Code of Practice For Earthing LV Networks and HV Distribution Substations IMP010011
Code of Practice For Earthing LV Networks and HV Distribution Substations IMP010011
Code of Practice For Earthing LV Networks and HV Distribution Substations IMP010011
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Purpose
The purpose of this document is 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, the Electricity at Work (EAW) Regulations 1989,
the Distribution Licences and The Distribution Code, by laying out the way in which CE will
develop efficient, co-ordinated and economical earth systems on the Low Voltage (LV)
networks and at High Voltage (HV) distribution substations.
This document supersedes the following documents, all copies of which should be
destroyed.
Ref
Version
DSS/010/011
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1983
Title
Code of Practice for Earthing LV Networks
Code of Practice for Earthing
This Code of Practice contains detailed policy and practice on earthing of LV Networks and
HV distribution substations. Section 3.20 provides basic information regarding HV and LV
earthing of distribution substations.
2.
Scope
The Code of Practice applies to all LV networks, services and HV distribution substations in
NEDL and YEDL the licensed distributors of CE Electric UK Funding Company Ltd (all
referred to hereafter as CE), and to the providers of connections to those networks.
It is not intended for this policy to be applied retrospectively with the exception of locations
identify as having inadequate earth networks or those where work is being carried out on
the network to satisfy the requirements of the ESQC Regulations 2002 as detailed in
section 3.3.2 of this code of practice.
All new networks shall be equipped with earth systems in accordance with this Document
3.0
Policy
3.1
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3.1.2 Requirements of The Electricity Safety, Quality and Continuity (ESQC) Regulations
2002.
The ESQC Regulations 2002 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 application of earthing shall be complied with, specifically:
Reg. No Text
Application to this policy
3(1)(b) distributorsshall ensure that their
This will be achieved by installation of earth
equipment is so constructedas to
systems to standard designs by trained
prevent dangeror interruption of supply, personnel.
so far as is reasonably practicable
6
Adistributor shall be responsible for the This will be achieved by installation of
application of such protective devices to systems and conductors to provide a low
his network as will, so far as is reasonably impedance path back to source for earth
practicable, prevent any current, including fault currents as far as is reasonably
any leakage to earth, from flowing in any practicable
part of his network for such a period that
that part of his network can no longer
carry that current without danger.
7(1)
A generator or distributor shall, in the
This will be achieved by installation of
design, construction, maintenance or
system conductors to minimise damage
operation of his network, take all
and accidental disconnection
reasonable precautions to ensure
continuity of the supply neutral conductor
8(1)
A generator or distributor shall ensure
This will be achieved by installation of earth
that, so far as is reasonably practicable,
systems and conductors of sufficient rating
his network does not become
and in such a manner as to prevent
disconnected from earth in the event of
disconnection
any foreseeable current due to a fault.
8(2)a
A generator or distributor shall, in respect This will be achieved by design of the
of any high voltage network which he
network
owns or operates, ensure that the
network is connected with earth at, or as
near as is reasonably practicable to, the
source of voltage but where there is more
than one source of voltage in that network,
the connection with earth need only be
made at one such point
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Part V11 Paragraph 24, Sections (4) & (5) of the ESQC Regulations states that: (4) 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.
(5) In this regulation the expression "new connection" means the first electric line, or the
replacement of an existing electric line, to one or more consumer's installations
Paragraph 24 of the ESQC Regulations specifically excludes the connection of the
distributors combined neutral and protective conductors to any metalwork in a caravan or
boat. Further information on locations where an earth terminal should not be offered are
given in Section 3.15 of this Code of Practice.
The general CE policy is to ensure that an earth terminal can be provided so that, when an
installation has been designed to the current edition of the IEE Regulations for Electrical
Installations (BS7671), a customer's fuse will operate in the event of an earth fault on the
customer's installation.
In order to ensure adequate current flows, the best method is to use a continuous metallic
return path between the customer's earth terminal and the transformer neutral point. The
soil is inadequate for the purpose of carrying earth fault current because of its high
resistance. Metallic water pipes, although often used in the past, are unsuitable because of
the increasing use of non-metallic pipes and inserts, and the potential high resistance to
earth.
The continuous metallic return path will be provided by cable sheath, by separate cable
conductor, by protective multiple earthing (PME) or by protective neutral bonding (PNB).
PME and PNB are means whereby the neutral conductor, apart from its normal function of
carrying load current, provides the metallic return path for earth fault currents from the
customer's premises to the substation transformer neutral. The adequacy of the return path
is determined by its loop impedance.
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In addition the use of protective multiple earths, interconnection of neutrals and good load
balancing where possible will normally ensure that, even if the neutral breaks, any
customer will be left with at least one earth connection on that portion of the neutral still
connected to the installation. This will in most cases tend to hold down the potential of the
neutral and consequently the voltage applied to the frames of customer owned metallic
apparatus, to a reasonable value.
3.1.3
3.1.4
3.2
Key Requirements
CE policy is that all networks shall be so constructed such that an earthing terminal can be
made available at every LV supply point and that this will provide a solid metallic return
from the earth terminal of the installation back to the substation transformer neutral
terminal. Earthing on all LV networks shall be carried out in accordance with the policy and
procedures detailed in this Code of Practice. These procedures all comply fully with the
ESQC Regulations (2002), and it will not be necessary for field staff to make a detailed
study of the regulations.
The details for implementing this policy are set out in the appropriate sections of this Code
of Practice. Matters relating to contact with customers are to be found in Appendix A and B
of this Code of Practice.
3.2.1
New Networks
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All new CE underground and overhead LV networks shall be constructed to the earthing
standard required by this Code of Practice so that normally all new customers can be
provided with an earthing terminal. (In a few special circumstances it may not be
appropriate to offer the terminal examples are discussed in section 3.15 of this document)
3.2.2
Existing Networks
The implementation of this policy ensures that CE LV customer can have a solid metallic
earth-fault return path provided by one of the following means:
(a)
(b)
The electrically continuous sheaths of separate neutral and earth (SNE) cables
(c)
The choice between these alternatives will depend on the nature of the existing distribution
system and the cost of providing the earthing terminal.
(In a few special circumstances it may not be appropriate to offer the terminal see sections
3.15 for details)
3.3
3.3.1
Implementation
New Networks
New underground and overhead networks shall be constructed in accordance with the
requirements of this Code of Practice. All customers on such networks will be offered the
use of a CE earth terminal connected to the neutral conductor.
New underground networks shall normally use combined neutral and earth (CNE) cables.
However, it shall be permissible to use separate neutral and earth (SNE) cables for
services or networks where this is beneficial. In particular, for customers whose
installations may require special attention such as those identified in 3.15 of this Code of
Practice, an economic evaluation should be carried out of the alternative methods of
earthing. For customers in this category, and if the network is suitable, we may offer an
SNE supply.
New overhead networks shall use aerial bundled conductors in accordance with the code of
practice NSP/004/041 LV ABC Networks.
PNB may be adopted for a single customer supplied via an underground or overhead
service from a substation. However it is acceptable to provide PME to a single customer
and this method should be adopted where practical.
3.3.2
Existing Networks
The earthing arrangements on LV networks should be checked to see whether it is to the
standard described in this Code of Practice whenever any of the following situations occur:
-
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(a)
Connecting a new substation into the network, or recovering a substation from the
network.
(b)
(c)
(d)
(e)
Customers' earthing facilities which are provided via metallic water pipes are about
to be rendered ineffective by the insertion of non-metallic pipes in distributors or
service pipes by the Water Company or plumbing contractors.
(f)
(ii)
If the network comprises cables having electrically discontinuous sheaths but the
number of discontinuities is small, the sheaths shall be made continuous by
bonding across the discontinuities.
(iii)
In all other cases PME shall be applied to the network in accordance with the
requirements of this Code of Practice.
3.4
3.4.1
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(a)
(b)
(c)
(d)
3.4.3
3.4.4
3.4.5
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(i)
(ii)
(iii)
CE staff may visit customers' installations for miscellaneous reasons such as the
reconnection of a supply. If the installation is found to be inadequate then the
procedure in Appendix B should be followed to notify the customer.
(iv)
3.6
3.7
3.7.1
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For PME networks, obligatory connections of the neutral conductor to earth electrodes or
neutral conductors of other distributors shall be made at the final insulated end of every
branch of the distributor. A multi-service line supplying up to four adjacent customers need
not be considered as a branch of the distributor for this purpose. Where a service
supplying up to four adjacent customers is 40 metres or more in length an earth electrode
must be installed and connected to the neutral at or near the last service termination.
