the requirements of the Electricity
Safety, Quality and Continuity Regulations 2002 and the Electricity at Work Regulations 1989. This
code of practice is driven by our Quality of Service (QoS) performance, specifically reduction in number
of faults, reduction in number of customers per fault and improvements in fault restoration times. The
document should be considered in conjunction with IMP/001/912 – Code of Practice for the Economic
Development of the HV system.
This document supersedes the documents detailed in Section 3.13
2
Scope
This document applies to both rural and urban HV circuits (main line and spur protection) with a
nominal operating voltage of between 1kV and 22kV. All other voltages are covered by DSS/007/001 –
Code of Practice for the Protection of High Voltage Networks (TS1).
This code of practice shall be applied to all HV system development including new connections, system
reinforcement and asset replacement. It is not intended to apply this code of practice retrospectively.
3
Policy
3.1
Assessment of Relevant Drivers
The key internal business priorities relating to the protection and control of HV circuits are:

Employee Commitment – achieved by developing a safe HV system to ensure that members of the
public and employees are not exposed to risks to their health as far as reasonably practicable;

Financial Strength – contribute towards maximising IIS rewards and minimising operating costs;

Customer Service – achieved by reducing the impact of fault incidents on customers

Operational Excellence – achieved by reducing the potential number of customer interruptions and
customer minutes lost
The external business drivers relating to the protection of HV systems are detailed in the following
sections.
3.1.1 Requirements of the Electricity Safety, Quality and Continuity Regulations
The Electricity Safety, Quality and Continuity (ESQC) Regulations 2002 impose a number of obligations
on Northern Powergrid, mainly relating to quality of supply and safety. All the requirements of the
ESQC Regulations that are applicable to the protection of HV circuits shall be complied with and the
Northern Powergrid distribution systems shall be designed to comply with these requirements.
Regulation 3 states that “generators, distributors and meter operators shall ensure that their equipment
is … so constructed, installed, protected (both electrically and mechanically), used and maintained as
to prevent danger, interference with or interruption of supply, so far as is reasonably practicable”. This
code of practice details the processes to be followed in order to ensure that the network is designed to
limit the number of people that are affected by a fault and hence comply with these requirements.
Regulation 6 states that “A … distributor shall be responsible for the application of such protective
devices to his network as will, so far as is reasonably practicable, prevent any current, including any
leakage to earth, from flowing in any part of his network for such a period that part of his network can
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no longer carry that current without danger.” This code of practice details the process to be followed in
order to ensure that the protection fitted to HV circuits meets these requirements.
3.1.2 Requirements of the Electricity at Work Regulations 1989
Regulation 5 of the Electricity at Work Regulations 1989 states: “No electrical equipment shall be put
into use where its strength and capability may be exceeded in such a way as may give rise to danger”
and places obligations on the business relating to the safety of plant and equipment used on the
distribution system. It requires that plant and equipment is designed and operated within the limits of its
capability.
3.1.3 Requirements of Distribution Licences
The Distribution Licenses held by Northern Powergrid contain a number of conditions to be complied
with which are relevant to system design. In particular, Standard Licence Condition 24 requires the
distribution system to be planned and developed to a standard not less than that set out in Engineering
Recommendation P2/6 (2006) – Security of Supply. This CoP requires that the HV distribution system
is designed to at least the standard required by ER P2/6.
In addition, Standard Licence Condition 21 refers to the Distribution Code and requires that “the
licensee must take all steps within its power to ensure that the Distribution Code in force …remains a
code approved by the authority that complies with … the requirement that the Distribution Code … must
be designed so as to permit the development, maintenance and operation of an efficient, co-ordinated,
and economical system for the distribution of electricity.” The code of practice requires that the HV
distribution system is designed in line with the Distribution Code and therefore in an efficient, coordinated and economical manner as required by the Electricity Act.
3.2
Key Requirements
The general objective in developing the protection for the HV network is to ensure the safety of public
and staff and to minimise QoS indicators through the management of the number of customer
interruptions, the number of customers affected and the restoration time of faults within the constraint of
the customers’ willingness to pay,
This code of practice helps ensure that all HV circuits are protected in a manner which:
3.3

Prevents, as far as reasonably practicable, danger to members of the public and staff;

Optimises network security and availability; and

Satisfies all other relevant obligations.
Background
3.3.1 Design Policy
The objective of this code of practice is to ensure safety and contribute to achieving our strategy to
deliver an economic level of reliability in line with the willingness to pay of our customers. This is
achieved through the management of the four variables:

Number of interruptions;

Customers per fault;

Restoration time, and

Cost.
Different design approaches can be used to improve these base measures and this code of practice
aims to detail the approaches to be used.
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3.3.2 Design Guidelines
This code of practice shall be read in conjunction with the relevant Engineering Recommendations and
other CE Electric UK documents including the following:

Code of Practice for the Economic Development of the HV System (IMP/001/912)

Code of Practice for the Application of Lightning Protection (IMP/007/011)

Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001)

Code of Practice for the Setting of Protection and Associated Equipment (TS16/17)
(DSS/007/007)
Design of HV circuits should be performed using the following design process:
Collect circuit
information
Identify main and spur lines, customer numbers and their distribution
Design protection for main line
- design source protection (see section 3.4):
- position pole mounted autoreclosers (section 3.4.1)
Design protection for spurs (see section 3.6):
- position auto sectionalising links (section 3.6.1)
- design required fuse protection (section 3.6.2)
Complete additional circuit protection:
- ferreoresonance assessment (section 3.7)
- position lightning protection (section 3.8.1)
- position fault passage indicators (section 3.8.2)
A summary of the process to be followed can be found in Appendix 1.
3.3.3 Network Configuration
The main purpose of the HV system is to distribute electricity in localised urban and rural areas in an
economic, efficient, safe and secure manner, meeting the needs of electricity supply to customers
currently and likely to be connected to it. The HV system supplies HV/LV substations, larger demand
and generation customers at HV, systems owned and operated by Independent Distribution Network
Operators (IDNOs) and is designed in line with IMP/001/912 – Code of Practice for the Economic
Development of the HV system.
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Urban HV systems normally comprise underground cables and looped ground mounted HV to LV
substations. Rural HV systems normally comprise of a mixture of underground cables, ground
mounted HV to LV substations and overhead lines with teed pole mounted HV to LV transformers.
The Northern Powergrid HV systems predominantly operate at 20kV and 11kV, with a small proportion
of lower voltage (5.25kV to 6.6kV) assets associated with specific customer supplies or small pockets
of old industrial areas of urban centres.
The majority of 20kV systems serve more sparsely populated rural areas, typically Northumberland
whereas the 11kV systems serve more urban areas such as Tyne and Wear, Durham, Cleveland and
most parts of Yorkshire.
3.4
Main Circuit Protection
Details of the process and requirements for source circuit breaker protection can be found in:

