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Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
Date: 28/07/2014

15 Measurements

15.1 Overview

The HV and LV (if applicable) earth resistance values shall be measured before and after
connection to the network to confirm that the design values have been achieved. This
section includes various methods for measuring the earth resistance, for more detailed
information, guidance and test equipment refer to ECS 06-0024.

All measurements shall be carried out in accordance with the Distribution Safety Rules and:

• HV insulated gloves and dielectric footwear (yellow wellies) shall be worn when handling
the test equipment.

• All connections to the substation or pole-mounted equipment shall be made before
connecting the leads to the test equipment.

• The test leads shall be temporarily disconnected from the test equipment and the ends
kept separated when moving probes during the test.

• Communication shall be maintained between operators at all times.

These measures are intended to reduce the possibility, or consequence, of test leads
becoming live under HV fault conditions.

This section includes the following test methods:

• Earth resistance measurement using the fall-of-potential method.
• Earth resistance measurement using the comparison method.
• Earth resistance measurement using a clamp meter.

15.2 Test Equipment

The following approved earth resistance measurement equipment, leads and accessories
are available from Norwich Instrument Services (NIS) and are compatible with the tests
detailed in this standard:

• DET4TD2 (battery) or DET4TR2 (rechargeable) is a basic four-terminal earth tester.

• DET4TC2 (battery) or DET4TCR2 (rechargeable) is a four-terminal earth tester with
selectable frequencies and greater sensitivity and also includes stakeless (clamp-on)
testing capability. Note: The clamp-on kit is an optional accessory.

15.3 Earth Resistance Measurement using the Fall-of-Potential Method

The most common method of measuring the earth resistance is the fall-of-potential method
using a four-terminal or three-terminal earth tester. Three points of contact are made with the
soil: the earthing system under test (connected to C1 and P1), the current probe (C2) and
the voltage probe (P2).

© UK Power Networks 2014 All rights reserved Page 51

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
Date: 28/07/2014

The method described below is suitable for the measurement of small earth electrode
systems associated with secondary substations and pole-mounted equipment. For larger
systems additional measurement points are required as detailed in ECS 06-0024.

1. Connect terminals C1 and P1 to the HV or LV earthing system under test.

2. Place the C2 current probe 50 metres away from the substation or pole.

3. Take three measurements by placing the P2 potential probe in line with the C2 probe at
25 metres (50%), 31 metres (62%) and 35 metres (70%) away from substation or pole
the as shown below11.

50% 62% 70% 100%

50m
P1 25m 31m 35m

C1

P2 C2

C1 P1 P2 C2 Disconnect P2 terminal

FOUR-TERMINAL when moving P2 probe
EARTH TESTER Operator to wear HV
rubber gloves

4. If the measured values are within 5% of the middle (31 metre) value and do not decrease
with distance, the value at 31 metres is the overall earth resistance. If the measurements
are more than 5% of the middle value the test should be repeated using a different
transverse, i.e. relocate the C2 probe at 90 degrees to the first test and measure the
potential using P2 along the same line. See examples below. If this does not provide a
satisfactory value the probe spacing should be doubled to 50, 62, 70 and 100 metres and
the test repeated again.

Example 1: Example 2:

• 9.6 ohms measured at 25m • 10.3 ohms measured at 25m
• 10.0 ohms measured at 31m • 12.0 ohms measured at 31m
• 10.4 ohms measured at 35m • 13.9 ohms measured at 35m

10 ohms x 0.95 (-5%) = 9.5 ohms and 12 ohms x 0.95 (-5%) = 11.4 ohms and
10 ohms x 1.05 (+5%) = 10.5 ohm 12 ohms x 1.05 (+5%) = 12.6ohm

The readings are within the range 9.5-10.5, The readings are outside the range 11.4 to
therefore the resistance of 10 ohms is valid. 12.6, therefore the resistance of 12.0 ohms
is not valid and the test should be repeated.

11 Other distances may be used provided the percentage distances are maintained. Page 52
© UK Power Networks 2014 All rights reserved

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
Date: 28/07/2014

15.4 Earth Resistance Measurement using the Comparison Method

If there is not sufficient space to carry out a fall-of-potential measurement (or if there is a
likelihood of buried metallic services along the route as in many urban locations) it is
possible to measure the earth resistance by comparison using an earth. This method uses
the wider earthing system as a reference point and assumes it has a very low value.

A four-terminal earth tester is used to measure the earth resistance of a local earthing
electrode.

1. Connect terminals C1 and P1 to the earthing system under test.

2. Connect terminals C2 and P2 to the reference earthing system as shown below.

3. The meter will display the sum of the reference earth and the test electrode; if the
reference earth is a large cable network the dominant resistance will be that of the
system under test. In other words, the actual value will be slightly less than the displayed
value.

