RCBO tripping — telling overload from earth leakage
An RCBO (combined RCD + MCB) keeps tripping and you need to know whether it's tripping on overcurrent/overload or on earth leakage — the fix is very different for each.
Safety first
An RCBO can trip for two very different reasons — earth leakage (shock risk) or overcurrent (overload/short). Don't keep resetting; identify which, and treat both as real.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Earth-leakage fault (RCD side)
Most likelyLeakage to earth on the circuit trips the RCD function — same causes as any RCD trip.
- 2
Overload (MCB side)
#2Too much load current for the rating trips the overcurrent function, usually after running a while.
- 3
Short circuit (MCB side)
#3A short trips the overcurrent function instantly and hard.
- 4
Neutral-earth fault
Least likelyA neutral-earth fault trips the RCD side, sometimes confusingly.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeNote the trip behaviour: instant on energising (short), after running a while (overload), or seemingly random/at switch-on (leakage). Use any trip indicator the RCBO provides.
A pattern (and indicator) suggesting overcurrent vs leakage.
Overcurrent pattern → treat as overload/short. Leakage pattern → treat as RCD trip.
If unclear, separate the two by testing load current and insulation.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
RCBO tripping
→ step 2 - 2decision
Does the pattern/indicator point to overcurrent?
Yes→ step 3No→ step 4 - 3decision
Is load within rating with no short?
Yes→ step 4No→ step 5 - 4decision
Is insulation to earth good (no neutral-earth fault)?
Yes→ step 6No→ step 7 - 5result
Overload/short — address the load/wiring (MCB side).
- 6result
Re-examine for cumulative leakage / appliances.
- 7result
Earth/neutral-earth leakage — repair it (RCD side).
Common mistakes apprentices make
- Treating every RCBO trip as 'earth leakage' (or vice versa).
- Ignoring the RCBO's own trip indicator if it has one.
- Resetting repeatedly into a short.
- Not separating load-current testing from insulation testing.
When to stop & escalate
Confirmed overload may need load redistribution or a rating review; confirmed leakage needs a wiring/appliance repair. Both are real — never bypass an RCBO.
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.
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Related faults
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.
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.
RCD won't reset at all
The RCD won't stay reset — the toggle won't latch up, or it trips instantly every time, so the circuit can't be restored.
Learn the theory
How the gear and circuits behind this fault actually work.