If the HV and LV earth electrodes have been separated at the substation the overall
resistance to earth of the HV earth electrode and that of the neutral conductor shall not
exceed 20 ohms. Measurements of resistance will be made by an approved method. If the
resistance of the HV earth electrode to earth exceeds 20 ohms then additional earth
electrodes shall be connected to reduce the resistance. If the resistance of the neutral
conductor to earth exceeds 20 ohms then additional earth electrodes shall be connected at
convenient points on the network to reduce the resistance. The method of installation of
earth electrodes shall be as described in 3.9.
If the HV and LV earth electrodes have been combined at the substation, because they
meet the conditions in Section 3.20.5 no measurement of neutral-to-earth resistance need
be made.
Drawing C952986 shows typical arrangements of LV network earthing.
3.7.2
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reduce the resistance. The method of installation of the earth electrodes shall be as
described in Section 3.9.
If there is any overhead line in the HV circuit between the ground mounted HV substation
and the source substation the HV electrode and the LV earth electrode shall be separated
unless the conditions in section 3.20.5 are met. The HV earth electrode and LV earth
electrodes shall not be combined at pole mounted transformers without written permission
from the Policy Manager.
Good load balance along an LV feeder will assist in containing neutral voltage rise in the
event of an open circuit neutral conductor.
Drawing C952986 shows typical arrangements of LV network earthing.
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Existing earthing arrangements of such networks are satisfactory and will not require
modification.
Any extensions or modifications including repairs to these networks will employ either CNE
or SNE cables, when SNE cable which has an insulated outer sheath is used an earth
electrode shall be installed at each service joint position.
When repairs or alterations are carried out on SNE networks, SNE cable may be used in
the following circumstances:
(a)
(b)
(c)
The neutral conductor of the CNE cable shall be connected to the neutral conductor and
sheath or earth conductor of the SNE cable at the transition joint. Further earth electrodes
shall be installed on the CNE cable section in accordance with Section 3.7.1.
Drawing C952994 Sheet 1 and C952997 Sheet 2 show typical arrangements.
The work completed above will result in the CNE cable section and that part of the SNE
cable lying between the substation and the CNE cable having been converted to PME. But
neither the branches of this SNE cable, nor any other distributors on the network will have
been converted unless the neutral conductor has been earthed at an appropriate position
on the branch or distributor.
Drawing C953019 shows typical applications of CNE cable
(ii)
The neutral conductor of the CNE cable shall be connected to earth electrodes or the
neutral conductors of other distributors at the end of every branch. At the transition joint it
will be connected to the neutral conductor and metallic sheath of the SNE cable.
Existing SNE customers may retain an SNE earth where the following conditions are met:
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The neutral conductor of the CNE shall be connected to the neutral conductor and metallic
sheath or earth conductor of the SNE cables at the transition joints.
Existing SNE customers on the SNE cable most remote from the substation may retain an
SNE earth where the following conditions are met:
The circuit is interconnected with another substation via SNE cable.
The SNE cable is 50 metres or more in length, whose metallic sheath is in close contact
with earth, as it will normally provide an adequate connection with earth (10 ohm or less) to
limit the rise of potential under open circuit neutral.
Where the SNE cable is less than 50 metres in length and not interconnected with another
substation then an earth electrode connected to the cable sheath shall be installed at each
service joint position and at the end of each branch or the cable shall be converted to PME
and the bonding checked at existing properties. Further earth electrodes need be
connected only to ensure that the overall resistance to earth of the neutral of the LV
network complies with Section 3.7.1.
The work completed above will result in the CNE cable section and that part of the SNE
cable section lying between the substation and CNE cable section having been converted
to PME. Any branches of the SNE cable nearer the substation and all the SNE cable more
remote from the substation will not have been converted to PME unless additional neutral
to earth connections have been made at appropriate positions.
Drawing C953019 shows typical applications of CNE cable.
(iv)
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Where it is necessary to overlay a section of SNE cable in an SNE network 3 core CNE
waveform cable will be used unless it can be shown to be more economic to use 4 core
SNE cable in which case 4 core SNE cable may be used. Where the SNE cable which is
replaced had a metallic sheath which is in close contact with earth the disconnected cable
will be retained and the metallic sheath used as an additional earth electrode to help
maintain the network earth. The disconnected cable must be marked up on the cable
records as an earth electrode.
Where 3 core CNE waveform or 4 core SNE waveform cable is used the following
additional procedure will be adopted:At suitable service joint positions (Approx. every 50m) cross-bond the copper earth screen
to the metallic sheath of the disconnected cable with 35mm sq cu insulated conductor
using a substantial sweated connection or an earth clamp which complies with the
requirements of BS951. This will help reduce the overall network earth impedance.
If 4 core SNE waveform cable is used the following procedure will be adopted:The tail end of the 4 core waveform cable will be cap-ended with an earth electrode to PME
standard. Both SNE and CNE earth facilities can continue to be made available along the
route in accordance with Section 3.8.1 (v).
(v) Provision of CE Earth Terminals on mixed CNE and SNE Cable Networks
CE earth terminals, connected to the neutral conductor, will normally be provided to
customers connected to CNE cable sections unless they are precluded under the ESCQ
regulations (boats and caravans) or are a Special Case as defined in Section 3.15. The
customers' installations must comply with the bonding and earthing conditions of Appendix
A before connection to the earthing terminal may be made.
Existing services on the SNE cable section may either be provided with, or may retain, CE
earth terminals connected to the sheath or earth conductor. When PME has been applied
to part of the network, all new services on the SNE sections of the network will be provided
by CNE service cables and the customers offered the use of a CE earth terminal
connected to the neutral; their installations must then comply with the PME bonding
conditions of Appendix A. If the metallic sheath of the SNE cable section is in close contact
with earth it will be necessary to bond the neutral conductor of the CNE service cable to the
neutral and metallic sheath of the SNE cable section only at the service joint. In cases
where the SNE sheath is insulated from earth or is less than 9 metres in length then an
earth electrode must be installed and connected to the neutral at the service joint, unless a
similar connection has previously been made on the branch concerned at a point farther
from the supply transformer. Note -The 9 metres of SNE cable is equivalent to an earth rod which would
need to be installed at the most remote point on the network or at the most remote service position to enable
PME supplies to be provided from the network.
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If CNE cables are used to replace existing lengths of SNE cables and existing SNE service
cables are transferred to the CNE cable, any existing CE earth terminals must be replaced
by earth terminals connected to the neutral conductor. The metallic sheath of the SNE
service cable shall then be bonded to the neutral conductor of the CNE cable at the service
joint. Customers' installations must then comply with the requirements of Appendix A.
3.8.2
3.8.3
3.8.4
to
Overhead Networks without Aerial Earth Wires and which have not been converted
PME or PNB
Existing earthing arrangements of such networks are unsatisfactory and shall be brought to
the required standard in accordance with an agreed programme or when any of the
opportunities in Section 3.3.2 arise.
PME shall be applied to the networks and the earthing arrangements at the substation
supplying the network shall be made to comply with the requirements of Section 3.20
where practical.
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The neutral conductor of the network shall be connected to an earth electrode, or to the
neutral conductor of another distributor, at the end of every distributor or branch to which
PME is to be applied. These connections shall normally be made at the final support of the
distributor in order that all customers may be offered a CE earth terminal.
Group services or under-eaves wiring supplying up to 4 adjacent customers need not be
considered as branches of the distributor for this purpose.
Additional earth electrodes shall be installed at intervals of not more than eight span
lengths along the route of a long overhead distributor. All neutral connections shall be
made using compression joints or duplicate mechanical connectors.
Where LV underground cables are connected in overhead lines the metallic casing of cable
boxes and any metallic cable sheaths shall be connected to the neutral conductor at both
ends of the underground cable sections. Line tap type neutral connections on tier type
cable boxes shall be replaced with compression lugs fitted with double locking nuts.
Where an LV isolator is installed in the run of an overhead line the neutral blade of the
isolator shall be removed and replaced by a bolted link or compressed jumper connection.
If the HV and LV earth electrodes have been separated at the substation, the overall
resistance to earth of the neutral conductor must not exceed 20 ohms. The measurement
of resistance shall be made by an approved method and additional earth electrodes
installed at convenient points on the network as necessary to achieve the required value.
If the HV and LV earth electrodes have been combined at the substation no further
measurements of neutral to earth resistance need be made.
3.8.5
3.9
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Where earth electrodes are installed at substations they shall consist of driven rods or
stranded copper conductor or copper strip, the driven rod is preferred. The earth
resistance of these electrodes will normally have to be measured at pole-mounted
substations and, in combination with HV cable sheaths etc, at ground mounted substations.
3.10
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or by connecting the sheath to the link box or pillar and to the neutral conductor at a
convenient position (e.g. a joint or service termination). If the metallic case of a link box or
pillar is not in contact with earth, either directly or by connection to a cable sheath which is
earthed, then an earth electrode must be installed and connected to it.
