AdvancedHigh risk

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.

Start fault-finding

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.

Full detail — causes, the why, and common mistakes.

Likely causes

Ranked from most to least likely.

1
Earth-leakage fault (RCD side)
Most likely
Leakage to earth on the circuit trips the RCD function — same causes as any RCD trip.
2
Overload (MCB side)
#2
Too much load current for the rating trips the overcurrent function, usually after running a while.
3
Short circuit (MCB side)
#3
A short trips the overcurrent function instantly and hard.
4
Neutral-earth fault
Least likely
A neutral-earth fault trips the RCD side, sometimes confusingly.

Reports are saved on this device to reflect what you actually find.

Premium fault tree

The full ranked causes, test sequence and flowchart for this fault are part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Testing sequence

Work through one test at a time. Expected reading and what each result means.

Test 1 of 3

Note 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.

Expected reading

A pattern (and indicator) suggesting overcurrent vs leakage.

Your actual reading

If it passes

Overcurrent pattern → treat as overload/short. Leakage pattern → treat as RCD trip.

If it fails

If unclear, separate the two by testing load current and insulation.

Often leads toRCD / RCBO keeps tripping Motor overload keeps tripping

View all expected readings at once

1. Note 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.

2. For a suspected overcurrent trip, measure the circuit load current against the RCBO rating and check for a short.

Load within rating and no short.

3. For a suspected leakage trip, isolate and insulation-test the circuit to earth (and for neutral-earth faults).

Good insulation to earth and no neutral-earth fault.

Full test sequence

The step-by-step test flow with expected readings for this fault is part of Sparkie Sidekick Pro.

Coming soon — no payment required during preview.

Fault-finding flowchart

The same logic as a decision tree.

1
start
RCBO tripping
→ step 2
2
decision
Does the pattern/indicator point to overcurrent?
Yes→ step 3No→ step 4
3
decision
Is load within rating with no short?
Yes→ step 4No→ step 5
4
decision
Is insulation to earth good (no neutral-earth fault)?
Yes→ step 6No→ step 7
5
result
Overload/short — address the load/wiring (MCB side).
6
result
Re-examine for cumulative leakage / appliances.
7
result
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.

Related faults

RCD / RCBO 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.

5 causes4 test steps

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Motors & overloads

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.

5 causes4 test steps

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RCD / RCBO faults

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.

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Learn the theory

How the gear and circuits behind this fault actually work.

Device

RCBO

Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.