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Document Number: EDS 06-0012
Date: 10/04/2015
ENGINEERING DESIGN STANDARD
EDS 06-0012
EARTHING DESIGN CRITERIA
Network(s):
EPN, LPN, SPN
Summary:
This standard details the criteria for designing earthing systems at all voltages.
Owner:
Allan Boardman
Date:
10/04/2015
Approved By:
Steve Mockford
Approved Date:
29/04/2015
This document forms part of the Company’s Integrated Business System and its requirements are mandatory throughout UK
Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset
Management. If you have any queries about this document please contact the author or owner of the current issue.
Applicable To
UK Power Networks
External
All UK Power Networks
G81 Website
Asset Management
Contractors
Capital Programme
ICPs/IDNOs
Connections
Meter Operators
HSS&TT
Network Operations
UK Power Networks Services
Other
THIS IS AN UNCONTROLLED DOCUMENT, THE READER MUST CONFIRM ITS VALIDITY BEFORE USE
Version: 3.0
Earthing Design Criteria
Document Number: EDS 06-0012
Version: 3.0
Date: 10/04/2015
Revision Record
Version
3.0
Review Date
10/04/2017
Date
10/04/2015
Author
Stephen Tucker
Why has the document been updated: Periodic review. Review date further extended to tie in with
the revision of national standards ENA TS 41-24 and ENA ER S34.
What has changed: No changes
Version
2.0
Review Date
31/03/2015
Date
11/03/2013
Author
Stephen Tucker
Review date extended to tie in with the review of national standards ENA TS 41-24 and ENA ER S34
Version
1.3
Review Date
Date
22/08/2012
Author
Stephen Tucker
Reviewed for publishing on G81 website
Version
1.2
Review Date
Date
28/09/2011
Author
Stephen Tucker
Reclassification of document from Earthing Design Manual Sections 1 and 2
Version
1.1
Review Date
Date
11/03/2011
Author
Version
1.0
Review Date
Date
31/03/2008
Author
John Lowe
Document rebranded
Stephen Tucker
Original
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Document Number: EDS 06-0012
Version: 3.0
Date: 10/04/2015
Contents
1
Introduction ............................................................................................................. 4
2
Scope ....................................................................................................................... 4
3
Glossary and Abbreviations ................................................................................... 4
4
Design Limits ........................................................................................................... 5
4.1
Touch and Step Potential Limits ................................................................................ 5
4.2
HOT or COLD Classification ...................................................................................... 6
4.3
Transfer Potentials .................................................................................................... 7
5
Risk Assessment ..................................................................................................... 7
6
References ............................................................................................................... 8
Appendix A – Legacy System Earthing Methods ............................................................. 9
A.1
EPN System Earthing Practice .................................................................................. 9
A.2
LPN System Earthing Practice ................................................................................ 10
A.3
SPN System Earthing Practice ................................................................................ 11
Appendix B – Definitions .................................................................................................. 12
Appendix C – Earthing Standards and Legislation......................................................... 20
C.1
Industry Standards .................................................................................................. 20
C.2
British and European Standards .............................................................................. 21
C.3
North American Standards ...................................................................................... 21
C.4
International Standards ........................................................................................... 21
C.5
Legislative Documents ............................................................................................ 21
Figures
Figure B-1 – Potential on Soil around Earth Rod – 3D View ................................................ 14
Figure B-2 – Potential on Soil Surface Near Earth Rod – 2D View ...................................... 16
Figure B-3 – Touch, Step and Transfer Potentials at an Electricity Substation .................... 18
Figure B-4 – Potential Contours on Soil Surface around Earth Rod .................................... 19
Tables
Table 4-1 – Sample Maximum Acceptable Touch and Step Voltages (ENA TS 41-24
Figure 2) .............................................................................................................. 5
Table 4-2 – Maximum EPR for a COLD Substation and for which a Transfer Voltage is
Permitted ............................................................................................................. 6
Table 4-3 – EPR above which Mitigation should be Considered by Third Parties .................. 7
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Earthing Design Criteria
Document Number: EDS 06-0012
Version: 3.0
Date: 10/04/2015
1
Introduction
This standard details the criteria for designing earthing systems at all voltages.
Definitions for the terms used and a catalogue of reference documents associated with
earthing practice are given in Appendix B and Appendix C.
2
Scope
This standard applies to the earthing at substations and networks across all voltage levels.
This document is intended for internal and external use.
