Company Directive

STANDARD TECHNIQUE: TP21OC/0

Earthing System Measurements - Part C

Equipment Earth Connections

Summary
This Standard Technique defines the requirements for verifying equipment earth
connections on earthing systems which are to be owned or adopted by National Grid
Electricity Distribution.

This document does not apply to earth connections between a substation and a terminal
tower.

Author: Mark Kneebone

Implementation Date: April 2024

Approved by

Carl Ketley-Lowe
Head of Engineering Policy

Date: 17th April 2024

Target Staff Group Network Services Teams, Engineering Trainers & ICPs

Impact of Change GREEN - The change has no immediate impact on working practices or
has been aligned to current working practices – Communication via a
monthly update of changed policy. Team Manager discretion on how the
changes are communicated to the team.

Planned Assurance checks Policy Assurance Specialists shall confirm whether the requirements
have been complied with during their sample checking of completed jobs

NOTE: The current version of this document is stored in the NGED Corporate Information Database. Any other
copy in electronic or printed format may be out of date. Copyright 2024 National Grid Electricity Distribution

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IMPLEMENTATION PLAN

Introduction

This Standard Technique defines the requirements for verifying equipment earth
connections on earthing systems which are to be owned or adopted by National Grid
Electricity Distribution.

Main Changes

This document is a new ST, however, it replaces parts of TP21O.

Impact of Changes

This Standard Technique is relevant to staff, Contractors and Independent Connection

Providers involved with the design / assessment of earthing systems.

Implementation Actions

Managers should notify relevant staff that this Standard Technique has been published.

There are no retrospective actions.

Implementation Timetable

The document can be implemented once being read and understood and can be utilised
from issue.

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REVISION HISTORY

Document Revision & Review Table

Date Comments Author

April 2024  New document Mark Kneebone

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Contents
1.0 INTRODUCTION ............................................................................................ 5

2.0 DEFINITIONS ................................................................................................. 5

3.0 REFERENCES ............................................................................................... 5

3.1 British Standards ............................................................................................... 6
3.2 Energy Networks Association ............................................................................ 6

4.0 OVERVIEW OF EQUIPMENT EARTH CONNECTION MEASUREMENTS .... 6

5.0 REQUIREMENTS ........................................................................................... 6

5.1 Method ............................................................................................................... 6
5.2 Restrictions ........................................................................................................ 7
5.3 Interpretation Of Results .................................................................................... 7
5.4 Sources of Measurement Error and Variation ................................................... 8
5.5 Test Results ....................................................................................................... 9

6.0 RISK ASSESSMENT & METHOD STATEMENT ......................................... 10

6.1 Risk Assessment ............................................................................................. 10
6.2 Method Statement ........................................................................................... 11

APPENDIX A SUPERSEDED DOCUMENTATION ........................................... 14

APPENDIX B RECORD OF COMMENT DURING CONSULTATION ............... 14

APPENDIX C ANCILLARY DOCUMENTATION ............................................... 14

APPENDIX D KEY WORDS .............................................................................. 14

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1.0 INTRODUCTION

This Standard Technique defines the requirements for verifying equipment earth
connections on earthing systems which are to be owned or adopted by National
Grid Electricity Distribution.

It is essential to verify that each item of plant and equipment is effectively

connected to the earth electrode system following the construction, modification or
repair of an electrical installation.

This Standard Technique shall be applied to all NGED substations at 33kV and
above. The measurement procedure can be optionally applied to substations at
lower voltages where required.

This document does not apply to earth connections between a substation and a
terminal tower.

2.0 DEFINITIONS

For the purpose of this document the following definitions are employed:

TERM DEFINITION
Earth Electrode A conductor or group of conductors in direct contact with the
soil and providing an electrical connection to earth.
Earthing Conductor A protective conductor which connects plant and equipment to
an earth electrode.
Earthing System The complete interconnected assembly of earthing conductors
and earth electrodes (including cables with un-insulated
sheaths).
Earth Potential The difference in potential which may exist between a point on
the ground and remote reference earth.
Reference Earth Part of the Earth, the electric potential of which is conventionally
taken as zero.
Electrical Installation An assembly of associated electrical equipment to fulfil a
specific purpose. For example: a 132kV, 66kV or 33kV
substation; a ground mounted distribution substation; auxiliary
equipment on an earthed pole; etc

3.0 REFERENCES

This document makes reference to, or should be read in conjunction with, the
documents listed below. The issue and date of the documents listed below shall
be those applicable at the date of issue of this document, unless stated otherwise.

