RCD trips only in wet weather or after wash-down
The RCD holds fine when dry but trips after rain, washdown, or in damp conditions — pointing to moisture creating an earth-leakage path somewhere outdoors or in wet areas.
Safety first
Water + electricity earth leakage is a shock hazard. Don't bypass the RCD. Isolate before testing damp circuits and treat wet areas with extra caution.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Water ingress into outdoor fittings/sockets
Most likelyRain or washdown gets into outdoor socket-outlets, lights, or junction boxes, creating a leakage path to earth.
- 2
Damaged/degraded outdoor cable
#2Cracked insulation or a damaged buried/exposed cable leaks once it gets wet.
- 3
Faulty outdoor appliance/equipment
#3An outdoor appliance with failing seals leaks to earth when damp.
- 4
Condensation in enclosures
Least likelyTemperature swings cause condensation inside fittings that leaks to earth.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Correlate trips with weather/washdown and identify which circuit(s) the RCD protects in wet areas.
A link between damp conditions and a particular outdoor circuit.
Suspect circuit identified — isolate it and split the load.
If indoor too, broaden the search to condensation/enclosures.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
RCD trips when wet
→ step 2 - 2decision
Does splitting the load isolate it to a wet circuit/appliance?
Yes→ step 3No→ step 4 - 3result
Dry/reseal/repair the wet circuit or appliance.
- 4decision
Does insulation testing reveal a damp/low circuit?
Yes→ step 5No→ step 6 - 5result
Cable/appliance leakage — repair and retest.
- 6result
Suspect condensation in enclosures — address sealing/ventilation.
Common mistakes apprentices make
- Bypassing the RCD because 'it only trips when wet'.
- Testing only when dry and finding nothing.
- Overlooking outdoor socket/light seals and gland integrity.
- Ignoring condensation inside enclosures.
When to stop & escalate
Water ingress into fixed wiring or fittings needs proper repair and resealing by a qualified person, then retesting. Never remove earth-leakage protection from outdoor circuits.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
RCD / RCBO keeps tripping
An RCD or RCBO trips repeatedly — immediately on reset, randomly during the day, or only when certain equipment runs. The earth-leakage protection is doing its job; something is leaking.
RCD won't reset at all
The RCD won't stay reset — the toggle won't latch up, or it trips instantly every time, so the circuit can't be restored.
Heater tripping the RCD / earth leakage
A heater circuit trips its RCD/earth-leakage protection — often when cold and first switched on, or once it's been damp — pointing to leakage to earth from the element.