RCBO trips the instant you try to reset it
The RCBO won't stay set — push it up and it trips straight back down, with nothing switched on. Different behaviour from nuisance tripping under load.
Safety first
An instant trip with no load usually means a genuine standing fault on the circuit, not a faulty device. Don't keep slamming it up — isolate and test the circuit dead before re-energising.
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
Standing active-to-earth fault on the circuit
Most likelyA dead short or solid leakage from active to earth is present even with all loads off, so the RCBO sees the fault the instant it closes and trips immediately.
- 2
Neutral-to-earth fault downstream
#2A neutral touching earth past the RCBO unbalances it as soon as it's set — a classic 'trips on reset with nothing on' fault.
- 3
Borrowed or crossed neutral with another circuit
#3If this circuit's neutral is shared with another circuit's neutral, return current flows through the wrong path and the RCBO trips immediately.
- 4
Failed RCBO mechanism
Least likelyLess common, but a device that trips with the circuit proven clear and disconnected may have failed internally.
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 ModeSafely isolate, disconnect the circuit conductors from the RCBO's load side, and try to reset it with nothing connected.
With the load side disconnected, the RCBO should set and hold.
Device holds with no load connected — the fault is on the circuit, not the RCBO.
It still trips with the load side disconnected — the RCBO itself is suspect.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
RCBO trips instantly on reset
→ step 2 - 2decision
Does it hold with the load side disconnected?
Yes→ step 3No→ step 4 - 3decision
Is insulation resistance to earth high on the dead circuit?
Yes→ step 5No→ step 6 - 4result
Fault is the RCBO itself — replace and retest.
- 5decision
Is the neutral independent (not borrowed/crossed)?
Yes→ step 7No→ step 8 - 6result
Standing active/neutral-to-earth fault — split the circuit to locate it.
- 7result
No circuit fault found — recheck device and terminations.
- 8result
Borrowed/crossed neutral — correct the neutral arrangement.
Common mistakes apprentices make
- Condemning the RCBO before disconnecting the load side to prove where the fault is.
- Forgetting a neutral-to-earth fault trips an RCBO just as readily as an active fault.
- Missing a shared/borrowed neutral between two circuits.
- Repeatedly resetting onto a standing fault instead of testing dead.
When to stop & escalate
If the standing fault is in a buried or inaccessible cable, escalate for fault location rather than guessing. A borrowed neutral can indicate non-compliant past work across multiple circuits — flag it for proper rectification.
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 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.
Lost or high-resistance neutral
Strange symptoms across a circuit or installation — voltages going high and low on different loads, flickering, equipment damage — pointing to a lost or high-resistance neutral.
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.