All neutral links should be fitted permanently in position by bolting even where the phase
links are removed for normal operation.
All metallic link box pits should also be bonded to the metal case of the link box and the
neutral conductor.
Where non-metallic link boxes are used in conjunction with metallic link box pits, there shall
be a bond between the metallic link box pit and the neutral conductor.
3.11
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The earth terminal and any external metalwork of street lighting fittings mounted on
buildings or wooden poles will be connected to the PME earth terminal. The only exception
is if the fitting is double insulated. Bond size will be 6mm2.
3.11.3 Roadside and Other Housings Accessible to the Public.
Examples are public telephones, pedestrian crossing bollards, and ticket machines.
Equipment of this type should be Class II (double insulated) or equivalent construction. No
mains derived earthing terminal is required, neither is a residual current device needed for
earth fault protection.
3.11.4 Existing Separate Street-Lighting Mains with Inadequate Earth-Return Facilities
The earthing arrangements on these distributors shall be brought to the standard required
by the Electricity Safety, Quality and Continuity Regulations 2002. This shall be done
either at the request of the Local Authority or as the result of the earthing improvements on
the electricity distribution network.
The sheaths of any underground street lighting cables and the metalwork of any link boxes,
pillars or pole boxes will be connected to the neutral conductor.
3.11.5 Street-Lighting Services from Electricity Networks with Inadequate Earth Return
Facilities
Where individual street-lighting services are provided from distributing mains with
inadequate earth-return facilities two courses of action are possible: (i)
Bring the earthing arrangements of the whole network up to the standard required
by this Code of Practice.
(ii)
Apply the Electricity Safety, Quality and Continuity Regulations 2002 to those parts
of the network from which street-lighting services are fed.
Generally the former course of action will be preferable as the benefit of improved earthing
facilities may be offered to all customers.
In some cases, however, the latter course of action may be necessary. On every branch
or section of a distributing main to which the relevant street furniture is connected the
neutral conductor shall be bonded to the non-current carrying metalwork of the relevant
equipment with the exception that metal doors of concrete and plastic columns shall not be
so bonded where the supply to the column is derived from a PME network. The neutral
conductor of the distributor will also be connected to earth or to the neutral of another
distributor at a point farther from the supply transformer than the most remote street
lighting column etc, at which point an earth terminal is to be provided.
In any case where looped services are provided between street lamps an earth electrode
shall be installed and connected to the neutral conductor at the final lamp position. It is not
permissible to consider metallic street furniture to be earth electrodes.
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armouring and metallic sheath of the service cable need not be bonded to the neutral
conductor.
This exception is permitted because the possibility of being able to touch the cable sheath
is extremely remote and for it to be at a potential there would have to be an open-circuit
neutral conductor fault in existence at the time.
3.12
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3.14
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expected to come into, electrical contact with earth and which are in easy reach of any
exposed non-current-carrying metalwork of the Developer's or Contractor's electrical
installation.
Construction site supplies shall be provided through a residual current device (RCD) which
must be provided by the site Developer or Contractor. The construction site earth
continuity conductor should be connected to an independent earth electrode which should
be a minimum of 9 metres away from any HV earth electrode or cable sheath which may
be on the site. The independent earth electrode should be of such a value that in the event
of an earth fault, the maximum sustained voltage between the earth continuity conductor
and the general mass of earth should not exceed the values prescribed in the current
edition of BS7671.
Alternatively the Developer could install a 1:1 isolating transformer of the appropriate rating
enabling a separately earthed neutral LV system to be created within the boundaries of the
site.
For large temporary supplies which require their own substation it will usually be possible to
provide an earthing terminal connected directly to the transformer neutral in accordance
with Section 3.12.1. For detailed requirements BS 7375 and BS 7671 should be consulted.
Where a CE PME earth terminal is provided for future use the earth terminal block shall be
rendered inaccessible so as to prevent unauthorised connection. A notice with inscription
"CE EARTH TERMINAL - NOT TO BE USED ON A TEMPORARY SUPPLY" shall be fixed
adjacent to the earth terminal block.
3.15.2 Farms, Milking Parlours, Pig Stys etc
A PME earthing terminal shall be given to farm premises and similar establishments
provided that the installation complies with the bonding requirements and conditions of
Appendix A.
There are areas of special risk where animals could be subjected to a difference in
potential between wet floors and bonded metalwork. In these situations the metalwork in
the entire area of any farm buildings shall be bonded to the earthing terminal. If the
customer wishes he may install a high sensitivity residual current device to provide
additional protection in the event of a fault on the installation within the wet area. This is
advisable where there is a bare concrete ground floor within the total area of the main
buildings. The CE earthing terminal may be used as the circuit breaker earth electrode
provided the whole of the metalwork within the main buildings is effectively bonded.
Where in remote buildings all extraneous conductive parts cannot be bonded to the
earthing terminal, the pipes and metalwork of isolated buildings, whether or not they have
an electricity supply, shall be segregated from metalwork connected to the PME earthing
terminal. Any supplies to such buildings should be controlled by a RCD and the associated
earth electrode and protective conductor shall be segregated from any metalwork
connected to the PME earthing terminal.
Particular care must be taken in areas where livestock are housed as they are sensitive to
very small voltages. A suitable metallic mesh shall be installed in the concrete bed of a
dairy and bonded in accordance with the PME requirements.
If PME is to be applied to an existing dairy the steel reinforcement in the floor should be
bonded. Alternatively if small voltage differences are unacceptable the area concerned
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should be protected by an RCD and the associated earthing system segregated electrically
from the remainder of the installation.
(b)
PNB Installations
Where a dedicated transformer is used to supply a single customer farm premises then
PNB shall be adopted and shall comply with the conditions of Section 3.12.2 and Appendix
A.
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A number of buildings with bar services have a 2kVA isolating transformer with a 110V
centre-tapped secondary winding to give 55V to earth. A supply is given via a flexible
armoured cable to delivery tankers which are fitted with 1hp pump motors.
The transformer does not however provide isolation for the centre-tap of the 110V winding
if it is connected to the PME earthing terminal. The risk from providing supplies outside the
equipotential zone is considered to be slight particularly as the time the tanker will be
connected to the supply will be short.
3.15.6 Quarries
A PME earthing terminal may be provided for the part of the installation associated with
permanent buildings such as offices provided that the buildings electrical installation wiring
conforms to BS 7671 and there is no interconnection with the earthing arrangements
associated with the plant used in the production work of the quarry.
3.15.7 Petrol Filling Areas
The HSE Notes of Guidance for the provision of Petrol Filling Stations exclude the use of
PME for the petrol filling area. PME can be provided for use in other buildings associated
with the premises. The developer may request us to provide an SNE earth for the
development. If the supply is to be given from an SNE network then an SNE earth can be
provided. The connection charge will include the additional costs of providing an SNE
service.
If the service is to be supplied from a PME network then the developer will have to provide
the necessary RCD protection for the petrol filling area.
3.15.8 Fairgrounds and Showgrounds
PME earth terminals will not be made available to such installations. A residual current
device must be installed by the customer.
3.15.9 Railway Service Areas
It is recognised that under starting currents or traction fault conditions, a voltage gradient
can appear on the traction return rail, which could be impressed on the supply cable. A
PME earth terminal will only be provided where the railway operator can confirm that a
voltage not in excess of 25V will be impressed on the distribution company earth terminal.
Note - Care is necessary with DC traction systems using earth return, as this can cause
accelerated corrosion to exposed (i.e. Hessian served) cable sheaths and earth electrodes.
3.15.10
The Electricity Safety, Quality and Continuity (ESQC) Regulations 2002 do not permit the
provision of PME earthing terminals for caravan and boat installations. This has the
following implications:(a)
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(b)
(c)
A PME supply shall not be extended to shower blocks due to the higher probability
of persons being barefooted at these locations unless a buried bonded metal grid
has been installed. This requirement also applies to other locations where toilet or
shower blocks have been provided for general public use where people are likely to
be barefoot e.g. beachside locations, parks etc. Where outside showers have been
provided then provision of PME earthing is not recommended as providing a reliable
equipotential cage may prove impractical.
(d)
The site wiring shall be two-wire, phase and neutral, supplied through residual
current devices, each one of which shall not supply more than 6 caravans. The
residual current devices must be supplied by the customer or site owner.
The caravan installation should comprise separate phase, neutral and earth
conductors. The earth conductor should be connected to an independent earth
electrode which may be common for up to 6 caravans. The earth electrode should
be a minimum of 3 metres away from any PME earth electrode which may be on
the site. The independent earth electrode should be of such a value that in the
event of an earth fault, the maximum sustained voltage between the earth continuity
conductor and the general mass of earth should not exceed 50 volts without the
residual current device operating. For values of resistance of the independent earth
electrode relating to various rated operating currents of residual current devices see
Drawing C953027.
