Why Your RCBO Keeps Tripping
An RCBO combines overcurrent and earth-leakage protection in one device, so the first job is working out which of the two is tripping it — the fault-finding follows from there.
An RCBO (residual current breaker with overcurrent protection) protects a single circuit against both overload/short-circuit and earth leakage. Because it does two jobs, a tripping RCBO can mean an overcurrent fault or an earth-leakage fault, and the two need very different approaches.
This guide explains how to tell which mechanism is operating, how to trace an earth-leakage fault by dividing the circuit, how to rule out cumulative standing leakage, and when the RCBO itself is the culprit. It assumes safe isolation before any investigation.
Key takeaways
- An RCBO trips on either overcurrent (overload/short) or earth leakage — identify which first.
- Frequent tripping under load points to overload; tripping regardless of load points to leakage.
- Trace earth-leakage faults by splitting the circuit and testing insulation resistance in sections.
- Cumulative leakage from several appliances can trip a healthy RCBO — unplug and reintroduce.
- Always safely isolate before opening anything, and re-test before returning to service.
Overcurrent or earth leakage?
Start by working out which mechanism is tripping. If the RCBO holds until you switch on a specific heavy load and then trips, and the trip is often immediate under load, suspect overcurrent — an overload or a short-circuit. If it trips at random, with light or no load, or the instant you energise the circuit, suspect earth leakage.
Some RCBOs have a flag or indicator distinguishing the two; where fitted, use it. Otherwise the behaviour under load is your best clue.
Isolate first, always
Before you probe, test or open anything, prove the circuit dead using safe isolation and an approved voltage indicator. A tripping device is not a substitute for isolation.
Chasing an earth-leakage fault
For a leakage trip, the reliable method is to split the circuit and test insulation resistance section by section. Disconnect the circuit, divide it at a convenient midpoint (a junction box or accessory), and test each half between line-and-earth and neutral-and-earth. The half that reads low contains the fault; halve it again until you have it.
Common culprits are moisture in an outdoor accessory, a nail or screw through a cable, a failed heating element, or a neutral-to-earth fault that only an insulation test reveals. Damaged cable and water ingress top the list.
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Cumulative leakage and appliances
A perfectly healthy circuit can trip an RCBO if the total standing earth leakage from connected equipment adds up beyond the threshold. IT equipment, some LED drivers and appliances with filters each leak a little; together they can nudge a sensitive device over the edge.
To test for this, unplug everything on the circuit and reintroduce loads one at a time, watching when the device trips or the measured leakage climbs. If no single item is faulty but the total is too high, the circuit may need splitting across two RCBOs.
Prove it before you hand back
Once you've fixed the fault, re-test insulation resistance and RCD operation and record the readings — TradePlanr lets you issue a Minor Works Certificate for the repair so the customer has proof the circuit is safe.
When it's the RCBO itself
Devices do fail. A nuisance-tripping RCBO with no measurable leakage and no overload, that trips on a healthy circuit and continues after everything is disconnected, may itself be faulty. Swap it for a known-good device of the same rating and type to confirm before condemning the circuit.
Equally, an RCBO of the wrong type (an AC type on a circuit with DC leakage from electronics) can nuisance-trip. Check the type is right for the load as well as the rating.
From guidance to action
Frequently asked questions
How do I know if it's overload or earth leakage?
If the RCBO trips when a specific heavy load switches on, suspect overcurrent. If it trips at random, with little or no load, or the instant the circuit is energised, suspect earth leakage. Some RCBOs indicate which mechanism operated.
Can appliances trip a healthy RCBO?
Yes. The standing earth leakage from several appliances — IT gear, LED drivers, filtered equipment — can add up beyond the trip threshold. Unplug everything and reintroduce loads one at a time to find the cumulative cause.
Could the RCBO itself be faulty?
Occasionally. If a circuit has no measurable leakage and no overload but the device still trips, swap it for a known-good RCBO of the same rating and type to confirm. Also check it's the correct type for the load.
Related guides
RCD Keeps Tripping: Causes and Fixes
A structured method for tracing why an RCD trips — from nuisance tripping to real faults.
No Power to Sockets: Fault-Finding Guide
A methodical route from the board outward — from a single dead outlet to a broken ring conductor.
Safe Isolation Procedure
The step-by-step safe isolation procedure, proving dead with a GS38 tester, and locking off.
