Unlike an overload trip, an RCD fault often persists with everything switched off, because the leakage is in the fixed wiring or in a permanently connected appliance such as an immersion heater, boiler or outdoor light. The good news is that the isolation process is logical and quick.
The isolation sequence
Switch every MCB on the affected side to off, then reset the RCD. If it holds, switch the circuits back on one at a time. The one that drops the RCD again is your faulty circuit.
- All MCBs off, RCD reset
- Restore circuits one by one, pausing between each
- Note the circuit that causes the trip
- Leave that circuit off and use the rest of the house normally
Usual suspects
Water ingress into outdoor sockets and garden lighting is the seasonal favourite. Elsewhere it is heating elements — showers, immersion heaters, washing machines and dishwashers — where the element has started to break down internally. Damaged cable from a picture hook or a floorboard nail is a close third.
What we do differently
We measure insulation resistance circuit by circuit rather than swapping parts. That gives a number, and the number tells us whether the leakage is in the cable, the accessories or the connected load. Where a board only has a single RCD serving many circuits, we will usually quote an RCBO board as the long-term fix so one fault stops taking out half the house.
Common questions
- Why does it trip more in wet weather?
- Moisture in outdoor accessories or in a garage or shed supply is the most common seasonal cause.
- Can I leave the faulty circuit off?
- Yes, that is the correct interim step, provided the circuit does not serve anything essential like heating controls or a freezer.
- Would RCBOs prevent this?
- They limit the disruption. A fault then only takes out its own circuit instead of everything on the RCD.
- How quickly can you attend?
- Non-urgent work is normally booked in the same working week, Monday to Friday, 8am to 5pm.
Run the isolation sequence, tell us which circuit drops it, and we can arrive already knowing where to start.
Related guides
- Why Your Fuse Box Keeps TrippingA consumer unit that trips is doing its job — the question is what it keeps objecting to.
- RCD vs RCBO: What the Difference Means at HomeThe difference is not protection quality — it is how much of your house goes dark when something leaks.
- Outdoor Sockets and Garden Lighting That TripGarden electrics live outdoors, and outdoors is where every weakness in an installation eventually shows.