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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
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The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.
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Testing sequence
Work through one test at a time. Expected reading and what each result means.
Full test sequence
The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.
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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.
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.