The main chassis of the caravan should be connected to the earth continuity
conductor and hence to the independent earth electrode.
Drawing C953027 also shows the recommended method of supplying a caravan
site and should also be used for yacht marinas and other similar installations such
as sites with certain types of chalets.
(e)
A PME earthing terminal may be provided for use in mobile homes or temporary
site offices which are constructed so that a person in contact with the general mass
of earth cannot touch any metalwork of the home or site office which would be
connected to a PME earthing terminal. Such mobile homes or site offices may be
treated in the same manner as permanent premises and provided with a PME
earthing terminal if the installation meets the bonding requirements of the relevant
British Standard.
Where the mobile home or temporary site office has exterior metalwork and cannot
comply with the above touch criterion, a PME earthing terminal shall not be
provided. Supply should be given through a residual current device, and an
independent earth electrode sited at least 3 metres away from any PME earth
electrode should be provided. Both of these items are the responsibility of the
customer.
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A PME earthing terminal may no longer be provided for use in a vehicle used by the
British Broadcasting Corporation, the Independent Broadcasting Authority or others
for the purpose of outside broadcasts as the blanket approval is withdrawn. An
RCD shall be provided by the customer.
High-Rise Buildings and Multi Occupancy Building Complexes
Engineering Recommendation G87 Guidelines for the Provision of Low Voltage Supplies to
Multiple Occupancy Buildings should be read in conjunction with the following paragraphs
which summaries the earthing requirements.
Networks within multiple occupancy buildings shall be designed on an SNE principle,
although incoming service connections from local PME distributors are permissible, as is a
PME earth terminal at the Intake Position. The appropriate main equipotential bonding
connections to structural steelwork and to metallic services should be made at this point of
connection, for details of the size of copper conductor required refer to appendix E. At
individual Customers Installations the main equipotential bonding between metallic
services, extraneous metalwork and the earth terminal shall be carried out in accordance
with BS 7671. This will ensure that no harmful potentials appear between earthed and
extraneous metalwork within the Customers premises under fault conditions. A single
Intake Position is the preferred design option for, amongst other reasons, avoiding
problems caused by the flow of neutral current through the building steelwork and
equipotential bonding between the Intake Positions. Note that a single Intake Position may
have more than one incoming service cable and cutout. If it is not possible to design on the
basis of a single Intake Position, then a non-PME connection option shall be considered (ie
shall be of the SNE type or no earth shall be provided from the network the landlord
making their own earth provision). All Intake Positions within a single building shall employ
the same earthing method. PME connections are not recommended in these situations due
to the risks associated with significant neutral current flow through structural steelwork and
fixings under fault or other conditions and the difficulty associated with providing an
alternative DNO-owned bond between Intake Positions.
In some steel framed multiple occupancy buildings such as a horizontal run of industrial or
commercial units where individual services are to be provided from a DNOs PME
distribution main it will be more difficult to avoid the problems identified earlier. There are
several ways of addressing this which will vary to the extent that they are able to mitigate
the problem.
Provide a single Intake Position to the building with a group metering position at a
communal point. A Customers separate neutral and earth cable could then be
supplied to each unit separately.
Individual PME connections are not recommended in these situations due to the risks
associated with significant neutral current flow through structural steelwork and fixings
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under fault or other conditions and the difficulty associated with providing an alternative
DNO owned bond between Intake Positions. In the case of multiple PME cutouts at a single
Intake Position, all earth terminals shall be bonded together. The size of the bonding
conductor shall be not less than that of the smallest associated service cable neutral
conductor. Such bonding conductors shall be coloured green-yellow with the addition of
blue markers to indicate that they will be carrying current.
Drawing C953070 shows the bonding requirements for an SNE rising main.
Where minor changes or extensions are carried out to existing complexes which have been
supplied with CNE mains safeguards have to be taken with regard to the connection of the
supply neutral conductor to earth at the extremities of the LV distribution systems in the
building complexes. It is essential that the PME earth shall provide a continuous metallic
path from the remote end of the distributor to the neutral at the substation.
The connections at the various ends of the neutral conductors shall be made in one or
more of the following ways:(a)
On a ring system the neutrals are made continuous at normally open points.
(b)
A bonding lead connecting the neutrals between the remote ends of two
distributors.
(c)
A separate earth conductor run from the remote ends of the distributors to an
earth electrode or the neutral at the substation.
(Note - Potential EMC problems may occur particularly if there is more than one PME
supply cable, where neutral return currents could flow through the metal frame of the
building appendix C provides guidance on minimising EMC problems)
Incoming gas and water services and structural steel work in contact with the general mass
of the earth will be bonded to the earth bar of the incoming distribution panel in accordance
with Appendix A.
3.15.12
Occasionally a service will terminate in a position remote from the building it supplies. In
this case the size of the PME bonding in the building supplied must be related to the size of
the incoming supply cable in accordance with appendix E. If the size of the circuit
protective conductor of the cable between the supply intake position and the building is
less than that of the PME bonding conductor, a suitable additional conductor will be
installed.
3.15.13
Under an open-circuit supply neutral condition the potential of an outside water tap will rise
above earth potential. A person coming into contact with the tap could receive an electric
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shock and the shock could be severe if that person were barefooted. The probability of
these two conditions occurring together is considered to be so small that the use of PME
where a metal outside tap exists is not precluded.
An insulating insert may be incorporated in the pipe to the outside water tap. However care
should be taken to ensure that simultaneous contact with metal pipework on each side of
the insert is not possible if there is likely to be a potential across the insert under this
condition
3.15.14
For the majority of situations it is acceptable for lightning protection to be connected to the
customer's earthing arrangements providing it has been designed and installed in
accordance with the latest version of BS 62035 "Protection against Lightning".
When periodic testing of the lightning protection system is undertaken, precautions are
necessary when breaking the link between the lightning protection electrode and the
customer's earth since the customer's earth may be at a potential above true earth.
3.15.15
Examples are Public Telephones, Pedestrian Crossing Bollards, Ticket Machines etc.
Equipment in this type of structure should be Class II or equivalent construction. No
mains-derived earthing terminal is required, neither is a residual current device needed for
earth fault protection.
The service arrangement to BT public telephone housings is covered in Engineering
Recommendation PO4/1. It is suggested that telephone housings owned by other
telephone operators should have similar service arrangements.
Other freestanding structures not of Class II construction such as small brick or metal
housings, or metal cabinets containing pumps, motors, controls etc, may be offered the
use of a PME terminal provided the supply is single phase and the maximum load does not
exceed 2kW. An earth electrode with a value not exceeding 20ohms should be supplied
and installed by the customer and connected to the earth bar.
If these conditions are not met, a PME terminal should not be offered. No earth terminal
shall be made available from the network, the customer will install their own earth and
protect the circuit be installing an RCD.
For other wall or pedestal mounted structures not of Class II construction which require a
single phase supply, the use of a PME terminal should not be offered. No earth terminal
shall be made available from the network, the customer will install their own earth and
protect the circuit by installing an RCD
For installations requiring three phase supplies where the load is balanced, PME can
normally be offered.
Crash barriers adjacent to street furniture should not be connected to a PME earth terminal
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2.
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3.19
Introduction
This section summarises the standard arrangements for the earthing and bonding of the
most common standard types of 20kV, 11kV and 6kV Ground-Mounted Distribution
substations owned and installed by CE.
Three common standard arrangements are provided in section 3.20. Where an HV
substation is to be sited within or adjacent to, a higher voltage substation (e.g. 132/33kV,
33/11kV, etc), the guidance of the Design Engineer must ALWAYS be sought before
proceeding with the construction process.
3.20.1.2
For a fault directly to earth or between HV and LV transformer windings, a path is required
to allow sufficient earth fault current to flow back to its source, operate protection and
disconnect the supply. An important part of the path is the earth electrode at the substation.
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For the majority of ground mounted substations, a combined HV/LV earth with will be
installed. (See provisos below)
The electrode will consist of bare conductor and earth rods installed around the HV
equipment and bonded to the substation equipment.
Exposed, earthed metalwork associated with HV plant (transformer, switchgear etc) is to be
connected to the bare electrode in and around the substation. The LV transformer neutral
is to be connected to the same electrode, or if separately earthed via insulated conductor to
the LV electrode system situated at least 9m away from the substation.
Where the HV and LV earths are combined an electrode will be laid for a minimum of 80m
out with incoming and outgoing HV cable routes. Where a new substation is teed into an
existing network the additional electrode will be laid the length of the HV trench where this
is less than 80m.
Bonding of the HV and LV earths can be achieved using a combined electrode or the fully
rated link within the LV distribution cabinet, which connects the LV neutral bar to the HV
switchgear earth bar.
