Safe Isolation
Why It Matters
Safe isolation is the single most important procedure in electrical work. Skipped or done badly, it can end in electric shock and death. This page gives the basic sequence. For the full procedure, use the sources below.
Fuller Guidance
- Electrical Safety First, Best Practice Guide 2 (Issue 4.3), Safe Isolation: Guidance on the management of electrical safety and safe isolation procedures for low voltage installations. The safe isolation flowchart is on page 19.
- NICEIC EIC pocket guide.
The Basic Sequence
- Get permission and identify the point of isolation. Tell the customer, and anyone nearby, that the supply is being isolated. Locate the circuit and the device that isolates it.
- Select approved test equipment. An approved voltage indicator and a proving unit, undamaged, with GS 38 style leads and probes.
- Prove the voltage indicator on the proving unit (or a known live source). It must read live.
- Switch off and isolate.
- Lock off and fit a warning notice. Use your own lock. Keep the key on you. Never leave it lying about.
- Test dead at the point of isolation, on the outgoing side, in every combination (see below).
- Prove the voltage indicator again on the proving unit.
- Only now start work.
Some training and assessment materials list steps 2 to 5 in a slightly different order. In every version, the tester is proved before and after the dead test.
The Dead Test
Single-phase: test L-N, L-E and N-E.
Three-phase: test all 10 combinations. A logical order is:
| Group | Tests |
|---|---|
| Phase to phase | L1-L2, L1-L3, L2-L3 |
| Phase to earth | L1-E, L2-E, L3-E |
| Phase to neutral | L1-N, L2-N, L3-N |
| Neutral to earth | N-E |
Every reading should be 0 V.
Common Mistakes
- Not proving the tester before and after. If you do not prove it, you do not know it works.
- Putting the key down, or leaving it where someone else can pick it up and re-energise the circuit.
- No warning notice. The notice tells others not to switch the circuit back on and gives IT and other users time to shut equipment down.
- Testing at the wrong position on the switch. Test on the outgoing side of the isolator.
- Isolating more than you need to. Only isolate the circuit you are working on, and leave other circuits energised where the job allows.
- Not checking the switches on the circuit. Make sure the item is isolated, not just switched off.
- Relying on slider switches. Some boards have sliders that disconnect the live side of a breaker. They are not suitable as the only means of isolation.
- Not accounting for other sources of supply, such as solar PV, batteries, generators or a UPS.