Helpful video reference. We use the video "Fault Find: Tripping RCDs / RCBOs" as the video reference here. It shows a working electrician's systematic approach to diagnosing earth-leakage and overcurrent trips on a real UK board, including the isolation steps and what to look for inside fittings.
1. Understand what an RCBO does
A standard circuit breaker (MCB) trips when a circuit carries too much current, protecting the cable from overheating. An RCD trips when current is leaking to earth, protecting people from electric shock. An RCBO (Residual Current Breaker with Overcurrent protection) does both in a single device, each protecting one circuit.
Because it combines two protection mechanisms, an RCBO can trip for two entirely different reasons. Diagnosing which one is happening is the first step.
2. Note exactly how and when it trips
Before you touch anything, observe the pattern:
- Trips the instant you switch it on, even with nothing plugged in: almost always a wiring fault or a faulty RCBO itself.
- Holds with nothing connected, trips when you plug a specific appliance in: that appliance is leaking to earth. The RCBO's earth-leakage function is operating correctly.
- Trips after a while or when the circuit is heavily loaded: likely an overcurrent trip. Something is drawing more current than the circuit is rated for, or there is a high-resistance fault heating up under load.
- Trips at the same time every day (e.g. when the dishwasher or immersion heater starts): that specific appliance is the likely cause, combined with any background leakage already on the circuit.
3. Identify the circuit and remove all loads
The RCBO label should identify the circuit it protects: "kitchen ring", "upstairs sockets", "garage", and so on. Go to that part of the house and either unplug everything from the sockets or switch all the lights off, depending on the circuit type.
For a lighting circuit, turn every light switch to off. For a socket circuit, physically unplug every appliance including anything that is usually left plugged in (phone chargers, televisions on standby, fridges if they are on this circuit).
4. Reset the RCBO
With everything disconnected, reset the RCBO. Some RCBOs need to be pushed fully down to off before they will latch back on, others have a test button that must be released first. Hold it in the on position briefly to make sure it latches.
If it trips again immediately with nothing connected, the fault is in the fixed wiring, a ceiling rose, a back-box connection, or the RCBO itself. Skip to step 7.
If it holds, the fault is in one of the loads you just removed. Continue to step 5.
5. Reintroduce loads one at a time
Plug appliances back in or switch lights on, one at a time. Wait about 30 seconds between each one. When the RCBO trips, the last thing you switched on or plugged in is the likely culprit. Unplug it and reset the RCBO. If it holds with everything else on, you have found the problem.
Make a note of which appliance caused the trip. It should not be used again until it has been PAT-tested or repaired by a qualified person.
6. Test the suspect appliance on another circuit
To confirm the fault is in the appliance and not in the socket or that section of cable, try the appliance on a socket protected by a different RCBO. If it trips that RCBO as well, the appliance has a genuine earth-leakage fault. Common causes: a worn cable where the live conductor is touching the metal casing, a failed capacitor in a motor, or water ingress into an outdoor tool or kettle base.
7. If the fault is in the fixed wiring
A wiring fault that trips an RCBO with nothing connected is harder to find and usually needs a professional. The most common causes in UK homes:
- Moisture in a light fitting or outdoor socket: water bridges the gap between live and earth. Let the fitting dry out fully, or replace it.
- A damaged cable: particularly in older homes where cables run under floorboards and someone has driven a screw through one.
- A cable buried in damp plaster or screed: condensation or a slow leak can cause the insulation resistance to drop enough to trip the RCBO.
- A loose connection arcing at a terminal: can cause an intermittent earth-leakage current.
An electrician can carry out an insulation resistance test (a quick, non-destructive test with a dedicated tester) to confirm or rule out cable damage, and can trace the circuit section by section.
8. Consider accumulated LED leakage
Modern consumer units typically use RCBOs set to trip at 30 mA of earth-leakage current. LED drivers often pass a small leakage current (commonly 0.5 to 3 mA per fitting) as a normal part of their operation. Individually, these are well within the safe limit. On a circuit with many fittings, they add up.
If you have recently replaced all the halogen lights on a circuit with LEDs and the RCBO has started tripping, this is a plausible cause. The fix is either to split the lighting circuit across two RCBOs, or to replace the LED fittings with low-leakage alternatives (the specification should show a leakage figure in mA).
When to call us
If the isolation steps above have not located the fault, or if the fault is in the fixed wiring rather than an appliance, that is the point to call. Richard covers fault-finding across Sandwich and east Kent, typically diagnosing and resolving RCBO trips in a single visit.
RCBO still tripping in Sandwich or east Kent?
Richard carries a full test kit and can trace circuit faults to the source. Based in Sandwich CT13, most fault-finding visits resolved same day.
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