Why Your RCD Keeps Tripping: 6 Causes and How to Fix Them

It is one of the most common calls I get in York. Frankie, my switch keeps tripping and I cannot work out why. It always happens at the worst possible time. Mid-shower. Halfway through dinner. Working from home on a deadline. The good news is that your electrics are doing exactly what they are supposed to. The bad news is that something is causing it, and it will not fix itself.

This guide walks through the six reasons I find on the vast majority of call-outs, how you can safely isolate the fault yourself before ringing anyone, and the point at which you should stop and pass it to a qualified electrician.

What an RCD does

An RCD (Residual Current Device) is a safety switch that disconnects the supply the instant it detects an imbalance between the live and neutral currents on a circuit. That imbalance means current is leaking somewhere it should not. Usually to earth, sometimes through a person. A 30 mA RCD opens in under 30 milliseconds at a 30 mA fault, which is fast enough to prevent a lethal shock ^[2].

You will find an RCD in one of three places in a home consumer unit:

• As a main switch protecting the whole board (older split-load layout from the early 2000s)
• As a dual RCD protecting two banks of circuits (17th Edition layout, common 2008 to 2018)
• As individual RCBOs, one per circuit (current BS 7671:2018+A4:2026 standard)

Which type you have matters for diagnosis. A single main-switch RCD tripping kills the whole house. A dual RCD kills half. An RCBO kills one circuit. How much of the house goes dark is the first clue to which board you are looking at.

The 6 most common reasons your RCD keeps tripping

1. A faulty appliance

This is the number one cause I find on call-outs, by a wide margin. Kettles, washing machines, dishwashers, tumble dryers and older fridges are the usual suspects. Inside the appliance, a heating element or motor winding slowly degrades over time and allows a tiny current to leak to the earthed metal casing. The appliance may run fine for weeks, then trip the RCD when cold, halfway through a cycle, or the moment a heating element switches on.

If the trip is intermittent and timed around one appliance's use, suspect that appliance first. The 6-step diagnosis below will find it inside an hour.

2. Moisture in an outdoor socket or outbuilding

Water and electricity do not get on. If the RCD trips after heavy rain, during a thaw, or when you plug in something outside, the fault is almost always moisture ingress. Common culprits: an outdoor socket with a perished rubber gasket, garden lights with cracked lenses, a damp shed with rodent-damaged cable, or a Christmas lighting transformer that has sat outside all summer.

The easy test is to unplug everything outside, reset the RCD, and see if it holds. If it does, you have found the circuit at fault.

3. A shower element or immersion heater failing

Electric showers and immersion heaters work hard. The sheathed element sits directly in the hot water, and over years the insulating mineral packing breaks down at one end. Once it does, water touches live, and the RCD sees earth leakage. Tell-tales: the RCD trips the moment the shower is switched on, or when the hot water cylinder fires up at 3am.

If the shower is more than ten years old and the element has failed, it is often cheaper to replace the whole unit than the element alone. Immersion heater elements are straightforward to swap once the cylinder is drained.

4. Cumulative earth leakage across multiple circuits

Every appliance leaks a tiny amount of current to earth by design, a few microamps each. Put ten or fifteen circuits on one shared 30 mA RCD, and the combined leakage can creep up to 15 mA on a cold, damp morning with every heater running. The RCD trips even though no single appliance is faulty.

This is known as nuisance tripping, and it is the main reason BS 7671:2018+A4:2026 favours one RCBO per circuit instead of one shared RCD ^[1]. If your consumer unit is from the 17th Edition era (pre-2019), there is a good chance this is what you are dealing with.

5. Deteriorating cable insulation

If your home was wired before the 1990s and has not had a rewire since, the rubber or early-PVC insulation on the cables is likely starting to break down. It goes brittle, it cracks at bends, and you get leakage-to-earth faults that worsen in hot weather or when a circuit is loaded up. This is the most serious of the six, and it usually needs an EICR to diagnose properly.

Warning sign: if the insulation resistance test on a circuit reads below 2 megohms, the cable is on borrowed time.

6. A faulty or ageing RCD itself

RCDs have a service life. Around 10 to 15 years in, the electromechanical trip coil can become over-sensitive, mechanically sticky, or fail a calibrated operational test. If an electrician tests the RCD and the trip time is drifting, or the monthly test button no longer fires the mechanism, the device itself is the fault and needs replacing.

Electrical Safety First recommends pressing the test button every three months ^[3]. If yours does not trip when you press it, call someone today, not next week.

