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IMP/001/911 Code of Practice for the Economic
Development of Low Voltage Networks
1.
Purpose
The purpose of this document is to state Northern Powergrid’s policy for the economic development of low
voltage networks. This document also describes key assumptions made in the preparation of this document and
sets out to provide clarity across the organisation with respect to evolving approaches and the alignment of
legacy practices across the group.
Recognising that distribution networks are developed primarily through new connections, the general objective
of this policy is to obtain a robust, economical and efficient network, taking into account the initial capital
investment, network losses, maintenance and operation costs over the life of the asset.
This policy helps to ensure the company achieves its requirements with respect to the Electricity Act 1989 (as
amended) (the Act), the Electricity Safety, Quality, and Continuity (ESQC) Regulations 2002 (as amended), the
Electricity at Work (EAW) Regulations 1989, the Distribution Licence conditions and the Distribution Code, by
laying out the way in which Northern Powergrid will develop efficient, co-ordinated and economical LV
networks.
The following documents have been superseded by this policy, all copies of which should be withdrawn from
circulation.
Ref
Version and Date
Document
IMP 001 911
V2.0 August 2012
Code of Practice for the Economic Development of Low Voltage
Networks.
2.
Scope
2.1.
General
The policy covers:

Northern Powergrid (Northeast) Ltd and Northern Powergrid (Yorkshire) plc, the licensed distributors of
Northern Powergrid, and to the providers of connections to those networks, including but not limited to
IUS;

LV network design including functional specifications for secondary (HV/LV) substations, network topology,
fusing, and basic network parameters such as earth loop impedance, but excluding equipment standards;
and

all new, reinforcement and replacement work on all mains and services.
This document does not establish, but instead co-ordinates with policy on:

earthing;

HV network design;

circuit ratings, both overhead and underground;
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
substation construction, including precautions against fire and flooding;

consideration of connection arrangements, including those for multi-occupancy premises and to embedded
‘independent’ networks, which are covered in IMP/001/010 – Code of practice for standard arrangements
for customer connections; and

customer metering arrangements.
This document establishes engineering policy; separate commercial policies will apply. This policy establishes
requirements for the development of networks, without considering how the associated costs would be shared
between connection and use of system charges.
2.2.
ICPs and IDNOs
Since this policy specifies the way in which Northern Powergrid’s distribution networks shall evolve, it applies to
all the providers of adopted extensions to those networks, whether IUS or an Independent Connections Provider
(ICP). Generally, extensions not compliant with this policy shall not be adopted by Northern Powergrid.
Connections to IDNO networks are addressed in IMP/001/010 – Code of practice for standard arrangements for
customer connections.
3.
Policy
3.1.
Assessment of Relevant Drivers
The key internal business drivers relating to LV design policy are:

safety,

financial; and

quality of supply.
The external business drivers relating to the economic development of the LV network and application of this
code of practice are detailed in the following sections.
3.1.1 Requirements of the Electricity Act 1989 (as amended)
The Electricity Act 1989 (as amended by the Utilities Act 2000) (‘the Act’) lays down the core legislative
framework for Northern Powergrid operations as a distributor. Specifically, it gives force to the ESQC
Regulations 2002, and in section 9 creates the key obligation to develop and maintain an efficient, co-ordinated
and economical system of electricity distribution. Discharge of this obligation shall be supported in this
document by providing guidelines on efficient development of the wider network.
3.1.2 Requirements of The Electricity Safety, Quality and Continuity (ESQC) Regulations 2002 1
The ESQC Regulations 2002 (as amended) impose a number of obligations on the business, mainly relating to
quality of supply and safety. All the requirements of the ESQC Regulations that are applicable to the design of
low voltage networks shall be complied with, specifically:
1
st
This includes The ESQC (Amendment) Regulations 2006 (No. 1521, 1 October 2006) and The ESQC (Amendment)
th
Regulations 2009 (No. 639, 6 April 2009)
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Reg. No
3(1)(a)
Text
…distributors…shall ensure that their equipment
is sufficient for the purposes for and the
circumstances in which it is used; and
Application to this policy
This requirement is absolute, and not subject
to a ‘reasonably practicable’ test. This policy
specifies that
 cables and transformers should not be
exposed to a duty beyond their
capability. This will be achieved by
defining permissible ratings; or
 equipment should not be exposed to
short circuit current in excess of its
capability. This will be achieved by
defining maximum prospective short
circuit current (PSCC).
3(1)(b)
…distributors…shall ensure that their equipment
is so constructed…as to prevent danger…or
interruption of supply, so far as is reasonably
practicable
In this policy, the prevention of danger will be
achieved through fusing (reg. 6) and defining
where the use of overhead lines and polemounted substations is permissible.
The prevention of interruption of supply, so far
as is reasonably practicable, will be achieved
by defining LV system topology (reg. 23(1))
and permissible limits on teed HV connections.
6
23(1)
24(4)
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 that part of his
network can no longer carry that current
without danger.
…[the] network shall be: (a) so arranged; and (b)
so provided, where necessary, with fuses or
automatic switching devices, appropriately
located and set; as to restrict, so far as is
reasonably practicable, the number of
consumers affected by any fault in [the] network
Unless he can reasonably conclude that it is
inappropriate for reasons of safety, a distributor
shall, when providing a new connection at low
voltage, make available his supply neutral
conductor or, if appropriate, the protective
conductor of his network for connection to the
protective conductor of the consumer’s
installation.
This will be achieved by defining permissible
fuses.
This will be achieved by defining low voltage
network topology in terms of permissible
numbers of customers per feeder.
This will be achieved by requiring new
networks to be developed in accordance with
Engineering Recommendation G12.
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Text
A distributor shall not give his consent to the
making or altering of [a connection from [that]
distributor's network to…another distributor's
network], where he has reasonable grounds for
believing that (a) the…other distributor's network fails to
comply with British Standard Requirements
or these Regulations; or
(b) the connection itself will not be so
constructed, installed, protected and used
or arranged for use, so as to prevent as far
as is reasonably practicable, danger or
interruption of supply.
Application to this policy
This will be achieved by applying this policy to
all network developments, including those
made by another distributor.
Unless otherwise agreed in writing…the voltage
declared in respect of a low voltage supply shall
be 230 volts between the phase and neutral
conductors at the supply terminals
This will be achieved by providing guidelines
for permissible voltage drop on low voltage
networks.
Note that this does not oblige Northern
Powergrid to check each installation, but
instead to take reasonable steps where there
are reasonable grounds for concern
3.1.3 The Health and Safety at Work etc. Act 1974
Section 2(1) states that ‘It shall be the duty of every employer to ensure, so far as is reasonably practicable, the
health, safety and welfare at work of all his employees’. Section 3(1) also states that ‘It shall be the duty of
every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that
persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or
safety’.
This is addressed in this policy through prescribing permissible:

substation locations;

overhead line locations; and

fusing.
3.1.4 The Environmental Protection Act 1990
Section 80 of the Environmental Protection Act 1990 makes it an offence not to comply with an abatement
order, and requires that such orders must be raised when a local authority is satisfied that a statutory nuisance
exists. Section 79 of that Act defines statutory nuisance, including noise and vibration.
This is addressed in this policy through prescribing permissible arrangements for substation construction.
3.1.5 Requirements of Distribution Licences
The Distribution Licences contain a number of conditions that are relevant to network design.
In particular, Standard Licence Condition 24 requires the distribution network to be developed to a standard not
less than that set out in Engineering Recommendation P2/6 (2006) – Security of Supply. Generally, LV networks
will fall within Class of Supply A (less than 1 MW), and therefore do not require alternate infeeds.
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The licences also provide for an incentive scheme for headline performance derived from the Information and
Incentives Scheme (IIS). Broadly, between 1 April 2010 and 31 March 2015, each customer interrupted costs
Northern Powergrid £5, and each hour that a customer remains interrupted costs a further £10.
This is addressed in this policy through minimising the numbers of customers per feeder on new networks and
providing for cost-effective interconnection on existing networks.
3.1.6 Requirements of the Distribution Code
Each Distribution Licence holder is required to hold, maintain and comply with the GB Distribution Code.
The Distribution Code covers all material technical aspects relating to connections to, and the operation and use
of, the Distribution Systems of the Distribution Network Operators. The Code is prepared by the Distribution
Code Review Panel and is specifically designed to:

permit the development, maintenance and operation of an efficient, co-ordinated and economical system
for the distribution of electricity; and

facilitate competition in the generation and supply of electricity.
The Distribution Planning and Connection Code specifies the technical and design criteria and the procedures
which shall be complied with in the planning and development of the distribution systems. It also applies to
Users of the distribution systems in the planning and development of their own systems in so far as they affect
the Northern Powergrid systems.
This code sets out principles relating to the design of equipment and its operating regime. Equipment on
Northern Powergrid systems and on Users’ systems connected to the distribution systems shall, where
appropriate, comply the standards laid out in Annex 1 of the Code.
The Distribution Code also gives force to a number of Engineering Recommendations. Those particularly
relevant to this policy, in terms of defining our obligations, are:

Engineering Recommendation P2/6: Security of supply

Engineering Recommendation G5/4 Planning levels for harmonic voltage distortion and the connection of
non-linear equipment to transmission and distribution systems in the United Kingdom.

Engineering Recommendation P26/1 The estimation of the maximum prospective short circuit current for
three phase 415V supplies.

Engineering Recommendation P28 Planning limits for voltage fluctuations caused by industrial, commercial
and domestic equipment in the United Kingdom.

Engineering Recommendation P29 Planning limits for voltage unbalance in the United Kingdom for 132kV
and below.
3.1.7 Retrospection
This policy shall be applied in full when designing new LV feeders, whether from new or existing substations.
However, as some of the requirements identified above are qualified by ‘so far as is reasonably practicable’,
there is no requirement to pro-actively review the entire network solely for the purpose of bringing it up to the
standards laid out here for new build.
In a small number of cases special connection arrangements, which are not strictly in accordance with the
documents policy, may be more appropriate and can be considered where there are benefits to both Northern
Powergrid and customers. Any such deviations shall be agreed with the Design Manager.
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Key Policy Requirements
LV distribution networks should be developed in an efficient and cost effective manner to deliver electricity to
the LV supply terminals of our connection customers whilst meeting the requirements of the Act and the
Licence.
The general objective in developing LV networks is to obtain a simple and robust minimum overall cost network,
taking into account the initial capital investment, system losses, maintenance and operational costs over the life
of the asset. Any development of LV networks should seek to improve the quality and reliability of the supply
we provide and to reduce potential customer minutes lost.
This policy is written to ensure that all new LV network developments are made in such a way as to:

prevent danger to the public and staff, and to minimise environmental pollution and statutory nuisance;

discharge the obligation under section 9 of the Act, and specifically to have due regard to future
requirements and network performance;

optimise network security and availability;

optimise power quality experienced by all connected customers; and

satisfy all other relevant obligations.
3.3
Basic LV Design Principles
3.3.1
Voltage
To deliver statutory supply voltage levels of 230/400V +10%/-6%, LV networks shall be designed not to exceed a
voltage drop on the main under normal running conditions, from a 240 V nominal base, of:

6% within 15 km of the primary at 11 kV, or within 30 km at 20 kV; or

4% beyond 15 km of the primary at 11 kV, or beyond 30 km at 20 kV.
With high penetrations of distributed generation, designers will need to consider the potential for voltage rise.
It may be necessary to reduce LV network impedance to stay within statutory supply voltage levels of 230/400V
+10%/-6%.
3.3.2
Voltage Disturbing Loads
LV design studies involving network disturbance analysis shall be carried out before connection of abnormal
loads, such as motors, welders and harmonic producing equipment. Reference shall be made to Engineering
Recommendations G5/4 (harmonics) and P28 (flicker) during such studies. Note that those recommendations
provide guidance as to the types and sizes of customer load that may be connected without detailed analysis.
3.3.3
Neutral and Earth Loop Impedance
3.3.3.1 Design Maximum Values of Loop Impedance
There are two paths for loop impedance that must be considered, specifically:

phase to neutral, which affects voltage fluctuation (flicker); and

phase to earth, which affects both fusing of the general network and the quality of the earth provided to
customers.
On CNE networks, these paths are the same, so the two issues will be considered together.
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It should also be noted that this section considers absolute maximum values for loop impedance. It is also
essential that fusing and phase to earth loop impedance be co-ordinated, and it may therefore be necessary to
limit network impedance in order to secure adequate fault clearance times.
Phase to neutral loop resistance on new networks should not exceed 250 mΩ including the transformer, main
and service, in accordance with Engineering Recommendation P5. To provide for the loop resistance of a typical
20m service, which is 50 mΩ, mains laid in advance of providing services should be designed to a maximum of
200 mΩ through transformer and main.
For many years LV networks were designed to a phase to neutral loop impedance limit of 400 mΩ. There is no
intention of investing significant capital on these existing networks to reduce loop resistance.
Although Northern Powergrid now has an obligation under ESQC regulation 24(4) to provide an earth when
providing a new connection (i.e. the first electric line, or the replacement of an existing electric line, to one or
more consumer's installations) at low voltage, the impedance of that earth is not specified. The ESQC
regulations, and their guidance notes, do not state a value.
Engineering Recommendation P23 gives nominal phase to earth loop impedance values to be provided to
customers for indicative purposes (800 mΩ for SNE and 350 mΩ for PME/PNB networks); it does not specify a
maximum value. Engineering Recommendation P23 is not a planning standard. It explicitly provides only
‘typical maximum values’ that can be quoted to customers in the absence of a specific assessment.
There is a clear difference between ‘typical’ and ‘maximum design’ values. There is no clear need to set a design
value for SNE services other than the 800 mΩ typical value quoted in Engineering Recommendation P23.
Conversely, Northern Powergrid already has a large proportion of CNE-capable networks designed to 400 mΩ
(rather than the 350 mΩ typical value quoted in Engineering Recommendation P23).
This gives maximum design values for single-phase services of:
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3
CNE service:
Phase-neutral and phase-earth loop
impedance
SNE service:
Phase-neutral loop impedance
SNE service:
Phase-earth loop impedance
250 mΩ
400 mΩ
-
400 mΩ
-
800 mΩ
As noted earlier, these values may have to be reduced to meet the fusing requirements.
These figures may be quoted to ICPs as ‘maximum design values’. Customer enquiries will generally involve the
‘typical maximum values’ discussed in Engineering Recommendation P23, which shall be quoted in such
circumstances.
3.3.4
LV Fault Levels
Equipment shall be specified to the following fault levels for to accommodate infeed from the higher network
and LV-connected generation are:

18 kA for single-phase connections up to 100 A, consistent with BS 7657;

30 kA for other connections; and

35.5 kA for equipment at general network substations.
These figures may be quoted to ICPs and customers as ‘maximum design values’, i.e. the performance standards
to which they should specify their equipment.
Typically, maximum values of fault infeeds are:

18 kA for single-phase connections up to 100 A, consistent with BS 7657;

