Why does my RCD keep tripping?

What an RCD is reacting to

A residual current device compares the current flowing out along the circuits it protects with the current coming back. When the two differ, current is leaking to earth through damaged insulation, through water, or through a person, and the RCD cuts the supply. A 30 mA RCD, the type fitted to protect people, is allowed to trip at any leakage above half its rating, so it reacts to quite small amounts.

A tripping RCD is usually doing its job. The question is what it is detecting. If you would rather hand the problem over straight away, electrical fault finding explains how a fault is traced.

Common causes

A faulty appliance

An appliance with a failing heating element, a worn motor or moisture inside can leak current to earth. Appliances that combine heat, water or motors, such as kettles, washing machines, dishwashers, tumble dryers and outdoor tools, are worth checking first. The trip often coincides with the appliance switching on or reaching a particular stage of its cycle.

Water and damp

Outside lights, garden sockets, sheds and garages are exposed to rain and condensation. Water in a fitting or junction box creates a path to earth, which is why trips during or after heavy rain often point to something outdoors. A leak from a bathroom or roof into a light fitting below has the same effect.

Small leakages adding up

Many appliances and electronic devices leak a tiny current to earth in normal use, well within their own limits. Where one RCD protects several circuits, those amounts add together. Wiring guidance recommends that normal leakage on an RCD stays below 30 per cent of its rating to avoid unwanted trips, and a board sharing one RCD across a busy kitchen, a home office and a utility room can approach that. The trips then seem random, because they depend on what happens to be switched on.

Damaged wiring

A nail or screw through a cable, a cable crushed under a floorboard, rodent damage in a loft, or old insulation that has broken down can all cause leakage. Trips that began just after building work or DIY are a strong clue. Faults where a neutral conductor touches earth also trip RCDs, and can make the trip appear to come from a different circuit to the one at fault.

The RCD itself

An RCD is a mechanical device and can wear or become oversensitive. That should be the last conclusion rather than the first: replacing a working RCD will not stop a genuine fault from tripping the new one.

Safe checks you can do

These checks involve nothing more than switches and plugs. Do not remove any covers, open the consumer unit, or touch anything that looks damaged, wet or scorched.

1. Note what happened. What was switched on, the time, whether it was raining, and whether the trip was immediate or came after a while.
2. Look at the board without opening it. Note which RCD has tripped and whether a circuit breaker has also switched off.
3. Unplug everything on the affected circuits. Switch appliances off and take the plugs out, including anything outside.
4. Reset the RCD once. If it will not stay on with everything unplugged, stop there. The fault is in the fixed wiring or in something wired in without a plug, such as an immersion heater, oven or shower, and it needs an electrician.
5. If it stays on, plug appliances back in one at a time, leaving a little time between each. If it trips when one particular appliance goes back in or starts up, leave that appliance unplugged and have it checked or replaced.
6. If everything is back on and it holds, keep a note of when it next trips. A pattern, such as rain, a time of day or one appliance’s cycle, is useful information for an electrician.

A reset or two while working through these checks is reasonable. Resetting it again and again to keep the power on is not: repeated tripping means the fault is still there.

When to stop and call an electrician

• the RCD will not reset with everything unplugged, or trips again straight away
• there is a burning smell, scorching, buzzing, or heat at the board, a socket or a fitting
• water is getting into a light fitting, socket or the board: switch off at the main switch if it is safe to reach, and keep clear
• the trips began after building work, a leak, or damage to a wall or floor
• the trips are intermittent and cannot be linked to an appliance
• someone has had an electric shock: if they are hurt, call 999

If something is dangerous or cannot wait, urgent electrical help explains what to do while you wait.

If the neighbours are off too

If nothing in the house works but the RCD and main switch are both on, and neighbouring houses have also lost power, the supply has failed rather than your installation. Report it by calling 105, which connects you to your local network operator.

What an electrician does to find it

Tracing an RCD trip is a matter of testing: measuring the insulation on each circuit, checking wired-in appliances, and separating parts of the installation until the leakage path is found. Intermittent faults can take longer, because the fault has to be present to be measured. Once the cause is fixed, it is also worth asking whether the board is sharing one RCD across more circuits than it should.

The difference between RCDs, RCBOs and older fuses is covered in Fuse box vs consumer unit.