QualifiedHigh risk

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.

Start fault-finding

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.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

  1. 1

    Blown fuse or open pole on one phase

    Most likely

    A single blown HRC fuse, open fuse-link, or a contactor pole not making leaves two phases live and one dead.

  2. 2

    Loose or burnt connection on one phase

    #2

    A loose terminal, burnt lug, or corroded joint creates an open (or high-resistance) on one phase only.

  3. 3

    Upstream supply loss of one phase

    #3

    The incoming supply itself has lost a phase (utility/distribution issue), affecting everything fed from it.

  4. 4

    Failed contactor pole or isolator contact

    #4

    One pole of a switching device isn't conducting, dropping that phase downstream of it.

  5. 5

    Broken conductor in the cable or motor lead

    Least likely

    A 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.

Test 1 of 3
1

Measure phase-to-phase voltages at the load/starter (L1-L2, L2-L3, L1-L3).

Expected reading

Three balanced phase-to-phase voltages.

If it passes

All three phases present at this point — the loss (if any) is downstream toward the motor.

If it fails

A low/zero reading involving one phase shows which phase is missing here. Work back upstream from this point.

Often leads toMotor overload keeps trippingHeater bank drawing uneven current across phases
View all expected readings at once
1. Measure phase-to-phase voltages at the load/starter (L1-L2, L2-L3, L1-L3).
Three balanced phase-to-phase voltages.
2. Step upstream toward the supply, measuring at each point (load side and supply side of fuses/contactor/isolator) to find where the phase reappears.
The missing phase is present on the supply side of a device but absent on its load side.
3. At the suspect device, isolate and check the fuse/pole/connection for that phase (continuity, tightness, condition).
Continuous fuse/pole and a tight, sound connection.

Fault-finding flowchart

The same logic as a decision tree.

  1. 1
    start

    Suspected single-phasing

    → step 2
  2. 2
    decision

    Are all three phase-to-phase voltages present at the load/starter?

    Yes→ step 3No→ step 4
  3. 3
    result

    All phases present here — loss is downstream toward the motor (cable/lead/connection).

  4. 4
    decision

    Working upstream, does the missing phase reappear on the supply side of a device?

    Yes→ step 5No→ step 6
  5. 5
    result

    Fault is at that device/joint: open fuse, non-making pole, or burnt connection. Repair/replace.

  6. 6
    result

    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.

Related faults

Motors & overloads

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.

5 causes4 test steps
QualifiedHigh risk
Heating elements & SSRs

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.

5 causes4 test steps
QualifiedMedium risk
General testing

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.

5 causes4 test steps
ApprenticeMedium risk

Learn the theory

How the gear and circuits behind this fault actually work.

Device

Induction motor

The workhorse AC motor — a rotating magnetic field in the stator drags the rotor around with it.

Device

Overload relay

Protects a motor from sustained over-current by tripping the control circuit if it runs too hot for too long.

Device

Phase failure / monitoring relay

Watches a three-phase supply and disconnects the load if a phase is lost, unbalanced, or in the wrong sequence.

Principle

Single-phasing

What happens when a three-phase load loses one phase — and why it's so damaging to motors.

Principle

Three-phase power

Three AC supplies offset in time, giving smooth power and a rotating field for motors.