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.
Safety first
Live board work — isolate and prove dead before touching terminals. A heavily loaded phase or neutral can be very hot; a loose neutral on a three-phase board is dangerous. This is licensed electrical work.
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.
Premium fault tree
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
Board badly unbalanced
→ step 2 - 2decision
Is one phase clearly higher / neutral high under load?
Yes→ step 3No→ step 4 - 3decision
Are single-phase loads concentrated on the heavy phase?
Yes→ step 5No→ step 6 - 4result
Balanced now — re-measure at peak load.
- 5result
Redistribute circuits across phases (licensed design work).
- 6decision
Is the neutral high from non-linear/harmonic loads?
Yes→ step 7No→ step 8 - 7result
Harmonic neutral current — design-level review.
- 8result
Investigate a fault/leakage raising that phase's current.
Common mistakes apprentices make
- Measuring at light load and missing the peak imbalance.
- Not measuring the neutral as well as the phases.
- Adding more load to the already-heavy phase.
- Ignoring harmonic neutral current on electronic-heavy boards.
When to stop & escalate
Re-balancing circuits across phases, and any harmonic/neutral-sizing concerns, is design-level licensed work. A loose or undersized neutral on a three-phase board is a safety priority to rectify.
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
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.
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.
Busbar or board connection running hot
A busbar joint, incomer, or board connection runs hot / discoloured — a high-energy fire-risk fault on a distribution board that must be addressed quickly.
Learn the theory
How the gear and circuits behind this fault actually work.
Three-phase power
Three AC supplies offset in time, giving smooth power and a rotating field for motors.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.
Power factor
How much of the current actually does useful work — and why a poor figure costs you capacity and money.
Real, apparent & reactive power (kW · kVA · kVAr)
Three different 'powers' on an AC system — what each one is, and why they don't simply add up.
Maximum demand
The realistic peak load a supply has to carry — the figure that sizes the mains, the main switch and the protection.