Determining Zs for a lighting circuit

Determining Zs for a lighting circuit

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The aim of this article is to explain why it is necessary to determine the value of earth fault loop impedance, and considers the different options available for the contractor when needing to obtain the measured value for a typical lighting circuit.

Note that this article particularly applies when performing periodic inspection and test of older installations which pre-date recent editions of BS 7671.

Typically, the earth fault loop path comprises the external impedance (Ze) and the impedance within the installation (R1 + R2). If the overall impedance of the fault path is too high the fault current
will be insufficient to meet the required disconnection time for operation of the protective device.

In order for the protective device to operate within the maximum permitted time of Table 41.1 of BS 7671, the earth fault loop impedance (Zs) for the circuit must not exceed the maximum earth fault loop impedance values for the protective device given in Tables 41.2 to 41.4 of BS 7671. This article will discuss the loop impedance testing of a lighting circuit only.

Obtaining the value of earth fault loop impedance for a lighting circuit can be determined by one of the following options:

  1. Adding the result of external earth fault loop impedance (Ze) test to the measured value of (R1 + R2) obtained at the furthest point of the circuit, or
  2. Taking a direct measurement of (Zs) using an earth fault loop impedance test instrument.

Although it may be more practical to take a direct measurement (option 2) on circuits such as the ring final circuit, carrying out such a test on a lighting circuit for example may not be generally suitable as it involves uneccessarily working on or near exposed live parts.

It is recognised that luminaire plug-in adaptors are available which facilitate a safe and reliable connection to live terminals during a test. However, the use of such adaptors may become irrelevant when a connection to earth is required, usually within the luminaire. The removal of covers for access to the earth terminal may inherently expose persons to live parts.

To conform to Regulation 14 of the Electricity at Work Regulations 1989, unnecessary work carried out on or near live conductors should be avoided wherever possible.

Regulation 643.7.3 of BS 7671 permits the value of earth fault loop impedance to be determined by means other than a direct measurement, we shall therefore, only discuss the use of option 1 in this article.

Measurement of (Ze)
Regulation 313.1 requires the value of (Ze) to be determined for every installation and is typically carried out during the initial veri cation of the installation and once during any subsequent periodic inspection and test.

Such a test verifies the presence of a means of earthing and provides the ohmic value of this which is expected to lie within the parameters for the intended earthing arrangement.

This process will however, involve working on or near live parts at the origin of the installation, in which all necessary precautions must be taken to prevent danger. Before carrying out the test, the installation must be totally isolated from the supply, and the means of earthing disconnected from all other protective bonding conductors within the installation. Guidance on an appropriate safe isolation procedure is outlined in the Electrical Safety First publication Best Practice Guide 2.

The test is typically carried out using a two-lead, as shown in Fig 1, or three-lead earth fault loop impedance test instrument, although the testing procedure may vary depending upon the instruments, manufacturer,

On completion of testing and before the installation is re-energised, the earthing conductor or other protective conductors that were previously disconnected to facilitate the measurement of Ze must be reconnected.

Measurement of (R1 + R2)
Before carrying out the (R1 + R2) resistance measurement, the relevant circuit must be isolated from the supply.

Such a test is carried out between the circuit protective conductor and the associated line conductor and measures the series resistance of both conductors for the circuit under test.

The purpose of measuring the (R1 + R2) for a circuit is:

a)  to obtain the highest resistance of the circuit under test at the most distant point or accessory of the circuit from the consumer unit, and

b) to verify continuity of the circuit protective conductor to each point and accessory

Furthermore, a correctly performed (R1 + R2) test allows for the verification of polarity.

Having temporarily connected together the line conductor and circuit protective conductor at the consumer unit, the instrument test leads can be connected to the terminals of the line conductor and the circuit protective conductor at the accessory. To allow for the worst case scenario of earth loop impedance to be determined, the highest value of (R1 + R2) resistance should be obtained at the accessory at the furthest point within the circuit.

Testing instrument

The values of (R1 + R2) are measured using an ohmmeter having a low resistance range, typically between (0 to 2 Ω or similar) or a multi-function test instrument set to the continuity range. It should be noted that the resistance of the test leads will be significant, therefore for maintaining accurate results, it will be necessary either to record the resistance of the test leads for subtraction from the overall measurement or use the auto-null function, where permitted, on the test instrument prior to carrying out the testing.

Determining the measured value of Zs

Having obtained the value of Ze for the installation and the (R1 + R2) for the furthest point of the lighting circuit we can calculate the value of Zs, as shown:


Zs = Ze + (R1 + R2)

– Zs is the earth fault loop impedance (Ω)

– Ze is the measured value of external impedance (Ω), and

– (R1 + R2) is the measured value of resistance (Ω) for the line conductor and protective conductor, at the most distant point or accessory from the distribution board or consumer unit.

However, where the lighting circuit is supplied from an additional sub-board located at a distance away from the origin, an allowance must be made for the impedance of the sub-mains cable, as shown below:

Zs = Zdb + (R1 + R2)

Zdb is the measured value of earth loop impedance (Ω) at the distribution board or consumer unit to which the final circuit is connected.

To satisfy the requirements of Chapter 41 of BS 7671 for the earth loop impedance, the measured value of Zs as described in the above examples, should not exceed 80% of the required values as given in Table(s) 41.2 to 41.4. For overcurrent protective devices not covered by those Tables, further information relating to the limiting values of Zs must be obtained from the device manufacturer.

Where the protective device is a non- delayed RCD, the maximum value of Zs can be found in Table 41.5 of BS 7671. Although the Zs values in Table 41.5 are intended for a TT system they may also be applied to a TN system.

Note that where an RCD is used to provide the necessary disconnection time in a TN circuit the values of Zs given in Table 41.5 may be used.


While it is a requirement to determine the earth fault loop impedance for every circuit within an installation, determining such a value for a lighting circuit should be obtained through other means than taking a direct measurement, thereby reducing any unnecessary contact with live terminals.

To comply with Regulation 14 of the Electricity at Work Regulations 1989, unnecessary work carried out on or near a live conductor should be avoided where ever possible.

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