3
Glossary and Abbreviations
Term
Definition
ASC
Arc suppression coil
EF
Earth fault
EPR
Earth potential rise
NER
Neutral earthing resistor
NEX
Neutral earthing reactor
FLC
Full load current
ROEP
Rise of earth potential
UK Power Networks
UK Power Networks (Operations) Ltd consists of three electricity
distribution networks:

Eastern Power Networks plc (EPN).


London Power Network plc (LPN).
South Eastern Power Networks plc (SPN).
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4
Design Limits
The object of earthing design is to ensure that a substation is safe with regard to step and
touch potentials and, in addition, to render it COLD, if reasonably practicable.
Consideration should be given to reducing system earth fault levels where this provides an
economical solution to the above.
The limits are based on those in ENA TS 41-24 and ENA S36-1.
4.1
Touch and Step Potential Limits
The values given in Table 4-1 are the touch and step potential limits applicable within or
adjacent to substations. For fault clearance times in excess of 5s a 75V touch voltage limit
applies for high voltage installations.
It is standard practice to specify a layer of 75mm crushed rock or chippings over the
substation site area not already covered by concrete or tarmac. As Table 4-1 shows, this
allows a higher value of safe touch potential within the area where this exists. A greater
thickness of chippings has minimal effect on the tolerable voltage calculation, but does
improve the integrity of the insulation. A layer of tarmac may be specified where higher
voltage limits are necessary. A specification for surface covering material is contained in
ECS 06-0022. In cases where the earth potential rise (EPR) is low and the touch and step
voltage limits for the on soil situation are easily achieved, then the electrical characteristics
of the chippings are not relevant and pebbles, sandstone or other material with the
necessary mechanical and civil engineering qualities may be used.
Table 4-1 – Sample Maximum Acceptable Touch and Step Voltages (ENA TS 41-24 Figure 2)
Fault Clearance
Time
Soil
Chippings (75mm- 150mm)
Touch
Step
Touch
Step
0.2s
1030V
3200V
1400V
4600V
0.35s
600V
1800V
800V
2600V
0.4s
480V
1500V
650V
2200V
0.5s
290V
890V
380V
1280V
0.6s
240V
670V
290V
980V
0.7s
195V
535V
250V
815V
1.0s
150V
450V
200V
640V
Outside the site area, where crushed rock is not used, the lower values are applicable. The
safety of the working environment is further improved by ensuring that staff wear approved
boots/shoes with adequate insulation and use insulating gloves when carrying out switching
operations using metal handles that are connected to the earthing system.
It is important to recognise that correct operation of protection devices and switches is
assumed in determining the fault duration and the acceptable potential differences. The
conductor and electrode cross sectional areas are based on a clearance time of 3s (with the
exception of 132kV, which is based on 1s). In the unlikely event of the clearance time for
backup protection exceeding this, the calculated back up protection operating time shall be
used for these calculations.
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Higher EPRs may occur at remote structures, such as lattice towers and potential grading
may be required there to control touch and step voltages if there is a higher than normal risk
to the public and third parties.
Step voltages on the soil within a substation are very rarely of sufficient magnitude to cause
concern. However, outside the substation humans with no footwear or animals are much
more susceptible to the effect of step voltages and due consideration of the possible impact
shall be made. Step voltages of 25V to 50V can be fatal to horses and cattle. Animals with
soft padded feet (such as dogs) are particularly susceptible. Particular caution is necessary if
the access routes to schools, stables, milking parlours or camp sites are near the substation.
No bare electrode should be installed in such areas and attempts should be made to restrict
the step voltage.
4.2
HOT or COLD Classification
The limits in Table 4-2 are to be used to classify a substation as either HOT or COLD, for
liaison with BT, other telecommunication operators, Network Rail and to comply with ENA
ER S36. These limits are not directly relevant to safety of operational personnel or the
public, which is determined by touch, step and transfer potential limits and covered in
Section 4.1. Where the EPR exceeds these values, a conductive earth path from the
substation to a customer's premises is not permitted unless calculations show that they are
lower than the applicable limit, or measures have been taken to control the voltages at those
premises. Measures may include the separation of HV and LV neutral earths or provision of
an isolation transformer – see later sections for further details.
Table 4-2 – Maximum EPR for a COLD Substation and for which a Transfer Voltage is Permitted
Substation Voltage
Transfer
Voltage
400kV, 275kV and 132kV
650V
66kV and 33kV
650V
High reliability lines with main protection that normally
operate within 0.15 seconds and always within 0.5
seconds and has back up protection
430V
Normal reliability lines or clearance times in excess of
0.2 seconds
20kV, 11kV, 6.6kV
Comments
430V
If a substation is classified as a COLD site, no further action is necessary. If it is HOT, further
investigation may be warranted to more accurately determine the EPR – this is covered in
more detail in the relevant earthing design standard (refer to Section 6 for specific
references). Once a site has a definite HOT classification, limit values given in Table 4-3 will
determine whether mitigation is necessary and enable calculation of the associated cost for
third party equipment. Note: A dialogue is necessary with third parties about the area
outside the substation where the Table 4-3 limit values are exceeded.
