Basic requirements of testing electrical installations

Insulation resistance is a dead test: a high d.c. test voltage is applied between conductors, so the circuit must be isolated. Earth fault loop impedance, RCD and prospective fault current tests need the supply connected, so they come after the dead tests.

Source: BS 7671:2018+A4:2026 (Part 6, initial verification sequence (paraphrased))

Inspection comes before testing and is normally done with the relevant part of the installation disconnected from the supply. It finds visible defects, such as damage, wrong cable or missing covers, before any test voltage or supply is applied. Doing it after live tests would expose people to defects that could have been seen. BS 7671, Part 6 (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, Regulation 642.1, inspection before testing (paraphrased))

Polarity testing checks that single-pole devices such as switches, fuses and circuit-breakers are connected in the line conductor, and that terminals such as socket and lampholder contacts are correctly connected. A switch wrongly placed in the neutral would leave the equipment live when switched off. BS 7671, Part 6 (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, polarity (paraphrased))

Dead tests are only safe on a circuit that has been securely isolated, locked off and proved dead with a proven voltage indicator. Just switching off a breaker leaves it open to someone switching it back on. Loop impedance is a live test and comes after the dead tests. HSE HSG85, safe isolation; EAWR 1989, regulation 13.

Source: HSE HSG85 Electricity at work: Safe working practices (Safe isolation and proving dead); Electricity at Work Regulations 1989 (Regulation 13, precautions for work on equipment made dead)

A ring final circuit relies on its line, neutral and protective conductors each forming a complete loop back to the board. If a ring is broken, it becomes two long radial legs and the cable may be overloaded even though everything seems to work. The test also shows up interconnections that make the readings uneven. BS 7671, Part 6, continuity of ring final circuit conductors (paraphrased).

Source: BS 7671:2018+A4:2026 (Part 6, continuity of ring final circuit conductors (paraphrased))