Safe working practices on electrical installations

A tester that fails silently would show 'dead' on a live circuit. Proving it on a known live source or proving unit before and after the test shows it was working when it read dead.

Source: HSE GS38 and HSG85 (Proving dead / safe isolation procedure)

Locking off with a personal lock, and keeping the key, secures the isolation so nobody else can re-energise the circuit while you are working on it. A caution notice should be fixed as well. Locking off does not prove the circuit is dead: you must still prove dead with a working voltage indicator. HSE HSG85, secure isolation.

Source: HSE HSG85 Electricity at work: Safe working practices (Secure isolation: locking off and caution notices)

A two-pole approved voltage indicator with GS38 probes and leads, proved on a proving unit or known live source before and after use, gives a reliable result. Non-contact pens are useful for a quick check but are not reliable enough to prove dead. A resistance range can be damaged by, and may not detect, a live supply. HSE GS38 and HSG85.

Source: HSE GS38 Electrical test equipment for use on low voltage electrical systems (Voltage indicators for proving dead); HSE HSG85 Electricity at work: Safe working practices (Proving dead)

First identify the circuit and its supplies, then isolate and secure the isolation with a lock, keep the key and post a caution notice. Only then prove dead with a voltage indicator that you prove before and after the test. Proving dead before locking off leaves a gap in which someone could switch the supply back on. HSE HSG85, dead working procedure.

Source: HSE HSG85 Electricity at work: Safe working practices (Working dead: paragraph 46 and Figure 4 (dead working procedures))

A multi-lock hasp lets each person fit their own lock and keep their own key. The isolator cannot be switched on until the last lock is removed, so nobody can be put at risk by someone else finishing early. A lock-out box or key safe can do the same job. HSE HSG85, secure isolation.

Source: HSE HSG85 Electricity at work: Safe working practices (Secure isolation: multiple locking hasp)