Why does my RCD trip at the same time every night?
How to use the clock to identify a timed load that nobody is switching on, and the small set of appliances that cause almost every trip at a fixed hour.
- Difficulty
- beginner
- Time
- 20 min
- Read
- 4 min
- Safety
- caution
Short answer
Something is switching itself on at that hour and leaking to earth. The usual suspects are an immersion heater on a timer, storage heaters on an off-peak switch, a freezer defrost cycle, an outside light on a timeclock, and a washing machine or dishwasher heating on delay start. Work out what runs then, run it deliberately during the day, and you will reproduce the trip.
Trips that happen with nobody in the room feel random and are usually the most solvable of all, because the clock has already narrowed the field for you. Almost everything in a house that switches itself on does so on a schedule, and almost everything that leaks to earth does so because it contains a heating element in contact with water.
Safety
Step by step
- Write down the exact time of each trip.Three data points is usually enough. If the times cluster within a few minutes, a timer or a control is involved. If they scatter, look at temperature-driven cycling instead, such as a freezer or a fridge.
- Check the off-peak or economy tariff switching time.If you are on Economy 7 or a similar tariff, the meter switches at a set hour. Anything on the off-peak supply — immersion heater, storage heaters, hot water — energises at that moment. A trip at exactly that time points there before anything else.
- Check the immersion heater and its timer.A failing immersion element leaks to earth long before it fails outright, and it leaks most at the start of a heating cycle. This is probably the single most common cause of an overnight RCD trip in a UK house.
- Check the freezer and fridge defrost cycles.Frost-free appliances run a small heater periodically to clear the evaporator. A deteriorating defrost heater leaks only during that cycle, which can be hours apart and roughly regular.
- Check anything on a delay start.Washing machines and dishwashers set to run overnight both heat water. A perished element or a damp motor shows up when the heat comes on, several minutes into the cycle rather than at the start.
- Check the timed outdoor and security lighting.A timeclock or a dusk sensor energises outdoor fittings at a consistent hour, and outdoor fittings are the ones most likely to have water in them.
- Reproduce it deliberately during the day.Once you have a candidate, run it on purpose while you are watching. An immersion heater boosted manually, a wash cycle started now, the outside lights switched on at the switch. A trip you can cause on demand is a diagnosis.
- Stop using the item and have it tested or replaced.An immersion element, a defrost heater and a washing machine element are all replaceable by a competent appliance engineer or electrician. Do not keep the appliance in service between trips — the leakage is what the RCD is protecting you from.
Tips
- Smart meter apps and any smart plug with energy monitoring will show you exactly what draws power at that hour, which shortcuts the whole exercise.
- If the trip time drifts by a few minutes each night, that is a thermostatic cycle rather than a clock — think fridge, freezer, or a heater with a room stat.
- Cumulative leakage across many appliances trips at the point where one more small load tips the total over 30mA. If nothing is individually guilty, that is what an electrician measures with a clamp meter.
Common mistakes
- Turning off the RCD overnight so it stops waking you up — That leaves a group of circuits with no earth leakage protection at all through the night, while the fault that caused the trips is still present.
- Blaming the appliance you were last using — The whole point of a fixed-time trip is that the cause is not something you touched. Following the clock finds it far faster than following your memory.
If it doesn't work
Trips at exactly the Economy 7 changeover time
Cause: Immersion heater or storage heater element leaking to earth — Fix: Leave the immersion or storage heating off and see whether the trips stop. Have the element tested and replaced.
Trips a few hours apart at slightly varying times
Cause: Frost-free fridge or freezer defrost heater — Fix: Run the freezer on a different circuit for a day if you safely can, or have it tested. A leaking defrost heater is a repair, not a wait-and-see.
Trips mid-way through an overnight wash
Cause: Washing machine or dishwasher heating element — Fix: Stop running it on delay start, run one cycle while watching, and have the element tested if it reproduces.
Trips at dusk
Cause: Outdoor lighting energising with water in a fitting — Fix: Isolate the outside lighting circuit and see whether the trips stop, then have it tested.
Questions people ask
Could it be the neighbours or the grid?
No. An RCD responds only to an imbalance on the circuits it protects, which are all inside your own installation. Voltage disturbances from outside do not trip an RCD.
The immersion heater is off at the switch — can it still be the cause?
Only if the switch genuinely breaks it. Many immersion switches break the live only, and if the fault is neutral-to-earth it will still leak. A more reliable test is to switch the immersion heater's own MCB off at the board.
Is it worth changing the board to RCBOs?
If the tripping turns out to be cumulative leakage across several circuits rather than one fault, then yes — separating circuits so each has its own 30mA device is the correct engineering answer. It is a consumer unit change and must be done by a qualified electrician with a certificate.