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.
Safety first
The monitoring relay is protecting the equipment from a supply problem. Don't bypass it to get running — find the supply fault it's detecting. Power can still be live on some phases.
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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Genuine phase loss or imbalance
Most likelyThe relay has correctly detected a missing or unbalanced phase and dropped out to protect the load.
- 2
Wrong phase sequence (after supply work)
#2A phase-sequence relay trips if phases were swapped during supply work, protecting direction-sensitive equipment.
- 3
Relay set too sensitively
#3Imbalance/voltage thresholds set too tight cause nuisance trips on normal supply variation.
- 4
Faulty monitoring relay
Least likelyThe relay itself has failed and is dropping out with a healthy supply.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure the three phase-to-phase voltages at the relay's sensing terminals.
Three balanced phase-to-phase voltages present.
Supply looks healthy — check sequence, settings, then the relay.
A missing/unbalanced phase confirms why it tripped — chase the supply.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Phase-monitor relay tripped
→ step 2 - 2decision
Are all three phase-to-phase voltages balanced and present?
Yes→ step 3No→ step 4 - 3decision
Is the phase sequence correct?
Yes→ step 5No→ step 6 - 4result
Missing/unbalanced phase — chase the supply fault (do not bypass).
- 5decision
Are settings sensible and does it reset on good supply?
Yes→ step 7No→ step 8 - 6result
Reversed sequence — correct the connections.
- 7result
Healthy — supply event may have been transient; monitor.
- 8result
Over-tight settings or failed relay — adjust or replace.
Common mistakes apprentices make
- Bypassing the monitoring relay to get running and exposing the load to single-phasing.
- Not checking phase sequence after supply or transformer work.
- Blaming the relay before confirming the supply is actually healthy.
- Setting thresholds so tight that normal variation nuisance-trips it.
When to stop & escalate
A genuine phase loss or sequence problem at the supply is upstream — raise it appropriately rather than working around the relay. Never disable supply-protection to keep production running.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
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 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.
Learn the theory
How the gear and circuits behind this fault actually work.