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The procedure, step by stepSix steps. Why prove the tester twice?
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Done on a board with the supply energised. The order matters.
- Identify what you're isolating. The client tells you the problem; you locate the circuit from the circuit chart on the board — sockets, cooker, lighting.
- Switch off in the right order. Each individual circuit breaker or RCBO first, then the RCD serving them, then the next group of breakers, then their RCD — and the main switch last.
- Lock off the device you've isolated at, fit your warning sign, and keep the key on you.
- Prove the AVI works — on a proving unit, or a known live supply.
- Test, in the order below.
- Re-prove the AVI on the proving unit or known live source. Only then is the system proved dead.
Why isolate circuit by circuit rather than just hitting the main switch? You can switch a main switch on load — that's what it's for. But dropping every circuit individually first is better practice.
⚡Proving unit or known live supply — either is acceptable. The proving unit is the safer option every time, because you aren't putting your fingers near live conductors to prove your tester.
✋Step 6 is not optional. Getting no reading proves nothing on its own — a dead tester gives you exactly the same result as a dead circuit. You only know the system is dead once you've shown the instrument still works afterwards.
Safe isolation doesn't go on the certificate. There's no box for it. It isn't one of your recorded tests — it's what you do before you start filling anything in.
Electrician's Guide · Section 7 · the full procedure ★ GN3 · safe isolation
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