Code of Practice for the Protection of High Voltage Networks (TS1) (DSS/007/001)

Code of Practice for the Setting of Protection and Associated Equipment (TS16/17)
(DSS/007/007)
Protection of the main circuit should be considered first. The protection of spurs should be excluded in
this section and protection of the main line should be implemented in the following manner.
3.4.1 Auto-Reclosers
3.4.1.1
Functional Specification
The functional specification for auto-reclosers shall be in line with the latest version of NPS/001/009 –
Technical Specification for 11kV, 20kV and 33kV Pole Mounted Auto-Reclose Circuit Breakers.
The protection functional requirements are as follows:

4 trips to lockout with any combination of tripping characteristics.

Over Current / Earth Fault (OC/EF) and Sensitive Earth Fault (SEF) sequences to be
independent of each other.

Selectable Sequence Co-ordination.

Individually adjustable Dead Times

Adjustable reclaim times

Sequence co-ordination.

Cold Load Pickup.

Magnetising Inrush Restraint.

Voltage Measurement on the incoming terminals with provision for an external Voltage
Transformer (VT) for measurement of voltage on the load terminals.
All auto-reclosers fitted onto main line circuits shall be equipped with remote control.
3.4.1.2
Design Rules
A summary of the design process can be found in Appendix 1.
Source Protection Zone
All HV circuits with greater than 1km of overhead line (including spurs) shall be equipped with autoreclose facilities either at the source or at another circuit breaker (see below).
The functionality and operation of the primary breaker should then be considered. If the breaker has a
multi shot capability, the use of this should be used.
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Note that consideration should be given to disabling auto-reclose on the primary breaker and installing
an auto-recloser at the beginning of the first main overhead line section where the following applies:

the first leg from the Primary substation consists entirely of underground cable which feeds
more than 50 customers;

the primary substation breaker does not have multi shot capability.
Primary CBs and Pole Mounted Auto Reclosers (PMARs) being used to provide the source zone autoreclose facilities should be set in accordance with section 3.4.1.3
Further Protection Zones - Location of PMARs
In determining possible locations for a PMAR the following should be considered:

the current capacity and breaking capacity of the auto-recloser; and

staff access requirements.
In general the provision of a PMAR on an overhead line distribution circuit will result in a reduction in
the number of customers per fault and the fault restoration times on that circuit. The improvement in
performance is a function of the location of the PMAR, and is greatest at a particular point in the line.
However, it may not be desirable to locate the recloser at this point, for example accessibility may be
difficult, and so it is necessary to estimate the benefit of a number of alternative locations and select the
most advantageous.
The performance benefit is a function of many parameters with wide tolerances, including fault rates,
outage times, customer distribution etc. To simplify the criteria for provision of PMARs, guideline rules
have been developed to describe the number and location of reclosers that should be installed on a
given circuit depending upon its make up and configuration. The objectives include:

No customer should experience more than four interruptions of three minutes or longer per
annum.

No more than 500 customers between remote control points (note that all main-line PMARs
are to be fitted with remote control).

No more than 2000 customers per circuit.

A maximum of three auto-reclose devices should be used in series at 11kV and four at
20kV.

No more than 200 customers between switching points.

