RCD trips the moment a specific load is switched on
The RCD is fine until a particular appliance/circuit is switched on, then it trips immediately — clearly pointing at that load or its switch-on behaviour.
Safety first
A load that instantly trips the RCD likely has an earth fault — a shock risk. Isolate it and test before using. Don't bypass the RCD to keep using the appliance.
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
Earth fault in the appliance
Most likelyThe appliance has a fault to earth that draws leakage the instant it's energised.
- 2
Heating element / motor winding leakage
#2An element or winding leaking to earth trips the RCD on switch-on.
- 3
High switch-on inrush/filter leakage
#3Some equipment has a switch-on earth-current transient (filters) that trips a sensitive RCD.
- 4
Damaged flex/connection on that load
Least likelyA damaged lead or connection on the appliance leaks to earth when live.
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.
Confirm it's that specific load by switching it in isolation and watching the RCD.
RCD trips reliably only when that load is switched on.
Confirmed culprit — isolate it and test the appliance.
If other loads also trip it, treat as a broader leakage issue.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
RCD trips on a specific load
→ step 2 - 2decision
Does only that load trip it (in isolation)?
Yes→ step 3No→ step 4 - 3decision
Is the appliance/flex insulation to earth healthy?
Yes→ step 5No→ step 6 - 4result
Other loads trip it too — treat as a broader leakage issue.
- 5result
Suspect switch-on transient/filter leakage — review RCD type/arrangement.
- 6result
Earth fault in appliance/element/winding — repair/replace.
Common mistakes apprentices make
- Continuing to use an appliance that trips the RCD by bypassing protection.
- Not insulation-testing the appliance and its flex.
- Assuming nuisance when there's a genuine earth fault.
- Overlooking a damaged lead on that specific load.
When to stop & escalate
An appliance with an earth fault must be repaired or taken out of use — not run on a defeated RCD. Transient/filter issues with sensitive RCDs may need an RCD-type review.
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.
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.
Motor with low insulation resistance to earth
An insulation resistance test on the motor reads low to earth — a sign the winding insulation is degraded, damp, or contaminated, risking trips and failure.