The clearance times quoted are for a correct operation of main protection. Operation under
back-up protection is not considered.
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Table 4-3 – EPR above which Mitigation should be Considered by Third Parties
Third Party
Equipment Involved
Protection Clearance
Time and Voltage Limit
Above 0.2s
0.2s or less
BT and Telecommunication
Operators
Isolation required on cable
termination
430V
650V
BT and Telecommunication
Operators
Attention to main trunk lines,
domestic properties, call boxes,
modems etc
1150V
1700V
Railways (Network Rail)
Attention to signalling and
communication cables
430V
1150V
Processing plants and
refineries
Attention to signalling, process
control and communication cabling
430V
650V
All reasonable endeavours should be used to reduce the EPR to below 5kV as the maximum
limit and to 1.7kV as a generally accepted upper limit at substations.
4.3
Transfer Potentials
Limits are not defined in the standards, but the substation earthing system design shall
ensure that transfer potentials exceeding 430V or 650V (as appropriate) do not exist. If
transfer potentials above this value can occur, then potential grading at the remote end must
be provided (refer to Figure B-1 and the remote touch voltage definition in Appendix B). The
potential grading should provide similar protection to that at the primary substation.
5
Risk Assessment
The use of risk assessment approaches to design, construction and maintenance are
recognised by the HSE and are to be included in the next version of ENA TS 41-24. The IEC
standard recognises the probabilistic nature of the factors which influence the risk of a shock
occurring. The factors include the level of fault current, its duration, the presence of human
beings and the proximity to sensitive third party equipment.
There are general cases where application of risk assessment can help in arriving at a
sensible, cost effective design. For example, if the amount of high voltage equipment at a
substation is minimal (a transformer within a durable, non-metallic enclosure supplied by
underground cable), the risk of a high EPR occurring at a time when this may introduce
danger may be so small as to become an insignificant aspect of the design. If however
protection operation relies on a fault thrower operation, then the EPR associated with this
operation must be taken into account. Work is presently taking place on risk assessment
methodologies at at national level and once completed will be incorporated into the
standards.
In special cases, specialist advice can be obtained to provide a fully documented risk
assessment. For example, the preferred electrode design may involve a perimeter electrode
outside the fence. Trees or buildings may prevent installation of this on one or two sides of
the site. They may also impede access to the fence. In this case it would be sensible to
install the perimeter electrode just inside the fence on these two sides and quantify the risk
of an excessive touch voltage occurring. However, there would be a requirement to monitor
site developments and if the trees or buildings were removed, a perimeter electrode should
be installed if the touch voltage requires it.
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6
References
EDS 06-0001
Earthing Standard
EDS 06-0013
Grid and Primary Substation Earthing Design
EDS 06-0014
Secondary Substation Earthing Design
EDS 06-0015
Pole-mounted Equipment Earthing Design
EDS 06-0016
LV Network Earthing Design
EDS 06-0017
Customer Installation Earthing Design
EDS 06-0018
NetMap Earthing Information System (internal document only)
ENA TS 41-24
Guidelines for the Design, Installation, Testing and Maintenance of Main
Earthing Systems in Substations
ENA ER S36
Procedure to Identify and Record HOT Substations
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Appendix A – Legacy System Earthing Methods
A.1
EPN System Earthing Practice
Transformer
Ratio (kV/kV)
Winding
Arrangement
HV
Winding
Earthing
132/33
Yd
Solid
LV Winding
Earthing
Maximum Fault
Current
Earth Tx & NER
1000A/Tx
or high
impedance
earthing Tx only
(older schemes
limited EF current
to full load current
based on Tx
rating)
132/11
Yy
Solid
NEX
700A/Tx
132/11
Yyy
Solid
Common NER or
NEX
700A/winding or
Tx
Earthing Tx &
NER or NEX
700A/Tx
132/11
Yd
Solid
Comments
