Contactor drops out on its own / won't stay latched
The contactor pulls in when you press start but drops out the moment you release the button, or randomly during running — the seal-in (latch) isn't holding it.
Safety first
A circuit that won't latch may be doing so because a protective device is operating. Don't defeat the seal-in to force it; find why it isn't holding.
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
Seal-in (hold-in) auxiliary contact not making
Most likelyThe latch relies on an aux contact across the start button. If it doesn't make, releasing the button drops the coil.
- 2
A protective device opening the circuit
#2An overload, e-stop, or interlock briefly opening will drop the latch — by design — so it won't stay in.
- 3
Marginal coil voltage
#3If the coil voltage sags after pull-in, the contactor can't hold and drops out.
- 4
Wiring fault in the latch branch
Least likelyA loose terminal or break in the seal-in wiring opens the hold path.
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.
Press and hold start: does it stay in only while held, then drop on release?
It should stay in after release if the seal-in is working.
If it holds after release, the latch is fine — chase the random drop-outs instead.
Drops on release points straight at the seal-in path.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Contactor won't stay latched
→ step 2 - 2decision
Does it drop the moment you release start?
Yes→ step 3No→ step 4 - 3decision
Does the seal-in aux contact make across the start button?
Yes→ step 5No→ step 6 - 4result
Holds after release — chase random drop-outs (protection/supply/wiring).
- 5result
Check latch wiring, coil voltage, and any protective device operating.
- 6result
Seal-in contact not making — repair/replace it.
Common mistakes apprentices make
- Strapping out the seal-in to 'make it stay in' instead of fixing it.
- Missing an overload that resets quickly and drops the latch each time.
- Overlooking marginal coil voltage that can't hold after pull-in.
- Not checking the aux contact actually bridges the start button.
When to stop & escalate
If a protective device is dropping the latch, treat that as the real fault and investigate it — don't bypass the latch. Persistent coil-voltage sag should lead to a control-supply review.
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
Auxiliary contact not making — seal-in or interlock fails
The contactor pulls in, but an auxiliary contact (used for seal-in, interlock, or status) doesn't change state, so the circuit won't latch, an interlock misbehaves, or status feedback is wrong.
Contactor chattering or buzzing instead of holding in
The contactor rapidly clicks/buzzes, pulls in and drops out repeatedly, or hums loudly without seating cleanly. Often comes with arcing noise and heat.
Motor overload keeps tripping
The thermal/electronic overload trips repeatedly, either on start or after the motor has run for a while. Resetting only buys you a short run before it trips again.
Learn the theory
How the gear and circuits behind this fault actually work.