Electrical isolation
Safe isolation, lock-off
The procedure for making electrical equipment dead before work: identify, isolate, lock off, prove dead with a tested voltage indicator, and control the key.
The Electricity at Work Regulations prohibit work on or near live conductors unless it is unreasonable for them to be dead, it is reasonable to work live, and suitable precautions are taken; in construction that means almost all electrical work is done dead. Safe isolation is the sequence that guarantees it: identify the circuit and its point of isolation; isolate it; secure the isolation with a lock and a unique key held by the person doing the work, with a warning tag; prove the circuit is dead at the point of work with an approved voltage indicator that has itself been proved on a known live source or proving unit before and after; and only then work. Re-energisation is by the person who locked off, after checking everyone is clear.
On a site with several trades, the permit-to-work system coordinates isolations so that one electrician's lock-off is not undone by another's reconnection, and so that the site manager knows what is dead and what is not. Temporary site electrics run at reduced voltage (110 V centre-tapped-to-earth) for portable tools, with RCD protection and regular inspection and testing of the distribution.
Where it comes from
- Electricity at Work Regulations 1989 regs.4, 12, 13 and 14.
- HSE guidance HSR25 (Memorandum of guidance on the Electricity at Work Regulations), GS38 (Electrical test equipment for use on low voltage electrical systems) and HSG85 (Electricity at work: safe working practices).
- BS 7671 (Requirements for electrical installations) and BS 7909 for temporary electrical systems.
Questions people ask
Why prove the tester before and after?
Because a tester with a flat battery or a broken lead reads dead on a live circuit. Proving it on a known live source before, and again after, shows it was working when it told you the circuit was dead.
Can a site manager isolate a circuit for a trade?
The person who will work on the circuit must control the isolation: their lock, their key. A site manager can operate the switch but should not be the one holding the only key.
Electrical isolation, handled on a phone
SiteLynx puts the RAMS, permits, inspections, audits, plant checks and records behind every term in this glossary on the crew's phones, signed on site, with a PDF at the end. 14 days free, no card.
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