Liquid NERs no longer
used. If required solid
ones are used
2 secondary windings
NEX 8.5Ω
New schemes
now high
impedance
earthing Tx only
132/11
Ydd
Solid
Earthing Tx &
NER or NEX
700A/winding
NEX 8.5Ω
New schemes
now high
impedance
earthing Tx only
Separate earthing Tx
for each winding
33/11
Dy and Yy
Unearthed
ASC, NEX or
NER
700A/Tx for
reactor or
dependent on
NER value
33/6.6
Dy and Yy
Unearthed
NER or NEX
450A approx/Tx
for reactor or
dependent on
NER value
33/3.3
Yy
Unearthed
Solid
2500A/Tx
© UK Power Networks 2015 All rights reserved
2 secondary windings
ASC systems
extremely common on
rural networks, most
fully rated (some short
term). Some ASC
earthing now uses
NEX instead of solid
earthing when ASC is
out of service. In
particular for Dy
transformers or Yy
fitted with Tertiary as
earth fault currents
can exceed 3 phase
under these conditions
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A.2
LPN System Earthing Practice
Transformer
Ratio (kV/kV)
Winding
Arrangement
HV
Winding
Earthing
132/66
Yy
132/33
LV Winding
Earthing
Maximum Fault
Current
Solid
NER
FLC
Yd
Solid
Earthing Tx &
NER
FLC
132/22
Yd
Solid
Earthing Tx &
NER
FLC
132/20
Yyy
Solid
Common NEX
1000A/winding
132/11
Yy
Solid
Solid
13.1kA
132/11
Yyy
Solid
Common NEX
2000A/winding
66/33
Yd
Unearthed
Earthing Tx &
NER
FLC
66/22
Yd
Unearthed
Earthing Tx &
NER
FLC
66/20
Yyy
Unearthed
Common NEX
1000A/winding
66/11(6.6)
Yy
Unearthed
Solid
13.1kA
33/11(6.6)
Dy
Unearthed
Solid
13.1kA
22/11(6.6)
Dy
Unearthed
Solid
13.1kA
Comments
2 secondary
windings
2 secondary
windings
2 secondary
windings
Note: 20kV is the new distribution voltage at Bankside. The transformer can accept a
primary voltage of 66kV or 132kV and will initially operate at 66/20/20kV but will later operate
at 132/20/20kV.
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A.3
SPN System Earthing Practice
Transformer
Ratio (kV/kV)
Winding
Arrangement
HV
Winding
Earthing
132/33
Yd
LV Winding
Comments
Earthing
Maximum Fault
Current
Solid
Individual
earthing Tx &
liquid NER
Typically FLC
contributed per
Tx
NER values raised
at some multipleTx sites, reducing
EFL to combined
rating of two Tx
Dy
Unearthed
* Solid
Typically 100
MVA (5250 or
8750A/Tx)
* Individual
exceptions exist
e.g. resistance
earthing at gas/oil
plants, and a few
short-time ASCs in
rural areas
Yy with delta
tertiary
Solid
Liquid NER
per winding
FLC per
secondary
winding
Some sites have
dual secondary
windings
11/6.6
Auto
Star-point unearthed
Inter-bus
(or phase
shifting auto)
Comparable
with Source
side network,
(whether Solid
or NER)
Inter-bus and
Inter-network
11(6.6)/3.5
Dy
Unearthed
Step-down
Auto
Star-point unearthed
11(6.6)/2.0
Single phase
doublewound
Unearthed
Unrestricted –
determined by
source and
step-down Tx
impedance
Found only in
small areas of
obsolete-type nonstandard networks
Grid
33/11(6.6)
Primary
132/11(6.6)
Grid
Step-down
Solid
Solid
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Appendix B – Definitions
Wherever possible, the definitions are taken from IEV chapter 195, with additional wording
added if necessary for clarification or for use within public power supply systems.
Auxiliary Earth Electrode – an earth electrode with certain design or operating constraints. Its
primary function may be other than conducting earth fault current to earth.
Chain Impedance – the steel foundation of a tower forms an earth electrode. When several
towers are connected together via the earthwire, a ladder network is formed. The overall
earth impedance of the circuit from a specified point is called the chain impedance.
Circulating Transformer Neutral Current – the portion of fault current which flows back to the
transformer neutral point via the metallic paths and/or the earthing system without ever
discharging into the soil.
Counterpoise – conductor or system of conductors, buried in the ground and electrically
connecting the footings of the supports of an overhead line.
Earth Electrode Current – this is the maximum value of current which the total earth
electrode may be expected to conduct, during the lifetime of the installation. In single
earthed neutral systems fitted with current limiting devices, the maximum earth electrode
current is limited by that device unless there are secure parallel circuits offering an
alternative current path to that provided by the earth electrode impedance.
