RCBO
Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.
What it is
An RCBO is a single device that does two jobs: it provides earth-leakage protection (like an RCD) and overcurrent protection (like a circuit breaker) for one circuit.
How it works
It contains both an earth-leakage sensing element and thermal/magnetic overcurrent elements. Either an earth-leakage fault or an overload/short can trip it.
Because one RCBO protects a single circuit, a fault on that circuit only trips that circuit — unlike a single RCD protecting many circuits, where one fault takes them all out.
Where it's used
Increasingly the default in modern boards — one RCBO per circuit gives both protections and limits the impact of a fault to one circuit. Telling whether an RCBO tripped on leakage or overcurrent is the key diagnostic step.
What it looks like when it fails
Same two families as its parents: leakage-style trips (moisture, a failing appliance, N–E fault) and overcurrent-style trips (instant = short, delayed = overload). Most units don't tell you which side operated — that's your first diagnostic job, because the fixes are completely different.
The device itself fails like an RCD does: won't reset with the circuit disconnected, or a dead test button. On models with a functional-earth pigtail, a pigtail that never got landed can compromise the device's operation — check it's connected per the maker's instructions.
How to test it
Split the question first: disconnect the circuit at the device — if it now holds, the circuit's the problem. Insulation-test the circuit (L–E and N–E) for the leakage family; clamp the running current against the rating for the overload family.
The device gets both parents' tests: an RCD tester for the leakage side (hold at half, trip at rated within the AS/NZS 3017 limits — verify the figures you test to), and it should carry its rated load without warming or tripping on the overcurrent side.
Gotchas
The handle looks identical after a leakage trip and an overload trip on most units — don't guess which it was; test the circuit and let the readings tell you. Some models have a trip-indicator flag worth learning to read.
Line/load orientation matters on some RCBOs, and the functional-earth pigtail (where fitted) isn't decoration. And never 'temporarily' swap an RCBO for a plain MCB to keep a circuit alive — that removes the shock protection, which is exactly what was tripping.
Safety first
Never bypass an RCBO's earth-leakage function. Identify whether it tripped on leakage or overcurrent — the fix differs.
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.
Related faults
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.
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.
A circuit breaker keeps tripping (domestic)
One circuit breaker keeps tripping — instantly on reset or after a load runs — and you need to tell an overload from a short or a faulty appliance.
Related definitions
RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.