Cause vs symptom, at a glance

If you can describe when the trip happens, you are halfway to the fault. Match the pattern below to the most likely cause.

Diagnose an RCD trip safely in 6 steps

1. Switch off and unplug everything on the affected circuit Go through the rooms served by the tripped RCD and unplug every appliance. Turn off hard-wired spurs at the local isolator. If the RCD feeds fridges or freezers, keep this step under an hour to avoid spoiling food.
2. Reset the RCD Push the RCD lever firmly up to the ON position. If it stays on, the fault is in one of the items you just unplugged. If it trips straight back down, skip to step 5.
3. Plug appliances back in one at a time Reconnect each item in turn and switch it on. Wait at least a minute. The RCD will trip on the faulty appliance, sometimes the moment it is plugged in, sometimes when an element or motor fires.
4. Isolate the faulty appliance Unplug it and leave it off. Reset the RCD and finish reconnecting the rest of the circuit. Replace, repair or scrap the faulty item. If it is a kettle or a toaster, a new one is cheaper than a call-out.
5. If the RCD will not reset with nothing plugged in, isolate circuits at the consumer unit Switch off every MCB on the affected RCD bank. Reset the RCD. Now switch each MCB back on one at a time. The circuit that trips it is where the fault lies, somewhere in the fixed wiring, a light fitting, an extractor fan or a spur.
6. If the RCD trips with every MCB off, stop and call an electrician With every circuit isolated, a tripping RCD means the device itself or the meter tails feeding it are at fault. That is not a DIY repair. Leave the RCD off, write down what you tested, and ring a registered electrician.

Nuisance trip vs genuine fault

Not every trip means something is dying. Sometimes the RCD is doing the electrical equivalent of clearing its throat. Here is how I tell the two apart on a call-out:

• Trips once, resets, then holds for months. Nuisance. Often a one-off surge, a brief damp spell, or an appliance that has since been unplugged. Monitor, do nothing yet.
• Trips repeatedly over a few days, then settles. Borderline. Worth an insulation resistance test on the suspect circuit at your next EICR.
• Trips in the same conditions every time (rain, shower on, heating cycling). Genuine fault. Needs diagnosis.
• Trips within seconds of reset, or will not reset. Definitely genuine. Do not keep trying to reset it.
• Trips during a thunderstorm. Surge-related. Genuine but often transient. If it keeps happening, fit a type 2 SPD in the consumer unit, now a default expectation under BS 7671:2018+A4:2026.

When to stop and call an electrician

How we charge for fault finding

Every fault find is billed by time on site, not a flat fee, because I don't know which of the six causes above you are dealing with until I have tested. I use a calibrated Megger tester for insulation resistance and RCD operation, so the time spent is diagnosis, not guesswork. You get the meter readings, the circuit the fault was on, and the specific component that failed.

Any remedial work, say replacing a faulty shower, swapping an RCD, or running a new radial for a failed spur, is estimated separately once the cause is identified. I never repair blind and I never bill for a job I haven't first talked through with you. Book a diagnostic call-out and I'll confirm the rate and timing before I come out.

Preventing repeat trips, and when an RCBO upgrade makes sense

A single shared RCD on a modern house with a fridge, a condensing boiler, an electric shower, LED downlights and a heat pump is a nuisance-trip waiting to happen. The fix is to replace the shared RCDs with individual RCBOs, one per circuit. That delivers three concrete benefits:

• A fault on one circuit only trips that circuit, and the rest of the house stays on.
• Cumulative leakage can no longer build up across circuits. Each RCBO only sees its own circuit.
• Fault finding is faster on subsequent call-outs, because the failed circuit announces itself the moment it trips.

An RCBO-per-circuit consumer unit upgrade is scoped on a site visit. The cost depends on circuit count, enclosure type, whether the existing tails and earthing need bringing up to current standards, and whether any cabling needs making good. Book a no-obligation survey and estimate.

What BS 7671:2018+A4:2026 says about RCD protection

The current edition of the IET Wiring Regulations, BS 7671:2018+A4:2026, came into force on 15 April 2026. For a domestic property it requires 30 mA RCD protection on:

• All socket outlets rated up to 32 A
• All final circuits supplying lighting
• Cables concealed in walls at a depth of less than 50 mm where not otherwise protected
• Mobile equipment used outdoors

The amendment also extends AFDD requirements in certain higher-risk accommodation and continues to favour RCBO-per-circuit as the preferred method of compliance ^[1]. If your consumer unit still uses shared RCDs protecting multiple circuits, it is not unsafe, but it is no longer the recommended layout for new or upgraded installations.