Earth Electrode Impedance – this is the impedance to the general mass of earth, or
reference earth, at a given frequency, of the following connected together: buried electrodes,
cable sheaths, tower lines, earthing at adjacent installations and all connected fortuitous
electrodes (such as steel reinforcing bars).
Earth Electrode Resistance – this is the resistive component of the Earth Electrode
Impedance.
Earth Electrode – this is a conductor or group of conductors in direct contact with the soil
and provides the conductive part in electrical contact with earth through which the fault
current flows to ground. It can be a rod, tape, steel reinforcing bar or the sheath of some
types of cable.
Earth Grid – for large area substations the earth electrode is normally run as a ring
surrounding all the plant and equipment within the substation. The outer ring is then
supplemented with cross-members run at 90 to each other, all connected together at
crossing points and at the outer ring. This is known as the earth grid. The earth electrode
area is the area occupied by the earth grid or electrode area.
Earth Nest – this is a collection of separate earth electrodes made up of rods and/or
conductors specifically installed to make contact with the general mass of earth and
connected together by a common metallic connection. The earth nest is to be kept free of
any plant earthing connections and should be provided with a test link that does not require
disconnection.
Earth Return Current – the proportion of fault point current which returns to source via the
ground.
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Earth Return Path – the electrically conductive path provided by the Earth between earthing
arrangements, for example between the two rods at A and B.
Earth Surface Voltage – the voltage between a specified point on the ground somewhere
near the rod and reference (true) earth.
Earthing Conductor – this is a conductor which connects plant, equipment and electrodes of
the earth electrode system. Examples include the above ground connections between
substation equipment and the earth grid, and the metallic sheath of an underground cable
which has an insulated serving.
Earthing Conductor Voltage – this is the voltage between the earthing conductor and
reference (true) earth.
Equipotential Bonding – this is the provision of electrical connections between conductive
parts to reduce the potential difference between them.
Exposed Conductive Part – conductive part of equipment which is not normally live, but
which can become live when basic insulation fails.
Extensive Earthing System – a large network of interconnected metallic buried or regularly
earthed parts that surrounds totally or partially the earthing system of the substation.
Fault Point Current (gross earth fault current) – the maximum value of current which could
flow at any fault point (NGT Definition).
Functional Equipotential Bonding – equipotential bonding for purposes other than safety.
Earth Potential Rise (EPR), Earth Potential Rise (UE) or Rise of Earth Potential (ROEP) –
these are terms in common use meaning the same as the Earthing Conductor Voltage. It is
the potential on exposed metalwork and the earth electrode or conductor during fault
conditions, relative to remote earth. It is the product of the total earth impedance and the
current that flows through this to ground. Note: that this value will differ at various points on a
large earthing system.
To help understand the definitions, a simple example is used. Assume two earth rods of
2.4m length have been driven into the ground in an open field about 50m apart and a current
is passed between them. Figure B-1 shows the potentials which are established on the
surface of the soil above one of the rods. The highest potential is directly above the earth rod
(at A) and it falls off towards true earth potential as one moves away from it. The current
flows through the ground between the two rods, taking a multitude of paths, but the current
density is greatest near each rod.
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Surface
60
Potential
( % of GPR)
45
A
30
15
0
20
0
Distance (m)
-20
-10
0
10
20
30
Distance (m)
D
E
Figure B-1 – Potential on Soil around Earth Rod – 3D View
Global Earthing System – an extensive earthing system for which touch, step and
transferred voltages are within tolerable limits. An example would be several 11kV
substations and their associated LV cable networks in close proximity to one another,
especially if the underground cable is of the hessian served type.
HOT Substation – a HOT substation is defined as a substation where the rise of earth
potential when maximum earth fault current flows, can exceed values specified in
ENA TS 41-24. Refer to Table 4-2 for the current values.
As a general rule, the 650V value normally applies to 132kV and higher voltage equipment.
It will also apply to 66kV and 33kV equipment if the protection clearance times and
dependability are such that it can be classified as high reliability. Note that the above criteria
determine the site classification, but not the requirement or otherwise to carry out mitigation
work. This is explained in more detail in Section 2 (design limits).
Independent Earth Electrode – an earth electrode located at such a distance from other
electrodes that its electrical potential is not significantly affected by electric currents between
Earth and other electrodes.
Insulation of operating location – a measure to increase the resistance between the floor (or
surface) at an operated location and earth, in such a way that no non-permissible voltages
can be picked up.
