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.
Safety first
Single-phasing can leave parts of a circuit live and overheat motors quickly. A 'dead' phase at one point may be live at another. Prove dead carefully and watch for hot equipment.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Blown fuse or open pole on one phase
Most likelyA single blown HRC fuse, open fuse-link, or a contactor pole not making leaves two phases live and one dead.
- 2
Loose or burnt connection on one phase
#2A loose terminal, burnt lug, or corroded joint creates an open (or high-resistance) on one phase only.
- 3
Upstream supply loss of one phase
#3The incoming supply itself has lost a phase (utility/distribution issue), affecting everything fed from it.
- 4
Failed contactor pole or isolator contact
#4One pole of a switching device isn't conducting, dropping that phase downstream of it.
- 5
Broken conductor in the cable or motor lead
Least likelyA broken core in a multicore cable or motor lead opens one phase to the load.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeMeasure phase-to-phase voltages at the load/starter (L1-L2, L2-L3, L1-L3).
Three balanced phase-to-phase voltages.
All three phases present at this point — the loss (if any) is downstream toward the motor.
A low/zero reading involving one phase shows which phase is missing here. Work back upstream from this point.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Suspected single-phasing
→ step 2 - 2decision
Are all three phase-to-phase voltages present at the load/starter?
Yes→ step 3No→ step 4 - 3result
All phases present here — loss is downstream toward the motor (cable/lead/connection).
- 4decision
Working upstream, does the missing phase reappear on the supply side of a device?
Yes→ step 5No→ step 6 - 5result
Fault is at that device/joint: open fuse, non-making pole, or burnt connection. Repair/replace.
- 6result
Phase already missing at incoming supply — escalate upstream (utility/distribution).
Common mistakes apprentices make
- Measuring phase-to-neutral only and missing which phase-to-phase pair is down.
- Replacing a blown fuse without finding why that phase opened (often an overload or fault current).
- Assuming the motor is faulty when it's actually being fed only two phases.
- Not stepping methodically from the load back toward the supply to localise where the phase disappears.
When to stop & escalate
If a phase is missing at the incoming supply, raise it with the supply authority/responsible person — it's upstream of your installation. A fuse that blows again after replacement indicates a downstream fault current to investigate before re-energising.
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.
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Related faults
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.
Heater bank drawing uneven current across phases
A multi-element heater bank pulls noticeably different current on each phase. Heating is uneven, output is low, or a phase reads much lower than the others.
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.
Induction motor
The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.
Overload relay
Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.
Phase failure / monitoring relay
Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.
Single-phasing
What happens when a three-phase load loses one phase — and why it's so damaging to motors.
Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.