Fault Finding, Testing, Verification and Commissioning
Testing follows a logical sequence: visual inspection, then dead tests, then live tests, then verification and commissioning. Dead tests include continuity of protective earthing, continuity of ring final circuits, insulation resistance, and polarity. Live tests include fault-loop impedance, RCD trip time and current, and functional testing.
Insulation resistance is measured with a high-voltage DC tester (typically 500 V for 230/400 V circuits), with a minimum acceptable value commonly 1 MΩ for a new installation, though lower values may be acceptable for existing installations with justification. Always isolate and prove dead before insulation testing, and disconnect sensitive equipment.
Fault finding uses a systematic approach: gather information, form a hypothesis, test, and confirm. Understand common faults: open circuit, short circuit, earth fault, high resistance joint, and reversed polarity. A trap is assuming a tripping RCD means a faulty RCD rather than a genuine earth leakage fault.
Worked example: an RCD trips intermittently. Test insulation resistance on each circuit; a reading below 1 MΩ on one circuit points to moisture or damaged insulation. Disconnect loads and retest to isolate.
Commissioning includes checking protective device settings, labelling, and documentation. Traps include testing with loads connected (giving false insulation readings), forgetting to prove the tester, and not recording results.