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Interference – unwanted electrical coupling between a primary circuit or earthing conductor
and a nearby secondary control or communication cable, occurring under network earth fault
conditions. This may occur in two main ways:


Conductive Interference – Voltages relative to earth imposed on the secondary circuit,
owing to the local rise-of-earth-potential in the vicinity of a substation earthing system
during a network earth fault. (This effect is the main risk associated with HOT
substations.)
Inductive Interference – Voltages induced between the ends of the secondary circuit,
owing to parallelism with a primary circuit carrying unbalanced current during a network
earth fault. (In practice this effect is typically confined to private pilots laid along the route
of long primary cable feeders.)
Lightning Conductor – a conductor appropriately placed on a structure to conduct lightning
current to an earthing arrangement.
Main Switchgear Earth Bar – the earth bar to which all the exposed metalwork of equipment
within a building or switch-room is connected. In substations with frame leakage protection
fitted, there will be two earth bars. Exposed switchgear metalwork will be connected to one
bar and the earth grid and incoming cable sheaths to the other. The two bars will be
connected at several points, each connection passing through a frame leakage CT.
Material Alteration – the work or change at an existing substation that determines whether
attention is required to its earthing. The factors include:





Construction activities involving more than direct replacement, on a like for like basis, of
one or several items of plant. For example, increase in transformer numbers or capacity.
The substation metal fencing arrangement is modified, particularly if new or extended
metal fences are used (it is envisaged that only the fence earthing will be reviewed for
this – not the overall substation earthing).
The earth fault level is increased significantly.
The construction of circuits supplying the site is changed and so increases the amount of
fault current returning via the soil. For example, if a tower line is replaced with insulated
trident type construction for environmental reasons.
A significant proportion of an existing Hessian served cable network is being replaced
using plastic sheathed cables.
Mid-Point Conductor or Electrode – a conductor electrically connected to the mid-point (often
between parts of an installation owned by different companies) and capable of contributing
to the distribution of electrical energy. This is a design concept often used by north American
companies, but does not represent good practice and should be discouraged.
Multi-Earthed Neutral Conductor System – a neutral conductor of a distribution line
connected to the earthing system of the source transformer and regularly earthed, for which
touch, step and transferred voltages are within tolerable limits.
Neutral Conductor – the conductor electrically connected to the neutral point and capable of
contributing to the distribution of electrical energy.
Neutral Point – the common point of a star connected poly-phase system or the earthed midpoint of a single phase system.
Parallel Earthing Conductor – a conductor laid along the cable route to provide a low
impedance connection between the earthing arrangements at the ends of the cable route.
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Path – any non-metallic material between the bare skin and the earthing system.
Potential Grading – control of the earth potential, especially the earth surface potential, by
means of earth electrodes.
Primary Earth Electrode – an earth electrode specifically designed or adapted for
discharging earth fault current to earth, often in a specific discharge pattern, as required by
the earthing system design.
Protective Equipotential Bonding – this is equipotential bonding for the purpose of safety.
Protective Conductor – a conductor provided for the purposes of safety (protection against
electric shock).
Power System Earthing – functional earthing and protective earthing of a point or points in
an electric power system.
Resistance between the body contact points and return paths – the resistance between the
bare skin of the contact point and the metal being touched or of the remote electrode to
which current through the body will return. It includes gloves, shoes, contact with surface soil
and the soil between the surface and metal electrode.
Soil Resistivity – the resistivity of a typical sample of soil. It is the specific electrical
resistance across two opposite faces of a 1m3 homogeneous sample. It is expressed in units
of ohm-metres.
GPR
Potential on exposed
metalwork
Ust1
Ust2
-10
0
10
Distance (m)
Figure B-2 – Potential on Soil Surface Near Earth Rod – 2D View
Step Potential (Us) – as illustrated Figure B-3 there will be potential differences established
on the surface of the soil when fault current is flowing. The magnitude of the potential
difference a person walking in the area would experience depends upon the orientation of
the feet with respect to the voltage contours and the distance between feet. Step voltage in a
particular direction is defined as the potential difference between two points a metre apart. It
is greatest adjacent to the electrode. Step potentials are those normally responsible for the
death of animals, such as cattle and horses, during fault conditions, where values as low as
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25V can cause a severe shock or death. Step potentials can be reduced by using potential
grading electrodes, in particular by installing the outermost electrodes at a progressively
greater depth.
Stress Voltage – the voltage appearing during earth fault conditions between an earth part or
enclosure of equipment or device and any other of its parts and which can affect its normal
operation or jeopardizes safety.
