Here is the short answer: your RCD is almost certainly working perfectly. It trips because it has measured a current leaking to earth somewhere — usually 30mA or more — and nine times out of ten the culprit is one appliance, one outdoor socket that has taken water, or the combined leakage of a dozen things that were all fine individually.
The instinct is to blame the device — it is the thing that moved. But an RCD is a measuring instrument with a switch on it, and a tripped RCD is a reading, not a fault.
Fifty years on the Villa Road counter means we have had this conversation many times, and it almost always ends the same way: a kettle, a washing machine, or an extension lead that has been living in the rain.
1. What an RCD is actually measuring
An RCD compares the current flowing out on the line conductor with the current returning on the neutral. In a healthy circuit those two figures are equal — everything that goes out comes back.
If some current is going somewhere else — damp insulation, a failing heating element, a person — the two figures no longer match. When the difference reaches the device's rating, typically 30mA, it disconnects. Thirty milliamps is deliberately small: below the threshold at which a shock through the chest is likely to be fatal. That is the whole reason the device exists.
So the question is never "why did the RCD trip?" It is "where is that current going?"
2. The seven real causes
- A faulty appliance. The most common by a distance. Heating elements are the usual suspects — kettles, washing machines, dishwashers, tumble dryers, immersion heaters — because an element beginning to break down leaks to earth long before it fails outright.
- Water where it should not be. Outdoor sockets, garden lighting, garage supplies, a leak into a ceiling void. This is the one that trips at 4am after rain and is fine by the time you look at it.
- Cumulative leakage. Every electronic device leaks a tiny amount to earth by design. Individually trivial; thirty of them on one RCD can sit close enough to 30mA that anything tips it over. Nobody suspects this one, because nothing is actually broken.
- A damaged cable. A nail or screw through a cable during a shelf installation is a classic, as is a rodent in a loft.
- A neutral-earth fault. Neutral touching earth somewhere — often a crossed connection in a socket or a light fitting after work has been done. Frequently trips the moment the circuit is energised.
- A genuinely faulty RCD. It happens, but it is last on this list for a reason. Suspect it only after everything else has been eliminated.
- Old-style split-load boards. Not a fault — a design consequence. One RCD covering half the house means one dodgy toaster takes out lights, sockets and freezer together. Modern boards use RCBOs so a fault drops one circuit only.
The good news is that you can usually identify which of these it is without any test gear at all.
3. The half-split method — twenty minutes, no instruments
This is what an electrician does first, and you can do it safely from outside the consumer unit.
- Unplug everything. Not "the things you suspect" — everything. Every appliance, charger and extension lead. Turn off immersion heaters and anything hard-wired with its own switch.
- Reset the RCD. If it still will not stay in with nothing connected, the fault is in the fixed wiring and you need an electrician. Stop here.
- If it holds, restore circuits one at a time. Switch one MCB back on, wait, then the next. When it trips, you have found the circuit.
- Now plug appliances back into that circuit one at a time, leaving a minute between each. The one that trips it is your answer.
- Watch for the delayed ones. A washing machine may only trip once it starts heating, twenty minutes into a cycle. If nothing trips immediately, run the suspects through a full cycle before clearing them.
Trade tip: if the trip happens overnight or in the early hours with nobody in the house, look at the things that operate on a timer or a thermostat — immersion heater, central heating, a freezer defrost cycle, an outdoor lighting timer. Intermittent trips that correlate with rain are almost always an external socket, a garden light, or an extension lead left plugged in outside. That is a five-minute diagnosis once you think of it and a fortnight of frustration if you do not.
4. Protecting an outdoor supply properly
If the culprit is an outdoor circuit — and it very often is — the fix is not just to dry it out. Give the outdoor supply its own local protection so a wet garden does not take the whole house down.
The OP9RCD is an IP66 13A 2-gang double-pole RCD switched socket in weatherproof black polycarbonate, Type A 30mA, 230V 50Hz. IP66 is the outdoor-grade rating — jets, not just splashes — and the integral RCD means the outdoor socket protects itself.
