Safe Isolation Procedure
Step-by-step guide to safely isolating electrical circuits
HSE GS38, Electricity at Work Regulations 1989, BS 7671
Safety Critical Procedure
Safe isolation must be carried out every time before working on or near electrical circuits. Approximately 30 fatal electrical accidents occur in UK workplaces each year, with ~1,000 non-fatal incidents reported (RIDDOR). As little as 50mA can cause ventricular fibrillation. The EAWR 1989 imposes absolute duties — they must be met regardless of cost or difficulty.
The 6 Steps
Identify the circuit(s) to be worked on using drawings, schedules, or labelling at the distribution board.
- Check circuit charts/schedules at the DB
- Confirm which circuits supply the work area
- Consider all sources of supply (multiple DBs, sub-mains, UPS, generators, PV)
- Never rely solely on labels — they may be inaccurate from unrecorded modifications
- Where possible, verify by checking the intended load turns off when the breaker operates
Switch off the supply using the appropriate isolator, MCB, or main switch.
- Use the correct isolating device for the circuit
- Ensure it provides full disconnection (all poles)
- For TT systems, both line and neutral may need isolation
- Never rely on an MCB alone if a separate isolator exists
Lock off the isolating device and attach a warning notice.
- Use a lock-off device (MCB lock, handle lock, or padlock)
- Attach a caution label: 'Danger — Do Not Switch On'
- Keep the key/unique lock on your person at all times
- If lock-off not possible, remove fuse and keep it
- For multiple workers: use a multi-lock hasp — each person applies their own padlock
Test your voltage indicator on a known live source to confirm it is working correctly.
- Use a GS38-compliant two-pole voltage indicator (not a multimeter)
- Test on a known live supply or a dedicated electronic proving unit
- Confirm the indicator shows voltage correctly across all ranges
- Check leads, probes, and batteries are in good condition
Test between all conductors and earth at the point of work to confirm the circuit is dead.
- Single-phase: test Line–Neutral, Line–Earth, Neutral–Earth (3 tests)
- Three-phase: complete the full 10-point test sequence (see below)
- Test at the point of work, not just the DB
- Also test at the point of isolation (outgoing side)
Test your voltage indicator again on a known live source to confirm it is still working.
- Re-test on the same known live source used in Step 4
- Confirm the indicator still reads voltage correctly
- This proves the dead reading in Step 5 was genuine
- If the indicator fails to light, treat the dead reading as invalid and repeat the entire process
“Prove — Test — Re-prove”
The golden rule: prove your tester works, test for dead, then prove your tester again.
Three-Phase 10-Point Test Sequence
Missing any single test is an AM2 exam “red line” failure
| # | Test | Purpose |
|---|---|---|
| 1 | L1 to L2 | Phase-to-phase potential |
| 2 | L1 to L3 | Phase-to-phase potential |
| 3 | L2 to L3 | Phase-to-phase potential |
| 4 | L1 to Neutral | Phase-to-neutral voltage |
| 5 | L2 to Neutral | Phase-to-neutral voltage |
| 6 | L3 to Neutral | Phase-to-neutral voltage |
| 7 | L1 to Earth | Line-to-ground isolation |
| 8 | L2 to Earth | Line-to-ground isolation |
| 9 | L3 to Earth | Line-to-ground isolation |
| 10 | Neutral to Earth | Confirm neutral not floating/borrowed |
GS38 Test Equipment Specifications
Use a dedicated two-pole voltage indicator, not a multimeter. Multimeters are susceptible to wrong range selection, and a flat battery can give a false “zero” reading on a live circuit.
| Requirement | Specification | Rationale |
|---|---|---|
| Exposed metal tip | ≤4mm (preferably ≤2mm) | Prevents accidental short-circuits between phases |
| Finger barriers | Mandatory on all probes | Prevents hand sliding onto live conductors |
| Lead rating | CAT III or CAT IV | Withstands high-energy voltage transients |
| Fusing (multimeters) | HBC fuse (usually 500mA) | Protects user if meter fails internally |
| Lead type | Captive leads (non-detachable) | Prevents exposing live connector pins |
EAWR 1989 — Key Regulations
| Regulation | Focus | Application |
|---|---|---|
| Regulation 12 | Means for isolation | Identifying and operating the correct switch-disconnector or fuse |
| Regulation 13 | Precautions for dead work | Applying personal padlocks and danger tags to isolation points |
| Regulation 14 | Live work prohibition | Mandatory proving dead before touching any conductive part |
| Regulation 16 | Competence | Operative must have skill to use GS38 equipment correctly |
Isolated conductors running parallel to live conductors can pick up induced (“phantom”) voltages — enough to light up a voltage indicator but typically lacking current capacity for a direct shock.
Solution: Use a dual-impedance voltage indicator (e.g., Drummond MTL20) with a low-impedance test button. If the reading collapses to zero under load, it was induced. If it persists, there is a hard voltage source — stop work and investigate.
- • Skipping the re-prove step — the tester could have failed between tests
- • Testing at the DB only — always test at the point of work
- • Using a non-contact voltage detector alone — NCVDs are for initial checks only
- • Using a multimeter — use a dedicated two-pole voltage indicator (GS38)
- • Not locking off — a 2024 survey found 25% of electricians rarely or never lock off
- • Using tape to “secure” a breaker — provides no physical barrier to reconnection
- • Forgetting other supplies — check for UPS, generators, PV, battery storage
- • Testing on the incoming side — this side remains live after isolation
These “red line” errors result in immediate failure of the AM2 practical assessment
- Failure to keep padlock key in personal possession
- Failing to prove tester on proving unit both before AND after the circuit test
- Testing on the incoming (live) side of the isolator
- Missing a single test point in the 10-point three-phase sequence
- Failure to attach a 'Danger: Men Working' notice