Where the systems are combined, the neutral-earth connection and LV earth electrode will
form part of the return path for HV fault current.
The HV and LV electrode systems can be combined only where the following two
conditions apply:(i)
The conditions in paragraph 3.20.5 are met (the substation is connected to a global
earthing system).
(ii)
The EPR at the substation is less than 430V during fault conditions.
The conditions can normally be satisfied if the substation is in an urban location, within an
all underground cable network and not the first substation on a circuit out from a primary
substation.
Where the HV and LV earths are not combined because the conditions (i) and (ii) above
can not be met (normally substations teed off an overhead line) earth resistance values not
exceeding those given in table 3.20.1.2 will be achieved. Separate HV and LV electrode
systems are required which are electrically segregated via a 9m separation in the soil, in
these situations no additional electrode will be run with the outgoing HV cable routes.
It is the companys policy to identify substations where HV and LV electrodes are not
combined, by fixing a label to the outside of the LV cabinet.
Where HV and LV earth electrodes are segregated the following maximum values shall
apply:Table 3.20.1.2
Maximum Value
System
Voltage kV
11
HV Earth
LV Earth
20
20
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3.20.2
3.20.2.1.
General.
(i)
Page
At new ground mounted substations in urban areas supplied from global underground
networks, the HV and LV electrode systems will normally be combined, i.e. a separate LV
electrode is not necessary. In all other areas, separate HV and LV electrode systems shall
be installed. Where these are kept separate, a warning notice is to be fitted and shall have
the following wording:-
CAUTION
Avoid making simultaneous contact between the high voltage metalwork earthing
and low voltage earthing systems.
3.20.2.2
Bonding.
All normally accessible metalwork will be connected together and to the HV earth, via
bonding conductors. This is to ensure that all adjacent, exposed metalwork remains at a
similar potential during fault conditions. The bonding conductor between main items of
plant and the electrode system could carry significant earth fault current and must be fully
suitable for this purpose see appendix E.
Exposed metalwork (transformer and switchgear enclosures, metal fencing, metal doors,
metal enclosures, LV enclosures) will be bonded together and to the main HV earth
electrode (bare electrode in and around the substation).
Doors, metal roofs etc., will not carry appreciable fault current, other than via a secondary
wiring fault, so smaller bonding conductors are permitted. In all cases the minimum size is
16mm2 copper or 35mm2 aluminium.
3.20.2.3
Control of Potential.
During an earth fault, protection must be provided for anyone who could be in contact with
exposed earthed metalwork. This applies particularly to those required to carry out fault
switching at the substation. This protection can be provided by keeping the operators
hands and feet at a similar potential.
This is achieved by laying bare HV electrode, which is connected to exposed metalwork
and switchgear, in the ground such that it is immediately beneath and just beyond the
position where the operator stands
The following general conditions apply:
For ground mounted substations supplied from an overhead network or substantive
underground network, a standard arrangement which provides potential grading and a
reasonably low resistance, is always required. The standard earthing arrangements in
section 3.20.7 should suffice without any further additional electrode.
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touch voltages. Such systems permit the division of the earth fault current in a way that
results in a reduction of the earth potential rise at the local earthing system. An example of
a global earth system is one associated with a city centres where the majority of cables
have metallic sheaths in direct contact with earth.
In urban situations it is desirable that ground mounted distribution substation HV and LV
earths are bonded together to achieve the lowest practical earth value in an environment
where installation and effective segregation of earths is often difficult or impossible.
Previously the only criterion that had to be met was the combined values of the high and
low voltage earths should be 1ohm or less this is no longer acceptable.
If HV and LV earths are to be combined there is an obligation for CE to ensure the potential
on the combined HV/LV earth electrode and in turn customers exposed conductive
metalwork does not rise to unacceptable levels (above 430V) under fault conditions. The
two main fault scenarios are:
EHV faults at the source primary substation causing the primary substation and its
HV cable system to become Hot.
The possibility of the earth potential rise (EPR) on the combined HV/LV earth electrode
rising above 430V can generally be ignored when:-
1)
ii.
iii.
At least 8kM of metallic sheathed cable in direct contact with the soil.
iv.
OR
2)
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ii.
iii.
iv.
At least 2kM of metallic sheathed cable in direct contact with the soil with a
continuous cable sheath connection back to the source primary substation.
v.
If the above conditions are met the HV and LV earths of urban ground mounted distribution
substation may be bonded together.
If any of these criteria cannot be met further investigation will be required.
If condition 1 or 2 cannot be met the actual earth return current at the proposed
distribution substation can be calculated and Tables 1 and 2 consulted using this
current in place of the source fault current.
If condition 3 cannot be met refer to the NEDL/YEDL Earthing Databases (It may
show that a primary substation is cold).
If after this further investigation the requirements for combining the HV and LV earth can
not be met the earths shall be separated with reference to the guidance in section 3.20.7.3
Table 1
Calculated Impedance of PILCSWA Cable Networks with metallic sheaths in contact with the soil Assumes No Continuous Metallic Sheath Path Back To Source
Resultant EPRs > 430V are shown highlighted in grey
Total*
Length of
PILCSW
A cable kM
2
4
6
8
10
3000A
2500A
2000A
1500A
1000A
500A
0.431
0.247
0.188
0.155
0.136
1293
741
564
465
408
1077.5
617.5
470
387.5
340
862
494
376
310
272
646.5
370.5
282
232.5
204
431
247
188
155
136
215.5
123.5
94
77.5
68
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*Metallic cable sheaths must be in direct contact with the soil.(hessian serving
allowed)
Cable in ducts or with plastic sheath do NOT count.
Cables following parallel routes do NOT count.
Metallic sheathed LV cables may only be counted if they are connected to a common HV/LV earthing
system
Uniform Soil Structure 200m
Table 2
Calculated Impedance of PILCSWA Cable Networks with metallic sheaths
in contact with the soil. Assumes Continuous Metallic Sheath back
to source substation (maximum 65% reduction - minimum 35% ground
return)
Resultant EPRs > 430V are shown highlighted in grey
Total*
EPR for Gross Source Earth Fault Currents of:
Length of
Impedance
PILCSW
ohms
3000A 2500A 2000A 1500A 1000A 500A
A cable kM
2
0.431
453
377
302
226
151
75
4
0.247
259
216
173
130
86
43
6
0.188
197
165
132
99
66
33
8
0.155
163
136
109
81
54
27
10
0.136
143
119
95
71
48
24
*Metallic cable sheaths must be in direct contact with the soil.(hessian serving
allowed)
Assumes continuous Sheath back to source primary substation.
Cable in ducts or with plastic sheaths do NOT count.
Cables following parallel routes do NOT count.
Metallic sheathed LV cables may only be counted if they are connected to a common HV/LV earthing
system
Uniform Soil Structure 200m
The following issues/cases should also be considered: 1) If the substation is connected to a modern HV cable network with all insulated cable
sheaths, e.g. PICAS, XLPE etc. In this case the HV and LV cable sheaths will make
no significant contribution to the HV earthing system. (They are generally the most
important factor in lowering earthing resistance in older systems with hessian
served metallic sheathed cables) Unless carefully managed there is a natural
tendency for such systems to have high resistances to earth and potential rises
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under fault conditions. The benefits of Global earthing tend to be lost overtime as
more insulated sheath HV and LV cable is installed and cables with metallic sheaths
in contact with the soil are removed.
2) In order to maintain a global earthing system the following action will be taken: o
In newly developing urban areas where a high proportion or possibly all the
network has or will be plastic sheathed HV and LV cables and where there is
higher than average soil resistivity (>200m) consideration will be given at
the network design stage to installing addition earth electrode in excess of
the 80m mentioned above at each distribution substation in such cases
guidance of the policy manager for earthing will be sort.
3) The use of land in which bare earth electrodes are placed should be considered.
Locations to avoid include sites where people may be expected to walk barefoot
such as caravan sites and pathways to beaches etc. Animals have a relatively low
step voltage (25V), hence routes where farm animals may be expected to pass or
congregate frequently such as gateways and drinking/feeding troughs etc should be
avoided.
4) Bare copper conductor can corrode in hostile soil environments causing earthing
systems to become ineffective. Soils containing sulphates, sulphides and acids
should be avoided.
5) HV cables with insulated sheaths or where the sheath is not in close contact with
earth will not contribute to the global earth. Examples are XLPE, aluminium - or
lead-sheathed cables with PVC oversheath, and cables laid in bitumen or
compound-filled troughing or pipes, or laid on cable trays. Hessian or jute serving is
not considered to form an insulated sheath.
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Construction Details
3.20.6.1
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(ii)
Extensible earth rods - copper clad to ENA TS 43-94. 1.2m long 16mm diameter
and / or deep driven 70mm2 stranded copper.
