RCD tripping caused by a neutral-to-earth fault
An RCD protecting several circuits trips, but the load that 'causes' it doesn't seem faulty. The real issue is a neutral touching earth somewhere on the protected group.
Safety first
A neutral-earth fault can be live and is easy to misdiagnose. Isolate before testing neutrals, and never lift a neutral on a live shared circuit — other circuits may be relying on it.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Neutral conductor contacting earth downstream
Most likelyA damaged neutral touching an earthed enclosure, or a neutral and earth crossed at an accessory, lets return current split between neutral and earth so the RCD sees an imbalance and trips.
- 2
Neutrals from different RCDs commoned together
#2If neutrals from circuits on different RCDs are linked, current returns through the wrong RCD's neutral, tripping one or both — often only when a particular load runs.
- 3
Moisture bridging neutral to earth
#3Water in a junction or fitting can create a low-resistance neutral-to-earth path that comes and goes with conditions.
- 4
Appliance internal neutral-to-frame leakage
Least likelyAn appliance leaking neutral to its earthed frame adds to the imbalance the RCD measures.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeIdentify every circuit on the tripping RCD and note what's running when it trips.
A pattern — it trips when a specific circuit/load is active.
You've narrowed it to a circuit/load group to test.
No pattern — the fault may be a wet/intermittent neutral-earth bridge.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
RCD trips — suspect neutral-earth fault
→ step 2 - 2decision
Does neutral-to-earth IR test low on a suspect circuit?
Yes→ step 3No→ step 4 - 3result
Neutral-to-earth fault located — trace for damage, water, or crossed N/E.
- 4decision
Are neutrals kept separate per RCD?
Yes→ step 5No→ step 6 - 5result
Circuit neutrals are clean — check for an appliance leaking neutral-to-frame.
- 6result
Commoned neutrals across RCDs — separate onto correct bars.
Common mistakes apprentices make
- Chasing an active-side fault when the imbalance is from the neutral.
- Lifting a shared neutral while it's live and dropping other circuits.
- Not realising two RCDs can fight over a commoned neutral.
- Overlooking moisture as the bridge that only faults in the wet.
When to stop & escalate
Commoned neutrals across RCDs usually reflect non-compliant past wiring that needs proper separation — flag and rectify rather than patch. Escalate intermittent wet-weather faults for cable location if they're in inaccessible runs.
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.
Was this guide helpful?
Related faults
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.
Lost or high-resistance neutral
Strange symptoms across a circuit or installation — voltages going high and low on different loads, flickering, equipment damage — pointing to a lost or high-resistance neutral.
RCBO trips the instant you try to reset it
The RCBO won't stay set — push it up and it trips straight back down, with nothing switched on. Different behaviour from nuisance tripping under load.