Earth Bar RParallel

P1 P2
C1
C2

R1 C1 P1 P2 C2 Reference Earth i.e.
Wider Network
Substation Earth Under Test FOUR-TERMINAL
(Disconnected from Network) EARTH TESTER

If RParallel << R1 then measured ‘earth loop’
resistance’ (RParallel + R1) approaches R1

15.5 Earth Resistance Measurement using a Clamp Meter

A clamp meter can also be used to carry out a comparative test by measuring between the
bundled electrode system and the wider cable sheath network as shown below. This test can
be used whether or not cables have been connected; however it is more accurate if the
cables are connected. It is of most value if there is no easy way of measuring the
contribution of the whole system. Note: This method relies on conductors being ‘bundled’
together to effectively reduce the test to a single point of connection between the earthing
system and the reference earth. If this is not done, the test current will circulate around the
local electrode system only and the result will be incorrect; readings below 0.5 ohms should
be treated with suspicion.

Earth Bar RParallel

Clamp Type Substation Connected to
Earth Tester Network via Cable Screens

R1 Reference Earth i.e.
Wider Network
Substation Earth Under Test
(Connected to the Network) If RParallel << R1 then measured ‘earth loop’
resistance’ (RParallel + R1) approaches R1

© UK Power Networks 2014 All rights reserved Page 53

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
16 References Date: 28/07/2014

EDS 06-0002 HOT Sites (internal document only)

EDS 06-0004 Earth Fault Loop Impedance Requirements (internal document only)

EAS 06-0011 Earthing Materials

EDS 06-0012 Earthing Design Criteria

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)

ECS 06-0022 Grid and Primary Substation Earthing Construction

ECS 06-0024 Earthing Testing and Measurements

EAS 07-0021 Signs and Labels for Operational Sites

EDS 07-0102 Secondary Substation Civil Design Standards

EDS 08-0121 Supplies to HOT Sites and National Grid Sites

EOS 09-0067 Theft of Substation Earthing (internal document only)

ECP 11-0503 Earthing Commissioning Procedure

11kV and LV Cable Jointing Manual

Overhead Line Manual

EM-MS-06-005-r3/ Instructions for Disconnecting Auxiliary Circuits in a Fuse Cabinet for a
EM-MS-06-009-r0 Hot Site

© UK Power Networks 2014 All rights reserved Page 54

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
Date: 28/07/2014

Appendix A – Purpose of Earthing

When an earth fault occurs on the electricity distribution network (see diagram below):

1. A large current will flow along the cable and return to the source via the cable sheath or
combined neutral-earth conductor and the general mass of earth.

2. The current will flow until the upstream protection operates.
3. The current flowing through the earth can cause a considerable rise in voltage (known as

the earth potential rise or EPR) on the earth and any earthed metalwork near the fault -
creating possible danger (touch and step voltages) to anyone in the vicinity.
4. This excessive rise in voltage may be transferred onto adjacent power and
communication cables creating possible danger to anyone who might be in contact with
them – this may be some distance from the actual fault.

Protection Direction of
Device Current Flow

Source Earth Load
Transformer Fault

Some current returns
through cable sheath

Some current returns
through the ground

Therefore the purpose of earthing is to:

1. To pass the fault current during an earth fault back to the system neutral and operate the
upstream protection.

2. To prevent dangerous voltages appearing at the substation and causing danger to staff
or the public.

3. To prevent dangerous voltages appearing on the customers LV neutral/earth.
4. To prevent damage to sensitive equipment (e.g. communications).

An EPR of less than 430V allows the HV and LV to be combined and is classified as a
COLD site. The EPR can be limited by reducing the fault current or the earth resistance at
the substation or site. If the EPR is greater than 430V the substation the HV and LV have to
be separated and the site is classified as a HOT site.

© UK Power Networks 2014 All rights reserved Page 55

Secondary Distribution Network Earthing Construction Document Number: ECS 06-0023
Version: 4.0
Date: 28/07/2014

Appendix B – LV Earthing Systems (PME)

PME – a form of TN-C-S system where the
neutral and earth are connected together at
the transformer. The supply cables use a
combined neutral and earth (CNE) cable
and the customer’s installation uses a cable
with a separate neutral and earth.

PNB – similar to PME except the neutral is RA
earthed at the customer end rather than Customer's Earth
the transformer. This arrangement is only
used to supply up to four customers in
remote locations.

TN-S – the neutral and earth are connected
together at the transformer and separate
neutral and earth (SNE) cables are used
throughout. Note: If any part of the network
is converted to PME, or the earth and neutral
conductors are bonded at any point beyond
the transformer, the system is TN-C-S.

TT - the neutral and earth are connected
together at the transformer. No earth
terminal is provided to the customer. The
customer has an independent earth
electrode to which any exposed metalwork of
the customer’s installation is connected. The
earth loop impedance is relatively high for
this arrangement and therefore a residual
current device (RCD) is usually required to
protect the customer's installation.

© UK Power Networks 2014 All rights reserved Page 56