22 kA for other connections; and

26 kA at substation LV busbars.
In the absence of site-specific assessment, and particularly in the absence of distributed generation, these
figures may be quoted to customers as ‘typical maximum values’, i.e. the likely infeed against which they should
perform their protection calculations.
2
including extensions of existing networks originally designed to the current 250 mΩ standard
3
these values are intended to be applied primarily to modifications of existing networks designed to the former 400 mΩ
phase-neutral loop impedance. They should not be applied to significant modifications to networks designed to the current
250 mΩ phase-neutral loop impedance: that is, this should not be seen as a relaxation permitting distributors to be
extended beyond the 250 mΩ limit
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Further detail is provided in Appendix A.2.
3.3.5
Design Loads and Equipment Capacity
3.3.5.1 Design Loads
For new connections, projected demand will be provided by the developer, which must include provision for
increased consumption over the life of the property. This will generally range from 0% for heating to 20% for
general domestic load over a notional 20-year window. Note that these values apply to individual properties.
The purpose of this allowance is to ensure that networks are designed to serve the foreseeable future needs of
those customers, not to make provision for as yet uncertain further connections. While designers shall have
regard to future network development, it shall be achieved through explicit additional provision.
For larger industrial/commercial customers, projected demand will be taken as agreed capacity at 0.9 power
factor.
The developer’s figures will be validated when assessing connection designs.
Pending detailed load research, the following indicative figures (which include a provision for increased
consumption over the life of the property) may be used when assessing demand on existing networks and also
when verifying the projected demand of a new development:
Customer type
ADMD
DEBUT
General domestic
2 kW (day)
6000 kWh URMC
0.5 kW (night)
Storage heaters
2 kW + 10% of installed heating (day)
900 kWh/kW restricted
2 kW + 60% of installed heating (night)
Direct-acting space heating (DASH)
1 kW + 50% of installed DASH load
no direct equivalent
Other electric heating
1 kW + 100% of installed load
no direct equivalent
These figures represent ultimate design load, allowing for organic growth. They will therefore be higher than
measured values. The DEBUT figures have been selected to give the equivalent demand as the ADMD method,
reflecting the fact that the load curve has evolved since the production of ACE 105.
DEBUT inherently corrects for small customer numbers. The equivalent can be provided by using the formula of
12 kW + (number of premises * ADMD).
3.3.5.2 Substation Transformer Capacity
4
To secure compliance with Engineering Recommendation P2/6, no transformer greater than 1000 kVA shall be
used for general networks. Larger units shall be used only to supply individual customers from dedicated
substations.
4
Engineering Recommendation P2/6 permits the use of the cyclic rating of a 1000 kVA transformer when serving groups
of customers.
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Maximum transformer size may also be limited by the need to constrain fault levels on networks with significant
amounts of embedded generation.
When assessing the rating of a new transformer, it shall be sized according to the developer’s estimate of
demand and consequential losses on the network, taking into account:

diversity between customers and feeders;

credible future development;

additional load from adjoining substations interconnected at LV, that might be picked up under fault
conditions;

the cost of losses; and

both cyclic capability and the impact of enclosure in accordance with IEC 354
For economic development of LV networks, the magnitude of iron and copper losses must be set against the
value of losses as quantified by the regulatory framework. On current equipment specifications, and under the
current settlement, this shows that the ‘economic’ rating is an ultimate design loading of 115% of nameplate
capacity for typical domestic load curves and transformers up to 1000 kVA.
This analysis also shows that 16 kVA single-phase pole-mounted transformers are uneconomic and shall
therefore not be used.
The 20% allowance for organic demand growth is similar to the likely additional transformer capacity above the
nameplate rating, as derived from IEC 354, and hence the initial network demand should not be designed to
exceed 95% of transformer nameplate rating.
When assessing the need to reinforce an existing substation to accommodate new demand, the capability of
existing transformers, particularly those serving industrial and commercial customers, shall each be taken on
their merits. According to BS 7735 (which incorporates IEC 354):
o

nameplate rating is based on a continuous load at 20 C ambient;

cyclic loading can allow uprating by up to 15% for typical domestic load curves;

each 1 C increase (decrease) in ambient temperature requires a 1% de(up) rating; and

enclosure adds about 5 C to effective ambient, and poor ventilation adds another 5 C.
o
o
o
These factors can have a material impact on the capacity of a transformer, and consideration shall be given to
these issues when assessing the capability of a transformer. The following examples illustrate the potential
impact:
o
o

domestic cyclic load with a winter peak (assumed 5 C external temperature, plus 5 C for the effects of
enclosure) allows up-rating to 125% of nameplate; and

continuous load with summer peak and poor ventilation (assumed 25ºC external temperature, plus 10 c for
the effects of an unventilated enclosure) requires derating to 85% of nameplate.
o
3.3.5.3 LV Mains
LV mains shall be designed and selected to meet the peak load requested by the customer, load growth and
credible future connections. Capabilities are laid out in the cable and overhead lines application guides.
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For economic development of LV networks, the magnitude of fixed and variable losses must be set against the
value of losses and the capital efficiency incentive provided under the regulatory framework. On current
2
equipment specifications, and under the current settlement, this shows that 185 mm cables are viable over
only a limited range of loadings. That is, the ‘economic’ rating is significantly lower than thermal capability until
2
the highest sizes are reached. Therefore, networks shall be laid out using 300 mm mains other than for short
2
tail-end spurs (e.g. cul-de-sacs), where 95 mm cable shall be used.
The network shall be laid out so as not to exceed the maximum permissible values for:

voltage drop;

permissible number of customers;

phase-neutral loop impedance; and

fusing, such that a fault at the end of the service will be cleared within 60 s.
In some circumstances, it may be found that the fuse necessary for 60 s clearance is below anticipated load
current, although real networks would generally be constrained by voltage limits before fusing limits are
reached. In such cases, then either loop impedance shall be reduced (e.g. by applying larger transformers) or
load shall be reduced by splitting feeders in the vicinity of the substation.
3.3.5.4 LV Services Termination Equipment
Distribution service termination equipment shall be designed and selected to meet the peak load requested by
the customer; potential for future load growth shall be discussed with the customer before design work starts.
The incoming side and busbars of three phase multi-way distribution units supplying multiple domestic
customers in one building shall be sized using an appropriate ADMD. Diversity should not be applied when
these units supply multiple commercial premises.
Further information is provided in IMP/001/010 – Code of practice for standard arrangements for customer
connections.
3.4
Standard LV Distribution Plant
Approved equipment shall be used throughout.
requirements.
3.4.1
This section will therefore deal only with functional
Substations
Substations shall normally be ground-mounted. Pole-mounted substations, whether part of an overhead line or
fed from an underground cable (totem pole or Inverted Pole Equipment (IPE)) shall be used only where an
overhead line would be acceptable on safety and amenity grounds. This includes open countryside up to the
edge of settlements, but excludes sites bounded by development.
To facilitate further development, at least one outgoing way per 150 kVA of nameplate transformer capacity is
required for ground-mounted substations. Pole-mounted substations equipped with 200 and 315 kVA
transformers shall generally be fitted with two outgoing ways. In addition, provisions for the connection of a
mobile generator to the LV circuits shall be provided.
To minimise the duration of interruptions, all ground-mounted substations of 315 kVA and above shall be fitted
with two fault-making load-breaking line switches or equivalent.
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LV Underground Cables and Overhead Lines
3.4.2.1 Underground Mains
LV underground mains shall be selected from approved three phase CNE cables and installed in accordance with
NSP/002 – policy for the installation of power cables. A PME network shall be established in accordance with
Engineering Recommendation G12.
When extending or replacing existing LV mains having a 4th core and sheath earth, including under fault
5
conditions, CNE mains shall be used and the feeder converted to PME in accordance with Engineering
Recommendation G12. It is not required to convert the entire network to PME, but any feeder from a
substation must be either PME or SNE along its entire length.
In co-operation with the local lighting authority, existing “switched” street lighting 5th core networks should be
converted to a permanently energised street lighting supply at the earliest opportunity and the records clearly
marked accordingly. Where this is not cost effective or agreement from the Lighting Authority cannot be
obtained, the CNE mains shall be laid with a separate single phase CNE service cable to continue the 5th core
street lighting supply.
The 5th core of LV mains cable supplying street lighting shall be energised from the same LV source as the phase
conductors within the main and this should normally be on blue phase. At link boxes and pillars the normal
open point of the 5th core shall coincide with the open points of the phase conductors of the main and shall be
“in phase” with any other energised 5th core across a linking point.
The phase conductors of three phase LV mains and services shall be jointed phase to phase, using core number
or colour as appropriate. The neutral or earth conductors of a three phase cable shall not be bonded to any
phase conductors. The practice of bonding a phase conductor to a neutral conductor in a three phase cable to
provide a large capacity single phase service is not permitted even for sections laid within the customer’s
boundary.
When extending single phase or split single phase networks with three phase mains cable, spare phase
conductors may be bonded to another phase at the source end to ensure that all conductors continue to be
energised and monitored. The bonded spare phase conductor should not terminate at any customers’ supply
points. It should be cut short and ended in the termination unit or in a joint box outside the premises. Records
shall accurately reflect the nature and location of any conductor phase bonding undertaken.
No load should be added to a bonded spare phase conductor of a three phase cable until the main is jointed out
normally and supplied from a three phase source.
3.4.2.2 Overhead Mains
New overhead mains should only be installed where underground mains are not economic or practical, as laid
out in more detail in section 3.10.2.
Where LV overhead mains are to be erected, fully-insulated conductors (e.g. ABC) shall be used. Such lines shall
be constructed in accordance with relevant specifications, notably the clearance requirements of ENATS 43-8.
For new mains, only conventional overhead lines shall be erected; new surface wiring or under-eaves mains are
not permitted.
5
th
When replacing existing LV mains having a 4 core and sheath earth as a result of a fault, SNE main can be used if
available.
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Where eaves mains (surface wiring) are to be replaced, it is not permitted that the cable remains in a ‘position
where it is likely to be damaged or where persons going about normal everyday activities could come into
contact with it’ (ENATS 43-8). This can be satisfied by placing the cable above the highest windows (i.e. directly
under the eaves), mounted on brick rather than barge boards or soffits.
If a suitable position on the wall cannot be achieved, then either:

where it is practicable to provide underground services, an underground main and services shall be
provided; or

where it is not practicable to provide underground services, an overhead main and services may be
provided.
Where eaves mains are retained, any infeed:
3.5

from underground mains shall be via a wall box fitted with 200 A cartridge fuses; or

from overhead mains shall be via a 200 A fused tee-off.
LV Network Security and Interconnection
Interconnection shall be provided to support the LV network of a substation fed from a tee, if it can be achieved
economically from a LV network with an independent HV source.
Otherwise, interconnection should be provided only where opportune so to do, specifically where:

no more than 50 metres of additional main is required; or

the LV cable can be laid in a common trench with the HV cable; or

the detailed criteria for supporting existing networks laid out in section A.4 are met.
Approved underground CNE link boxes (this does not include ‘Haldo’ street lighting pillars) shall generally be
used to interconnect underground LV networks. LV pillars shall be used only in areas with high water tables and
these should be located to minimise the risk of vehicular damage and vandalism. Points of interconnection on
overhead mains shall be provided using approved pole mounted LV fuses.
When planning to joint out the LV mains of a distribution substation which is to be permanently recovered, a
link box shall be provided near the existing site, maximising the opportunity to minimise the number of joints
that might otherwise be required (i.e. using terminations to the link box where straight joints or pot ends might
otherwise be used).
3.6
LV Network Protection & Control
3.6.1
LV Mains
Networks shall be fused to clear faults at the end of services in 60 s or, if networks are laid out in the absence of
detailed knowledge of services, to clear faults at the end of the main in 30 s. This requirement will be satisfied if
the loop impedance doesn’t exceed the values stated in the following:
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Fuse
Rating (A)
Loop impedance to end of
service (mΩ)
Loop impedance to end of
main (mΩ)
Capacity (kVA)
315 or less
290
250
215
400
230
200
275
500
180
160
330
of
32
6
Note:1 new networks shall be laid out with loop impedance to the end of the service not exceeding 250 mΩ.
Note:2 it may be necessary for the loop impedance to be lower than the maximum value stated in 3.3.3.1 to
ensure that the fuse will operate within the permitted time
Fuses shall also be no greater than specified in OPS/103/005 - Standard Fuse Data, to secure discrimination with
HV protection. Specifically, this may require the use of 200 or 160 A fuses with pole-mounted transformers.
Where connections to individual customers with a capacity above 135 kVA (200 A cut-out fuse) are provided,
designers shall have regard to discrimination with substation fuses.
Where material modifications are being made to existing feeders, the fusing on those feeders should be
reviewed to meet the requirements of this policy.
Underground networks shall not be fitted with section fuses. On overhead networks, one set of 200 A section
fuses shall be provided in series with the substation fuse. This shall be located at the customer-weighted centre
point (subject to loading). Eaves mains (surface wiring) shall be protected by 200 A fuses; any upstream section
fuses shall be deemed to satisfy this requirement.
More detail is provided in section A.3.
3.6.2
Fifth Core
Legacy networks, both overhead and underground, may include switched ‘fifth’ or even ‘sixth’ core conductors
to which public lighting is connected. There are many different examples of how the conductors are currently
controlled, including:

control gear in substation

control gear in outside cabinet in substation wall;

separate street lighting pillar at substation;

separate street lighting pillar at the link box; and

making certain street lamps into control columns.
To minimise operational inconvenience to Northern Powergrid and Public Lighting Authority’s (PLA),
arrangements that embed PLA control gear in Northern Powergrid assets shall be removed as soon as
practicable, in agreement with the lighting authority. Either existing service connections shall be transferred to
a phase conductor, or a PLA column shall be converted to control the fifth core conductor.
6
2
Taken as 3 x 230 V x fuse rating (or, in the case of 500 A fuses, the capability of a 300 mm waveform)
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Customers’ Services
Connections shall be in accordance with the Code of Practice – Standard Arrangements for Customer
Connections (IMP/001/010), which includes multi-occupancy buildings.
3.7
LV Network Configuration
The LV network should normally be developed as a network of radial mains supplied from a distribution
substation placed near to the load centre. Where economic, an interconnector to an adjacent distribution
substation shall be provided, as discussed in more detail in sections 3.5 and A.3.
LV customer connections shall be provided using a service termination unit on their premises, which is
connected to a nearby LV main using a dedicated service cable. Each customer premises should have only one
supply point at which all supply cables should terminate. Customers requiring very large three phase LV loads
may be supplied by more than one LV cable directly from a single distribution substation on their site, such that
these cables do not enter the public highway nor cross third party land.
3.8
Substation Location
Despite the use of modern equipment in small enclosures an HV/LV substation is a relatively expensive item.
Many kilometres of LV cable can be installed for the price of a single network substation. Because of this, it is
essential that maximum utilisation is obtained for each substation.
Substation optimum utilisation is however a function of the load density of the area to be supplied. For general
gas heated housing estates it may be possible to obtain an LV network supply radius of up to 700 metres
whereas for estates with off-peak electric heating the radius may be limited to as low as 200 metres with a 1000
kVA transformer installed and highly utilised. Town centre load densities should enable good utilisation of
substations equipped with the larger network transformers. Industrial estate load densities vary considerably
from place to place and with time.
To maximise substation utilisation and minimise network losses substation sites shall be as near as possible to
the load centre, taking account of credible scenarios for future development. At the very least, unless fully
utilised by the first phases of development, the substation shall be placed towards the extremity of the
development towards the areas to be developed.
Securing an appropriate location for the substation to reflect credible future development will require dialogue
between the licensee and the connections provider.
To facilitate ongoing maintenance, the connection of generation, and eventual replacement or reinforcement,
free access shall be provided to the substation site, with sufficient space for vehicles to be parked without
unduly obstructing traffic. Where generation is most likely to be needed, i.e. on sites with a single HV infeed,
suitable safe access for generators (vehicle- or skid-mounted) shall be secured.
All substations shall be situated such that Northern Powergrid staff or their contractors can gain direct access at
any time using only operational keys.
To satisfy planning constraints, the substation should be sited on or behind the building line. This will generally
secure the minimum required distance of 1m from the back of the footpath required to allow doors to be
opened without causing obstruction.
All substations must avoid creating a statutory nuisance, which would otherwise arise primarily due to noise &
vibration or electro-magnetic interference. This requires a minimum distance of 3m from new indoor
substations, or 6m from new pole-mounted substations, to adjacent dwellings.
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Substation accommodation
Ground-mounted substations shall be enclosed in accordance with the Policy for the Enclosure of Ground
Mounted Distribution Substations - IMP/009, and precautions taken against the risk of flood, fire and explosion
in accordance with the policy for Flood Risk Mitigation at Substation Sites – IMP/001/012 and the policy for Fire
Mitigation at Operational Premises - DSS/031 (which includes criteria for the use of fixed CO2 installations).
Due to the potential for statutory nuisance, the increased risk of fire, and generally poor ventilation (leading to
lower effective transformer capability), substations integral to other buildings should be avoided. It is
permissible for substations to share a party wall (so long as constructed of brick or similar material with no
breach that might permit the spread of fire) with industrial or commercial buildings. The use of common
trenches between the substation and customers’ switch rooms is no longer permitted; any ducts shall be sealed
with an approved fire-resistant material.
If an integral substation cannot be avoided, the following precautions must be undertaken at the developer’s
expense:


