Earth Fault Loop Impedance Testing: Zs Testing Procedure and Accepted Values

Earth fault loop impedance (EFLI) is the total impedance of the complete circuit that earth fault current must travel through if a line-to-earth fault occurs at any point in an electrical installation. Understanding and correctly measuring this impedance is fundamental to verifying that protective devices will disconnect within the required time under fault conditions — which is the primary mechanism by which BS 7671 protects against electric shock from indirect contact.

The earth fault current loop consists of: the line conductor from the point of fault back to the supply transformer; the transformer winding itself; and the return path from the transformer neutral (star point) via the earthing system and the protective conductor (CPC) back to the point of fault. The lower the impedance of this loop, the higher the fault current and the faster the protective device operates.

BS 7671:2018+A4:2026 requires that the earth fault loop impedance Zs must not exceed the value corresponding to the required disconnection time for the circuit concerned. The required disconnection times are 0.4 seconds for final circuits up to 32A supplying socket outlets or equipment accessible to the public, and 5 seconds for distribution circuits and final circuits supplying fixed equipment (with some exceptions).

The distinction between Ze and Zs is important for both testing and fault finding. Here is a clear breakdown:

For a typical TN-C-S (PME) supply in the UK, Ze is typically 0.35Ω or less. For a TN-S supply it is typically 0.8Ω or less. For a TT supply, Ze can be several ohms because the return path relies on the earth electrode and the distributor's earth, both of which have significant resistance. This is why RCDs are mandatory on TT installations — the Zs is too high to achieve disconnection within the required time using overcurrent devices alone.

Follow this procedure for measuring Zs at the furthest accessible point of a circuit. The circuit must be energised for this test.

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BS 7671:2018+A4:2026 Appendix 3 contains tables of maximum earth fault loop impedance values for different protective devices and disconnection times. The key tables are:

These tables are incorporated into the schedule of test results on an Electrical Installation Certificate or EICR. The measured Zs and the design Zs (maximum permitted) must both be recorded for each circuit.

Assessing whether a measured Zs is acceptable requires comparing the measured value against the table maximum corrected for conductor temperature:

On an initial inspection (EIC), a circuit that fails the Zs test must be rectified before the certificate is issued. On an EICR, a failing Zs is coded as C2 (requires urgent attention) if it represents a genuine risk of the device not operating within the required disconnection time.

When Zs readings exceed the permissible maximum, a systematic investigation is needed. The most common causes are:

Earth fault loop impedance testing is one of the most time-consuming tests on an inspection. Here are practical tips to work efficiently without compromising accuracy: