Interlocks
Logic that prevents an unsafe or impossible combination of states — like two contactors closing together.
What it is
An interlock is control logic (electrical, mechanical, or both) that stops a dangerous or impossible combination of states from happening — by making one action positively prevent another.
How it works
Electrically, an interlock often uses normally-closed auxiliary contacts: energising contactor A opens a contact in contactor B's coil circuit, so B can't energise while A is in. Mechanically, a physical bar can block both devices operating together.
The forward/reverse and star-delta interlocks are classic examples — they prevent a phase-to-phase short. Guard and safety interlocks stop a machine running while a guard is open.
Why it matters
Interlocks are safety and protection features. A stuck or mis-wired interlock can hold equipment off (annoying) — but defeating one to get running can cause a dangerous fault or injury. Always understand what an interlock protects before touching it.
When it misbehaves — and how to prove it
The usual call: machine won't start, everything 'looks' fine. Somewhere in the permissive chain an interlock isn't satisfied — a guard door a few millimetres off its switch, a worn aux contact, a key not fully home, or an upstream condition genuinely not met. Intermittent stops mid-cycle point at a marginal interlock switch being shaken open by vibration.
Prove it by walking the chain with a meter, in the order the control diagram shows — the circuit is live while you do this, so licensed work and due care: across each satisfied interlock contact you should read about zero volts — the one dropping real voltage is the open link. Never bridge contacts to find the break; the meter finds it without defeating anything. If the open interlock turns out to be doing its job, the fault is the condition it's watching — not the interlock.
Safety first
Never defeat an interlock to get running — it prevents dangerous faults or injury. Confirm what it protects and fix the real cause.
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
Forward/reverse interlock locking out both directions
Neither direction will run — the interlock that stops both contactors closing together appears to be holding everything off, so no movement at all.
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
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
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.
Star-delta starting
Starts a motor in star (lower current) then switches to delta (full power) once it's up to speed.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.