Phase failure / monitoring relay

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.

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.

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.

Nine times out of ten a 'failed' monitoring relay is reporting a real supply problem — a lost phase, sagging volts, imbalance, or a rotation swap after supply-authority work or a generator changeover. The machine won't start and the relay's LED tells you why.

Actual relay failures: the output contact never closes with a demonstrably healthy supply, chattering when the supply sits right on a threshold, and settings drift on old units.

Measure all three phases at the relay's own terminals — all present, balanced, and (with a rotation meter) in the right sequence. If the supply checks healthy and the output still won't close, compare its settings (imbalance %, voltage thresholds, delay) against what you measured.

Healthy supply, sensible settings, output still open — substitute the relay. They're cheap, and substitution settles the argument faster than anything else.

After any supply work, a sequence-sensing relay legitimately locks everything out because the rotation really did swap. That's the relay doing its job — fix the rotation, don't bridge the relay.

A supply that hovers near a threshold gives maddening intermittent lockouts — measure at the worst time of day, not the best. And never jumper it out to get running: it's the only thing standing between a single-phasing event and a cooked motor.