RCD keeps tripping: how to find the cause yourself

An RCD trips because it has detected current leaking to earth — around 30 milliamps is enough. It is not being oversensitive and it is not "faulty" merely because it keeps going. Something is leaking, and in about twenty minutes you can usually establish which circuit without any tools at all.

What an RCD is actually measuring

It compares the current flowing out on the live with the current returning on the neutral. Those should match. If a few tens of milliamps go missing, that current is finding another route to earth — through insulation that has broken down, through water, or through a person. The RCD cannot tell which, so it disconnects. That is the entire point of the device, and it is why "just replace the RCD" is almost always the wrong instinct.

There are two distinct situations behind almost every complaint, and they need different responses:

Cumulative leakage is genuinely under-diagnosed. Every switched-mode power supply — every TV, laptop charger, LED driver, induction hob — leaks a small amount to earth by design. Thirty of them on one RCD can sit just under the threshold on a normal day and cross it when the dishwasher starts. Nothing is broken. The board is simply arranged badly.

The elimination test

This identifies the circuit. Do it with the RCD reset and everything else off, and work methodically — the value is entirely in doing it in order.

1. Switch off every MCB (the individual circuit breakers) on the affected side of the board.
2. Reset the RCD. If it will not stay on with every circuit off, the fault is in the board or the incoming arrangement — stop, and call someone.
3. Switch the MCBs back on one at a time, waiting a few seconds after each.
4. When the RCD trips, you have found the circuit. Note which MCB it was.
5. Now unplug everything on that circuit and repeat. If the circuit holds with nothing plugged in, the fault is in an appliance, not the wiring.
6. Plug items back in one at a time. The one that trips it is the culprit.

The usual culprits, roughly in order

• Immersion heater or shower element. When these fail they fail to earth, and they do it reliably. A prime suspect if the trip coincides with hot water or a shower.
• Water in an outdoor circuit. Garden lighting, an outside socket, a pond pump, a shed supply. Frequently seasonal — it starts after heavy rain and stops in a dry spell, which is a strong tell. See garage and outdoor electrics.
• A failing appliance. Washing machines, dishwashers, tumble dryers, kettles. Anything combining water and a heating element.
• A damaged cable. A screw or nail through a cable during shelving or flooring work. Often appears days or weeks later as damp gets in.
• Old lighting circuits. Rubber-insulated or early PVC cable in lofts, where heat and decades have degraded the insulation.
• A borrowed neutral. Two circuits sharing a neutral, usually from historic alterations. Common in houses part-rewired at some point — which in Harlow's post-war new-town stock is most of them.

Why a new consumer unit is usually the wrong purchase

People frequently conclude that the board is the problem and buy a new one. A new board does not fix a leaking circuit — and a modern one, with a separate RCBO per circuit, will detect the same fault more reliably and trip that circuit rather than half the house. The fault is still there.

There is one honest exception, and it is about arrangement rather than repair. If your problem is cumulative leakage on a dual-RCD board, moving to per-circuit RCBOs genuinely solves it, because the leakage is then divided across many devices instead of summed on two. That is a real reason to change a board — see what a board change costs and why all-RCBO is worth specifying. But diagnose first: paying £600 to rearrange a board when one shower element was at fault is a bad trade.

Two things not to do

• Do not leave the RCD off to keep the power on. That removes the protection that exists to stop an earth fault killing someone. If you must keep a freezer running, run it from a different circuit temporarily, not from an unprotected one.
• Do not let anyone "swap the RCD" as a first move. RCDs do fail, and a nuisance trip with nothing connected can be a failed device — but that is a conclusion reached after testing, not before. Ask what leakage was measured, and on which circuit.

When to stop and call someone

Call an electrician if the RCD will not reset with every circuit off; if a circuit trips with nothing plugged in; if there is any burning smell, discoloured accessory or warm socket; or if the elimination test gives a different answer each time, which usually means an intermittent fault and is the hardest kind to chase. Diagnosis is charged separately from the repair for a good reason — nobody can say in advance how long an intermittent fault takes to find. What you can do is arrive having already narrowed it to one circuit, which is the difference between a one-hour visit and a three-hour one.

If the installation turns out to be failing on several circuits rather than one, that is a different conversation: read EICR certificate cost to get it assessed properly, and whether you actually need a rewire before anyone quotes you for one.