Additional auto-reclose protection zones should be installed where the potential annual
gain in customer interruptions (CI) and customer hours lost (CHL) as calculated by
approved software meets the following requirements (calculated for DPCR5 IIS incentive
rates):
(CI saved) + 2*(CHL saved) > 100
Any situation outside of these parameters should be referred to the Design Manager for guidance.
3.4.1.3
Application of Settings
Selecting protection settings on overhead line circuits is a balance between speed of operation to
reduce the amount of energy released at the point of fault and the deliberate insertion of time delays to
create grading points and so reduce the number of customers affected by a fault isolating only the part
of the network necessary to clear the fault. There is also the need to ensure that spur-line fuses and
sectionalisers operate as intended and a delay may be required for this.
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For new overhead lines that are of short or medium length the deliberate delaying of protection to
create grading points shall be employed but should be restricted to one grading point. Standard
settings will be adopted on short and medium length lines.
Where an existing short or medium line is being redeveloped the minimum time delay policy set out in
the preceding paragraph shall be implemented.
For new lines that fall into the long category the protection shall be graded using both time and current
settings. Opportunity shall be taken whenever an existing long line is being refurbished or reconfigured
to implement this protection policy.
In the case of overcurrent and earth faults, fuses and sectionalisers should, where possible,
discriminate with up-stream protection. It is accepted that in some cases 100% discrimination can not
be achieved and in most cases discrimination with the SEF settings we normally use will not be
possible.
General
Where the protection in use is currently instantaneous, fault sequencing should not be used however,
the facility to employ this in the future should be included to allow for its use should the current drivers
on quality of supply change.
Where sequencing is currently employed in conjunction with
instantaneous settings with the intention of minimising the number of transient interruptions seen by
customers, then this should be maintained. Where auto-reclosers are connected in series and delayed
protection is enabled then fault sequencing should be used on the recloser with the time-delay settings.
Due to the different protection characteristics available on each of the reclosers currently in use and on
those proposed for purchase, only units of the same fault interruption type and fitted with configurable
relays having suitable characteristics shall be installed in series.
Overcurrent Settings
These should be set in accordance with DSS/007/007.
Earth Fault Settings
These should be set in accordance with DSS/007/007.
Note that earth fault settings should be a minimum of 80 amps and discriminate with any fuses and
sectionalisers (20% margin on actuating current) installed in the zone.
Sensitive Earth Fault Settings
All HV circuits greater than one span of overhead line shall be equipped with SEF protection.
The operational policy for try-in post a confirmed SEF protection operation is currently under review at a
national level in conjunction with the HSE and therefore the auto-close arrangements shall continue to
be consistent with the operational procedure applied by the control centre for each circuit.
Where the control room operational procedure is to wait at least 30 minutes prior to try-in of the circuit
then the SEF protection shall be set to inhibit a re-closer sequence so that SEF operation causes a trip
and lockout of the circuit breaker.
Where the control room operational procedure is to initiate a try-in immediately after an earth fault
operation then the SEF protection shall be set to initiate an autoreclose sequence. Where a separate
SEF sequence is available it should be selected to 2 trips to lockout.
Short and medium line lengths
For protection purposes a line will be classified as short if the phase to phase or three phase fault
current at the most remote point on the feeder is 800A or greater at 11kV or 1600A or greater at 20kV
and of medium length if the fault current at the most remote point on the feeder is 400A or greater at
11kV or 800A or greater at 20kV. Other than the setting for the source Inverse Definite Minimum Time
(IDMT) relay which will be calculated, settings at all other relaying points will selected from a standard
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list approved by the Technical Services Manager and issued in DSS/007/007 – The setting of protection
and associated equipment.
The operating sequence for all source zone auto-reclose protection equipment on the HV system will
be 2 instantaneous trips followed by one IDMT trip to lockout with the IDMT set to a 0.7 second
clearance time in accordance with DSS/007/007. Where there is one pole mounted circuit breaker in
series with the source circuit breaker the pole mounted circuit breaker will be set to 4 instantaneous
shots to lock out. Where there is a second PMAR in series with the first, the first auto-reclose circuit
shall be set to 4 instantaneous shots to lockout with a 120millisecond delay and sequencing selected
for each trip. The second most remote PMAR shall be set at 4 instantaneous trips to lock out. Any
further down stream grading points will be achieved through the use of sectionalisers with different
pulse settings to trip i.e. grading shall be achieved by trips to lockout. Diagram 1 below illustrates the
basic configuration for the maximum number of circuit breakers in series.
3 Stage Auto-Reclose Scheme - Diagram 1
Ground Mount Auto
Reclosing Circuit
Protection
2 Instantaneous
Pole Mounted
Auto-Recloser
Pole Mounted
Auto Recloser
Protection
4 Definite Time
Each 120millisec
Delay and sequencing
Protection
4 Instantaneous
No Delay
SEF 16 Amp
7.5 Second Delay
SEF 16 Amp
5 Second Delay
Protection
3 Instantaneous
Opens in third
Dead Time
1 IDMT
SEF 20 Amp
10 Second Delay
Sectionaliser
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Long Lines
Long lines where the phase to phase or three phase fault current at the most remote point from the
source is below 400A at 11kV and below 800A at 20kV will have each protection point individually
calculated to ensure that the customers at the end of the lines are adequately protected. Additional
protection points may be added to provide further sectionalisation where this is considered
economically beneficial. Each additional protection point will require increasing the time delay of the
instantaneous settings of the relevant protection by 120milliseconds to ensure grading. The maximum
permitted time delay of the instantaneous protection at the source circuit breaker is 360millisec. Where
time delays are used in conjunction with instantaneous settings protection sequencing will also be
used.
Auto-Reclose Sequences
The dead time and reclaim time for auto-reclose relays at source protection points with solenoid or
motor charged circuit breakers will be set as detailed below:
For multishot relays
Dead Time
10.0 seconds
Reclaim Time
7.5 seconds
The dead time and reclaim time for pole mounted auto-reclose relays not at source protection locations
will be set as detailed below:
For multishot relays
Dead Time
5.0 seconds
Reclaim Time
10.0 seconds
The Use of Magnetising Inrush Settings
Magnetising inrush restraint settings will not normally be applied at protection points where
instantaneous protection is activated however the use of instantaneous protection settings may cause
problems when attempting to restore supplies after a transient fault due to high transformer
magnetising inrush currents in some networks. Where problems are identified due to the transformer
capacity on a section of network the magnetising inrush restraint feature shall be activated.
Network conditions where consideration should be given to applying a magnetising inrush restraint
setting are:PMARs with a 400A setting will have a magnetising inrush restraint setting applied if the installed
transformer capacity is above 5MVA.
PMARs at positions in the network with lower fault levels and a 200A setting will have a magnetising
inrush restraint setting applied if the installed transformer capacity is above 2.5MVA.
Notes:

Gas-filled Vacuum Recloser (GVR) PMARs do not have facility for selecting a magnetising
inrush restraint setting.

All MVA values relate to summated transformer capacity not load.
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Arc Suppression Coils
Arc Suppression Coil (ASC) earthing works in conjunction with the existing overhead line protection.
The coil will minimise the fault current for all single phase to earth faults whilst it is not bypassed. If the
fault current disappears within bypass time no other action is initiated and the supply will not be
disconnected, resulting in customers experiencing fewer interruptions and an improved quality of
service. After a set period of time, the coil will be bypassed which will allow the protection to deal with
interruptions caused by permanent single phase to earth faults and interruptions due to all other types
of faults.
During the time a single-phase to earth fault is on the system and before the ASC is bypassed most
faults will fully displace the system neutral. This will cause a large unbalance in the capacitive leakage
to earth between the three phases. Standard earth fault relays (i.e. non-directional) will detect this
unbalance as earth fault current. Normal earth fault settings are usually too high to be affected by this
unbalanced current however, SEF settings can be in range and care must be taken to set the bypass
time within SEF operating time where this occurs.
3.5.1.1
Functional Specification
The functional specification for the installation of new arc suppression coils shall be as follows:










3.5.1.2
An automatically tuned ASC with associated controller. The HV network connected to the
primary substation determines the coil size required. The actual coil rating chosen will be
50% greater than the initial requirement, in order to future-proof the site and allow for
operational switching.
Shunt circuit breaker with auto-opening and auto-closing protection scheme.
Standby earth fault switching scheme as required to compensate for the lower earth fault
levels with a single Neutral Earthing Resistor (NER).
Fitting of disturbance recorders as necessary.
Replacement / addition of neutral earthing resistors as necessary. Liquid filled resistors in
a poor condition will be replaced with dry type resistors.
Protection panel for housing of controller and associated protection scheme
Four isolators to allow for maintenance of the ASC equipment and transformer outages
Balancing of single phase spur lines as necessary to reduce the standing coil voltage to an
acceptable level.
SCADA alarm of coil operation
SCADA control of by-pass CB.
Design Rules
A summary of the design process can be found in Appendix 1.
ASCs are currently installed to cover a substantial part of the overhead line network. When any
changes are made to feeders out of a site which contains an ASC, then the impact of the changes on
the ASC should be considered.
ASCs will be installed at selected worst performing primary substations. These investments are driven
purely by quality of service performance. Installation is only beneficial at sites where the feeders are
made up from a high percentage of overhead line (i.e. cable forms typically no greater than one third of
the network). The performance of the feeders out of the sites will be assessed and the decision made
to apply ASC earthing to those sites which have the highest number of overhead line faults and the
greatest length of normally connected overhead lines.
The expected benefits should then also be considered. As the more effective sites are equipped with
these installations, the per site benefits can reasonably be expected to decrease. The utilisation of
ASC earthing is expected to provide a 10% reduction in permanent faults on overhead lines and to
reduce short interruptions by 50% at the selected substations.
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Spur Protection
Where economical, all spurs, apart from those in the final zone, need to be individually protected at
their point of connection to the main line.
Major spur – 5km or more of overhead line. Major spurs should be protected by auto-reclosers
wherever possible provided this does not compromise the main line grading points.
Medium spur – Spurs between 0.5km and 5km in length. Medium spurs should be protected by an
sectionaliser unless prospective fault currents coupled with clearance times are too high for the spur
conductors.
Minor spur – Spur with less than 0.5km of overhead line. The fault risk associated with these is
relatively small and it is difficult to justify the costs of retrofitting protection to existing sites. Therefore
where there are no existing links, minor spurs should be left solid. Where solid links are currently fitted
and in new locations, minor spurs should be protected by fuses unless they are in the final auto-reclose
section where an sectionaliser should be used. During an overhead line rebuild the opportunity should
be taken to protect all minor spurs with fuses or sectionalisers as appropriate.
The following table shows the appropriate protection to be used at the main line connection point.
Spur Type
Source A/R Zone
Middle A/R Zone
Final A/R Zone
Major Spur
A/R
A/R
3 shot A/S
Medium Spur
2 shot A/S
2 or 3 shot A/S
2 or 3 shot A/S
Minor Spur
2 shot A/S or fuse
2 shot A/S or fuse
2 shot A/S
*Check fault currents and clearance times are acceptable for the conductor sizes in use. If not suitable
for the use of sectionalisers then fuse, or exceptionally, a recloser should be used.
Diagrams presenting an overview of the protection to be applied to spurs can be found in Appendix 4.
Consideration should then be given to adding further discrimination on major and medium spur lengths.
3.6.1 Auto-Sectionalising Links
3.6.1.1
Functional Specification
The functional specification for auto-sectionalising links shall be in line with the latest version of
NPS/001/032 – Technical Specification for 11kV, 20kV, Pole Mounted Auto-Sectionalising Links
(ASLs).
With regard to the protection functional requirements, these are as follows:

ASL’s are designed to recognise a pre-determined sequence or fault currents and then, during
a period when the upstream protective device is open, to disengage and drop-out to the
isolating position.

All actual sectionalisers deviate to varying degrees from the ideal characteristics required of a
sectionaliser. The deviations can take the form of requiring a pre-charge from load current to
operate correctly or requiring the fault current to be on for a minimum time. Any such
deviations will be determined at the time of tendering to ensure that the devices remain suitable
for the locations where they are to be fitted.

The ASL’s shall have load current ratings available of between 15 and 112 Amps.

ASL’s shall have a reclaim time of 25 to 30 seconds

ASL devices are required to be capable of either two or three shot operation.
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Design Rules
A summary of the design process can be found in Appendix 1 and a guide to calculating the required
settings found in Appendix 5.
The location of the sectionalisers on an overhead circuit selected for auto-reclosing shall comply with
the following guidelines but, when applying them, consideration must be given to local conditions
affecting access. Note that additional rules may be introduced from time to time to cover any
restrictions as to where the types of sectionalisers being purchased can be used.

Normally spurs with more than 5km of overhead line shall be protected by an auto-recloser
unless they are in the final main-line protection zone in which case a 3-shot sectionaliser
should generally be used

Normally spurs between (0.5 km) and 5km shall be protected by an sectionaliser.

In new locations, where solid links are currently fitted and during a major rebuild of the
main line normally spurs less than 0.5 km long shall be fused however consideration shall
be given to the installation of a sectionaliser if the number of connected customers exceeds
50. Note that minor spurs in the final protection zone should be protected by a 2-shot
sectionaliser as fuses will not operate with all instantaneous shots on the auto-recloser.

Where spurs less than 0.5 km are solidly connected to main lines then these should
generally be left solid. However, the opportunity should be taken during an overhead line
rebuild of the main line to apply protections to all short spurs.

Fuses should not normally be used to protect spurs containing TSG’s.

Where a spur is located in the first protection zone from the primary, sectionalisers should
be limited to a maximum of two counts.

Care must be taken to ensure that the maximum fault current and prospective clearance
times are compatible with the conductor sizes in use. Particularly in the source protection
zone this may limit the use of sectionalisers and fuses will have to be used instead.

Care must be taken to ensure that devices are only used within their fault rating e.g. new
sectionalisers are limited to 10kA maximum through fault current for one second.

Care must be taken to ensure that the manufacturer’s recommendations on the
discrimination between actuating currents and minimum auto-reclose relay settings are
maintained., e.g. the AB Chance 25 amp (load current) rated sectionaliser has an operating
current of 1.6 times this, i.e. 40 amps and the recommended minimum recloser setting is 50
amps

Details of the fault rating of small overhead conductors are shown in the section on design
rules in Appendix 1.
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3.6.2 Fuses
3.6.2.1
Functional Specification
The functional specification for pole mounted expulsion fuses shall be in line with the latest version of
NPS/001/004 – Technical Specification for 11kV, 20kV and 33kV Pole Mounted Expulsion Fuses and
Solid Links.
3.6.2.2
Design Rules
A summary of the design process can be found in Appendix 1 and a guide to calculating the required
settings found in Appendix 5.
3.6.2.3

Fusing shall be used in the first two protection zones only.

