Isolator / disconnector
A switch whose job is safe isolation — visibly and securely disconnecting a circuit for work.
What it is
An isolator is a switch designed to safely disconnect a circuit or piece of equipment from the supply so it can be worked on. Its purpose is isolation, not regular switching of load.
How it works
It provides a clear, reliable break in the supply — often lockable so it can be secured off during work (lockout). Some are load-breaking; pure isolators may be intended to be switched off-load.
Isolation is only safe when you also prove dead at the point of work — an isolator can be mis-labelled, or another supply path can exist (a borrowed neutral, a second feed).
Where it's used
Main switches at boards, local isolators next to motors and air-conditioners, and lockable isolators for safe maintenance. Central to lockout/tagout procedures.
Safety first
Isolating is not the same as proving dead. Always lock off where possible and prove dead at the point of work — labels can be wrong and second supplies can exist.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
No supply at a socket-outlet or point
A socket-outlet or point is dead — nothing plugged in works — while other points may be fine. A bread-and-butter 'trace it back' fault.
Distribution board circuits mislabelled / wrong breaker isolates wrong circuit
Switching off a labelled breaker doesn't isolate the expected circuit (or isolates the wrong one) — a schedule/labelling problem that's a real safety hazard for anyone relying on it.
E-stop circuit won't reset / machine won't start
The machine won't start because the emergency-stop circuit won't reset — the safety relay stays dropped out, holding everything off, even with all e-stops apparently released.
Related definitions
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Distribution & sub-mains
How power is split from the main board into final circuits and sub-boards, with protection at each level.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.