Safe isolation procedure: step-by-step guide
Safe isolation is the most important thing you will ever learn as an electrician. Get it right every time and you go home every time. This is the standard UK procedure, in order, with the test combinations and equipment you need.
What you need
- An approved two-pole voltage indicator meeting HSE guidance GS38 (not a multimeter or a non-contact "volt stick").
- A matching proving unit (or a known live source).
- A lock-off device and padlock suitable for the isolator or circuit breaker, with a unique key you keep on you.
- Warning notices / danger tags.
The safe isolation procedure
- Identify the circuit or equipmentUse drawings, charts and labels to find the correct point of isolation. Get permission to isolate and warn anyone affected.
- IsolateSwitch off at the correct isolator, switch-disconnector or circuit breaker. A functional switch is not a means of isolation.
- Secure the isolationLock it off, keep the key on your person, and fit a warning notice. If the device can't be locked, remove fuses or use another secure method.
- Prove the voltage indicatorCheck it works on the proving unit or a known live supply.
- Test that the circuit is deadTest between every combination of live conductors and earth at the point of work (see the tables below).
- Re-prove the voltage indicatorProve it again on the proving unit to show it was working throughout the test.
- Begin workOnly now is it safe to start. If you leave and come back, check the isolation is still in place.
Test combinations
Single-phase (3 tests)
| Test | Between |
|---|---|
| 1 | Line – Neutral |
| 2 | Line – Earth |
| 3 | Neutral – Earth |
Three-phase (10 tests)
| Between lines | Lines to neutral | Lines to earth | Neutral to earth |
|---|---|---|---|
| L1 – L2 | L1 – N | L1 – E | N – E |
| L1 – L3 | L2 – N | L2 – E | |
| L2 – L3 | L3 – N | L3 – E |
GS38 test equipment requirements
HSE guidance note GS38 sets out what test probes and leads should look like to reduce the risk of shock, short circuits and arc flash:
- Probes with finger barriers or shaped to stop fingers slipping onto the tip.
- Exposed metal tips of no more than 4 mm — 2 mm or less, or spring-loaded retractable tips, are preferred.
- Leads that are well insulated, a distinctive colour, long enough but not excessively so, and undamaged.
- Fused leads or current-limiting resistors, as appropriate.
Common mistakes
- Testing before proving the tester, or forgetting to re-prove afterwards.
- Only testing line to neutral and missing the other combinations.
- Isolating the wrong circuit because the chart is out of date — always test at the point of work.
- Leaving the key in the lock or not using a warning notice.
- Relying on a non-contact voltage detector to prove dead.
- Forgetting other sources of supply: borrowed neutrals, solar PV, batteries, generators or a second supply.
Do safe isolation step by step on iZapp’s 3D Testing Bench — prove, isolate, lock off, test dead and re-prove on a virtual board. Free, on your phone.
Frequently asked questions
What is the safe isolation procedure?
It is the method used to make sure a circuit or piece of equipment is dead before you work on it: identify the circuit, isolate it, secure the isolation with a lock and warning notice, prove your voltage indicator on a known source, test that the circuit is dead between all conductors, then re-prove the voltage indicator.
Why do you prove the tester before and after?
Proving the voltage indicator before and after testing shows it was working throughout. If it failed during the test, a "dead" reading could be false — and that is how people get killed.
How many tests are needed on a three-phase supply?
Ten: L1–L2, L1–L3, L2–L3, L1–N, L2–N, L3–N, L1–E, L2–E, L3–E and N–E.
Can I use a multimeter for safe isolation?
No. HSE guidance GS38 recommends a two-pole voltage indicator designed for the job, used with a matching proving unit. Multimeters can be set to the wrong range or fail in ways that give a false dead reading.
What does GS38 say about test probes?
Probes should have finger guards and well-insulated leads, with exposed metal tips of no more than 4 mm (2 mm or less, or spring-loaded retractable tips, are preferred) to reduce the risk of short circuits and arc flash.
Last updated 2026-10-14. This guide summarises standard UK practice and HSE guidance GS38 for training purposes. Always follow your employer's procedures and the current guidance.