Earth Resistance, Loop Impedance, and RCD Testing
Domain focus
This domain covers the measurements that prove a fault will be cleared in time: earth electrode resistance, earth fault loop impedance (Zs), and residual current device (RCD) performance. Questions usually hinge on method validity and on applying the correct acceptance rule.
Earth resistance
Fall-of-potential drives a current stake 30-40 m from the electrode and places the potential stake at 62% of that distance. Repeat with the potential stake moved ±10%; readings should agree within about 5%. Clamp-on meters work only when a parallel earth return path exists and are invalid on an isolated rod.
Loop impedance
Zs = Ze + R1 + R2. Compare the measured value with the maximum permitted for the protective device in the wiring regulations table. Because tabulated maxima assume conductors at operating temperature, many specifications apply an 80% rule: measured Zs must not exceed 0.8 × the tabulated maximum.
RCD testing
At rated residual current (1× IΔn) the device must trip within 300 ms; at 5× IΔn within 40 ms; at 0.5× IΔn it must not trip. Record the actual trip time, not just pass/fail.
Worked example
A 32 A type C MCB at 230 V has a tabulated maximum Zs of 0.72 Ω. The 80% rule limit is 0.72 × 0.8 = 0.576 Ω. Measured Zs is 0.62 Ω. Against the raw table it appears to pass, but under the specified 80% rule it fails, so the circuit needs a longer CPC or a different device.
Traps that cost marks
- Measuring loop impedance at a socket with appliances connected, creating parallel paths.
- Using a clamp meter on a single rod with no return path.
- Confusing 30 mA and 300 mA RCD criteria.
- Reading the table at a different ambient or conductor temperature than the one specified.
- Taking one fall-of-potential reading with no stake-spacing check.