Structural Earth Electrode – a metal part which is in conductive contact with the earth directly
or via damp concrete, whose original purpose is not earthing, but which fulfils all
requirements of an earth electrode without impairment of the original purpose. Examples
include pipelines, sheet piling, foundations, reinforcing bars in concrete and the steel
structure of buildings.
Substation Main Earth Bar – this is a central conductor to which all earthing and bonding
conductors are connected. It is normally situated above ground but may be within a cable
trench. It was a feature of older substation designs, but is not generally used now.
Resistance Area (of a rod or electrode) – the area around it where most of the voltage drop
occurs. For a 2.4m rod in soil of uniform resistivity, this would be the area within 4 to 6m
radius of the rod, for a moderate current flow. The radius would increase if the magnitude of
the current increased or in a non-uniform soil which has a high resistivity lower layer.
Touch Potential (Ut) – Figure B-2 shows the potential on the surface of the soil near rod A,
from the side and in two dimensions. If, during the time that fault current flows, a person
were to touch the rod (or any exposed metalwork connected to it), then the potential
difference experienced between hands and feet is termed the touch voltage. This is
illustrated in Figure B-3, where UST1 is the touch voltage above the rod and UST2 the touch
voltage some distance away. It is clear that it increases rapidly with distance from the rod,
before levelling out. In calculating this value it is assumed that the feet are 1m away from the
metalwork being touched. Touch voltages are normally reduced by using potential grading
electrodes.
Transfer Potential – at position D in Figure B-3, the potential on the soil surface is higher
than at true earth, but significantly lower than that at rod A. If the wire of an insulated
conductor was connected to rod A and extended to position D, there would be a potential
difference between the end of the wire and the surface of the soil at D. If a person was able
to touch the wire, whilst standing at D, an electric shock may be experienced. The potential
difference at D is termed the “transfer potential”. The transfer potential in AC systems has a
maximum value equal to that of the rise of earth potential, such as at rod A. Where the
potential rise exceeds a certain value, then precautions are necessary to prevent excessive
transfer potentials. These are described later. The precise definition is as follows:
A potential rise of an earthing system caused by a current flowing to earth, transferred by
means of a connected conductor (for example a metallic sheath or pipe) into areas with little
or no potential rise relative to reference earth. This results in a potential difference occurring
between the conductor and its surroundings.
True or Reference Earth – with respect to each rod, is the potential on the surface of the soil
a significant distance away, i.e. outside the resistance area, such as point E in Figure B-1.
Because it is outside the influence area of the rods, the electrical potential there is
conventionally taken as zero.
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Voltage
gradient
across
site
Touch Potential
(UST)
Touch
Potential
(UST)
Earth
Potential
Rise, EPR
(UE)
Step Potential
(USS)
Transfer Potential
(UTST)
(transferred
Cable sheath
source voltage for
earthed at
touching if sheath
substation
not earthed at
remote end)
Earthing
Electrode
S1
S2
S3
Earthing
Electrode
Potential grading earthing
electrodes (eg ring earth
electrodes), each
connected to the earth
electrode
Remote Touch Potential
(UTSTE)
(transferred source
voltage for touching if
sheath is earthed at
remote end as well)
Earthing Electrode
Cables having a continuous metal sheath
insulated throughout but exposed at both ends
Figure B-3 – Touch, Step and Transfer Potentials at an Electricity Substation
Having described the standard terms and potentials for a simple example, Figure B-3 (which
is taken from the European Standard) illustrates the potentials for a typical substation
situation. The European symbols and descriptions are included for reference and the
generally accepted UK equivalent term is included in brackets. As can be seen from the
UTSTE symbol, it is necessary to consider connected earth electrodes at a remote point and
this is precisely the arrangement where a remote distribution substation is connected to a
primary substation via a plastic sheathed 11kV cable.
Underground Cable Route Earth Electrode – earth electrode usually laid along the cable
route, protected if required against corrosion, to provide earthing along its route.
Zone of Interest or HOT Zone. In Figure B-4, the potentials on the surface of the soil around
the earth rod have been represented as equipotential lines. This is the same effect as would
occur for a substation, except that the lines would follow the shape of the electrodes
installed, particularly nearer the site, so would not normally be circular until a significant
distance away. If the rise of earth potential at the substation exceeds one of the ITU or other
agreed limits, then the equipotential line coinciding with these (e.g. 430 or 650V contour)
would need to be identified. The area enclosed by the appropriate contour line is known as
the zone of interest (sometimes referred to as the HOT zone). Special precautions may be
necessary where there are any buried metallic services situated within the zone of interest.
