Three-phase outlet / appliance not working (one phase missing)
A three-phase outlet or appliance (welder, large machine, commercial kitchen gear) isn't working right — a missing phase at the socket leaves it under-powered or not starting.
Safety first
Three-phase sockets carry significant power. Isolate and prove dead. A missing phase can leave parts live and damage 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
One phase lost (fuse/breaker/connection)
Most likelyA blown fuse, tripped single-pole, or loose connection drops one phase to the socket.
- 2
Socket / plug wiring fault
#2A fault or wrong connection at the three-phase socket or plug.
- 3
Circuit protective device partially operated
#3One pole of the protection has operated, dropping a phase.
- 4
Upstream single-phasing
Least likelyA phase is missing further upstream (board/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.
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 all three phases (phase-to-phase) at the outlet.
Three balanced phase-to-phase voltages.
All present — the appliance/plug or its internal wiring is the fault.
A phase missing — work back to find where it's lost.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
3-phase outlet not working
→ step 2 - 2decision
Are all three phases present at the outlet?
Yes→ step 3No→ step 4 - 3result
All present — the appliance/plug/wiring is the fault.
- 4result
A phase missing — trace it back (fuse/pole/connection/board).
Common mistakes apprentices make
- Measuring phase-to-neutral only and missing the down phase-pair.
- Assuming the appliance when a phase is missing at the socket.
- Overlooking a single-pole that's operated.
- Not tracing the phase back methodically.
When to stop & escalate
Three-phase socket/circuit work is licensed. Upstream single-phasing may extend to the supply (authority) — see single-phasing fault-finding.
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
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.
Power point (GPO) completely dead
Nothing plugged into a power point works, while other outlets are fine. A classic trace-it-back fault on a single GPO or the run feeding it.
Main switch / incomer not making on all phases
A board's main switch or incomer isn't passing all phases — downstream gets partial supply (single-phasing) or nothing — pointing at the switch contacts or its terminations.
Learn the theory
How the gear and circuits behind this fault actually work.