Why does my RCD trip when it rains?

An RCD trips on current leaking to earth. Water is a conductor. The correlation with weather is not a coincidence and it is not the RCD being oversensitive — it is the RCD reporting a real fault that only exists when the fitting is wet, which is exactly the condition in which an electric shock outdoors would be most dangerous.

Safety

Step by step

1. Confirm the correlation before hunting.Note the date and time of each trip against the weather. Rain, or the first frost thaw, or a hosepipe or pressure washer being used, all point the same way. Trips in dry weather point somewhere else entirely.
2. List everything on the installation that lives outside.Outdoor sockets, garden lights, security lights, pond pumps, hot tubs, a shed or garage supply, an EV chargepoint, an outside tap heater, Christmas lights left up, and any extension lead run out through a window or door.
3. Isolate the outdoor circuits first, at the consumer unit.If there is a labelled garage, shed, garden or outside lights circuit, switch that MCB off and leave it off through the next wet spell. If tripping stops, you have identified the circuit without touching anything outside.
4. Unplug anything outdoors that plugs in.Pond pumps, plug-in garden lights and seasonal lighting come out at the socket, indoors where the socket is dry. A single wet plug-in item is by far the commonest cause and the easiest to prove.
5. Look, without opening anything.From outside, look for a fitting with condensation inside the lens, a cracked or missing gasket, an outdoor socket with its flap missing or propped open, a light mounted so its cable enters from above rather than below, and a security light on a wall where water runs down.
6. Check where cables enter the building.A cable that runs down a wall and into a hole with no drip loop carries water inside along its sheath. This is a common and completely invisible cause. The fix — reforming the entry with a drip loop and sealing it — is small work but it is not owner work if it means disturbing a fixed cable.
7. Suspect a buried or clipped cable if nothing on the surface explains it.Damaged armoured cable to a shed, a cable nicked by a spade years ago, or a joint in a buried run that was never made in a proper enclosure. Only insulation resistance testing finds this, and it needs an electrician.
8. Leave the affected circuit off and get it tested.Once you know which circuit it is, that is genuinely all you can safely establish. Give the electrician the circuit, the weather correlation and the list of what is on it — that turns a long diagnostic visit into a short one.

Tips

• A fitting full of condensation is already failed even if it has not tripped yet. Water inside an outdoor light is not cosmetic.
• Outdoor sockets have a hinged flap for a reason. A plug left in permanently with the flap propped open is an outdoor socket with no weather protection at all.
• If the trips began after a hedge cut, fence job or digging, say so — a nicked cable is then the first thing to test rather than the last.

Common mistakes

• Deciding the RCD is faulty because it only trips in bad weather — That pattern is the strongest possible evidence that the RCD is working. Replacing it would remove the only thing standing between a wet live part and whoever touches it.
• Opening an outdoor fitting to dry it out — It is an outdoor accessory on a live circuit, often at height, often wet. Even isolated, reassembling it to its original IP rating is skilled work — a gasket seated wrong makes the next failure worse.
• Using an indoor extension lead outside as a workaround — An indoor lead has no weather rating at all and puts unprotected connections on wet ground. It converts a tripping problem into a shock risk.

If it doesn't work

Questions people ask