(iii)
Bonding conductor for connecting roof and metal walls of external enclosure 2
stranded copper, PVC covered, flexible 16mm cross sectional area.
(iv)
(v)
(vi)
3.20.6.2
Installation of Electrode.
The standard electrode arrangement will be installed during the civil work associated with
construction of the substation. Where it can be reasonably assumed that the soil resistivity
will be greater than 200m (i.e. in arrears where the soil is made up primarily of slate,
shale or rock), prior to, or during the civil works, soil resistivity readings should be taken.
Once interpreted, these can be used with tables 3.20.7.2 and 3.20.7.3 to provide an
estimate of the resistance value. If the calculated resistance is too high, the guidance in the
tables should be followed to reduce it. Normally this is by installing longer vertical rods.
Alternatively, or in addition, earth electrode can be installed in one of the HV cable
trenches, provided this is 9m from any planned LV electrode.
The LV electrode can be laid in an LV cable trench, providing that 9m separation is
maintained between this electrode and any HV metalwork, HV hessian served cables and
the HV electrode. Guidance is provided in Table 3.20.7.3 concerning the amount of
electrode required to achieve the necessary LV resistance value.
Care must be exercised when driving earth rods because of the possibility of puncturing /
damaging cables or other services buried below the cable trench level. This is especially
important when rods are installed after the completion of cable laying. Cable locating tools
and techniques must be used in addition to consulting accurate cable records, to avoid
damaging such cables/services.
Where earth conductors are laid adjacent to HV cables, the backfilling medium will be
dictated by the cable thermal requirements. It is standard practice to lay the earth
conductor on the bottom of the trench, below the cables, so it is in direct contact with the
surrounding soil. Ideally there should be 0.5m clearance between hessian-covered or plain
tape-armoured conductors and the earthwire, but this can be reduced to 0.2m when the
cable or earthwire has an insulated outer sheath.
Horizontal electrode adjacent to the substation door (and any other earth electrodes
forming part of measures to reduce the effect of touch voltage) are to be bedded in suitable
backfill, then covered with a layer of crushed rock or chippings or concrete slabs.
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Under no circumstances are earth electrodes to be installed in coal ash (or similar
material), which is normally very corrosive to copper.
3.20.6.3
20/11/6.6kV Cables
The earth sheaths (and armours, if present) are to be terminated via a flexible connection
onto the switchgear earth bar that interconnects the transformer and HV switchgear. The
flexible connection shall be sufficiently long to accommodate earth passage fault indicators
2
or automation transmitters and shall be of 50mm (minimum) copper. It is important that
these connections are of low resistance and secure against loosening by vibration etc. The
resistance of the connection and joint is to be measured using a micro-ohmmeter and will
be below 100.
(ii)
(iii)
(iv)
Additional protection is provided to protect operators, staff and the public against
possible excessive voltage rise, where applicable.
Standard construction drawings, tables which give the S/S electrode resistance and where
necessary a separate LV electrode resistance for a range of uniform soil conditions and
notes of guidance are given for three typical ground mounted substation designs.
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Earthing Arrangement
Drawing
Number
C944571
C951997
C944570
C944575
Arrangement
No.
1
2
3
4
25 or less
50 or less
100 or less
Arrangement
4
2.2
4.3
8.5
150 or less
13.0
12.4
9.7
12.7
200 or less
17.3
16.6
12.9
500
43.2
41.4
32.2
Typical Soil
Resistivity (m)
Soil Type
Loam
Chalk
Clay
Sand/Gravel/Clay
mix
Sand/Gravel/Clay
mix
Slate/Shale/Rock
mix
42.2
Where the HV and LV earth are separated additional HV earth electrode to the standard
design will be required for soil resistivities in excess of around 200m to maintain a 20
maximum HV earth resistance value.
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Arrangement 1
Horizontal Electrode
Only (Length Required)
25
5m
50
6m
100
12m
150
20m
200
28m
500
75m
Arrangement 2
3m Vertical Rods + 5m
Horizontal Electrode Segments
3m Rod x 7 plus
7 x 5m = 35m Horizontal
Key
Key 3m Vertical Rod
5m Horizontal Electrode
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Arrangement 1 - Ground Mounted Substation with close coupled switchgear and Combined HV/LV
Earth
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Arrangement 2 - Ground Mounted Substation with close coupled switchgear and separate HV/LV
Earth
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Arrangement 3 - Steel Kiosk Type Ground Mounted Substation with Combined HV/LV Earth
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Arrangement 4 - Ground Mounted Substation with separate HV switchgear and Combined HV/LV
Earth
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Pole-Mounted Substations
The cost of installing earth electrodes at an isolated pole-mounted substation and
associated LV network to achieve a resistance to earth value capable of restricting the EPR
to less than 430V under HV fault conditions will normally be excessive.
Therefore, the HV earth electrode at the pole-mounted substation shall always be
separated from the LV earth electrode.
The HV earth electrode and the LV earth electrode shall each have a resistance to earth
less than 20 ohms; the value will be measured by an approved method.
A minimum distance of 9 metres will be maintained between any uninsulated parts of the
HV and LV earth electrodes which are below ground level. The required separation will
normally be obtained by earthing the LV neutral conductor one span away from the
substation, Drawing C953076 Fig A. (over page) Connections to earth electrodes will be
insulated from earth to a depth of at least 600mm below ground level using the conductors
specified in Appendix E.
Consideration must be given to maintaining HV/LV earth physical segregation including
taking into account the presence of metallic sheathed cables.
If a metallic sheath LV cable is connected at the substation its metallic sheath must be
connected to the neutral conductor. Separation of the electrodes in this case may be
achieved by connecting the transformer tank to a remote earth electrode by a green
coloured insulated lead as shown in Drawing C953076 Fig B. The HV earth electrode lead
on the pole will be spaced at least 75mm from the LV earth-lead and the sheath of the LV
cable.
If, at an existing pole-mounted substation, a common HV and LV earth electrode is
installed and its resistance to earth is greater than 1 ohm then the HV earth shall be
separated from the LV earth. The existing earth electrode shall normally be used for the
LV earth and a new electrode shall be installed for the HV earth.
Pole mounted substations supplying mobile phone base stations on transmission towers.
shall be designed and installed in accordance with DSS/022/001 (Code of Practice for Low
Voltage Connections to Mobile Telephone Base Stations with Antennae on High Voltage
Structures)
Where copper theft has occurred on a site the above ground bonding should be replaced
with aluminium conductor in line with Note 3, 4 and 7 of appendix E.
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Document Reference
2
Version
4.
References
4.1
Internal
IMP/010/011
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4.2
External
BS 951: Electrical Earthing. Clamps for Earthing & Bonding. Specification
BS 62035 - Code of practice for protection against lightning
BS 7361 Cathodic Protection.
BS 7375: Code of practice for Distribution of electricity on construction and building sites
BS7671 - Requirements for Electrical Installations (IEE Wiring Regulations) - Current
Edition
Engineering Recommendation G59/1- Recommendations for the Connection of Embedded
Generating Plant to the Public Electricity Suppliers Distribution Systems (Amendment 1
(1992) & Amendment 2 (1995))
Engineering Recommendation G78 - Recommendations for the provision of LV
Connections to Mobile Telephone
Base Stations with Antennae on High Voltage
Structures.
Engineering Recommendation G83/1 - The connection of small-scale embedded
generators (Up to 16A per phase) in parallel with public low voltage distribution networks
Engineering Recommendation G87 Guidance for the Provision of Low Voltage Supplies to
Multi Occupancy Buildings
Engineering Recommendation G12/3 - National Code of Practice on the Application of
Protective Multiple Earthing
Engineering Recommendation G14 - Protective Multiple Earthing; Recommended
Principles for Testing to ensure Correct Polarity
Engineering Recommendation G39/1 - Model Code of Practice covering electrical safety in
the planning, installation, commissioning and maintenance of public lighting and other
street furniture
Engineering Recommendation P04/1: Services to British Telecom public telephone
housings
Engineering Recommendation P24, 1984: AC traction supplies to British Rail (Addendum
No.1 1990)
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ENA Technical Specification 41-24: Guidelines for the design, installation, testing and
maintenance of earthing systems in substations
ENA Technical Specification 43-94: Earth rods and their connectors
Guidance Note GS 24 - Electricity on Construction Sites
Guidance Note HS(G)41 - Petrol Filling Stations: Construction and Operation
The Electricity Safety, Quality and Continuity Regulations 2002.
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Definitions
Arial Bunched Conductor (ABC) refers to a single bunched LV overhead cable
construction
"Bonding Lead" means a conductor by which items of metalwork are connected together
electrically, not normally for the purpose of carrying current, but so as to ensure a common
potential.
