Large motor start trips the distribution board
Starting a large three-phase motor (lift, pump, compressor, aircon) trips the board feeding it or dips the supply — an inrush/coordination issue rather than a running fault.
Safety first
Repeated start attempts stress the supply and equipment. Isolate before working. Licensed work; coordinate with any plant owner before testing starts.
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.
- 1
Protection not rated/charactered for the inrush
Most likelyThe breaker curve trips on legitimate motor starting current.
- 2
Reduced-voltage starter not working
#2A star-delta/soft starter that's failed lets full inrush through (see those faults).
- 3
Weak/loaded supply sagging on start
#3The supply dips on inrush, tripping protection or other equipment.
- 4
Multiple large loads starting together
Least likelyCoincident starts exceed the board's capability.
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.
Confirm it trips only on start (runs fine once going) and which device trips.
A start-only trip on a specific device.
Start-only → look at the protection characteristic and starter.
If it also trips running, treat as an overload/fault, not inrush.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Motor start trips board
→ step 2 - 2decision
Does it trip only on start (runs fine once going)?
Yes→ step 3No→ step 4 - 3decision
Is the starter limiting inrush and the protection suited to starting?
Yes→ step 5No→ step 6 - 4result
Trips running too — treat as overload/fault, not inrush.
- 5decision
Do starting current/time and supply stay within limits?
Yes→ step 7No→ step 8 - 6result
Failed starter or wrong protection — rectify/select correctly.
- 7result
Within limits — investigate intermittent behaviour.
- 8result
Supply sag / coincident starts — stagger starts / review supply.
Common mistakes apprentices make
- Upsizing protection to stop start trips instead of fixing the start.
- Not noticing a failed reduced-voltage starter.
- Ignoring coincident large starts on one board.
- Repeatedly attempting starts and overheating the motor.
When to stop & escalate
Starting method, protection coordination, and supply capacity are design-level licensed work. Coordinate staggered starting and any supply upgrade properly rather than defeating protection.
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
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
Star-delta starter not transitioning to delta
A star-delta (wye-delta) starter starts the motor in star but never switches to delta — the motor runs weak/slow, or trips, because it stays in the starting connection.
Equipment tripping the supply on startup surge
Switching on a piece of equipment trips an upstream breaker or causes a momentary dip — the inrush/startup surge is exceeding what the protection or supply can ride through.
Learn the theory
How the gear and circuits behind this fault actually work.