What is the earth fault impedance?

Earth fault loop impedance is the path followed by fault current when a low impedance fault occurs between the phase conductor and earth, i.e. “earth fault loop”. Fault current is driven round the loop by the supply voltage. The higher the impedance, the lower the fault current will be and the longer it will take for the circuit protection to operate. So in short it is the impedance of the earth fault current loop starting and ending at the point of earth fault. This impedance is abbreviated to Zs.

The earth fault loop impedance can be used with the supply voltage to calculate the earth-fault current, and hence, to properly determine earth cable size.

External earth loop impedance (Ze)

Regulation 313.1 requires a number of characteristics of the supply to an installation to be determined, including the nominal voltage to earth (U₀), the earth loop impedance of that part of the system external to the installation (Ze), and the prospective short-circuit current at the origin of the installation.

The value of external earth loop impedance (Ze) measured or otherwise determined in accordance with Regulation 313.1 may differ from the applicable typical maximum value declared by the electricity distributor, which is usually:

• 0.8 Ω for TN-S system
• 0.35 Ω for a TN-C-S system
• 21 Ω plus the resistance of the installation earth electrode for a TT system.

Earth loop impedance of final circuits

For each final circuit and distribution circuit, it must be confirmed that the value of line-earth loop impedance (Zs) is low enough to achieve automatic disconnection of supply to the circuit within the relevant maximum time specified in Regulation Group 411.3.2 in the event of an earth fault.

Table 1 gives the maximum disconnection times permitted for final circuits and distribution circuits in TN and TT systems at a nominal voltage to earth (U₀) of 230 V. When checking that the value of Zs is sufficiently low to achieve disconnection within the required maximum time, account must be taken of characteristics of the protective device used for automatic disconnection.

For commonly used overcurrent devices, this is usually done by checking that the measured value of Zs at the electrically most remote part of the circuit is not more than 80 % of the applicable maximum value given in Tables 41.2, 41.3 of 41.4 of BS 7671.

For overcurrent devices not covered by those tables, another reliable source of information on the limiting values of Zs must be consulted, such as the manufacturer’s data.

Where the protective device is a nondelayed RCD, the maximum value of Zs can be found from Table 41.5 of BS 7671. The Zs values in that table are intended for a TT system but may also be applied to a TN system. These Zs values not only meet the disconnection time requirements of BS 7671, they also meet the condition RA × I∆n ≤ 50 V given in Regulation 411.5.3 (ii) for a TT system. (RA is the sum of the resistances of the earth electrode (to Earth) and the protective conductor connecting it to the exposed conductive-part. I∆n is the rated residual operating current of the RCD.)

For RCDs not covered by Table 41.5, the maximum value of Ze can be determined from information given in Table 3A in Appendix 3 of BS 7671, by using the formula given on the same page as that table. In addition, for a TT system, Zs must be low enough to meet the condition RA × I∆n ≤ 50 V, mentioned above.

Consequences when performing earth loop impedance tests

For new installations, earth loop impedance testing should present few operational problems during the initial verification process, as the installation will not have been put into service.

However, for an installation that is in service, there may be serious consequences for the user of the premises if, for example, computer data is lost or a home life support system switched off as a result of an inadvertent interruption of supply during the test, such as might be caused by the unintended operation of an RCD.

Inadvertent disconnection of a circuit, group of circuits, distribution board or even a whole installation could occur if an RCD operates when an earth fault loop impedance test is carried out. As a result, a number of methods have been developed to minimise the likelihood of an RCD operating during such a test.

One such method, calculation, is described below. Another method is to measure the earth loop impedance with a loop test instrument that supplies a test current sufficiently low not to trip the RCD, such as 15 mA.