"Branch Lines" means any electric line through which energy may be supplied to 5 or more
customers from any distributing main
Business Owner means the manager (or his representative) within the business
responsible for the business process
Combined Neutral and Earth (CNE) reffers to a network where the earth return and
neutral return are provided by the same conductor
"Customer" means any body or person supplied or entitled to be supplied with energy by
CE and will include those customers on un-metered supplies.
"Customer's Installation" means the electrical apparatus under the control of the customer
on the customers premises together with the wiring connecting this apparatus to the supply
terminals. A meter and cutout owned by CE shall not form part of the customers
installation.
"Customer's Premises" means any area or building in the sole occupation of the customer.
"Distributing Main" means the portion of any main which is used for giving supply to branch
lines, service lines and multi-service lines for the purpose of general supply.
Distributor means a person who owns or operates a network, except for a network
situated entirely offshore or where that person is an operator of a network within the
meaning of Part I of the Railways Act 1993
"Earth-Continuity Conductor" means a conductor including any clamp connecting a part of
an installation or other accessible metalwork which is required to be earthed, to another
such part or to the supply neutral conductor.
"Earth Electrode" means a metal rod, plate or strip conductor buried in the earth for the
purpose of providing a connection with the general mass of earth.
"Earthing Lead" means the final conductor by which the connection to the earth electrode is
made.
"Earthing Terminal" means the terminal provided by CE and directly connected to the
supply neutral conductor at the supply terminals.
Earth Potential Rise (EPR) means the rise in potential from a zero earth reference value
during an earth fault on an ajacent HV or EHV circuit.
EHV Means 33kV, 66kV or 132kV
Global Earth means an equivalent earthing system created by the interconnection of local
earthing systems made up of earth electrodes and cables with metallic sheaths in direct
contactwith earth that ensures, by the proximity of the earthing systems, that there are no
dangerous touch voltages
HV High Voltage means above 5.5kV up to and including 20.5 kV
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"Multi-service Line" means any electric line through which energy may be supplied to two,
three or four adjacent customers from any distribution main or branch line or substation.
"Protective Multiple Earthing (PME)" - the technique of using the supply neutral conductor
of a LV distribution system for earthing of services to customers and street lights or other
street furniture.
Protective Neutral Bonding (PNB) refers to the situation where there is only one point in a
network at which consumer's installations are connected to a single source of voltage, in
such a case the supply neutral conductor connection to earth may be made at that point, or
at another point nearer to the source of voltage.
Residual Current Device (RCD) means a mechanical switching device or association of
devices intended to cause the opening of the contacts when the residual curent attains a
given value under specified conditions
"Service Lines" means any electric line through which energy may be supplied to a
customer from any distributing main on branch line or directly from a substation.
Separate Neutral and Earth (SNE) refers to a network where the earth return and neutral
return are provided by separate conductors.
TN-C means a systems where the neutral and protective functions are combined in a
single conductor throughout the system. Earthed concentric wiring or Earthed Sheath
Return Wiring are both examples of a TN-C system also known as CNE systems.
TN-S means a systems where there are separate neutral and protective conductors
throughout the system. A system where the metallic path between the installation and the
source of energy is the sheath and armouring of the supply cable is a TN-S system also
known as SNE systems.
TN-C-S means a systems where the neutral and protective functions are combined in a
single conductor but only in a part of a system. Systems with PME applied to them are
examples of a TN-C-S system , where the distribution system between the source of
energy and the installation is TN-C amd the installation itself is TN-S. PNB is also a variant
of a TN-C-S system also known as a mixed system.
TT means a system that has one or more points of the source of energy directly earthed
and the exposed and extraneous-conductive parts of the installation are connected to a
local earth electrode or electrodes that are electrically independent of the source earth(s).
An example would be where the customer provides his own indpenddent local earth and
provides earth fault protection by use of a RCD
Where Practical in the context of this document means where it is physically and
technically possible to achieve within the budgetary context of the work programme or
capital scheme that the activity is taking place under.
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Jim Paine
Date
11/05/2012
Sign
Date
Andy Ellam
21/05/2012
Dave Miller
Ged Hammel
Glen Hodges
Network Investment
Engineer
Overhead Line Standards
Manager
Programmes and Strategic
Design Leader
Sign
Date
Dave Miller
23/05/2012
Ged Hammel
20/06/2012
Glen Hodges
20/06/2012
6.5 Approval
Approval is given for the content of this document.
Mark Nicholson
Sign
Date
Mark Nicholson
06/07/2012
6.6 Authorisation
Authorisation is granted for publication of this document.
Sign
Director of Asset
Mark Drye
Mark Drye
Management
Date
10/07/2012
Appendix A Customer
Installation to Document
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Customers' Installations
Customers installations can only be connected to a PME terminal if the bonding conforms to
BS7671 (IEE Wiring Regulations)
1.
Appendix A Customer
Installation to Document
2
Version
2.
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3.
4.
Warning Notice
A warning notice shall be fixed at the customer's service position indicating that the earth
terminal is PME.
When a new PNB earth terminals is installed the appropriate label will be fixed at the service
position. If a supply to a single customer employing PNB is later extended to become a PME
network feeding a number of customers, then the PNB warning notice at the original customer's
premises shall be replaced by the PME type.
Appendix A Customer
Installation to Document
2
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5.
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their earthing installation is not to current standards and that they need to contact their
own electrician to rectify the situation the letter in sample letter 3 below will be left with
the customer.
Notes:
Note [1] With reference to Regulation 26 of the ESQC Regs
26(1) permits the DNO to issue a notice requiring the customer to rectify defects within 10 days
(the guidance notes cite an example where there are live conductors sticking out from a
consumer unit).
26(2) If the work hasn't been done within 10 days (for the example in the guidance notes) we
can disconnect - and leave another letter.
26(3) We can disconnect without giving notice on the grounds of safety. It's difficult so see how
an inadequate earth could be more dangerous than live wires sticking out of a consumer unit,
and hence it probably would not be appropriate for us to disconnect immediately on the grounds
of safety assuming the earth has been defective for some time. So it would seem reasonable
to give notice of disconnection and provide a reasonable time for the defects to be rectified.
26(4) We can reconnect as soon as the work's been done.
10 working days is discussed in the guidance notes for live exposed conductors, a no earth
situation will have a lower probability of causing harm as it needs another event such as a faulty
appliance to cause harm. 21 days is suggested as a reasonable time for customer to make
arrangements with an electrician without unduly delaying correction of the defects.
Note [2]
The Earthing Policy Manager will keep a record of the occurrences where an earth is not
available from the network or where the network earth is inadequate to determine if they are
isolated incidents or if there are areas of the network which need remedial attention.
The Services Programme Manager needs advance warning so that he can inform his
contractors that there may be an urgent request for an earth terminal from the customers
electrician. The Services Programme Manager will inform the Earthing Policy Manager of any
request for an earth, the remedial action taken and the state of the network.
There are 3 likely scenarios:
1. The customer contacts a qualified electrician whom makes a request for an earth
terminal on behalf of the customer. In this case if an earth can be provided for the
customer with work on the service or cut-out position it will be provided via scheduled
work.. If the provision of a network earth needs substantial work on the LV main cable
to provide, the electrician will be advised that an earth cannot be provided from the
network and advised to make his own earth arrangements. In this case the Services
Programme Manager will inform the Earthing Policy Manager of the work required to
provide an earth and the fact that the customer has been informed to provide their own
earth. The defective network will be brought up to specification through a work
programme.
2. The customers electrician provides a customer earth and corrects the defects without
requesting a Network earth.
3. The customer does not take any action to remedy the defects.
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Note [3]
The business owner will not send letter 2 until 28 days after the notification date to allow for a
letter to be received from the customer if the work was completed on the 21st Day.
Note [4]
The company may be criticised if customers have their supplies disconnected, however if there
is an incident at the property and a person is electrocuted, or there is a fire resulting directly
from the defective earth installation that the company was aware of the company would have
failed in it,s duties under the ESQC Regs. However the customers least likely to respond to the
notification are the old or vulnerable in society, the policy may need to be adjusted to provide
some form of support for these customers. The company can not take any responsibility for
work on the customers network.
Note [5]
If there is a high earth loop impedance on the network it is an indication that there is a fault is
on the earth system. Where the system is CNE this could result in an open circuit neutral/earth
resulting in wide spread damage. Such faults should be dealt with by repairs with the
appropriate level of priority placed on the actual fault condition. It is important that the Earthing
Policy Manager is informed of every occurrence of earth or neutral/earth fault on the network.
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Dated
To the occupier,
A representative from CE attended your property on the date of this letter and established that
the earth system for your electrical installation was defective.