Normally spurs of less than 0.5 km length that are solidly connected to a main line should
be left solid.

Spurs of less than 0.5 km in length that are fitted with solid links shall be fused however
consideration shall be given to the installation of a sectionaliser if the number of connected
customers exceeds 50.

Spurs containing TSG’s should not be fused unless this is unavoidable because of fault
level and conductor size factors..

Any existing fuses protecting ground mounted transformers shall be replaced with pole or
ground mounted circuit breakers.

Care must be taken to ensure that devices are only used within their fault rating.
Application of Settings
The maximum fuse size should be restricted to 50A for minor spurs to allow for grading to successfully
occur. The fuse size should be matched to the size of transformers being protected.
3.7
Ferroresonance
Ferroresonance is a term used to describe series resonance in an alternating current circuit consisting
of non-linear inductance (the transformer) and capacitance (the HV underground cable). In a
distribution system, this condition is most likely to occur when lengths of underground cable are
connected in an overhead system. The ferroresonant state can occur when energised and deenergised phases occur in the same section of line leading to the generation of overvoltages. When
this occurs, damage to both customers’ equipment and the distribution system can result.
Ferroresonance is also possible during system fault conditions where blown fuses leave the system
supplied single phase.
Tests indicate that the condition is most likely to occur during de-energising.
approximately 5% load is required to dampen ferroresonance.
However, only
The failure of surge arresters under these conditions is well known. It should be noted that with gapped
surge arresters this effect could go unnoticed as the resonant overvoltage would be below the
sparkover level of an 11kv surge arrester. However, in the case of metal oxide surge arresters some
failures will occur. When a line is refurbished then gapped or porcelain surge arresters should be
replaced. The failure mode with a polymeric housed surge arrester of proven short circuit capability is
not a violent explosion but nevertheless injury could be caused if personnel are near to the failing
arrester.
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Some conditions when ferroresonance can occur are:

During single phase switching operations.

Operation of a single-phase over-current protective device.

When one or two phases of a three phase circuit become open-circuited (blowing of a fuse,
breaking of a line conductor, etc).
The increase in the number of incidents of ferroresonance reported is due to modern trends in circuit
parameters:

Increased use of underground cables in rural systems.

More three phase transformers on rural systems.

Highly seasonal three phase loads resulting in some transformers operating at very light
load.

Improved material design of transformers (lower excitation currents).
Although measures to protect against ferroresonance should be considered, these measures will
normally only be applied where the line is new build or significant alterations have occurred.
3.7.1 Design Rules
A summary of the design process can be found in Appendix 1.
Where possible, avoid using HV underground cable in rural situations. Ferroresonance will not occur
where the system being energised or de-energised comprises entirely of HV overhead lines.
Where HV underground cable exists or is installed, the following rules shall be adopted:

Where there are more than 10 transformers in the circuit to be energised (or de-energised),
ferroresonance is unlikely to be a problem and no action is required.

Where there are less than 4 transformers in the circuit, each transformer should be equipped
separately with solid links at the transformer position. These links will be removed before the tee is
disconnected and replaced after the tee is re-connected.

Where there are between 4 and 10 transformers in the circuit, a 3 phase switching device, for
example an isolator, should be installed at the tee off position in addition to any spur protection
(sectionalisers). In exceptional circumstances (eg fault level above the rating of sectionalisers) an
auto-recloser can be used in this position. In this case this should be set to instantaneous
protection with the number of shots to lockout set to mimic a three or two shot sectionaliser as
appropriate.
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3.7.2 Critical Cable Lengths to avoid Ferroresonance
Critical cable lengths have been calculated for both the 11kV and 20kV systems. The following tables
indicate the maximum aggregate cable length in section without the risk of Ferroresonance:
Critical 11 kV Cable Lengths to Avoid Ferroresonance
Aggregate
transformer
capacity
5
10
15
25
50
100
200
315
500
750
1000
1250
1600
0.04
0.06
1.2
2.4
3.6
6.0
12.0
23.8
47.7
75.0
119.0
178.7
238.0
298.0
381.2
1.1
2.1
3.3
5.5
11
22.2
44.4
69.9
111
166.6
222.1
276.6
355
Cable Lengths in Metres
0.1
95 mm
0.83
1.6
2.5
4.2
8.3
16.7
33.4
52.6
83.5
125.3
167.0
208.0
267.0
0.9
1.7
2.5
4.0
8.0
16.0
32.0
50.0
75.0
115.0
155.0
194.0
248.0
185 mm
300mm
0.7
1.4
2.0
3.4
6.6
12.0
23.0
35.0
55.0
90.0
125.0
156.0
200.0
0.5
1.1
1.7
2.7
5.4
11.0
22.0
34.0
52.0
82.0
110.0
137.0
175.0
185 mm
300mm
0.21
0.42
0.6
1.04
1.98
3.6
6.9
10.5
16.5
27.0
37.5
46.8
60.0
0.15
0.33
0.51
0.81
1.62
3.3
6.6
10.2
15.6
24.6
33.0
41.1
52.5
Critical 20 kV Cable Lengths to Avoid Ferroresonance
Aggregate
transformer
capacity
5
10
15
25
50
100
200
315
500
750
1000
1250
1600
0.04
0.06
0.36
0.72
1.08
1.8
3.6
7.14
14.31
22.5
35.7
53.61
71.4
89.4
114.36
0.33
0.63
0.97
1.64
3.23
6.5
13.0
20.5
32.5
48.7
65
81.1
104.1
Cable Lengths in Metres
0.1
95 mm
0.25
0.48
0.75
1.26
2.49
5.01
10.04
15.78
25.05
37.5
50.1
62.4
80.1
0.27
0.51
0.75
1.2
2.4
4.8
9.6
15.0
22.5
34.5
46.5
58.2
74.4
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Table for XLPE triplex cable
Aggregate
transformer
capacity
5
10
15
25
50
100
200
315
500
750
1000
1250
1600
3.8
11kV cable lengths in Metres
95 mm
185 mm
300 mm
2.8
5.7
8.5
14.2
28.3
56.7
113
179
283
425
567
709
907
2.2
4.4
6.7
11.1
22.2
44.4
89
140
222
333
444
555
710
20kV cable lengths in Metres
95 mm
185 mm
1.8
3.7
5.5
9.1
18.3
36.5
73
115
183
274
365
457
585
1.2
2.5
3.7
6.2
12.4
24.9
50
78
124
186
249
311
398
1
2
3
5
9.9
19.9
40
63
100
149
199
249
318
Ancillary Circuit Protection
3.8.1 Lightning Protection
Lightning protection should be applied in line with IMP/007/011 – Code of Practice for the Application of
Lightning Protection.
3.8.2 Fault Passage Indicators
3.8.2.1
Functional Specification
The functional specification for fault passage indicators shall be in line with the latest version of
NPS/001/014 – Technical Specification for overhead line Fault Passage Indicators (FPIs).
With regard to the protection functional requirements, these are as follows:


Initially the FPI’s shall be supplied preset as follows:o
Default current settings 21A
o
Delay 150 mS
As an example the Bowden Pathfinders should be preset to:
o
SEF and Instant 21 A
o
Inrush 98 mS, Instant 148 mS, IDMT 150 mS
o
3 hour flash time and 60 seconds line trip
o
Type D (transient fault reporting enabled)
o
The above corresponds to setting codes of 99-16-207-2

The FPI’s shall be designed such that earth fault sensitivity levels can be amended from its
default value to a user entered value. If fitted with GSM/GPRS communications, this
amendment shall be achievable via the remote signalling system.

FPI’s shall be designed to discriminate between magnetic inrush current and fault current.
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Design Rules
A summary of the design process can be found in Appendix 1.
Locations for fitting pole mounted FPIs:

As near as possible to manual switching points

As near as possible to both ends of cable sections

Major spurs that are not protected with an auto-recloser – as near as possible to the point of
connection with the main line.

To ensure that no more than 500 customers are between monitored points

To ensure that there is no more than 3km of main line without an FPI
Pole mounted FPIs should not be installed on poles:

With underground cables

With transformers

With double circuit lines

At tee-off positions

Closer than 300m to 275 – 400kV lines

Closer than 150m to 132kV lines

Closer than 100m to 66kV lines

Closer than 50m to 33kV lines

Closer than 100m to 25kV overhead conductors
All pole-mounted FPIs used on main lines shall be capable of indicating back to Network Management
System (NMS) that they have operated.
3.9
Urban Circuits
Urban circuits are generally all underground (up to 1 km of overhead line is permitted) and will be
equipped with standard overcurrent and earth fault protection in accordance with the Code of Practice
for the Protection of High Voltage Networks (TS1) (DSS/007/001). Settings should be as the Code of
Practice for the Setting of Protection and Associated Equipment (TS16/17) (DSS/007/007). Circuits
with in excess of one span of overhead line will be equipped with SEF protection. Auto-reclose is not
normally enabled. Where practical, all overhead spurs on urban circuits shall be fused.
On circuits normally supplying in excess of 2,000 customers consideration should be given to installing
intermediate protection when this can be easily achieved, e.g. when replacing switchgear at a
distribution substation and there is space available for a feeder CB. The economics of doing this
should be checked against the interruption performance benefit that can be obtained using approved
software tools.
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3.10 Assumptions
It is anticipated that all calculations regarding design of protection for HV circuits will be carried out
using GROND. A set of standard assumptions for use in these calculations can be found in Appendix 2
of this document.
Several aspects of the policy may be subject to subsequent review. This includes section 3.7 on
Ferroresonance and the parts of section 3.4.1.2 relating to SEF protection.
Improved interruption performance has been evaluated using the IIS incentive rates for the DPCR5
period (2010/11 to 2014/15). This policy will be reviewed should there be a significant change in these
values after DPCR5.
3.11 Implementation and Monitoring
The main accountabilities for implementation and monitoring of this policy lie with:
Designation
Responsibility
Design Manager
The manager appropriate to the part of the network where
the policy is being applied, who is accountable for the
implementation of this policy.
Will ensure responsible
persons are appointed to implement this policy
Protection and Technical Services
Manager
The manager appropriate to the part of the business where
the policy is being applied, who is accountable for the
implementation of this policy.
Will ensure responsible
persons are appointed to implement this policy
Policy Manager
The manager who is accountable for the derivation of this
policy
Policy Production Manager
Responsible for monitoring compliance of all business
divisions with this policy
3.12 Planned Policy Review
This policy shall be proposed for review on a biennial basis or at any time when external or internal
influences drive a change in policy e.g. a change in legislation, or learning points from the initial
implementation stage.
The following responsibilities shall apply to policy control and review:
Designation
Responsibility
Publication Manager
Responsible for issuing a quarterly report to the Policy
Production Manager (or representative) detailing policies
scheduled for biennial review within the next six months
Policy Production Manager
Responsible for assessing the continued applicability of this
company policy and for amending this document and
communicating any changes in policy.
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3.13 Superseded Documentation
This document supersedes the following documents, all copies of which should be destroyed.
Ref
Version
Title
DD.554
DSS/007/010
Dec 2000
Draft (Sept 1999)
Code of Practice for the protection of HV rural lines
The protection of 11kV overhead lines
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References
4.1
External Documentation
4.2
Page
19 of
Reference
Title
Version and date
EAW Regulations
The Electricity at Work Regulations 1989
ESQC
Regulations
The Electricity Safety, Quality and Continuity
Regulations 2002
Statutory Instrument
1989 No 635
As amended at the
date of issue of this
policy. Statutory
Instrument 2002 No.
2665
Internal documentation
Reference
Title
Version and date
IMP/001/912
Code of Practice for the Economic Development of
the HV System
The Protection of High Voltage Networks (TS1)
The Setting of Protection and Associated Equipment
(TS16/17)
Standard for the Application of Tele Auto-Reclose to
the 66kV and 33kV Systems
Code of Practice for the Application of Lightning
Protection
Technical Specification for 11kV, 20kV and 33kV
Pole Mounted Auto-reclose Circuit Breakers
Technical Specification for 11kV, 20kV and 33kV
Pole Mounted Expulsion Fuses and Solid Links
Technical Specification for Overhead Line Fault
Passage Indicators
Technical Specification for 11kV, 20kV Pole Mounted
Auto Sectionalising Links
An Investment Appraisal for Arc Suppression Coil
Earthing
An Investment Appraisal for Under Performing HV
Feeders
An Investment Appraisal for the Protection of 11kV
Overhead Lines
V1.0; Jan 2009
DSS/007/001
DSS/007/007
DSS/007/009
IMP/007/011
NPS/001/009
NPS/001/004
NPS/001/014
NPS/001/032
05/61
13808
12109
V2.0; Mar 2000
V2.0; Jun 2001
V1.0; Mar 2001
V1.0, Aug 2006
V2.0, Feb 2008
V2.0, Dec 2007
V2.0, Jan 2008
V1.0 Dec 2010
V3.0, Mar 2005
V5.0, Mar 2008
V5.0, Dec 2008
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Definitions
Term
Definition
ASC
ASL
CoP
EF
FPI
GVR
HSE
HV
IDMT
Interruption
NER
NMS
OC
PMAR
QoS
Arc Suppression Coil
Automatic Sectionalising Link
Code of Practice
Earth Fault
Fault Passage Indicator
Gas-filled Vacuum Recloser
Health and Safety Executive
High Voltage (between 1,000 and 22,000 Volts)
Inverse Definite Minimum Time
A supply interruption with a duration in excess of three minutes
Neutral Earthing Resistor
Network Management System
Over Current
Pole-mounted auto-recloser
Quality of Service; referring to interruptions performance in terms of CI and
CML
Sensitive Earth Fault
A supply interruption with a duration of less than three minutes
Triggered Spark Gap
Voltage Transformer
SEF
Short Interruption
TSG
VT
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Authority for issue
6.1
Author
Page
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I sign to confirm that I have completed and checked this document and I am satisfied with its content
and submit it for approval and authorisation.
Sign
Claire Thomas
6.2
Asset Management Engineer
Claire Thomas
Date
16/12/2010
Policy Sponsor
I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and
submit it for approval and authorisation.
Sign
Andrew Ellam
6.3
Policy & Risk Manager
Andrew Ellam
Date
16/12/2010
Technical Assurance
I sign to confirm that I am satisfied with all aspects of the content and preparation of this document and
submit it for approval and authorisation.
Sign
Programmes & Strategic
Design Leader
Protection & Technical
Services Manager
Glen Hodges
Jim Paine
6.4
Date
Glen Hodges
17/12/2010
Jim Paine
16/12/2010
CDS Assurance
I sign to confirm that this document has been assured for issue on to the DBD system.
Sign
Sean Johnson
6.5
CDS Administrator
Sean Johnson
Date
16/12/2010
Approval
Approval is given for the content of this document.
Sign
Mark Nicholson
6.6
Head of System Strategy
Mark Nicholson
Date
16/12/2010
Authorisation
Authorisation is granted for publication of this document.
Sign
Mark Drye
Director of Asset Management
Mark Drye
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Date
22/12/2010
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Appendix 1 – Summary of Design Process
1. Collect circuit information and identify main and spur lines, customer numbers and distribution.
2. Design main line protection:
2.1. Source Circuit Breaker
2.1.1.
If protected circuit has less than 1 km of overhead, no auto-reclose and settings as per TS1;
else
2.1.2.
Source CB does not have auto-reclose facilities. Fit PMAR as close as possible to the start of
the first section of overhead line; else
2.1.3.
Source CB should be set for auto-reclose unless there is a section of underground cable
greater than 500m in length or supplying grater than 50 customers prior to the first section of
overhead line. If so then consider placing the auto-recloser at the first section of overhead line
rather than at the primary.
2.2. Position additional auto-reclosers on the circuit in line with the following guidelines using the approved
software modelling tool to assist in achieving the optimal solution:

No customer should experience more than four faults per annum

Aim for no more than 500 customers between remote control points

Aim for no more than 2000 customers per circuit

Aim for a maximum of 200 customers between switching points

In general, a maximum of three auto-reclose devices should be used in series at 11kV and four at
20kV
2.3. Consider the impact of the changes on any arc suppression coil fitted at the primary substation.
3. Design spur protection
3.1. Define the type of spur (major, medium or minor) and refer to table in section 3.6 for detailed
protection requirements
3.2. If spur is longer than 0.5 km then protect using a sectionaliser as long as the fault rating at the point of
installation is below the rating of the conductor. Small conductor within the first protection zone may
need to be protected by a fuse where fault ratings are exceeded. See appendix 5 for the detailed
rules on sectionaliser locations and calculation of sizes.
3.3. If spur supplies more than 50 customers then consider protecting using an sectionaliser
3.4. If spur contains any triggered spark gaps then avoid using fuses unless fault level and conductor size
considerations force the use of fuses.
3.5. If spur is located in the final protection zone then check the benefits of fitting a sectionaliser. Where
there are a low number of customers in the final protection zone then it will be unlikely that spur line
protection can be justified.
3.6. Otherwise protect with a fuse.
4. Consider protection of transformers.
5. Ensure circuit is protected against ferroresonance (where new build or a significant alteration only)
6. Design lightning protection in line with IMP/007/011 and re-check position against 3.4
7. Position fault passage indicators as required.
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Appendix 2 – Summary of Modelling Assumptions for use in calculations
Fault rates to be applied when using GROND application for CI & CML performance figures.
All studies should be carried out using the Light, Medium & Heavy fault performance figures for both
Underground cable and Overhead Lines as follows:
Light
Medium
Heavy
Underground
0.05
0.05
0.05
Overhead
0.1
0.08
0.07
All transient fault rates per km are set at zero.
Value per CI and CHL to be set as follows:
Type
Value (£)
CI
5
CHL
10
Restoration times to be set as follows:
Switching
Time (Mins)
Manual
80
Manual/EFI
80
Tele
10
Auto
2
Recloser
1
Repair times to be set as follows:
Type
Time (Mins)
Underground
480
Overhead
480
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Appendix 3 – Approved software modelling tools
Functionality
Approved Software Tool
Network configuration and plant position
GIS
Load flow and fault currents
DINIS
Predicted Fault frequency and protection position
planning
GROND
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Appendix 4 – Overview of protection arrangements to be used on spurs.
No PMAR, AR at Source
Fuses (≤50A)
ASLs 2 shot
CB
AR 2 shot + IDMT
One PMAR, AR at Source
Fuses (≤50A)
ASLs 2/3 shot
ASLs 2 shot
CB
AR 2 shot + IDMT
PMAR
4 shot
More than One PMAR, AR at Source
Fuses (≤50A)
ASLs 2 shot
CB
2 shot Inst +
IDMT
ASLs 2/3 shot
PMAR
4 shot
PMAR
4 shot
PMAR
4 shot
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More than One PMAR, AR at Source. Sectionalizer in Main Line
Fuses (≤50A)
*ASLs 2 shot
CB
AR 2 shot +
IDMT
ASLs 2/3 shot
PMAR
4 shot
PMAR
4 shot
ASLs 2 shot
Sectionalizer
3 shot (PMAR with
AR disabled)
Additional PMAR to allow Source CB AR to be Disabled.
Fuses (≤50A)
ASLs 2 shot
CB IDMT
PMAR
2 shot +
IDMT
ASLs 2/3 shot
PMAR
4 shot
* Check compatibility of sectionalisers with spur-line fault current and clearance time capabilities.
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Appendix 5 – Calculation of Sectionaliser and Fuse Ratings.
Sectionaliser Location
Care needs to be taken where sectionalisers are proposed to be installed particularly within the first
protection zone to ensure that any small section conductor is adequately protected.
Where the primary protection is instantaneous and two shot sectionalisers are to be installed to protect
small section conductor of 0.0225Cu or less then sectionalisers must not be installed where the fault
level exceeds 5kA. All locations above this level must be protected by either a pole mounted recloser
or fuse.
Where the primary protection is time delayed then sectionalisers must not be installed where the fault
level is above the following levels for the type of conductor being protected:
0.017Cu – Fault level must be below 2kA in all zones
0.0225Cu – Fault level must be below 2kA in all zones
50AAAC – Fault level must be below 4.5kA in all zones
0.05Cu – Fault level must be below 5kA in all zones
Where sectionalisers are proposed in areas where the fault level is in excess of the conductor rating
then consideration should be given to altering the primary protection on the circuit from IDMT to
instantaneous.
Calculation of Sectionaliser Rating
The pick-up current of the sectionaliser (Actuating Current) must be greater than twice the full load
current of the circuit. The full load current of the circuit will be diversified where 2 or more transformers
are connected.
Full load current must take account of three phase and single phase connections along the tee under
consideration. The maximum full load current can be calculated either:


