The global insulation resistance test, three phase and the periodicCan you test every circuit in one go?

The global test

Instead of circuit by circuit, you can test a group of circuits at once: main switch off, the circuit breakers on, one probe on the outgoing side of the main switch or busbar, the other on the earth bar (then the neutral bar). Every circuit fed from it is tested in one go.

• It’s quicker. But the reading is all those circuits in parallel
• It means switching on RCBOs and AFDDs — and an AFDD RCBO costs well over a hundred pounds. Leave those circuits out and test them on their own.

Three phase, and the periodic

Three phase is ten tests at 500 V: each line to neutral, each line to earth, each pair of lines, and neutral to earth.

On a periodic you often don’t know what’s connected, and you can’t go through someone’s home looking behind every cupboard. One cautious approach some inspectors take is to test live conductors to earth only, at 250 V, and N/A the live–live column — with the reason written in the remarks. That only applies where the equipment can’t be disconnected (Reg 643.3.2); where it can, disconnect it and test as normal.