Why does my RCD keep tripping? Ten causes and what is safe to check yourself

What an RCD is actually doing

A residual current device compares the current going out on the live with the current coming back on the neutral. If more than 30 milliamps goes missing, it assumes that current is going through something it should not be, possibly a person, and switches off in a fraction of a second, typically under 40 milliseconds on a serious fault. That is why it trips: it has detected a leak to earth, not an overload.

A breaker (MCB) that trips is different. That is too much current, usually a short circuit or too many things on one circuit. If your MCB trips rather than the RCD, the causes are different and this guide is not quite for you.

The ten most common causes

In rough order of how often we find them on Manchester callouts:

• –A faulty appliance, especially anything with a heating element: kettle, washing machine, dishwasher, tumble dryer, immersion heater
• –Moisture in an outdoor socket, garden light or shed supply after rain
• –A cable damaged by a screw, nail or drill, often weeks after a shelf went up
• –A damp bathroom fan or light fitting
• –A fridge or freezer with a failing compressor or defrost heater
• –Cooker or hob elements breaking down
• –Rodent damage to cables in a loft or under floors
• –A worn RCD that has become over-sensitive with age
• –Too many appliances with small natural leakage adding up on one RCD, common on old split-load boards
• –A neutral-to-earth fault from a badly made connection in a junction box or socket

What is safe to check yourself

You can do the following without touching any wiring. Switch off and unplug every appliance on the circuits the RCD protects. Reset the RCD. If it stays on, plug appliances back in one by one, switching each on, until it trips. The last thing you plugged in is your culprit.

If the RCD will not reset with everything unplugged, switch off all the MCBs under it, reset the RCD, then switch the MCBs back on one at a time. The one that trips it tells you which circuit has the fault. Write that down; it saves your electrician time and you money.

When to stop and call someone

Stop if you smell burning, see scorch marks, or a socket or switch is warm. Stop if the RCD trips with nothing plugged in and all MCBs off, which means the fault is at the board. And stop if it has tripped more than twice in a day and you cannot find a pattern. Repeatedly resetting an RCD that is doing its job is how a small fault turns into a fire.

If you are in Greater Manchester or Cheshire East, fault finding with us is £95 for the first hour and most RCD faults are found within it. Out of hours, the same number goes to our 24/7 emergency team.

How an electrician finds it

We isolate the suspect circuit and test insulation resistance between live, neutral and earth with a 500-volt tester. A healthy circuit reads in the hundreds of megohms; the minimum acceptable is 1 megohm and a faulty circuit reads well below that. We then split the circuit at sockets or junctions until the reading isolates the faulty section or accessory. It is methodical rather than clever, which is why it usually takes less than an hour.

If the RCD itself is the problem, we test its trip time and current. A worn RCD is replaced, or if the board is old and has one RCD for half the house, we will usually recommend an RCBO board so one fault only takes out one circuit.

Questions people also ask