Safe isolation, safety management and safe working practices
Making dead safe
Safe isolation is a sequence, not a single action, and the sequence is examined closely.
- Identify every source of supply to the part to be worked on - parallel supplies, a second consumer unit, PV inverters, generators, UPS, emergency lighting, control circuits and shared or borrowed neutrals.
- Notify affected persons and agree a shutdown.
- Isolate. Remember the consumer unit main switch does not isolate the meter tails or the distributor's cut-out.
- Secure the isolation with a lock and caution notice, and keep the key.
- Prove the voltage indicator before use (proving unit or known live source), prove dead at the point of work on every conductor, line-to-line and line-to-earth, then re-prove the indicator - the prove-test-prove routine.
- Only then apply portable earths or dismantle.
Traps. Fitting a lock-off and then testing at the board rather than at the actual point of work; using an untested indicator; forgetting that a single-pole device can leave a conductor at a dangerous potential; assuming that switching off proves dead. On a TN-C-S supply the neutral is a PEN conductor and must be treated as live.
Safety management. A risk assessment and method statement, suitable PPE, arc-flash awareness, barriers and permits to work are expected. Live testing must be justified and carried out with instruments of the correct overvoltage category.
Rule of thumb. If you cannot prove your voltage indicator on a proving unit, you have not proved dead - stop.