MCB (circuit breaker) keeps tripping
A circuit breaker trips repeatedly — instantly on reset, or after a load runs for a while — and you need to tell a short from an overload from a faulty breaker.
Safety first
A tripping breaker is protecting against a real fault current. Don't keep resetting into a short or upsize the breaker. Isolate and investigate.
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
Short circuit (instant trip)
Most likelyA short trips the breaker's magnetic (instantaneous) element the moment it's energised.
- 2
Overload (trips after running)
#2Sustained current above rating trips the thermal element after a delay.
- 3
Earth fault on the circuit
#3An earth fault can draw enough current to trip an MCB (and any RCD).
- 4
Wrong breaker rating/characteristic
#4An under-rated breaker or wrong curve trips on normal load/inrush.
- 5
Faulty breaker
Least likelyAn aged or faulty breaker can trip below its rating.
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.
Note when it trips: instantly on reset (short), after a delay/under load (overload).
A pattern separating short from overload.
Instant → short; delayed → overload. Proceed accordingly.
If random, suspect an intermittent fault or a faulty breaker.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
MCB keeps tripping
→ step 2 - 2decision
Does it trip instantly on reset (vs after a delay)?
Yes→ step 3No→ step 4 - 3decision
Is the circuit free of shorts/earth faults?
Yes→ step 4No→ step 5 - 4decision
Is load within rating with the correct breaker?
Yes→ step 6No→ step 7 - 5result
Short/earth fault — locate and repair before re-energising.
- 6result
Suspect a faulty breaker — replace and re-test.
- 7result
Overload or wrong breaker — reduce load / fit correct breaker.
Common mistakes apprentices make
- Repeatedly resetting into a short.
- Upsizing the breaker to stop tripping instead of fixing the fault.
- Not distinguishing instant (short) from delayed (overload) trips.
- Overlooking an earth fault as the trip source.
When to stop & escalate
A confirmed short or earth fault needs a qualified repair and retest before re-energising. Never fit a larger breaker to defeat protection; rating changes must suit the circuit's design.
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.
RCD / RCBO keeps tripping
An RCD or RCBO trips repeatedly — immediately on reset, randomly during the day, or only when certain equipment runs. The earth-leakage protection is doing its job; something is leaking.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Learn the theory
How the gear and circuits behind this fault actually work.