Topic 3 of 5
Safe isolation
Safe isolation is how you make sure a circuit is dead before you touch it. Get it wrong and the result can be fatal, so it's tested in every practical assessment.
The procedure
- Get permission and identify the circuit or equipment to be worked on.
- Isolate it at a suitable point, such as the MCB or isolator.
- Secure the isolation: fit a lock, keep the key on you and attach a warning label.
- Select an approved voltage indicator that meets HSE guidance GS38.
- Prove the indicator works on a proving unit or known live source.
- Test the circuit is dead between every combination of conductors.
- Re-prove the indicator on the proving unit to show it didn't fail during the test.
Only then is the circuit proved dead and safe to work on.
Tests to prove dead
| Supply | Tests |
|---|---|
| Single phase | L–N, L–E, N–E (3 tests) |
| Three phase | L1–L2, L1–L3, L2–L3, each line to N, each line to E, and N–E (10 tests) |
GS38 test equipment
- A two-pole voltage indicator, not a multimeter set to volts or a neon screwdriver.
- Probe tips with no more than 4 mm of exposed metal, ideally 2 mm or less, or spring-loaded retractable shrouds.
- Finger guards on the probes and well-insulated leads, in good condition.
Exam tip: remember "prove, test, prove". Proving the tester before and after is what makes the test trustworthy.
Mini test
6 questions. Pick an answer to see if you're right and why.