Why Does My RCD Keep Tripping?
An RCD trips when it detects current leaking out of a circuit to earth, the sort of leak that could pass through a person. So when your RCD keeps tripping, something on the circuits it protects is letting current escape: most often a faulty appliance, then water or damp in an outdoor or wet area circuit, then the small leaks from lots of electronics adding up. Occasionally the RCD itself is the fault.
This post is only about the RCD, the switch usually marked with a "T" or "Test" button. If the switch going off is a circuit breaker for a single circuit, our guide on why your electric keeps tripping covers overloads and short circuits as well. And if the RCD will not reset at all, the steps below still help, but it is also a job for an emergency electrician rather than something to leave overnight.
What an RCD actually detects
An RCD (residual current device) compares the current going out on the live conductor with the current coming back on the neutral. On a healthy circuit they match. If some of that current finds another path to earth, through a damaged cable, a wet fitting or a person, the two no longer balance and the RCD switches off.
Most domestic RCDs are rated at 30 mA, a tiny current compared with the 13 A a socket can supply. Electrical Safety First says RCDs can disconnect the supply in as little as 40 milliseconds and calls them lifesaving devices. BS 7671, the IET Wiring Regulations, requires 30 mA RCD protection for most sockets up to 32 A and for cables buried in walls.
That sensitivity is the point. It also means an RCD reacts to leaks far too small to blow a fuse or trip a circuit breaker, so a trip does not always come with any other symptom.
The likely causes, in order
1. A faulty appliance
This is the most common reason by a distance. Anything with a heating element is a prime suspect: kettles, washing machines, dishwashers, ovens, immersion heaters and tumble dryers. Elements can let moisture in as they age, and the leak often shows up the moment the appliance heats. Damaged flexes, cracked plugs and garden tools are next on the list.
2. Water, damp and outdoor circuits
Outdoor sockets, garden lighting, pond pumps and garage or shed supplies are exposed to rain and condensation. If the RCD trips in wet weather or first thing on a damp morning, suspect the outdoor circuits first, but do not open or handle a wet socket or fitting: leave that circuit switched off at the consumer unit and have it dried out and checked. Inside, a leak from a bathroom, a flat roof or a washing machine hose getting into a light fitting or back box can do the same thing.
3. Cumulative leakage (nuisance tripping)
Many modern appliances leak a small amount of current to earth as part of normal operation, particularly computers, TVs, chargers and anything with electronic filtering. On their own they are harmless. Put enough of them on one RCD and the total can creep close to its trip level, so a small extra leak, or a kettle switching on, tips it over.
This is common in older split load consumer units where one RCD protects half the house. The fix is not a less sensitive RCD. It is spreading circuits across more RCDs, or moving to individual RCBOs so each circuit has its own protection.
4. A wiring fault
A nail or screw through a cable, a neutral touching earth in a fitting, or a neutral from one circuit borrowed by another can all cause RCD trips that no appliance explains. These faults usually show up when the RCD trips with everything unplugged.
5. A failing RCD
RCDs are mechanical devices and can weaken with age, tripping at lower leakage than they should, or failing to trip at all. An electrician can measure how fast and at what current an RCD trips with a dedicated tester.
The unplug everything test
You can narrow down an appliance fault safely yourself, without taking any covers off:
Switch off and unplug everything on the circuits the RCD protects, including hard wired items with a switched spur such as the cooker, immersion or extractor fan, where you can.
Reset the RCD.
If it now holds, plug appliances back in and switch them on one at a time, waiting a minute or two between each. Heating appliances may need to warm up before they trip it.
When it trips, that last appliance is your suspect. Leave it unplugged and have it checked or replaced.
If the RCD will not reset with everything unplugged, switch off all the circuit breakers under that RCD, reset it, then turn the breakers back on one by one. The breaker that trips the RCD points to the circuit with the fault.
If step 5 finds a circuit, stop there. Leave that breaker off, use the rest of the house as normal, and have the circuit tested by an electrician. Do not take off socket fronts, light fittings or the consumer unit cover to look for the fault yourself.
Why intermittent RCD tripping needs an insulation test
RCD tripping that happens every few days, with no pattern you can link to an appliance, is the hardest kind to find, and the unplug test often shows nothing. It usually means a leak that comes and goes with temperature, moisture or load.
An electrician will run an insulation resistance test on each circuit, which puts a test voltage across the conductors to find breakdown that a normal meter will not see. Leakage clamp meters can also measure the standing leakage on each circuit to show whether cumulative leakage is the real story. The same tests form part of an EICR, which is worth having if the installation has not been inspected for some years.
Using the RCD test button
The test button creates a small, deliberate imbalance to check the trip mechanism works. Electrical Safety First says RCDs can be regularly checked this way, and that if the button does not trip the device, a registered electrician should investigate. Pressing it switches off everything that RCD protects, so expect clocks and routers to reset.
An RCD that trips on its test button can still have a slow or weak trip. Only a proper RCD test with an instrument confirms it disconnects fast enough.
When RCD tripping is an emergency
Most RCD trips are an inconvenience rather than a crisis. Call an electrician straight away if:
the RCD will not reset even with every appliance unplugged and every breaker off;
you can smell burning, or a fishy, hot plastic smell, near sockets, fittings or the consumer unit (our post on a fishy or burning smell in the house explains why);
anyone has had a shock, however mild;
water has got into a fitting, ceiling or the consumer unit;
you see scorching, melting or hear crackling.
Never hold an RCD in the on position or tape it up. It is tripping because it has found something.
RCD questions, answered
Can I replace an RCD that keeps tripping myself?
We would not. Even with the main switch off, the incoming supply cables and the terminals they feed stay live inside the consumer unit, so this is a job for a registered electrician. Swapping the RCD also rarely cures the problem, because the leak is usually somewhere else. Replace it only once testing shows the RCD itself is faulty.
Why does my RCD trip during thunderstorms?
A nearby lightning strike or supply disturbance can cause a brief surge that some RCDs react to. An occasional trip in a storm that resets straight away is usually nothing more. Frequent trips need looking at.
Why does my RCD trip at night?
Timed loads are the usual answer: an immersion heater on an off peak tariff, a dishwasher on delay, outdoor lights on a timer, or a freezer defrost cycle. Match the trip times to what switches on.
If your RCD keeps tripping around Shifnal, Telford, Wolverhampton or the wider West Midlands and you cannot find the cause, contact our team and we will test the circuits and find the leak.
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