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3.1 British Standards

NUMBER TITLE
BS EN 50552 Earthing of power installations exceeding 1 kV a.c.

3.2 Energy Networks Association

NUMBER TITLE
ENA TS 41-24 Guidelines for the design, installation, testing and
maintenance of main earthing systems in substations

4.0 OVERVIEW OF EQUIPMENT EARTH CONNECTION MEASUREMENTS

It is essential to verify that each item of plant and equipment is effectively

connected to the earth electrode system following the construction of a new
electrical installation, or modification of an existing one.

Plant and equipment must be sufficiently bonded to the earthing system to ensure
correct operation and that the potential difference created under fault conditions is
within the safe limits.

A sufficient bond between equipment and the earthing system can be confirmed if
the resistance between the central test point (or points) on the main earthing
system and individual items of earthed equipment does not exceed 20mΩ.

5.0 REQUIREMENTS

5.1 Method

The procedure is based upon the principle of measuring the resistance between a
central test point (or points) on the main earthing system and individual items of
earthed equipment using a four-terminal portable micro-ohmmeter and associated
insulated test leads and test clamps/clips/probes.

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 Figure 1: Test Arrangement

Resistance Meter
(4 Wire)

C1 P1 P2 C2

Plant under
test
Equipment
bonding
under test

Central test
point Main Electrode
System
Once it has been established that an item of plant in adequately bonded, it may be
itself be used as a new central test point to avoid the use of unduly long test leads
when testing adjacent equipment.

It is necessary to measure in the µΩ or mΩ range with a recommended injection

current of at least 100mA. The probable path of the injected current should be
considered and, where the substation uses a bus-zone protection scheme, care
should be taken to ensure that any test current does not produce enough current
to operate protection systems.

5.2 Restrictions

This test method shall not be used to check the bonding between a substation and
a terminal tower.

Care must be taken to ensure that the path of test current cannot operate system
protection. Joint resistance measurement shall not be performed:

 Across switchgear framework/cubicles/bonding bar and substation earth

electrode where the switchgear is fitted with frame-leakage protection.

5.3 Interpretation Of Results

The expected resistance between two effectively bonded points in an earthing

system should be low. It is determined by:

 Series resistance of the earthing conductor

 Series/parallel resistance of earth electrode
 Series/parallel resistance of joints/connections.

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 It will largely depend on the length of conductor/electrode involved and its cross-
sectional area. Using the resistance per meter for the appropriate conductor below
an anticipated bonding resistance can be calculated. An addition of 50µΩ for each
joint should be included in the calculation. Table 1 below gives example values.

Table 1: Conductor Resistance per meter at 20oC

Conductor Resistance per meter without joints (μΩ)

50mm x 6mm copper tape 57

50mm x 4mm copper tape 86
50mm x 3mm copper tape 115
25mm x 4mm copper tape 172
25mm x 3mm copper tape 229
120mm2 stranded copper 143
70mm2 stranded copper 254
2
35mm stranded copper 509

Worked example:

A circuit breaker is bonded to the earth electrode system using 20m of 50mm x
4mm copper tape with two joints in the bonding conductor.

(20m x 86µΩ) + (2 joints x 50µΩ) = 1,820µΩ or 1.82mΩ

Consequently, bonding measurements should be in the high micro-ohms or low

milliohms range rather than ohms.

If the measured value is less than 20mΩ then this implies adequate bonding, a
measured value above this threshold will require further investigation.

5.4 Sources of Measurement Error and Variation

Paint, scale or oxide coatings on conductors may affect the accuracy of the
resistance measurement. Conductors shall be abraded to expose clean surfaces
for connections.

Micro-ohmmeters are designed for a specific lead resistance. If higher resistance

leads are used the injected current may be reduced and may cause signal-noise
problems that may reduce the accuracy and/or repeatability of the resistance
measurement.

When using an adjacent item of plant as a known good test point additional
resistance may be included in the measurement. In the event that the pass limit is
exceeded the measurement should be repeated between the item of plant under
test and the main earthing system.

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5.5 Test Results

The following spreadsheet shall be employed for recording the results of

Equipment Earth Connections.

TP21OC Test Results

The measured value of equipment connection along with its reference should be
recorded in the construction/project file.

