Three-phase supply voltage high, low, or unbalanced
Measured supply voltages are out of the expected range or unbalanced between phases — causing equipment trips, poor performance, or protection operating across the board.
Safety first
Measuring incoming supply is live work — isolate where you can, use proven equipment, and treat as licensed. Voltage issues can damage equipment; act before re-energising sensitive loads.
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
Supply-authority voltage / network issue
Most likelyThe incoming supply itself is high/low/unbalanced (network or transformer tap).
- 2
Unbalanced loading raising imbalance
#2Heavy single-phase loading drags one phase voltage down relative to others.
- 3
Loose/high-resistance supply connection
#3A poor connection on one phase causes volt-drop/imbalance under load.
- 4
Neutral problem skewing phase voltages
Least likelyA neutral fault makes phase-to-neutral voltages swing.
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 and phase-to-neutral voltages at the board, at light and heavy load.
Balanced voltages within the expected range at both load levels.
If within range, the original symptom may be elsewhere.
Out of range / unbalanced — note whether it's load-dependent.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Supply voltage out of range
→ step 2 - 2decision
Is it out of range / unbalanced even unloaded?
Yes→ step 3No→ step 4 - 3result
Supply-side issue — escalate to the authority with readings.
- 4decision
Are connections/neutral sound and loading balanced?
Yes→ step 5No→ step 6 - 5result
If sound and still out of range unloaded, it's the supply.
- 6result
Loose connection, neutral fault, or imbalance — rectify.
Common mistakes apprentices make
- Measuring only unloaded and missing load-dependent volt-drop.
- Assuming the supply when it's installation loading/connections.
- Overlooking a neutral problem skewing readings.
- Re-energising sensitive equipment before resolving it.
When to stop & escalate
A supply that's genuinely out of range unloaded is the supply authority's responsibility — raise it with measurements. Installation-side imbalance/connections are licensed work.
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
Lost or high-resistance neutral
Strange symptoms across a circuit or installation — voltages going high and low on different loads, flickering, equipment damage — pointing to a lost or high-resistance neutral.
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.
Learn the theory
How the gear and circuits behind this fault actually work.