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Date: 10/04/2015
30
KEY
5 =1000V
15
Rod
4 = 650V
3 = 430V
2 = 200V
0
1 = 100V
-15
-15
0
15
30
M ETRES
Figure B-4 – Potential Contours on Soil Surface around Earth Rod
Note: For other earthing terms associated with operational work e.g. circuit main earth, local
earth, portable earth etc reference should be made to the distribution safety rules.
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Appendix C – Earthing Standards and Legislation
C.1
Industry Standards
ENA TS 41-24 – Guidelines for the Design, Installation, Testing and Maintenance of Main
Earthing Systems in Substations (Note: This document was produced in 1992, has been
partly updated and due for a complete rewrite. As it is the most up-to-date ENA standard, it
is used as the prime document. Requirements considered likely in the revised version have
been included wherever considered prudent.)
ENA TS 43-94 – Earth Rods and their Connectors
ENA TS 43-89 – Auxiliary Equipment on LV, 11kV and 33kV Overhead Line Supports (Note:
Draft not formally issued.)
ENA ER C55 – Insulated Sheath Power Cable Systems
ENA ER G12 – Requirements for the Application of Protective Multiple Earthing to Low
Voltage Networks
ENA ER G59 – Recommendations for the Connection of Embedded Generators to Electricity
Distribution Systems
ENA ER G75 – Recommendations for the Connection of Embedded Generating Plant to
Public Distribution Systems above 20kV or with Outputs over 5MW
ENA ER G78 – Recommendations for Low Voltage Supplies to Mobile Phone Base Stations
with Antennae on High Voltage Structures
ENA ER G83 – Recommendations for the Connection of Small-scale Embedded Generators
(up to 16A per phase) in Parallel with Public Low Voltage Distribution Networks
ENA ER P20 – Earthing Policy for Customers’ Installations
ENA ER P23 – Customers Earth Fault Protection for Compliance with IEE Wiring
Regulations
ENA ER P24 – AC Traction Supplies to British Rail (Addendum No1 1990)
ENA ER S34 – A Guide for Assessing the Rise of Earth Potential at Substation Sites (Note:
Most of the approaches used in S34 are considered robust and included in this document.
Guidance on cables and other equipment not available when S34 was written has been
included where possible.)
ENA ER S36 – Procedure to Identify and Record HOT Substations
ENA ETR 128 – Risk Assessment for BT operators working in a ROEP Zone
ENA ETR 129 – Risk Assessment for Third Parties using Equipment Connected to BT Lines
NGTS 3.1.2 – Substation Earthing
ACE 19 – Induced Voltages in Auxiliary Cables
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C.2
British and European Standards
BS EN 50522:2010 – Earthing of Power Installations Exceeding 1kV AC
BS EN 61936-1:2010 – Power Installations Exceeding 1kV AC Part 1. Common Rules
BS 7354:1990 – Design of High Voltage Open Terminal Substations (Note: There is a
special section giving guidance on the design of EHV substation earthing.)
BS 7375 – Distribution of Electricity in Construction and Building Sites
BS 7430:2012 – Code of Practice for Protective Earthing of Electrical Installations
BS 7671:2008 incorporating Amendment No 1: 2011 – Requirements for Electrical
Installations (IEE Wiring Regulations Seventeenth Edition)
CCITT/ITU – International Telegraph and Telephone Consultative Committee directives
concerning the protection of telecommunication lines against harmful effects from electric
power and electrified railway lines.
C.3
North American Standards
IEEE Standard 81 (1991) – Testing of Soil, Soil Resistivity and Electrode
IEEE Standard 80 (2000) – Guide for Safety in AC Substation Grounding
IEEE Standard 655 (1987) – Guide for Generating Station Grounding
C.4
International Standards
IEC/TS 60479-1 (2005) – Guide to Effects of Current on Human Beings and Livestock
(Note: This standard is internationally recognised and forms the basis of calculation for
estimating safe touch, step and transfer potentials. At the time of writing, there are some
changes in this standard and it is still being reviewed by BSI and ENA in terms of its
relevance to limit values used in the UK.)
IEC 60909 (HD533) – Short Circuit Calculation in Three Phase AC Systems
IEC 61201 – Touch Voltage Threshold Values for Protection Against Electric Shock
IEV 60050-195 – International Electro-technical Vocabulary: Earthing and Protection against
Electric Shock (Chapter 195)
C.5
Legislative Documents
HSE Booklet, Memorandum of Guidance on the Electricity at Work Regulations 1989
The Construction (Design and Management) Regulations 1994
The Electricity, Safety, Quality and Continuity Regulations 2002
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