Tracing earth faults

Does the RCD trip immediately on switching on, or only when a particular appliance is switched on? A constant earth fault points to the installation or permanently connected equipment; an intermittent fault often points to a particular appliance or moisture.

Switch off all circuit breakers downstream of the RCD and switch it on. If it stays on, the fault is in one of the circuits. If it still trips with no load, the fault may be in the RCD itself or in the fixed wiring leading to it.

Switch the circuits on one at a time. The circuit that trips the RCD contains the fault. Note which one — the rest of the fault-finding now concentrates on that circuit.

Disconnect the loads on the identified circuit one at a time (appliances, luminaires, socket outlets), and find the point where the fault disappears. Typical culprits are faulty heating elements, damp outdoor sockets and damaged flexes.

Make the circuit dead and measure the insulation resistance between the live conductors and PE. A value below 1 MΩ confirms an insulation fault. Remember to disconnect sensitive electronics so the test voltage does not damage them.

If the RCD trips without a single clear fault, the cause may be accumulated operational leakage current from many appliances. If it exceeds 0.3 × I_ΔN (9 mA on a 30 mA circuit), the load should be spread across several RCDs.

Correct the fault — replace defective equipment, dry damp joints, renew damaged cable — and repeat the insulation resistance test and RCD test to confirm that the circuit is in order again. Record findings and remedial work in the documentation.

Never reset an RCD that trips by »bypassing it« — it is signalling a real fault that must be found.