Inspection, Testing and Certification
Sequence, results and paperwork
Inspection, testing and certification follows a fixed order, and the reason for that order is a favourite examination question. Inspect first, with the supply isolated, checking the installation matches the design and BS 7671. Then carry out the dead tests: continuity of protective conductors (including main protective bonding), ring final circuit continuity (r1, rn and r2 end-to-end), continuity of separate CPCs, and insulation resistance at 500 V DC - 1 MΩ minimum for a 230 V circuit, or 0.5 MΩ at 250 V for SELV/PELV. Dead tests come first because they reveal faults that would be dangerous the moment the supply is energised.
Only then energise and carry out the live tests: polarity, earth fault loop impedance (Zs must not exceed the tabulated maximum for the device), prospective fault current, RCD operation on both the test button and an instrument (½×, 1× and 5× rated residual operating current), phase sequence where relevant, and functional checks.
Record results on a schedule of inspection and test results and issue the correct certificate: an Electrical Installation Certificate for new work, a Minor Electrical Installation Works Certificate for a like-for-like replacement or single-circuit addition, and an Electrical Installation Condition Report for periodic condition reporting. Certificates are signed by a competent person and must be accompanied by the schedules.
Traps: insulation resistance measured with sensitive equipment still connected; polarity treated as a live test only when it is also verified dead; a Zs above the maximum recorded and accepted without investigation; certificates issued without schedules.
Rule of thumb: on a healthy ring final circuit, r1 + r2 measured at a socket is about one quarter of the end-to-end (r1 + r2); a much higher reading suggests a spur or a break.