Three-phase equipment running hot from supply imbalance
Three-phase equipment (motors, heaters, large gear) runs hotter than expected or nuisance-trips, traced to a voltage/current imbalance between phases rather than the equipment itself.
Safety first
Hot equipment and live measurement — isolate where you can, use proven equipment, treat as licensed. Imbalance can shorten equipment life quickly.
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
Voltage imbalance from unbalanced loading
Most likelyHeavy single-phase loading drags one phase voltage down, unbalancing three-phase equipment current.
- 2
Loose/high-resistance connection on one phase
#2A poor connection raises that phase's volt-drop, creating imbalance under load.
- 3
Supply-side imbalance
#3The incoming supply itself is unbalanced (authority).
- 4
Developing single-phasing
Least likelyA partially-failing phase (fuse/connection) is on its way to single-phasing.
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 voltages and the equipment's per-phase currents under load.
Balanced voltages and currents within a small percentage.
If balanced, the heat is from the equipment/loading, not imbalance.
Clear imbalance — determine whether it's installation or supply.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
3-phase running hot (imbalance)
→ step 2 - 2decision
Are phase voltages and currents balanced under load?
Yes→ step 3No→ step 4 - 3result
Balanced — heat is from equipment/loading, not imbalance.
- 4decision
Is the imbalance present at the supply (vs your installation)?
Yes→ step 5No→ step 6 - 5result
Supply imbalance — escalate to the authority.
- 6result
Installation — check loading/connections; watch for single-phasing.
Common mistakes apprentices make
- Blaming the equipment when the supply/connections are unbalanced.
- Measuring voltage only, not per-phase current.
- Overlooking a loose connection on one phase.
- Ignoring a developing single-phasing condition.
When to stop & escalate
Supply-side imbalance is the authority's; installation loading/connection rectification is licensed work. Persistent imbalance shortens equipment life — address it.
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 distribution board badly unbalanced
One phase of a three-phase board runs much hotter / higher current than the others — nuisance tripping on that phase, a hot neutral, or a warm phase conductor at the board.
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.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.
Learn the theory
How the gear and circuits behind this fault actually work.