Helpful video reference. GSH Electrical, a UK specialist training channel run by Gary Hayers (a former installation electrician and college lecturer) with more than 170,000 subscribers, demonstrates the single-phase safe isolation procedure in "Electrical safe isolation procedure - Single Phase". The video is a training aid for City and Guilds Level 2 and 3 programmes and shows the test sequence clearly. Watch it alongside this guide to see the steps performed in practice.
1. Identify the correct circuit at the consumer unit
Find the MCB or fused switch labelled for the circuit you intend to work on. Consumer unit labels are often handwritten, faded or simply wrong — so confirm the label is correct before relying on it.
If you are not certain, plug a lamp into a socket on the target circuit. Switch MCBs off one at a time until the lamp goes out. Note which one caused it. Switch it back on and confirm the lamp lights again (confirming this is the correct breaker, not a coincidence). Then switch it off properly. For lighting circuits, turn a light on the target circuit on, then switch MCBs until it goes out.
2. Check your voltage indicator before switching anything off
This step is skipped more often than any other, and it is the most important one. Before switching off the target MCB, take your approved voltage indicator to a known live source — a socket on a different circuit that you are confident is energised. Test it. The indicator must show live.
This proves the instrument is functioning before you rely on it to tell you a circuit is dead. A faulty indicator that reads dead on a live circuit is more dangerous than no indicator at all, because it gives false confidence.
Your voltage indicator must comply with HSE guidance document GS38. That means two-pole probes with protective shrouding, finger guards, self-retracting or recessed tips, and current-limiting resistors in the leads. A neon screwdriver, a simple continuity tester, or an unprotected multimeter probe do not meet GS38.
3. Switch off the MCB
Switch the target MCB to the off position. For a fused consumer unit (old rewireable board), remove the fuse carrier and pocket it. Do not leave it on top of the board where someone can put it back in.
Do not rely on a switched fused connection unit, a pull cord switch, or a wall switch to provide isolation. These are not isolators in the BS 7671 sense — they do not provide full disconnection of all live conductors in a fail-safe way. Isolation must be at the MCB or fused isolator in the consumer unit.
4. Apply a lock-off or fit a warning notice
Fit a lock-off clip to the MCB to prevent it being switched back on while you are working. Most modern MCBs accept a clip-on padlock shackle or a purpose-made lock-off device. If you work as an electrician, carry these as standard.
For domestic DIY where you are the only person in the property and you can see the consumer unit from where you are working, a clearly written warning notice taped over the MCB is acceptable. If there is any chance someone else could access the board — a child, a housemate, a builder working elsewhere in the house — a physical lock-off is the only safe option.
Tell anyone in the property that you have isolated a circuit and ask them not to touch the consumer unit while you are working.
5. Prove live on the known source
Return to the known live socket (on the different circuit you tested in step 2). Test it with your voltage indicator again. It must still read live.
This confirms the instrument is still working after you have moved around and after any time that has passed since step 2. An instrument that gave a live reading in step 2 but has since failed or drained its battery will read dead on everything — including a live circuit. This second live check catches that failure before you rely on the dead reading in step 6.
6. Test the circuit to prove dead
Go to the point where you intend to work — the socket face, the switch terminals, the junction box, or the luminaire connections. Test between:
- Line (brown) and neutral (blue) — should read 0 V
- Line (brown) and earth (green/yellow) — should read 0 V
- Neutral (blue) and earth (green/yellow) — should read 0 V
All three readings must be zero. A reading between neutral and earth when the MCB is off can indicate a wiring fault that needs investigation before any work continues. Do not proceed if you see any reading other than zero between any pair of conductors.
7. Prove live on the known source one more time
Return to the known live socket and test it again. It must read live.
This final check — live, dead, live — is the complete proving sequence. It brackets the dead test with two live confirmations, eliminating the risk that the instrument failed between steps 2 and 6 without your noticing. It takes less than thirty seconds and it is what every qualified electrician does without exception.
8. Begin work
Only after completing the live-dead-live sequence should you open terminals, remove faceplates, or cut cable. Keep the lock-off in place or the warning notice visible throughout the job. If you need to leave the work area — even briefly — re-prove dead when you return if there is any possibility the circuit could have been restored.
When to call us
Correct safe isolation equipment — a GS38-compliant voltage indicator, lock-off clips, a calibrated multifunction tester — costs over £100 to buy properly. For a single job, it may make more sense to call Richard. Small jobs in Sandwich start at £10 per 10 minutes, and Richard brings calibrated test equipment and the right protective kit as a matter of course.
Need electrical work done safely in east Kent?
Richard carries approved test equipment and follows the correct safe isolation procedure on every job — from a socket swap in Sandwich to a full board change in Deal.
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