An RCD that trips and won't go back on is an annoying way to start the day, but it's the safety device doing exactly what it's designed to do — there's an earth fault somewhere and it's refusing to reconnect until you fix it. Here's the systematic way to find the culprit.
Step 1: Switch Off Every MCB on the RCD
Look at your consumer unit. The RCD is the larger switch (usually labelled "RCD" or "30mA"). Below it sit the MCBs (the smaller switches for each circuit). Switch all the MCBs that sit downstream of the RCD to OFF.
Now try to reset the RCD. If it stays on, brilliant — the fault is in one of the circuits, and we just need to find which.
If it still won't reset with all MCBs off, the RCD itself is faulty or the fault is upstream of the MCBs. Skip to Step 4.
Step 2: Switch MCBs Back On One by One
With the RCD now reset and stable, switch each MCB back ON one at a time. Wait 5-10 seconds after each one. The MCB you flick that causes the RCD to trip again is the one with the fault.
Once you've identified it, leave that MCB off (so the rest of the house has power) and call an electrician. You've already done half my job for me.
Step 3: Common Faults on the Identified Circuit
Once we know which circuit, the most common causes are: water in an outdoor socket or garden light (especially after rain); a faulty appliance — kettle, iron, immersion heater element, dishwasher heating element; a damaged cable behind a wall (mouse damage, screw through cable); aged immersion heater wiring on the immersion circuit; or a tracking fault inside an old socket.
Quick test you can do: unplug everything on the affected circuit. Reset the RCD. Plug each item back one at a time. If it trips when you plug something in, you've found a faulty appliance — replace it.
Step 4: If the RCD Won't Reset Even with Everything Off
Two possibilities: 1) The RCD itself has failed mechanically and needs replacing (£15 part, 10 minutes to swap). 2) The fault is in the wiring upstream of the MCBs — between the RCD and the meter. This is rarer but worth checking, especially in older split-load consumer units where the busbar can develop a fault.
Either way, this is electrician territory. Don't leave the RCD bypassed or wedged on — that defeats the safety mechanism entirely.
When the Trip Is "Random" — Intermittent Earth Faults
Sometimes an RCD trips at completely random times — once a week, once a month — and resets fine afterwards. These intermittent faults are nearly always: outdoor sockets letting in damp, a freezer's defrost cycle, an immersion heater element starting to fail, or a heat pump / water heater's thermostat cycling.
Tracking these requires plug-in monitors that log the trip events. I bring those if you've got an intermittent issue — usually solve it in 1-2 days of monitoring rather than weeks of guessing.
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