What are the usual causes?
| Cause | Typical clue |
|---|---|
| Faulty appliance | Trips when one particular appliance is plugged in or switched on (kettles, washing machines, immersion heaters, outdoor equipment) |
| Insulation fault | Trips after rain or in damp weather; outside sockets, lights and garden circuits are common culprits |
| Neutral-to-earth (N-E) fault | Trips when load is applied on a circuit, even a small one; often after recent work, a nail or screw through a cable |
| Cumulative earth leakage | Trips at random on a board where one RCD covers lots of circuits with electronic loads (IT, LED drivers, appliances with filters) |
| Over-sensitive or failing RCD | Trips with little load and no fault found elsewhere; a ramp test shows a low trip current |
| Wrong RCD type for the load | Electronic loads with DC components; the IET points to Table A53.1 in BS 7671 for selecting RCD types |
How do I find the fault, step by step?
- Ask questions first. When does it trip? After rain? When something’s switched on? Any recent work, new appliances or drilling?
- Unplug everything on the affected circuits and switch off fixed appliances at their isolators. Try to reset.
- Holds? Plug things back in one at a time until it trips. That’s your faulty appliance.
- Still trips? The fault is in the fixed wiring or a fixed appliance.
- Split the circuits, remembering MCBs only switch the line. Switch off all MCBs under the RCD, reset it, then turn them back on one by one. If one circuit’s line trips it, that’s where to look. But a single-pole MCB leaves the circuit’s neutral connected to the shared RCD neutral bar. With an N-E fault, the RCD can trip whenever any load under that RCD is used, whichever MCB you’ve switched off, so this step can point you at the wrong circuit. Check too whether any RCBOs are single- or double-pole.
- Isolate properly before finding an N-E fault. Isolate at the main switch, lock off and prove dead. Then disconnect each suspect circuit’s neutral (and line) from the bar and insulation-test that circuit on its own: L-N, L-E and N-E, with sensitive equipment disconnected. A low N-E reading points to the fault. Compare readings against the minimum values in BS 7671 for the test voltage you’re using. Alternatively, reconnect circuits one at a time, line and neutral together, until the RCD trips.
- Measure the leakage (a live measurement). A leakage clamp meter around line and neutral together reads the difference, which is the leakage. Clamping circuits inside a board means working with the cover off and live parts nearby. The HSE says live work “must only be done after it has been determined that it is unreasonable for the work to be done dead”, and needs precautions to reduce the risk. Only do it where it’s justified, with a risk assessment, suitable insulated tools and PPE, and where you can reach the conductors safely. The meter tails or an accessible circuit outside the board are safer places to clamp.
- Ramp-test the RCD if nothing else shows up, to see if it’s tripping well below its rating.
How do I check for cumulative earth leakage?
BS 7671 limits the accumulated protective conductor current downstream of an RCD to 30% of its rated residual operating current, so 9 mA on a 30 mA RCD. The IET explains this under Regulation 531.3.2 (numbered as in BS 7671:2018+A1:2020; check the current numbering in your copy of Amendment 4).
That 9 mA goes quickly: the IET notes Class I fixed motor-operated appliances are allowed up to 3.5 mA of leakage each under their product standard. Three or four items with filters on one RCD can be enough.
- Clamp around line and neutral together to see where the leakage is (a live measurement: see step 5 above).
- Watch out for parallel earth paths. The IET suggests clamping around line, neutral and protective conductor together to check for leakage returning another way.
- Fix it by dividing the circuits. The IET says “suitable division of circuits, and/or the use of isolation transformers, can help address unwanted operation of RCDs.” In practice that often means RCBOs on individual circuits rather than one RCD covering several.
How do I know if the RCD itself is the problem?
An RCD must not trip at up to half its rated current. The IET says a typical 30 mA RCD trips at around 18 to 23 mA. So a ramp test result in that region is normal, and the leakage on the circuit is more likely the issue.
If it trips well below half its rating, or fails its time tests, it needs replacing. Disconnect loads before you test, and test on both polarities.
What shouldn’t I do?
- Don’t upsize the RCD or move circuits off RCD protection to stop the tripping.
- Don’t leave a “temporary” bypass in place.
- Don’t insulation-test with electronic equipment connected: LED drivers, smart switches, boilers and appliances can be damaged or give false readings.
When is it a job for the DNO?
If the RCD trips across the whole installation and you’ve ruled out everything downstream, check the supply side, particularly on TT systems where the earth electrode and supply conditions matter. If you suspect a fault on the supply, contact the DNO.