Fault-finding when your RCD keeps tripping

Helpful video reference. The Tools4Sparks channel's "Electrician Life" series shows a working UK electrician tracing a fault on a circuit with a tripping RCD, using exactly the systematic approach described in this guide. The video is filmed in a real domestic property and covers the half-and-half elimination method well. Watch on YouTube.

1. Work out which device has tripped

Open the consumer unit door. Look for the device that has dropped to the OFF position. In modern boards this will be one of the following:

• RCD (residual current device). Usually a wider switch, often marked 30mA. Protects several circuits on one side of the board.
• RCBO (residual current breaker with overcurrent). Looks like a standard MCB but has a small test button. Protects one circuit independently.
• MCB (miniature circuit breaker). Protects against overcurrent only, not earth faults. If just an MCB has tripped without an RCD, the fault is a short or overload rather than earth leakage.

Write down which circuit is affected. The inside of the consumer unit door often has a list, though in older properties you may have to work it out by process of elimination.

2. Unplug everything on the affected circuit

Go round the house and unplug every appliance on that circuit. Sockets circuits cover a floor or ring. A lighting circuit will not have plugged-in appliances but may have table lamps plugged into sockets fed from the same RCD.

Do not skip this step and just try to reset. If the fault is in an appliance and you reset with it still plugged in, the RCD trips again immediately and tells you nothing useful.

3. Attempt a reset

With everything unplugged, push the tripped device firmly back to the ON position. A healthy RCD clicks solidly into place and stays there. If it springs back immediately even with nothing connected, the fault is in the fixed wiring, not an appliance. Skip to step 6.

If it holds, you have confirmed an appliance fault. Move to step 4.

4. Reintroduce appliances one at a time

Plug items back in one by one. Plug in, wait five seconds, watch the board. No trip? Move to the next appliance. When one causes a trip, that is your faulty item. Unplug it and reset the board. The rest of the circuit should now work normally.

Common culprits: washing machines, dishwashers, fridges and freezers with ageing compressors, storage heaters, garden power tools left plugged in over winter, and anything with a heating element.

5. Check for damp on the circuit

If unplugging appliances does not isolate the fault, or if the fault came on suddenly after heavy rain, look for moisture. Outdoor sockets, garden lighting, conservatory circuits and kitchen sockets near the sink are the usual suspects. Damp in a back box will cause an RCD to trip without a fault in any appliance.

If you find a damp socket or fitting, do not use it. Leave it off and call an electrician to inspect, dry out, and if necessary replace the fitting.

6. The half-and-half method (for sockets circuits)

If you want to narrow down the fault location on a ring or radial before calling an electrician, you can try this: mentally split the sockets on that circuit into two groups. Plug a lamp into each socket in group one only, leave group two empty. Reset. If it holds, the fault is in group two. Keep halving until you find the area. This is mostly useful if you are waiting for an electrician and want a rough location to speed up the visit.

When to call us

If you have worked through the above and cannot find the cause, or if the fault is clearly in the wiring rather than an appliance, that is where Richard comes in. Fault-finding callouts across east Kent start at £65 for the first hour. Most straightforward wiring faults are found and fixed in a single visit.