Electrical isolation is the process of disconnecting a circuit or piece of equipment from its power source and securing it so that it cannot be accidentally re-energised. This is a fundamental safety procedure that must be followed before any electrical work, testing, or maintenance takes place. The safe system of work for electrical isolation follows a strict sequence: identify, isolate, secure, prove dead, and earth where necessary. Shortcuts in this process have caused fatal electrocutions.
Key Hazards
Electrocution from working on circuits assumed to be dead but still live
Re-energisation by another person while work is in progress on the circuit
Stored electrical energy in capacitors discharging after the supply is isolated
Incorrect identification of circuits leading to the wrong circuit being isolated
Control Measures
Identify the exact circuit or equipment to be isolated using accurate circuit drawings and labelling.
Isolate the circuit at the correct isolation point and lock off with a personal padlock.
Attach a danger tag with your name, date, and reason for isolation to the locked-off device.
Prove the circuit is dead using a voltage indicator that has been tested on a known live source.
Test your voltage indicator on a proving unit before and after proving the circuit dead.
Earth the conductors where necessary, particularly on high-voltage systems after proving dead.
Inform all persons who may be affected by the isolation before carrying out the procedure.
Never rely on another person's isolation — always apply your own lock and prove dead yourself.
Follow the re-energisation procedure including removing earths, checking the work area, and removing locks.
Emergency / Rescue
If someone receives an electric shock, do not touch them while in contact with the source. Isolate the supply or use a non-conductive object to separate them. Call 999 and begin CPR if they are not breathing.
Remember
The safe isolation procedure is: identify, isolate, lock off, prove dead, then earth if required.
Always prove dead using a voltage indicator tested on a known live source before and after.
Apply your own personal lock and danger tag — never rely solely on someone else's isolation.
Stored energy in capacitors can deliver a fatal shock even after the supply has been disconnected.
Incorrect circuit identification is a common cause of working on live circuits — verify carefully.
Re-energisation by another person is prevented by using your own personal lock on the isolator.
Applicable Legislation: Electricity at Work Regulations 1989 · HSG85 (Electricity at Work — Safe Working Practices) · Health and Safety at Work etc. Act 1974 · BS 7671