Safe Isolation of Circuits
Proving a circuit is dead
Safe isolation of a circuit is a practical procedure with a fixed sequence, and the assessment watches your hands as much as your answers.
Identify the circuit and every source of supply to it - including PV inverters, standby generators, UPS, dual-tariff boards and adjacent circuits sharing a neutral. Select a device suitable for isolation; a functional switch is not an isolator. Isolate, lock off with your own padlock, attach a warning notice, and keep the key on your person. Where several people are working, use a multi-lock hasp or a permit to work.
Prove the voltage indicator on a proving unit, test the conductors at the point of work between all pairs, then prove the indicator again. Test the enclosure and any exposed metalwork as well as the terminals. Only then begin work.
Traps appear as scenarios. A borrowed neutral means an isolated lighting circuit can still have a live neutral. A shared enclosure may hold other live circuits, so a barrier and warning notice are needed even after your circuit is dead. Working live at a board with a multimeter instead of a dedicated voltage indicator is a fail. Leaving a lock-off without a notice, or leaving the key in the lock, is also a fail.
Remember that isolation must not remove earthing or bonding from equipment that remains in service, and that a device used for isolation must be capable of being secured against reconnection.
Rule of thumb: the isolation is only as good as the last test - if the tester cannot be proved after the test, discard the result and start the procedure again.