Lighting circuit trips its breaker
The breaker (or RCBO) feeding a lighting circuit trips — either the moment it's switched on, or after the lights have been on a while. Resetting holds for a bit or trips straight back.
Safety first
A circuit that trips on a short or earth fault is telling you something is wrong — don't strap it across or fit a bigger breaker. Isolate before opening fittings, and treat the circuit as live until proven dead.
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
Short or earth fault in a fitting or accessory
Most likelyA pinched active in a batten holder, a failed LED driver shorting internally, or water in an outdoor fitting can drop a dead short that trips instantly on switch-on.
- 2
Driver/transformer inrush exceeding a sensitive breaker
#2Banks of LED drivers draw a brief high inrush. On a circuit with many drivers and a low-rated or fast curve breaker, the surge alone can trip it as the lights are switched on.
- 3
Earth leakage building up (on an RCBO)
#3Multiple drivers each leak a small EMC/filter current to earth. Together they can creep past the RCBO's leakage threshold, especially in damp conditions.
- 4
Damaged cable from a fixing screw or rodents
#4A screw through a cable above a ceiling, or chewed insulation, can short active-to-earth or active-to-neutral and trip the moment the circuit is energised.
- 5
Overloaded circuit (too many fittings added)
Least likelyExtra runs added over time can push a lighting circuit past the breaker rating so it trips thermally after running a while, not instantly.
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 ModeNote whether it trips instantly on switch-on, or only after running — and whether it's a plain MCB or an RCBO (overload vs leakage).
A clear pattern: instant trip suggests a short/earth fault or inrush; delayed trip suggests overload or heat.
Pattern points you at the likely group of causes before you open anything.
No pattern yet — gather more by switching individual light groups where the layout allows.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Lighting circuit trips its breaker
→ step 2 - 2decision
Does it trip instantly on switch-on?
Yes→ step 3No→ step 4 - 3decision
Does an isolated section show low insulation resistance?
Yes→ step 5No→ step 6 - 4result
Delayed trip — suspect overload or heat. Check load vs rating and look for a warm/loose connection.
- 5result
Short/earth fault located in that section — inspect the fitting/run for damage or water.
- 6decision
Many LED drivers on a sensitive breaker?
Yes→ step 7No→ step 8 - 7result
Likely driver inrush/leakage. Escalate the design — don't oversize protection.
- 8result
Recheck for an intermittent fault that only shows under load or movement.
Common mistakes apprentices make
- Fitting a higher-rated breaker to 'stop the tripping' instead of finding the fault.
- Assuming it's the breaker when a single water-affected outdoor fitting is dropping the short.
- Testing the whole circuit as one and never splitting it to corner the fault.
- Ignoring that it's an RCBO — chasing an overload when it's actually accumulated earth leakage.
When to stop & escalate
If the inrush of a large LED bank is genuinely beyond the protection and the design needs reviewing, escalate rather than oversizing the breaker. Stop if you find damaged cabling in an inaccessible location or signs the circuit was overloaded by additions beyond its rating.
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.
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Related faults
A circuit breaker keeps tripping (domestic)
One circuit breaker keeps tripping — instantly on reset or after a load runs — and you need to tell an overload from a short or a faulty appliance.
Downlights cutting out then coming back
Downlights switch off after a while then come back once cooled — classic thermal cut-out behaviour, usually from heat build-up around the fitting or driver.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.