for the safety of staff:

substations should be at ground level with unrestricted 24 hour access from the public highway. If this
cannot be achieved the agreement of the Design Manager should be sought. When deciding if
alternative access arrangements are acceptable, facilities for bringing Northern Powergrid’s heavy
lifting equipment (e.g. a HIAB wagon) should be considered; and

the creation of a confined space is not permitted, and there must be free natural ventilation of the site.
This will generally preclude basement substations.
to minimise statutory nuisance:

the substation chamber shall be of dimensions that attenuate, rather than amplify, transformer hum.
At 100 Hz, and assuming 330 m/s as the velocity of sound, this means that dimensions approaching
multiples of 3.5m should be avoided;

low-loss, and hence low noise, transformers, shall be used;

the transformer shall be mounted so as to damp vibration (e.g. via the use of anti-vibration pads);

ventilation shall be arranged so as to conduct noise away from the site;

a minimum of 3m clearance shall be provided between all conductors and any chamber likely to be
used by the customer for electronic equipment; and

the chambers immediately adjoining the substation chamber shall not be such that they will, or might
credibly at some time in the future, be used as dwellings. For example, in blocks of flats, only
communal areas are permitted adjacent to the substation chamber.
3
Stand-alone substation buildings shall, wherever practicable, have an internal volume less than 29 m to avoid
the need to obtain planning permission.
Where future development is more likely (e.g. in town centres or commercial/industrial estates), substation
sites should, where possible, be large enough to accommodate a second transformer and associated HV and LV
switchgear.
3.10
Routing of Circuits
When routing new mains cables, to discharge the obligation under section 9 of the Act, consideration shall be
given to the potential for future network extension to cater for load development.
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This means that cable size and route shall have due regard to credible future developments as well as to
2
immediate need. For example, it may be appropriate to run a 300 mm cable to support a second phase, even
though immediate connections might only require a lower capability. This will require dialogue between the
licensee and, where appropriate, the connections provider.
To assist compliance with ESQC regulation 23(1), networks shall be arranged such that the average number of
customers per feeder shall not exceed 100. The maximum number of customers on any one feeder shall not
exceed 120.
LV mains shall be installed in the public highway. Care should be taken when accepting routes in ‘service strips’
and ‘mews courts’ as these may not become part of the public highway. Where adoption is assured it is
preferable to install mains in ‘service-strips’ rather than footpaths.
LV mains shall not be routed within the boundary of private properties and shall not be installed in footpaths at
the rear of properties with only pedestrian access. Services (whether overhead or underground) shall be run
only within the boundaries of the properties each supplies, i.e. a service to any one property shall not cross land
belonging to another.
Each premise shall have only one point of connection, as addressed in more detail in the Code of practice for
standard arrangements for customer connections (IMP/001/010).
Single-phase loads shall be equally distributed (balanced) across all three phases of the LV main. This will
minimise voltage unbalance and reduce losses.
3.10.1 Underground Mains
In the interests of public safety and network resilience, new LV mains and services will generally be
underground.
When routing new mains cables, to discharge the obligations under section 9 of the Act and ESQC regulations
3(1)(b) and 23(1), consideration shall be given to:

the potential for future network extension to cater for load development;

the possibility of an incident of inadvertent contact damaging more than one cable at the same time;

the need for free access to cables; and

the adverse impact on circuit capability if cables are in undue proximity.
This means that HV and LV cables from substations should be separated as far as it is economic so to do. For
example, it may be appropriate to run one HV cable and two LV cables in each of two common trenches, one
either side of the road, to balance economies in construction with effective ongoing operation.
3.10.2 Overhead Mains
In all cases an underground cable option is preferred unless uneconomic or impractical. New LV overhead lines
shall only be erected in rural areas and where routes can be selected to minimise the impact on the
environment and local amenity and where:

the main would not create a high risk site, as defined in DSS/023 – Risk Assessment of Overhead Lines;

the site would not create other public hazard, e.g. crossing of railways or high-speed roads; and

the cost of underground mains would be disproportionate, because either:
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
the length of mains per customer would be abnormally high; or

significant excavation and reinstatement costs might otherwise be incurred .
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7
Where LV overhead mains are to be installed, fully-insulated conductor shall be used. Unless disproportionate
costs would otherwise be incurred, services from overhead mains and pole-mounted transformers shall be run
underground, both for the same goals of safety and reliability as apply to mains, and also to support discharge of
the obligation under section 9 of the Act by facilitating future undergrounding of the network.
Any new LV overhead mains shall not be installed under any overhead lines operating at a higher voltage.
3.10.3 LV Services
Connections shall be provided using a service termination unit on the customer’s premises, which is connected
to a nearby LV main using a dedicated service cable.
All new connections, even from overhead lines, shall be made via dedicated underground services. Looped
services are no longer permitted.
The developer will, in accordance with NJUG Volume 2, install a service tube with a draw-wire and covered by
tile tape as per NSP/002 from each service position in a straight line either:

normal to an underground main; or

to a point as close as practicable to the nearest pole supporting an overhead main.
The service and tube shall be laid in the land of the final owner of the property serviced. For outdoor meter
positions the cable shall be mechanically protected by a ‘hockey stick’ or equivalent to the cabinet. Designers
2
shall have regard to the de-rating required when ducts are installed in cavity walls: for example, a 25 mm single
2
concentric cable in an external duct and a 35 mm single concentric in an internal duct are both capable of just
over 80 A.
When replacing or re-locating existing assets, dedicated underground services as described above shall be
provided wherever possible.
3.10.4
Street Furniture Services
Wherever commercially viable, street furniture shall be connected by a single direct service as for any other
customer.
Where this is not viable or agreement from the Public Lighting Authority cannot be reasonably obtained, it is
permitted to loop street lighting columns via the authorities own cable network up to a total power
consumption of 500W from one service connection onto a LV main. A sub-fuse rated at 25A shall be installed in
8
the first (Control) column and co-ordinated with the impedance to the last lighting column such that a fault
shall be cleared in 5s. A 25 A PLA fuse will so protect a loop impedance up to 1 ohm.
7
For example, if road crossings or significant lengths of cable within an existing footpath were required. For infill
developments, the establishment by the developer of civil infrastructure will generally reduce these costs to a
level where underground distributors remain economic
8
th
One method of integrating 5 core wiring with a street lighting control column is detailed in drawing
C1010662, which shows the internal column wiring arrangement and drawing C1010669 which shows a
th
breeches joint with 5 core termination (drawings referenced in section 4.2).
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For larger connections, two-and three-phase connections are permissible, although any connection anticipated
to be above 500W shall be metered in accordance with the Electricity (Unmetered Supply) Regulations 2001.
As for other connections, it is the developer’s responsibility to provide a suitable enclosure for Northern
Powergrid apparatus. This shall take into account public safety by providing secure accommodation, and the
safety of staff by permitting ready (controlled) access from ground level, arranged so as to reduce the danger
from passing traffic. Enclosures adjacent to high-speed roads are not permissible. Suitable keys etc. shall be
provided free of charge by the developer.
3.11
LV Network Earthing
To discharge the obligation under ESQC regulation 24, the LV network shall be developed in accordance with
Engineering Recommendation G12, to enable Northern Powergrid to offer a PME Earthing terminal at all new
and existing customers’ supply points. For individual connections on existing networks, the earthing policy will
provide further guidance.
3.12
Assumptions
This policy is aimed at ensuring that Northern Powergrid’s networks are extended in such a way at all voltages to
provide a safe and secure supply of electricity to all customers in line with the requirements of legislation.
The following assumptions have been made, but will be required to ensure the effective delivery of this policy
into the business:

