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.
What it looks like when it fails
Weathered outdoor isolators — on air-conditioners, pumps, pool gear — are a top single-phasing culprit: one pole burns or corrodes open while the other two keep conducting, and the motor downstream hums and trips. Water gets in, terminals corrode, and lids crack in the sun.
Mechanisms fail too: the handle moves but the contacts don't follow it — in either direction. That's why the handle position proves nothing on its own.
How to test it
Dead: continuity through each pole with the isolator closed (effectively zero ohms each), then confirm every pole is genuinely open with it off. Compare poles — one reading different to its mates is the fault.
Under load: volt-drop across each pole — millivolts on a healthy pole; a pole dropping real volts (or a warm/discoloured terminal) is burning inside. On a suspected single-phasing job, measure phase-to-phase on both sides of the isolator and compare.
Gotchas
The handle is not the proof — mechanisms fail with the handle happily saying OFF, labels lie, and second supplies exist (a borrowed neutral, control power from another board, solar backfeed). Prove dead at the point of work, every time.
Know whether it's a load-break isolator before opening it under load — pure off-load isolators aren't built to break current and can arc. And on the diagnosis side: before condemning a motor or its starter, check the cheap grey box on the wall next to it.
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.