Indoors, the equivalent for a single high-risk outlet is a local RCD socket.
The CU9RCD is a 13A 2-gang double-pole RCD switched socket, 30mA Type A, IP20, white, with captive terminals. In a workshop or garage the M9RCD does the same job in powder-coated steel, which survives being knocked by a wheelbarrow rather better than moulded plastic.
5. Protecting a single hard-wired circuit
Where the thing you want to protect is wired in rather than plugged in — an outside light, a garage supply, a pond pump, a boiler — an RCD fused spur is the right device.
The CU6RCD is a 13A RCD fused spur unit, 30mA Type A, IP20, double pole, in white thermoset resin. It gives one circuit its own 30mA protection and its own fuse, so a fault at the far end of a garden trips that spur, not the board.
Trade tip: note "Type A" on all four of these. Type AC devices — the older standard — can be blinded by the DC leakage that modern electronics, LED drivers, EV chargers and inverter appliances produce, so they may fail to trip when you need them. Type A handles pulsating DC components, and if you are replacing an RCD device anywhere it is the sensible minimum in 2026.
6. What to do about cumulative leakage
If the half-split test finds no single culprit but the RCD trips when enough things are on, you have cumulative leakage rather than a fault. Nothing to repair — but something to fix.
The answer is board design: splitting the load across more RCDs, or moving to RCBOs so each circuit has its own device. That is a consumer unit job, notifiable, and where this article hands over.
RCD Tripping, Summarised
| Symptom | Likely cause | First move | WME pick |
|---|---|---|---|
| Trips when one appliance runs | Faulty element | Isolate that appliance | — |
| Trips after rain | Water in an outdoor circuit | Isolate outdoor sockets | OP9RCD |
| Trips overnight, nobody home | Timed or thermostatic load | Check immersion, heating, freezer | — |
| Trips only when lots is on | Cumulative leakage | Board redesign — electrician | — |
| Trips with everything unplugged | Fixed wiring fault | Stop. Call an electrician. | — |
| Workshop or garage outlet | Harsh environment | Local RCD protection | M9RCD |
| Hard-wired outdoor circuit | Needs its own device | RCD fused spur | CU6RCD |
Where DIY stops — and here it stops at the cover
Everything in section 3 is safe: unplugging appliances, resetting an RCD and switching MCBs are all designed to be done by the occupier, from the front of the board, with the cover on.
Taking the cover off a consumer unit is not DIY. Parts inside remain live with every switch off, because the incoming tails are not isolated by anything you can operate. Replacing an RCD, adding RCBOs, changing a board or tracing a fixed-wiring fault is notifiable under Part P and needs a registered electrician to test and certify it. If the RCD will not reset with everything unplugged, that is the moment to pick up the phone. Knowing where DIY stops is the mark of doing it properly.
Expert Help in the West Midlands
Villa Road, Birmingham, since 1975. We will talk you through the half-split test at the counter — it costs nothing and often saves a call-out. And when the answer is an electrician, we say so, rather than selling you a part that was never going to fix it.
Browse plug sockets including RCD-protected outlets, or the wider wiring accessories range. Get in touch if you would like us to spec local RCD protection for an outdoor or workshop supply.
Frequently Asked Questions
Why does my RCD keep tripping?
Because it is detecting current leaking to earth, usually 30mA or more. The most common causes are a faulty appliance — typically something with a heating element — water in an outdoor circuit, or the combined leakage of many electronic devices on one RCD.
How do I find out what is tripping my RCD?
Unplug everything, reset the RCD, then restore circuits one at a time and appliances one at a time. If it will not reset with everything unplugged, the fault is in the fixed wiring and needs an electrician.
Can I replace an RCD myself?
No. Consumer unit work is notifiable under Part P and there are live parts inside the unit that are not isolated by any switch you can operate. It needs a registered electrician who will test and certify the work.