the current business goals for the security, safety and continuity of operation of the networks do not
change; and

the current range of approved plant and materials does not change.
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References
4.1
External Documentation
March 2014
Page
20
of
Reference
Title
Version and date
The Act
The Electricity Act 1989 (as amended by the
utilities act 2000, Energy act 2004)
HASAWA 74
The Health and Safety at Work etc. Act 1974
EPA 90
The Environmental Protection Act 1990
ESQC regulations
The Electricity Supply, Quality and Continuity
Regulations 2002
Statutory Instrument
2002
No.
2665
including
amendment
regulations in 2006
(SI 2006 No. 1521)
and 2009 (SI 2009
No. 639)
EAW regulations
The Electricity at Work Regulations 1989
Statutory Instrument
1989 No. 635
UMS regulations
The Electricity (Unmetered Supply) Regulations
2001
Statutory Instrument
2001 No. 3263
Distribution licence conditions
Electricity distribution consolidated standard
licence conditions
Guaranteed standards of
performance
The Electricity (Standards of Performance)
Regulations 2001
2001/3265
PD 88-5
Low-voltage fuses. Guidance for the application
of low-voltage fuses
1: 2010
Part 5: Specification of supplementary
requirements for fuse-links for use in AC
electricity supply networks
BS 7657
Specification for cut-out assemblies up to 100 A
rating, for power supply to buildings.
1: 2010
BS 7671
British Standard Requirements for Electrical
Installations
1: 2001
th
(The 16 Edition of the IEE Wiring Regulations)
The Distribution Code
The Distribution Code of Great Britain
21: Dec 2014
ERA 69-30
Current rating standards
1989
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Reference
Title
Version and date
Engineering Recommendation
G5
Planning levels for harmonic distortion and the
connection of non-linear equipment to
transmission and distribution systems
4-1: 2005
Engineering Recommendation
G12
Requirements for the application of protective
multiple earthing to low voltage networks
3: 1995
Engineering Recommendation
P2
Security of Supply
6: 2007
Engineering Recommendation
P5
Design of low voltage underground networks for
housing estates
5: 1987
Engineering Recommendation
P23
Consumers’ earth fault protection for compliance
with the IEE wiring regulations
1: 1991
Engineering Recommendation
P25
The short-circuit characteristics of Public
Electricity Supplier’s low voltage distribution
networks
1: 1996
Engineering Recommendation
P26
The estimation of the maximum prospective
short-circuit current for three phase 415V
supplies
1: 1985
Engineering Recommendation
P27
Current rating guide for high voltage overhead
lines operating in the UK distribution system
1: 1986
Engineering Recommendation
P28
Planning limits for voltage fluctuations caused by
industrial, commercial and domestic equipment
in the United Kingdom
1: 1989
Engineering Recommendation
G5/4-1
Planning level
distribution.
of
Harmonic
and
voltage
2005
ENATS 09-8
Impregnated paper insulated 600/1000 V cable
with three solid aluminium phase conductors and
aluminium sheath/neutral conductor (consac)
4: 1977
ENATS 12-6
Time fuse links for use with current transformer
releases on circuit breakers
1: 1973
ENATS 12-8
The application of fuse-links to 11 kV/415 V and
6.6 kV/415 V underground distribution networks
2: 1986
ENATS 37-1
415 V AC switchgear, control gear and fusegear
2: 1977
ENATS 37-2
Low voltage distribution fuse boards
4: 2004
ENATS 43-8
Overhead line clearances
3: 2004
ENATS 43-13
Aerial bundled conductor insulated with crosslinked polyethylene for low voltage overhead
distribution
2: 1990
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Reference
Title
BS 7735 (IEC 354)
Guide to loading
transformers
IEC 853
Calculation of cyclic and emergency current
ratings of cables
1
IEEE 1584
Guide for
Calculations
A: 2004
NJUG Volume 2
NJUG guidelines on the positioning of
underground utilities apparatus for new
development sites
22
of
Version and date
of
Performing
oil-immersed
Arc-Flash
power
Hazard
1 :1994
Issue 4: 29 October
2013
Internal documentation
Reference
Title
Version and date
DSS/023
Risk assessment of overhead lines
1.1: October 2003
DSS/031
Policy for fire mitigation at operational premises
2.1: May 2010
NSP/002
Policy for installation of distribution power cables
1.1: May 2010
IMP/009
Policy for the enclosure of ground mounted
distribution substations
2: May 2012
IMP/001/012
Code of practice for flood mitigation at
operational premises
2: March 2012
IMP/001/010
Code of practice for standard arrangements for
customer connections
4: July 2012
C1010662
Public lighting termination for 5 core control
network, 16mm CNE copper service cable
C1010669
Mains service breeches joint with 5 core
connection, PILC main 5 cores with CNE single
service
th
th
CAUTION! - This document may be out of date if printed
A, 16 August 2012
A, 16 August 2012
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Definitions
Term
Definition
ADMD
After Diversity Maximum Demand
CNE
Combined Neutral/Earth (TN-C)
DNO
Distribution Network Operator
ER
(ENA) Engineering Recommendation
ERA
ERA (Electricity Research Association) Technology limited
ENA
Energy Networks Association
ENATS
ENA Technical Standard
HSE
Health & Safety Executive
ICP
Independent Connections Provider
IDNO
Independent Distribution Network Operator
IEC
International Electrotechnical Commission
IEE
The Institution of Electrical Engineers
IEEE
The Institute of Electrical and Electronic Engineers, inc.
IIS
Information and Incentives Scheme
IUS
Integrated Utility Services
LVN
Low Voltage Network
Public Lighting Authority
Northern Powergrid
Northern Powergrid (Northeast) Ltd and Northern Powergrid (Yorkshire)
plc.
PME
Protective Multiple Earthing
PSCC
Prospective Short Circuit Current
SNE
Separated Neutral/Earth (TN-S)
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Authority for Issue
6.1
CDS Assurance
Page
March 2014
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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.
Lynn Donald
6.2
Sign
Lynn Donald
CDS Administration
Date
21/03/14
Author
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.
Review Period - This document should be reviewed within the following time period.
Standard CDS
review of 3 years
No
Non Standard Review Period & Reason
Period: One Year
Reason: Major review of IMP documents planned for 2015
Sign
Ian Fletcher
6.3
System Planning Engineer
Ian Fletcher
Date
21/03/14
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.
Alan Creighton
Mick Walbank
Derek Fairbairn
6.4
Asset Management Engineer
System Planning Manager
Customer Connections Design Manager
Sign
Alan Creighton
Mick Walbank
Derek Fairbairn
Date
21/03/14
21/03/14
21/03/14
Sign
Mark Nicholson
Date
21/03/14
Mark Drye
Date
21/03/14
Approval
Approval is given for the content of this document.
Mark Nicholson
6.5
Head of System Strategy
Authorisation
Authorisation is granted for publication of this document.
Sign
Mark Drye
Director of Asset Management
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Appendices
This section provides further detail on the preceding statements of policy, to:
A.1