Figure 2: Example test results sheet

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6.0 RISK ASSESSMENT & METHOD STATEMENT

6.1 Risk Assessment

HAZARD PROBABILITY CONTROL MEASURES

Electric shock or Low  No testing if lightning is likely
burns as a result of
earth potential rise  No testing whilst fault switching is
being undertaken

 No testing if insulators are damaged

 Use of insulated mats, gloves and

footwear

Electric shock or Low  One person in control of testing

burns from test
voltages / currents  Radio communication between earth
tester operator and personnel who
move remote current and voltage
probes (who may be out of sight /
earshot)

Electric shock or Low  Avoid test probe route parallel with

burns from induced overhead lines, if possible
voltages from
nearby power lines  If not possible, maximize separation
between test probe route and
overhead line

Electric shock or Low  Ensure condition of test equipment

burns due to and leads are satisfactory prior to use
damaged test
equipment or leads
Slips, trips and falls Medium  Maintain awareness of surroundings
whilst undertaking measurements

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6.2 Method Statement

6.2.1 Equipment

The following test equipment is required in order to perform joint resistance

measurements:

Figure 3: Measurement instruments

 Four-terminal portable micro-ohmmeter (e.g. Megger DLRO10)

 Insulated test leads – minimum 1.5mm2

 Test clamps, clips, probes

 Emery cloth or wire brush

6.2.2 Test Arrangement

Figure 1 shows the test arrangement to be employed for carrying out equipment
bonding measurements.

6.2.3 Safety Precautions

When performing equipment bonding measurements, the following precautions

shall be taken:

 Comply with applicable safety rules

 Conduct Site Specific Risk Assessment and communicate risks to people at

risk in accordance with ST: HS20A.

 All testing under immediate control of one person.

 Communication between earth tester operator and personnel who move

'remote' trailing leads.

 Personnel wear Class 1 rubber gloves and the additional protection of

insulating safety footwear.

 No testing if lightning likely (e.g. lightning risk warning Category 1).

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  CONTROL notified and no work if relevant fault switching planned/under way
(e.g. either on substation under test or, where cable is present, ANY point on
the network fed from the source primary substation AND INCLUDING any
primary substation run in parallel).

 If relevant lightning or fault switching occurs while testing, the testing must
cease immediately.

 Ensure condition of test equipment is satisfactory prior to use.

6.2.4 Method

1 IDENTIFY earth conductors to test.

2 INFORM CONTROL before commencing work. Confirm risk of lightning

not Category 1 and that no fault switching will be performed.
Ensure earth conductor to test has visible connection to earth
and check visually that system insulators associated with
earthed structure where earth connection will be made are
undamaged. If in doubt then prove the earthed structure DEAD
using procedure in ST: OS4B.

3 PLACE micro-ohmmeter between the earth conductors under test.

4 SWITCH the micro-ohmmeter on and allow to self calibrate/stabilise as

per manufacturers instructions.

5 REMOVE scale/oxide coating to expose clean surfaces for connection if

copper conductor.

6 CONNECT the C1 and P1 terminals of the micro-ohmmeter to the central

test point as shown in Figure 1 by the test leads.

NOTE Micro-ohmmeters are designed for a specific lead

resistance – if higher resistance leads are used the injected
current may be reduced and may cause signal-noise problems
that may reduce the accuracy and/or repeatability of the
resistance measured.

NOTE It is important that the leads are not commoned at the

micro-ohmmeter terminals as the lead resistance would be
included in the measurement. Furthermore, do not connect the
potential probe to the current probe if separate test leads are
used.

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 7 CONNECT the C2 and P2 terminals of the micro-ohmmeter to the
equipment under test as shown in Figure 1.

NOTE It is important that the leads are not commoned at the

micro-ohmmeter terminals as the lead resistance would be
included in the measurement. Furthermore, do not connect the
potential probe to the current probe if separate test leads are
used.

8 MEASURE the resistance. Select a suitable scale as necessary. If the

micro-ohmmeter has the facility to reverse the test current
polarity then re-test – in this case calculate the average of the
two measurements for interpretation.

9 SWITCH the micro-ohmmeter off.

10 DETERMINE if the equipment bonding is satisfactory.

11 RECORD the resistance of the equipment bond and its location in the
construction/project file/.
12 REPEAT from item 7, radially, for all nearby earthing conductors, as
required.

13 REPEAT from item 1 for other reference points until whole site checked,
as required.

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 APPENDIX A

SUPERSEDED DOCUMENTATION

This is a new document and no document is superseded by its issue.

APPENDIX B

RECORD OF COMMENT DURING CONSULTATION

ST: TP21OC/0 - Comments

APPENDIX C

ANCILLARY DOCUMENTATION

POL: TP21 - Fixed Earthing Systems

APPENDIX D

KEY WORDS

Earth; Earthing; Measurement; Test; Bonding; Equipment Connections

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