The short answer
An RCD (residual current device) trips when it detects current leaking to earth — this is its designed purpose, not a fault. Persistent RCD trips typically indicate a faulty appliance with degraded insulation causing earth leakage, moisture in a socket, switch or external fitting, ageing cable insulation that has become leaky, or in rarer cases a faulty RCD that has become overly sensitive or unreliable. The RCD is not the problem — it is protecting you from a problem. Finding and fixing the underlying cause is the correct response. An RCD that trips repeatedly and you cannot trace to an appliance needs a registered electrician.
An RCD that keeps tripping is doing its job — but it is telling you something in the installation or connected to it has an earth-leakage fault. Here is how to identify the cause and what the different tripping patterns suggest.
RCD trips — key facts
- What an RCD detectsCurrent leaking to earth (earth-fault current)
- Trip thresholdTypically 30 mA for domestic RCDs
- Most common causeFaulty appliance with degraded insulation
- Other common causesMoisture ingress, ageing wiring insulation
- If no clear causeRegistered electrician required
What an RCD does and why it trips
An RCD (residual current device) continuously compares the current flowing out on the live conductor with the current returning on the neutral. Under normal conditions these are equal. If current is leaking — flowing to earth through a fault, a person, or degraded insulation — the two values become unequal. When the difference exceeds the RCD's rated operating current (typically 30 milliamps for a domestic circuit RCD), the device disconnects the circuit in a fraction of a second.
This makes the RCD one of the most important protective devices in a modern consumer unit. A 30 mA leakage at mains voltage represents a potentially lethal shock current; disconnecting in less than 40 milliseconds (the BS 7671 requirement at that operating current) can prevent electrocution. When an RCD trips, it has done exactly what it is designed to do — the question is what caused the leakage.
Faulty appliance — the most common cause
The most frequent cause of a persistent RCD trip is a faulty appliance whose internal insulation has broken down, allowing a small current to leak to earth whenever it is plugged in and switched on. This leakage may not be enough to trip an MCB (overload device), but it is detectable by the RCD.
Commonly implicated appliances:
- Washing machines and dishwashers: water, heat and vibration degrade the insulation on internal wiring over time. Heating elements in particular can develop earth faults.
- Tumble dryers: similar degradation mechanisms; heating elements and motors are common fault points.
- Electric showers: the combination of high voltage and water makes these susceptible over time.
- Power tools: particularly older tools or those with damaged cables or worn carbon brushes (brush-motor tools generate electrical noise that can, in aggregate, cause nuisance trips on some RCDs).
- Fluorescent and LED driver units: some electronic ballasts and LED drivers have internal capacitors that create a small leakage current as a normal byproduct. In isolation this is not enough to trip an RCD, but multiple such devices on the same RCD can accumulate to a tripping level.
To identify a faulty appliance, unplug everything on the affected RCD's circuits, reset the RCD, then reconnect items one at a time. The appliance that causes the next trip is the culprit. Stop using it until it has been repaired or replaced by a competent person.
Moisture and damp
Moisture entering the electrical installation creates a conductive path to earth that an RCD detects. Sources include:
- External sockets and weatherproof fittings: sockets in gardens, on garages or under canopies can allow moisture entry if their seals degrade or their covers are left open.
- Bathroom and kitchen fittings: light fittings, extractor fans and shaver sockets in bathrooms can allow steam ingress, particularly if the IP rating is not appropriate for the location.
- Roof or plumbing leaks above electrical fittings: water tracking down through ceiling roses, wall switches or junction boxes.
- Condensation in outdoor installations: particularly in outbuildings, garages or lofts where the temperature varies and moisture condenses inside enclosures.
A trip that occurs specifically in wet weather, after a shower runs, or following heavy rain often points to moisture ingress. A registered electrician can identify the entry point and make the installation weathertight.
Ageing wiring insulation
The insulation on electrical cables does not last forever. In properties with older wiring — particularly properties built before the 1980s that may have rubber-insulated or early PVC wiring — the insulation can harden, become brittle and crack. Cracked insulation develops leakage paths to earth, particularly when circuits are first switched on (the initial thermal expansion of the cable can open small cracks).
If an RCD trips when the heating comes on, or when circuits in a specific part of the house are first used in cold weather, degrading insulation in older wiring is a plausible cause. A registered electrician can test insulation resistance to identify affected circuits and assess whether rewiring is needed.
| Tripping pattern | Likely cause | Suggested action |
|---|---|---|
| Trips when specific appliance connected | Faulty appliance (earth leakage) | Identify by elimination; stop using appliance |
| Trips in wet weather / after rain | Moisture in outdoor or bathroom fitting | Call electrician — find and seal moisture ingress |
| Trips when heating or cold circuit first used | Ageing insulation on older wiring | Call electrician — insulation resistance test |
| Trips randomly, no clear pattern | Accumulated leakage from multiple devices, or developing fault | Call electrician — measure total leakage |
| Trips immediately on reset with nothing connected | Wiring fault or faulty RCD | Do not keep resetting; call an electrician |
RCD tripping patterns and likely causes. Sources: Electrical Safety First guidance; BS 7671.
When the RCD itself may be faulty
An RCD can itself become unreliable through age or through repeated operation under high fault currents. Signs that the RCD device itself may be the problem include:
- It trips with no appliances connected and the insulation resistance of all circuits tests correctly.
- The test button no longer trips it, or it trips when the test button is pressed but cannot be reset.
- It trips spuriously in patterns that do not correspond to any appliance or environmental factor.
A registered electrician can test the RCD with a purpose-built tester to confirm whether it is operating correctly. An RCD that does not trip on its test button at the rated current is not providing the protection it should and should be replaced. For a modern domestic board, this is part of the overall consumer unit assessment — if the board is old and the RCD is suspect, a board replacement may be the most practical resolution.
Frequently asked questions
Is it safe to keep resetting an RCD?
Resetting an RCD once you have identified and removed the cause — a faulty appliance — is fine. If the RCD keeps tripping with no clear cause, or trips again immediately after you reset it, do not keep resetting. The device is detecting earth leakage that needs to be found and fixed by a registered electrician.
Can an RCD trip for no reason?
RCDs do not trip without a cause — they respond to a measurable imbalance in the current between live and neutral conductors. What can appear like a random or inexplicable trip may be due to accumulated leakage from multiple electronic devices, a slowly developing wiring fault, or occasional moisture ingress. The cause exists; it may just require testing to find.
Why does my RCD trip when I turn the lights on?
If the trip correlates specifically with switching on lighting circuits, the cause is likely in those circuits — degrading insulation in older wiring or a lamp fitting with a leakage fault. An RCD trip at 30 mA means current is finding a path to earth; testing the insulation resistance of the lighting circuits will identify where.
Sources & further reading
- Electrical Safety First — your consumer unit and circuit breakers
- Electrical Safety First — what to do if your electricity trips
- IET — BS 7671 18th Edition wiring regulations overview
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.