provide designers with a fuller background to the policy, allowing them to make informed site-specific
investment appraisals;

facilitate future review of this policy; and

explain to ICPs and others the basis for the policy decisions.
Voltage
The distribution network from Grid Supply Points to service termination shall be designed and managed to
deliver statutory LV supply voltage levels of 230\400V +10/-6%. HV and LV voltage control policies must
therefore be co-ordinated for an holistic assessment of voltage regulation but, to avoid the need for full voltage
studies for each scheme, general guidelines are presented here.
What at first appears to be a 16% (+10/-6%) band becomes a 12% band on the combined HV and LV mains after
allowing for:

a 1.5% dead-band on Automatic Voltage Control (AVC) equipment at the primary substation; and

a 1% drop for the typical 20m service.
Regulation across a fully loaded HV/LV transformer is about 5%. However, as this is largely reactive, the
effective contribution is around 2%. Therefore:

where regulation on the high voltage network is around 4%, which is the case for most urban networks and
for substations close to the primary on rural networks, a 6% voltage regulation is permissible on the LV
main.

where regulation on the high voltage network is around 6%, which is the case for substations far from the
primary on rural networks, a 4% voltage regulation is permissible on the LV main.
Broadly speaking, in the absence of embedded generation at HV, voltage regulation will not exceed 4% within 15
km of the primary at 11 kV, and 30 km at 20 kV. Therefore, within those ranges, a 6% voltage regulation is
permissible on the LV main: beyond those ranges, a 4% regulation is permissible on the LV main.
A.2
LV Fault Levels
ESQC regulation 3 requires Northern Powergrid to ensure that equipment is sufficient for the purposes for and
the circumstances in which it is used. Amongst other issues, this requires Northern Powergrid to assess network
fault levels and define maximum prospective short circuit current (PSCC), to ensure that equipment is specified
to withstand reasonably anticipated duty.
ENATS 37-1 requires LV switchgear and fusegear to withstand 43.3 kA for 0.2s; 36 kA for 1.0s; or 21.6 kA for
3.0s, dependant on protection characteristics. ENATS 37-2 requires LV fuse boards to withstand only 18 kA for
9
10
0.5s for single transformers up to 500 kVA and 35.5 kA for 0.5s for larger units . BS 7657 has superseded
ENATS 12-10, and requires that single phase cut-outs up to 100 A be rated at 16 kA short-circuit current.
9
the paper actually quotes 800A, or about 600 kVA
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Engineering Recommendations P25 and P26 give guidance as to estimating PSCC for LV connections as for
Engineering recommendation P23 and earth loop impedance, they do not mandate maximum fault levels.
However, Engineering recommendation P5 does create a requirement to control loop impedance to manage
fault levels, specifically that, at the point of connection of any service cable to the low voltage distributor, the
phase to neutral loop impedance should not be less than (0.00825 + j 0.0125) ohms to give a phase maximum
design prospective short circuit current of 16 kA at that point, assuming a nominal 240 V.
These standard values may be compared to likely network conditions, using typical impedance values from
Engineering Recommendation P28:

a 1600 kVA transformer close to a high fault level primary gives a PSCC at 433V of around 36 kA;

a 1000 kVA transformer close to a high fault level primary gives a PSCC at 433V of around 26 kA;
2

20m of 300 mm cable after such a transformer reduces PSCC at 433V to around 22 kA; and

a further 10m of 25 mm cable reduces PSCC at 433V to around 12 kA.
2
It is appropriate to use these values for all sites, as HV network configuration will change during the life of the
substation and the fault level infeed from the higher network may therefore approach the maximum values
used here. Similarly, while a 500 kVA transformer close to a high fault level primary reduces the PSCC at 433V to
around 13 kA, the potential for transformers to be replaced means that the higher values should be used.
These figures can be compared to the assumed maximum values quoted in Engineering Recommendation P25
and Engineering Recommendation P26, of:

16 kA at the junction of a single-phase service with the main;

18 kA at the junction of a three-phase service and the main; and

25 kA at the LV fuse board.
It can be seen that, in practice, slightly higher values can be found on the network than assumed in those
engineering recommendations.
In accordance with the section 9 obligation, the contribution of customers connected at LV to these fault levels
must also be addressed, and specifically the potential spread of generation. If we assume that 500 kVA of
generation with a 10% impedance were connected to a typical network, then we might reasonably expect an
additional fault level infeed of 5 MVA. This equates to an additional short-circuit current of about 7 kA at 433 V,
giving a potential PSCC of around 33 kA for a 1000 kVA transformer, or 20 kA for a 500 kVA unit.
Similarly, the PSCC at connections close to a large transformer on a generation-rich network might be expected
to rise. If the PSCC at the LV fuse board rose to 33 kA, the fault level for a three-phase connection would be
expected to rise to around 30 kA, but still be 16 kA for a single-phase 100 A connection.
These figures require application of the higher ENATS 37-2 standard (35.5 kA) to all LV fuse boards, and also
confirm that most credible networks will remain within those limits.
Where more than 500 kVA of LV-connected generation is expected on a network, it may be necessary to reduce
the size of transformer to constrain the contribution to fault level from the HV network. Individual studies will
be required for each such scheme but, as noted earlier, standard values should still be used for the fault level
infeed from the higher network.
10
up to 1600A, or about 1000 kVA: 50 kA rating is required for the largest units, but these are approved only for individual
customer connections and not for use on the general network
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In summary, ruling fault levels to accommodate infeed from the higher network and LV-connected generation
are:

16 kA for single-phase connections up to 100 A, consistent with BS 7657;

30 kA for other connections; and

35.5 kA for equipment at general network substations.
These figures may be quoted to customers as ‘maximum design values’.
Similarly, fault infeed from the higher-voltage network can be taken as:

16 kA for single-phase connections up to 100 A, consistent with BS 7657;

22 kA for other connections; and

26 kA at substation LV busbars.
In the absence of site-specific assessment, these figures may be quoted to customers as ‘typical maximum
values’.
A.3
Fusing
LV networks are protected:

electrically by fuses generally to BS 88 part 5. LVCBs are, and will be for the foreseeable future, used only at
the point of connection to individual customers and not to control circuits; and

mechanically by construction and installation.
We have a policy for mechanical protection, through construction and installation that will avoid danger so long
as the asset remains undisturbed.
The issue for electrical protection is whether networks are fused simply to protect the asset, or also to protect
those who may disturb the asset (including Northern Powergrid’s own staff). This is a question of what is a
reasonable level of electrical protection to provide.
The ESQC regulations require Northern Powergrid to:

prevent danger, so far as is reasonably practicable; and

ensure that no current (including earth faults) flows that the network cannot carry without danger, i.e. to
protect the asset.
There are three categories of persons to protect:

the general public;

staff, when operating on the asset; and

other persons working in the vicinity of Northern Powergrid’s assets.
Taking actions to prevent danger must pass the ‘reasonably practicable’ test, i.e. not present a cost that is
grossly disproportionate to the benefits.
The danger of inadvertent contact is primarily associated with electric shock and arc flash. To avoid the danger
of electric shock, the IEE wiring regulations (BS 7671) suggest setting protection to secure clearance within 5s.
To avoid the danger of arc flash, IEEE 1584 recommendations for arc flash protection are to limit the product of
fault current and clearance time to less than 7000 A-s (for 440 V networks an d a working distance of 0.38m).
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Both these sets of guidelines are intended primarily for installations in premises. It can be argued that they are
less applicable to public networks.
The baseline for network protection is the specific instructions for construction and installation in the ESQC
regulations, which will certainly protect the general public:

an electrically continuous metallic screen connected with earth, or some form of mechanical protection,
shall be provided for underground cables so that any tool will make contact with that protection or screen
before it can make contact with any live conductors (reg. 13);

underground cables shall also be kept at such depth or be otherwise protected so as to avoid damage or
danger (reg. 14); and

the height above ground of any overhead line, at the maximum likely temperature of that line, shall not be
less than that specified in the regulations (reg. 17).
If this were deemed to be all that were reasonably required to avoid danger, it would leave protecting the
integrity of the cable itself as the main purpose of fusing. ENATS 12-8 requires protecting only the main, but it is
worth considering protecting the service as well.
Options can therefore be summarised as:

prevent danger through fusing to 7000A-s or for a 5s clearance; or

prevent danger through mechanical protection and protect the asset through fusing to a longer time.
It is theoretically possible to fuse LV networks with phase-earth loop impedance up to 0.25 ohm (the limit for
flicker) to clear the bulk of faults within 5s, and a fault on the end of the network within 10s, by using 250 A
units. However, this would disproportionately restrict utilisation of cable and transformer capability, and is
therefore not reasonably practicable.
As we cannot reasonably fuse to clear faults within 5-10s, we cannot use fuses to protect against inadvertent
contact and arc flash. BS 88-5 curves rise steeply between 10s and 100s, so fusing standards become arbitrary,
as no one setting is markedly safer than any other. That is, it makes little difference whether fuses would permit
an arc to persist for 30s or 100s, as either could cause serious injury. Further, in practice, faults are likely to burn
clear without causing operation of the fuse. It is therefore reasonable to set a standard that balances flexibility
in design and operation, secure in the knowledge that the system is reasonably safe (primarily due to
construction and installation). Effective utilisation of cables and transformers requires that 400 A fuses be used
in the majority of situations, leading to a standard of clearing faults at the end of services in 60s or, if networks
are laid out in the absence of detailed knowledge of services, clearing faults at the end of the main in 30s. This
requirement can be achieved under the circumstances below:
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Fuse
Rating (A)
Loop impedance to end of
service (mΩ)
Loop impedance to end of
main (mΩ)
Capacity
11
(kVA)
315 or less
290
250
215
400
230
200
275
500
180
160
330
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Where point demands above 135 kVA are connected, the cut-out fuse will be greater than 200 kVA. This will not
discriminate effectively with a 400 A fuse at the substation. Therefore, point demands above 135 kVA shall
generally be connected to discrete feeders, and the following fusing adopted to provide discrimination:
Distribution capacity (kVA)
Cut-out fuse (A)
Substation fuse (A)
Maximum loop impedance to cut-out
(ohms)
135
200
400
0.23
170
250
500
0.18
215
315
630
0.14
275
400
800 (preferred)
0.11
275
400
630 (by exception)
0.14
500
800
0.11
300
12
The discussion above applies to ground-mounted transformers from 200 kVA upwards and pole-mounted
transformers from 100 to 315 kVA. Rural networks fed from transformers of 50 kVA or less shall be controlled by
160 A fuses, as:

anything larger would be superfluous, as the transformer capability would be no more than 100 A
continuous; and

anything smaller would not discriminate with domestic 80 A cut-out fuses.
The one exception to this 60s rule is for eaves mains (under-eaves wiring). As these assets are installed on
buildings, and there is greater potential for inadvertent contact, best practice is to apply the 5s rule in BS 7671.
Assuming that many existing and refurbished installations will have loop impedance slightly higher than the 0.25
ohms required on new build, a 200 A fuse (in a wall box) will:
11
12

generally clear in around 5s;

discriminate with 80 or 100 A cut-out fuses; and
2
Taken as 3 x 230 V x fuse rating (or, in the case of 500 A fuses, the capability of a 300 mm waveform)
2
The continuous capacity of a 300 mm waveform laid in proximity to another similarly loaded cable is around 430 A,
rather than the 465 A of a single cable quoted on the specification sheet. It may be possible to use higher capacities
where load profiles and installation conditions permit.
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discriminate with 400 A substation fuses.
For other situations, consult Technical Services for advice.
A.4
LV Network Security and Interconnection
The key factor here is the obligation under ESQC regulation 3(1)(b) to ensure that equipment is so constructed
as to prevent danger or interruption of supply, so far as is reasonably practicable. LV interconnection allows
Northern Powergrid to minimise the impact of both faults and planned outages on all or part of an LV network.
To assess the value of interconnection, we must assess the likely causes of outages. These include:

HV faults;

LV faults;

HV planned outages; and

LV planned outages
For substations looped into HV rings, the only fault that would lead to a sustained interruption that might be
mitigated by LV interconnection is disruptive failure at the HV/LV substation, which is rare.
Conversely, substations fed by HV tees do create a need for LV interconnection to support the LV network under
HV fault conditions. Such interconnection is of value only if provided from an independent HV source.
Given the restrictions on the number of customers per LV feeder laid out in section 3.10, the impact of LV
interconnection on overall restoration times for LV faults will be limited for new networks. For existing
networks, this may not be the case and, as will be explored later, the case can be made to providing
interconnection.
Planned HV circuit outages have a similar impact to faults, so the need for interconnection arises primarily for
substations fed from teed circuits. Improvements in asset specification (e.g. the use of SF 6 insulation) and
maintenance techniques (e.g. live-tank oil sampling) reduce the number of planned HV plant outages, but do not
eliminate them. Therefore, the need for interconnection arises primarily where dead-tank maintenance is still
required. The requirement for LV planned outages is rare, particularly on underground networks where most
work is carried out live.
Therefore, interconnection should be provided only where opportune so to do, specifically to support:

the LV network of a substation fed from a tee, if interconnection can be found from a LV network with an
independent HV source;

the LV networks of existing substations where plant requires dead-tank maintenance; and

existing LV distributors with poor fault performance.
As broad guidelines:

no more than 50 metres of additional cable should be laid to provide interconnection from any LV network;

to support substations under a wide range of conditions, as large a cable size as practicable should be
applied. This may be higher than would otherwise be required; and

interconnection will not normally be provided between substations with interdependent HV sources, unless
it becomes as easy to install a link box as two pot-ends (or, on existing LV OHL networks, where a section
point is as easy as two terminal poles one span apart).
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Approved underground CNE link boxes should normally be used to interconnect underground LV networks. LV
pillars shall be used only in areas with high water tables and these should be located to minimise the risk of
vehicular damage and vandalism. Points of interconnection on overhead mains shall be provided using approved
pole mounted LV fuses.
When planning to joint out the LV mains of a distribution substation which is to be permanently recovered,
consideration shall be given to installing a link box rather than jointing the mains through or terminating them
with insulated ends.
When planning to rebuild a distribution substation, the costs of installing interconnection to adjacent networks
shall be compared to the cost of generation (if practicable) and the impact of the headline performance (IIS)
incentive scheme. Most value from interconnection comes through sectioning, as a suspect cable cannot be
back-fed.
A spreadsheet, illustrated below, is available from Asset Management to calculate the amount of cable that can
be justified to provide interconnection, so long as the existing feeder is sectioned at or about the mid-point.
Additional costs may be justified for circuits with poor fault histories by substituting the site-specific fault rate.
standing data
rate of permanent faults on LV UG networks
discount rate
capital incentive rate
evaluation period
'IIS' value of one customer-hour saved
restoration time saved through interconnection
installed cable costs
variable data
length of existing feeder (km)
number of customers on existing feeder
additional cable that may be economically justified
A.5
8
6.90%
39.00%
10
6
6
50
2
200
208
Network configuration
This policy requires the number of customers on LV feeders to be limited to an average of 100 and a maximum
of 120. Network data shows that only 4% of existing feeders would breach this criterion, suggesting that it can
readily be achieved without entailing excessive cost:
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Version:-
Date of Issue:-
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March 2014
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