No-volt release
After a power cut, equipment stays off until deliberately restarted — preventing dangerous auto-restart.
What it does
No-volt release ensures that if the supply is lost and then returns, machinery doesn't restart on its own — someone has to press start again. It prevents a machine springing back to life unexpectedly after a power cut.
How it works
It's a natural feature of the start/stop seal-in circuit. When the supply drops, the contactor coil de-energises and the seal-in contact opens. When power returns, the coil has no path to energise (the start button isn't pressed and the seal-in is open), so the contactor stays off until start is pressed.
Why it matters
It's a key safety behaviour — imagine a saw or conveyor restarting by itself when power returns while someone's clearing it. This is one reason you must never strap out the seal-in: doing so can defeat no-volt release as well.
When it misbehaves — and how to prove it
Two opposite complaints, two very different answers. 'It doesn't come back on after a power cut' is no-volt release working correctly — explain it, don't fix it. 'It restarted by itself after the outage' is the dangerous one: somewhere the circuit has been given memory it shouldn't have — a maintained selector or 2-wire device where 3-wire control was intended, or a strapped-out seal-in. Find it and restore proper 3-wire control.
The third complaint — drops out on every brief dip or flicker — is a sensitivity problem: a control transformer sagging under load, a marginal coil, or loose control connections. Prove the behaviour deliberately: kill the supply for a second and watch. Stays off until start is pressed = correct. Anything else, trace which path re-energised the coil.
Safety first
Never defeat the seal-in/no-volt release — auto-restart of machinery can injure someone. Restore the circuit properly instead.
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
Contactor drops out on its own / won't stay latched
The contactor pulls in when you press start but drops out the moment you release the button, or randomly during running — the seal-in (latch) isn't holding it.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Related definitions
Start/stop circuit (seal-in)
A momentary start button that latches a contactor on, held by its own auxiliary contact until stop is pressed.
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.