Testing, Inspection, and Troubleshooting
What this domain tests
The order and method of inspection and testing, instrument use, interpretation of results, and the diagnostic reasoning used to locate faults.
Core rules
- Sequence: visual inspection first, then continuity of protective and bonding conductors, ring final circuit continuity, insulation resistance, polarity, earth fault loop impedance, prospective fault current, RCD operation and functional testing.
- Insulation resistance: 500 V DC, minimum 1 MΩ for a 400/230 V circuit, measured between live conductors and between live conductors and earth, with electronics, SPDs and dimmers disconnected.
- RCD: trip within 300 ms at rated residual current and within 40 ms at 5 × IΔn; test both half-cycles where the instrument allows.
- Test instruments must comply with GS38: fused or fused-probe leads and finger guards, with a proving unit for the voltage indicator.
- Certification: an Electrical Installation Certificate for new work and a Minor Works Certificate for small alterations, both with schedules of test results.
Worked example
A lighting circuit gives 2.5 MΩ insulation resistance. It passes the 1 MΩ minimum, but if similar circuits on the board read 100 MΩ this outlier points to moisture, cable damage or a shared neutral. Record the value, then investigate the outlier rather than passing the whole board on one number.
Traps
- Insulation testing with SPDs or electronic drivers still connected, giving false lows or damage.
- Measuring Z_e with main bonding connected, so parallel paths give a misleadingly low value.
- Confusing prospective fault current with earth fault loop current.
- Certifying an installation without completing the schedule of test results.