Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.
What it is
A monitoring relay protects three-phase equipment by watching the supply and dropping out (removing the control supply) if something's wrong — a lost phase, bad imbalance, or reversed phase sequence.
How it works
It continuously measures the three phases. If a phase goes missing, the voltages become badly unbalanced, or the rotation is reversed (after supply work), it operates its output contact to disconnect the control circuit and protect the load.
It's doing its job when it trips — single-phasing in particular can quickly overheat a motor, so the relay stops that happening.
Where it's used
Protecting motors and direction-sensitive equipment, especially where single-phasing or a reversed supply would cause damage. Don't bypass it to keep running — find the supply fault it's detecting.
Related faults
Phase-failure / monitoring relay has tripped the circuit
A phase-failure or phase-sequence monitoring relay has dropped out and is holding the control circuit off, stopping the equipment — even though the panel looks powered.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Motor runs in the wrong direction
The motor runs but the wrong way — pump runs backwards, fan blows the wrong way, conveyor reverses. Usually a phase-rotation issue after wiring or supply work.
Related definitions
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Single-phasing
What happens when a three-phase load loses one phase — and why it's so damaging to motors.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.