Summation of the ‘Transformer Rating Amp’ values in tables below; or
Summation of the separate three phase and single phase transformer ratings and using
 11kV – 5.3A per 100kVA (3-ph) and 9.1A per 100kVA (1-ph)
 20kV – 2.9A per 100kVA (3-ph) and 5.0A per 100kVA (1-ph)
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Example
Full load current of tee = 40Amps
Diversified current = FLC/1.5
Minimum Actuating Current
40/1.5 = 26.6 Amps
26.6 x 2 = 53.3 Amps
Next highest sectionaliser actuating current is 57.6A (from table in DD554)
Sectionalisers must be specified on with continuous rating and number of shots (2 or 3) therefore a 36A
rated unit is required (from table below)
Actuating Current (A)
Continuous Rating (A)
179.2
112
89.6
56
64
40
57.6
36
48
30
40
25
32
20
Transformer single phase currents for use in calculating sectionaliser and fuse sizes are shown in the
following tables:
Transformer Rating
Voltage
Phases
kVA
Amp
Linegear 2000 Rating
Pole
Ground
Mounted
Mounted
Substation
Substation
Three
Phase
25
50
75
100
150
200
300
315
500
800
1000
1.3
2.6
4.0
5.3
7.9
10.5
15.8
16.5
26.5
42.4
53.0
25 A
25 A
25 A
25 A
25 A
25 A
Single
Phase
Three
Wire
25
50
75
100
150
167
200
225
375
580
2.3
4.6
6.8
9.1
13.6
15.2
18.2
20.5
34.1
52.8
25 A
25 A
25 A
25 A
40 A
40 A
40 A
Single
Phase
Two Wire
5
15
25
0.5
1.4
2.3
25 A
25 A
25 A
11 kV
10 A
12 A
15 A
25 A
25 A
40 A
65 A
65 A
15 A
15 A
25 A
25 A
30 A
40 A
65 A
LV Fuse Rating
Pole
Ground
Mounted
Mounted
Substation
Substation
100 A
100 A
160 A
200 A
315 A
400 A
100 A
160 A
315 A
315 A
400 A
400 A
400 A
100 A
160 A
200 A
CAUTION! - This document may be out of date if printed
160 A
200 A
250 A
400 A
400 A
500 A
630 A
630 A
315 A
400 A
400 A
400 A
400 A
500 A
630 A
Document reference DSS/ 007 / 010
Version:- 1.1
Date of Issue:- Jan 2011
Transformer Rating
Voltage
Phases
kVA
Amp
Linegear 2000 Rating
Pole
Ground
Mounted
Mounted
Substation
Substation
Three
Phase
25
50
75
100
150
200
300
315
500
800
1000
0.7
1.5
2.2
2.9
4.3
5.8
8.7
9.1
14.5
23.2
29.0
25 A
25 A
25 A
25 A
25 A
25 A
Single
Phase
Three
Wire
25
50
75
100
150
167
200
225
333
375
580
1.3
2.5
3.8
2.0
7.5
8.4
10.0
11.3
16.7
18.8
29.1
25 A
25 A
25 A
25 A
25 A
25 A
25 A
Single
Phase
Two Wire
7.5
15
25
0.4
0.8
1.3
25 A
25 A
25 A
20 kV
6A
10 A
10 A
15 A
15 A
25 A
40 A
40 A
10 A
10 A
10 A
15 A
15 A
15 A
25 A
40 A
Page
30 of
LV Fuse Rating
Pole
Ground
Mounted
Mounted
Substation
Substation
100 A
100 A
160 A
200 A
315 A
400 A
100 A
160 A
315 A
315 A
400 A
400 A
400 A
100 A
160 A
200 A
CAUTION! - This document may be out of date if printed
160 A
315 A
315 A
400 A
400 A
500 A
500 A
630 A
315 A
400 A
400 A
400 A
400 A
500 A
500 A
630 A
30