Inspection, Testing, and Fault Diagnosis
Inspection, testing and fault diagnosis covers the sequence of verification, the instruments used, and how to interpret a reading that does not match the design.
Test sequence
- Visual inspection before any instrument is used: identification, damage, IP rating, fire sealing, barriers, correct device ratings, documentation.
- Dead tests: continuity of protective and bonding conductors with a low-resistance ohmmeter, continuity of ring and radial conductors, insulation resistance at 500 V DC with a minimum of 1 MΩ, and polarity.
- Live tests: earth fault loop impedance, RCD trip time (≤300 ms at IΔn and ≤40 ms at 5IΔn) plus test-button operation, phase sequence and functional checks.
- Record results, compare them with the design values and issue the certificate.
Fault diagnosis
Work from the symptom. A 30 mA RCD tripping instantly usually indicates a N–PE fault or a damaged cable; a delayed trip suggests moisture or cumulative leakage from several circuits. Split the installation at the board, test each circuit, then split the circuit at accessible points. An open neutral shows as low or fluctuating voltage with equipment that dims when another load switches on.
Worked example
An RCD trips only when the kitchen circuit is energised. Disconnect the circuit and test insulation: L–N = 200 MΩ, L–PE = 0.3 MΩ. The fault is to earth, not between live conductors, so inspect sockets and appliances on that circuit; the reading localises the fault and justifies further sectional testing.
Traps
- Insulation testing with electronics, SPDs or dimmers still connected.
- Using a multimeter instead of a loop tester for Zs.
- Testing an RCD with load connected.
- Certifying an installation without dead-test results.