1

C&G 2395-01

Level 3 Award in the

Periodic Inspection, Testing and

Certification of Electrical Installations

Earth Fault Loop Impedance Tests

Inspections are made to verify that the installed electrical equipment is in

compliance with Standards, and in accordance with the regulations

Prior to the testing, you must check the instrument is fit for purpose.

There are two methods for testing the continuity of protective

conductors.

It is unacceptable to simply record a result, it needs to be reasonable.

The insulation resistance test is used to verify that the insulation of

conductors, accessories and equipment is acceptable (has a high

resistance).

Site applied Insulation test is quite rare and is only carried out when

insulation has been applied on site.

A polarity test is carried out to determine that the line conductor only is

switched, to ensure the correct operation of accessories, and to ensure that

in Edison Screw lamps, the centre-contact is connected to the line

conductor.

Revision

describe the earth fault loop impedance paths for TN-S, TN-C-S and TT

systems

describe the methods of carrying out earth fault loop impedance tests in

terms of the tests used for measuring actual earth fault loop impedance

and methods of calculation of earth fault loop impedance from given data

and measurement of conductor impedance

explain how the results should be compared with relevant data such as

Table 41.2, 41.3 and 41.4 of BS 7671.

Outcomes of this Session

Regulation 612.8.1 requires that the effectiveness of the measures for

fault protection by automatic disconnection of supply is verified for TN-

S, TN-C-S and TT systems.

This verification can be determined by measuring or calculating the

earth fault loop impedance of the circuits within an installation.

Earth Fault Loop Impedance

This is the newer type of TN-S system.

This is a more traditional

arrangement and is found in

many older installations.

Typical TN-S Supply Earth (Terre) and Neutral Separate

Equivalent Earth Fault Loop for a TN-S Supply

The external earth loop depends on many factors such as length and cross-sectional

area of the cable.

The external fault loop is labelled Ze and the internal earth fault loop path is labelled

(R1+R2).

When combined give the overall impedance of the fault current path. ZS= Ze + (R1+R2)

A TN-S system usually has

an external earth loop

impedance of around 0.8 Ω.

quoted by the distributor for

both single and three phase

supplies.

Typical TN-C-S Supply Earth (Terre) and Neutral Combined through part of the System

This system has a combined neutral and

earth in the supply cable, but at the

consumers installation they separate, and

are separated throughout the rest of the

installation.

Examples of this type of supply

arrangement are PME (Protective Multiple

Earthing) or PNB (Protective Neutral

Bonding)

Equivalent Earth Fault Loop for a TN-C-S supply

A typical value of a external loop impedance would be a maximum of 0.35 Ω

for single-phase supplies. For three-phase supplies the figures vary.

This system has no earth provided by

the distributor, just the normal current

carrying conductors.

The consumer must supply their own

earth via an earth electrode and RCD

installed to 411.5.3.

A typical figure for the external earth

fault loop would be 21 Ω at the

distributors transformer. This value

covers the resistance of the neutral to

earth, the impedance of the transformer

winding and the line conductor.

TT Supply Earth to Earth (Terre - Terre)

TT Supply

TT systems are most commonly found on farms and villages and are fed from an

overhead supply system. They can also be found on petrol station forecourts.

The loop on the distributor’s side passes via the general mass of earth.

Testing Earth Fault Loop Impedance

Direct measurement using an earth fault loop impedance tester

Measurement of (R1+R2) and adding to a known value of Ze

The most common form of testing for the earth fault loop impedance is

using a earth fault loop impedance tester.

You can also determine the earth fault loop impedance by calculation

The earth fault loop test makes sure that the protective devices operate

within the required time. (anything from 0.05 s to 5 s depending on where

and what you are testing)

There are a number of ways in which the earth fault loop impedance can be

determined.

Zs=Ze+(R1+R2)

Test Points for Earth Fault Loop Impedance Testing

When testing sockets, all socket-outlets

should be tested, and the worse case result

recorded (the highest Zs value).

When testing lighting circuits, the end point

(furthest point) of the circuit should be

tested. Again, all lights on the circuit should

be tested, and the worse case result (the

highest Zs value) recorded.

Earth Fault Loop Impedance Testing is a

live test and the requirements of EAWR

Regulation14 apply. Be aware of the increased

risk to others

Two-lead Instrument at a D.B

Testing at the end point gives you the

total loop impedance of that circuit. This

is labelled Zs.

You will also need to test at the intake

position and at each subsequent

distribution board.

This is the Earth Loop Test with a two

lead instrument such as a Megger.

One lead is connected to the

incoming feed with the other

connected to the Main Earthing

Terminal (MET)

Earthing and main protective

bonding conductors should be

disconnected for the test!

Three-lead Instrument at a D.B

This is the Earth Loop Test with three

leads such as a Robin/Fluke. One

lead is connected to the neutral or

neutral block, one to the line, and the

other connected to the MET.

Instrument set for ZS

Earthing and main protective bonding conductors should be

disconnected for the test!

Motor circuits Guidance Note 3 does not permit testing at the

terminals of a motor.

You must test on the supply side of the motor.

Zs R1 + R2

The total earth-fault-loop impedance will be a combination

of the impedance readings and those from the continuity

test (R1 + R2).

Testing for Zs at the end of the circuit

You should also;

Make sure that you are taking the power for the meter from the circuit that you

are testing.

Remove the main equipotential bonding conductors.

Checking the Values

Once measured, the earth-fault-loop impedance should be checked;

For TN systems there are a number of options:

For standard thermoplastic circuits, the values in Appendix B of the

On-Site Guide and Guidance Notes 3 can be used.

The designers own calculated figures.

Table 41.2, 41.3 and 41.4 of BS 7671, after being corrected for

temperature. (20oC – 70oC) – 1.2!!!!!

Using the `rule of thumb’ method. (80% of max.values BS7671)

Remember: Read the exam paper question carefully and use the

method requested as per the given scenario!!!

The IET Guidance Note 3 provides a ‘rule of thumb’ to operate if you don’t want

to work through all the factors. This is now agreed to be 80%. This reduction is

an allowance for the fact that a measured Zs value is measured at an ambient

conductor temperature of 20oC. However, when under full load, the conductor

temperature may rise to a maximum operating temperature of 70oC, raising the

final actual Zs value.

This is done by:

Determining the new ‘corrected’ maximum tabulated value and comparing

with the measured value.

The new ‘corrected’ maximum tabulated value must be greater than the

measured value (measured @ 20oC) in order to satisfy ADS!

Rule of Thumb

Earth Loop (Zs) Test problems!

An Earth Loop Impedance test instrument may cause an RCD, RCBO,

or a 6 A Type B circuit breaker to trip when a circuit is under test.

For an RCD, there are a number of solutions to this problem:

Use a meter that has No-trip setting or effective D-Lok function

Replace the RCD for the duration of the test (not a preferred option) with a

circuit-breaker

Measure the external impedance (Ze) and add this value to the measured

value of (R1 + R2)

You must not short-out the RCD!!

Similar solutions can be put in place to reduce the

tripping of a 6 A BS EN 60898 type B circuit-breaker.

Earth Loop Impedance (Zs)

An actual measured value of Zs would generally be

lower than the calculated value due to the following

factors:

• The presence of parallel paths due to the

connection of protective bonding conductors

• Parallel paths through the steel

conduit/trunking containment system

• The effect of the steel wire armour being in

parallel with a conductor used as a cpc.

End of chapter 6