Why your RCD keeps tripping

Updated 19 July 2026

What an RCD is looking for

A residual current device compares the current flowing out on the line conductor with the current returning on the neutral. In a healthy circuit they match. If around 30 milliamps goes missing, it is leaking to earth, possibly through a person, and the device disconnects in well under 40 milliseconds.

That sensitivity is the point. It is also why a slightly damp outside light or a tired immersion heater element can bring the whole board down.

The safe way to narrow it down

1. Switch off every circuit breaker protected by the RCD that tripped.
2. Reset the RCD. If it will not stay on with every circuit off, the fault is in the board itself or the device has failed. Stop here and call us.
3. Switch the circuits back on one at a time, waiting a few seconds after each. The one that drops the RCD is your suspect circuit.
4. Unplug everything on that circuit and try again. If it holds, plug appliances back one at a time until it trips.
5. If it trips with nothing plugged in, the fault is in the fixed wiring and needs testing with an insulation resistance meter.

The usual culprits

• Outside circuits after rain. Garden lights, pond pumps and outbuilding supplies are the most common single cause.
• Immersion heaters and showers. Heating elements fail to earth as they age, often tripping only when they reach temperature.
• Washing machines and dishwashers. Small amounts of leakage that only appear mid cycle.
• Cumulative leakage. Several appliances each leaking a little, adding up to the trip threshold on a shared RCD. This is exactly what individual RCBOs solve.
• Damaged cable. A screw or nail through a cable during other work, sometimes months earlier.

Why one faulty kettle takes out your whole house

On older split load boards, a single RCD protects half the circuits. One fault anywhere on that half disconnects all of it, which is why people come home to a warm freezer and a dark hallway. Replacing the board with individual RCBOs means only the faulty circuit goes off. It is the single most useful upgrade in most older Sussex homes.