These defects are listed below:
The safe operation of your electrical installation and portable appliances rely on an effective
earth, the defects identified could give rise to danger.
Under the Electricity Safety Quality and Continuity Regulations 2002 a Distribution Network
Operator cannot knowingly supply electricity to a defective installation. Regulation 26 gives the
authority to disconnect defective installations that could give rise to danger.
This letter gives notice of disconnection for reasons of safety. .
What you must do now.
You must contact a qualified electrician and have them rectify the defects to your electrical
installation listed in this letter, within 21 days of the date of this letter.
When the work is completed inform the **** in writing.
If the defects listed in this letter are not rectified within 21 days of the date of this letter. Your
supply will be disconnected for reasons of safety
Signed on behalf of YEDL/NEDL
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Dated
To the occupier,
A representative from CE attended your property on the ****** established that the earth system
for your electrical installation was defective.
The defects identified were .
You were also left notification that electrical supply to your property would be disconnected after
21days if the defects, had not been rectified by a qualified electrician.
Please confirm by return of post that the defects have now been rectified.
If you do not confirm that the defects have been rectified or make contact by phone or letter to
the contact point above the electricity supply to your property will be disconnected for reasons
of safety.
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Dated
To the occupier,
A representative from CE attended your property on the date of this letter and established that
the earth system for your electrical installation was defective.
These defects are listed below:
The defects identified could give rise to danger in certain situations such as a fault on an
electrical appliance within your property.
What you must do now.
You must contact a qualified electrician and have them rectify the defects to your electrical
installation listed in this letter.
Signed on behalf of YEDL/NEDL
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Dated
To the occupier,
A representative from CE attended your property on the date of this letter to disconnect your
electrical supply because you have failed to carry out remedial work on the earth of your
electrical installation.
You were given a notice of disconnection on ***** which included details of the defects.
The disconnection has been carried out on safety grounds under authority of The Electricity
Safety Quality and Continuity Regulations 2002, Regulation 26.
Your property will not be reconnected until you have confirmed in writing that the defects have
been corrected.
Appendix C Guidance on
the Control of EMC
Problems to Document
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Three tests are carried out (see Drawing C953068). In the first test, the resistance of Electrode
A to the general mass of earth is measured by an earth megger or the equivalent, according to
the instructions issued with the instrument; let this resistance be Rae. In the second test, the
same measurement is carried out on Electrode B; let the result be Rbe. In the third test, the
resistance between A and B is measured let this be Rab.
If the resistance areas of the two electrodes are adequately separated then Rab must be equal
to or greater than 0.9 (Rae + Rbe).
It is important when measuring Rae and Rbe to ensure that the voltage and current leads are
run out to adequate distances.
2.
1
=
1
Rae
Rbe
1
+ Rce
......
It is important that when measuring each electrode resistance to earth that voltage and current
leads are run out to adequate distances to ensure that there is no overlap between the
resistance.
3.
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The individual values obtained should be noted and compared to the overall value obtained by
measurement. The clip on meter measures the value of the electrode under test in series with
the rest of the connected electrodes. The measured value will only be accurate if the other
electrodes in parallel have a low value compared to the one under test. Figures D.1 and D.2
show the arrangement for a pole type substation and the electrical equivalent circuit.
Figure D.13: Current Flows for a Loop Resistance Measurement of the LV Earth
Rline is the resistance of the LV overhead earth wire and pole earths, in parallel.
RLV is the earth resistance of the substation LV earth.
The resistance measured by the clamp-on earth tester (Rtotal) will be the sum of the resistance in the loop,
i.e. (RLV + Rline). It is assumed that the resistance of the return path (R line) is small enough to be
considered negligible against the larger RLV value, so that Rtotal is approximately equal to RLV. If a
relatively low resistance return path is not available, then this type of measurement cannot be made
accurately.
Appendix G General
Drawings to Document
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LOCATION
Overhead HV bonding
EARTHING
AND
BONDING
CONNECTIONS
32mm2 stranded copper, green PVC
(2mm radial thickness) insulated
conductor
32mm2 stranded copper, black PVC
(2mm radial thickness) insulated
conductor
32mm2 stranded copper, Black PVC
insulated
70mm2
stranded
copper,
PVC
insulated conductor or insulated copper
strip 3mmx25mm
70mm2 stranded copper, green/yellow
PVC insulated conductor
70mm2 stranded copper (2 x 35mm2),
green/yellow PVC insulated conductor
for cable sizes above 70 mm2 for
smaller size cable at least equivelant to
the phase conductor with a minimum
size of 16mm2
25mm2, stranded copper, green/yellow
PVC insulated conductor
Connection location
At customers premises, connection
between
service
neutral
and
companys earthing terminal
At customers premises connection
between the companys earthing
terminal and the earth bar of the
consumer unit.
Table 2
Copper
equivalent
crosssectional area
Green/yellow 16mm2 or half the
size of the Companys neutral
meter tail, whichever is the larger
Green/yellow 16mm2 or half the
size of the Companys neutral
meter tail, whichever is the larger
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Main bonding equipotential connections between earthing terminal and all metallic structures (See
table below).
70mm
Notes:
1) Supplementary local bonding conductors and circuit protective conductors in the customer's
installation should be of the size specified in the current edition of BS7671 (IEE Wiring Regulations)
2) Conductors on poles above 3m need not be insulated
3) Where copper theft has occurred on a site the above ground bonding should be replaced with
aluminium conductor (see note 6), with suitable bi-metallic joints at the interface between the
aluminium and the copper in the ground.
4) All earthing and bonding connections where buried direct in the ground shall be of copper conductor.
5) To avoid possible dangerous ground potentials an earth electrode shall be installed so that its
topmost point will be at least 600 mm below ground level. The insulation of the HV or LV earthconductor shall be retained up to the earth electrode terminal.
6) The copper equivalent cross-sectional areas of Supply neutral conductors for typical DNO cables are
as follows:
35 mm2 Al CNE 22 mm2
95 mm2 Al CNE 60 mm2
185 mm2 Al CNE 116 mm2
2
2
300 mm Al CNE 116 mm
2
300 mm Cu CNE 150 mm2
4mmx25mm Cu tape 6mmx25mm Al tape
4mmx40mm Cu tape 6mmx40mm Al tape
7) The aluminium equivalent for copper earth conductors and bonds are listed below:
4mmx25mm Cu tape 6mmx25mm Al tape
4mmx40mm Cu tape 6mmx40mm Al tape
70mm2 or 95mm2 PVC covered copper - 120mm2 PVC covered aluminium
Appendix G General
Drawings to Document
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Appendix G General
Drawings to Document
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Contents
Contents
Page Number
1.
Purpose
2.
Scope
3.
Policy
3.1
3.1.1
3.1.2
1
2
3.1.3
3.1.4
3.2
3.2.1
3.2.2
Key Requirements
New Networks
Existing Networks
5
5
6
3.3
3.3.1
3.3.2
Implementation
New Networks
Existing Networks
6
6
6
3.4
3.4.1
3.5
3.6
3.7
3.7.1
3.7.2
9
9
10
3.8
3.8.1
3.8.2
3.8.3
3.8.4
11
11
14
14
3.8.5
3.9
15
3.4.2
3.4.3
3.4.4
3.4.5
2
5
5
7
8
8
8
8
15
15
Appendix G General
Drawings to Document
2
Version
3.10
3.11
3.11.1
3.11.2
3.11.3
3.11.4
3.11.5
3.11.6
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19
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20
20
20
21
21
22
22
23
23
23
23
23
28
28
28
29
29
29
3.18
3.19
23
25
26
26
27
27
29
30
30
30
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3.20.1.1
3.20.1.2
Introduction
General Earthing Requirements
30
30
3.20.2
3.20.2.1
3.20.2.2
3.20.2.3
31
31
32
32
3.20.3
32
3.20.4
Customer Substations
32
3.20.5
33
3.20.6
3.20.6.1
3.20.6.2
3.20.6.3
Construction Details
Materials
Installation of Electrodes
20/11/6.6kV Cables
38
38
38
39
3.20.7
3.20.7.1
3.20.7.2
3.20.7.3
3.20.7.4
3.20.7.5
Standard Arrangments
Standard Arrangement Drawings
Standard Arrangment Resistance Values
Separate LV Earth Arrangments
Standard Earthing Arrangemts for Brick or Similar Build Substations
Standard Earthing Arrangemts for Steel Framed or Kiosk Substations
39
40
40
41
42
43
3.20.8
Pole-Mounted Substations
48
4
4.1
4.2
References
Intrernal
External
50
50
50
Definitions
52
54
Appendix A
Customers' Installations
55
Appendix B
58
Appendix C
65
Appendix D
66
Appendix E
68
Appendix F
Contents
70
Appendix G
General Drawings
73
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