Nuisance Tripping And Finding The Fault

A safety switch that trips is not faulty. It is detecting current going somewhere it should not, and it is the only reason that current did not go through somebody.

Which means the answer is never to reset it repeatedly and hope. The fault is there, and the job is to find which circuit and which item.

What the device is actually detecting

A safety switch compares the current flowing out with the current coming back. If a small amount is missing, it is leaking to earth, and the switch opens in a fraction of a second.

That leakage can be a genuine fault in one appliance, or it can be the sum of several small leakages across a circuit, each of them individually below the threshold. The second case is why a circuit can run for years and then start tripping without anything obvious changing: one more small leak tipped the total over.

Which is a different thing from a circuit breaker tripping, which is an overload or a short rather than a leakage. Knowing which device operated narrows the problem immediately, and the basics are in [safety switches and electrical testing](/blog/safety-switches-and-electrical-testing).

The usual causes, in order of likelihood

Moisture. The single most common cause. Water in an external power point, a light fitting in a wet area, a leaking roof above a ceiling rose, a flooded sub floor, or an exterior outlet without a working cover. Which is why tripping that starts after rain or after a leak is a water problem before it is an electrical one.

One failing appliance. A heating element on its way out in an oven, a kettle, a dishwasher, a washing machine or a hot water unit. Elements fail by leaking to earth, and they frequently do it only when hot, which is why the trip happens part way through a cycle.

A damaged lead or a damaged fitting. A cord crushed by furniture, a nicked cable, a cracked plug.

Water in a light fitting or an exhaust fan, particularly in a bathroom where the fan discharges into a roof space and the moisture comes back down.

Pests, because rodents chew cable insulation, and a chewed cable in a roof space is a classic intermittent fault. That connects to [pest entry points and the trades that close them](/blog/pest-entry-points-and-the-trades-that-close-them).

Accumulated leakage, with no single item at fault, which is the hardest version and the most common on an older installation with a lot on one circuit.

A failing safety switch, which does happen and is the last thing to suspect rather than the first.

How it is actually found

By elimination, in a defined order, with a meter rather than by guesswork.

Unplug everything on the affected circuit, reset, and see whether it holds. If it holds, the fault is in an appliance and it is found by plugging items back one at a time. This step alone resolves a large share of these calls and a tenant or an owner can do it before anybody attends.

If it trips with nothing plugged in, the fault is in the fixed wiring or the fixed fittings, and that is an insulation resistance test circuit by circuit.

Split the circuit to localise it, testing each section to find which run carries the leakage.

Measure the standing leakage on each circuit, which identifies the accumulated case and tells you how much headroom is left before the next small fault trips it again.

Check the obvious water paths in parallel: external outlets, wet area fittings, the roof space above any ceiling fitting, and the sub floor.

An intermittent fault is the difficult one, because it has gone by the time anybody arrives. Those are found by recording when it happens: after rain, when the oven is on, when the washing machine drains, at a particular time of day. That pattern is the diagnosis, and it is worth writing down before calling anybody.

What it is not

It is not a reason to replace the safety switch with a higher rating or to bypass it. A device that is tripping is a device that is working, and defeating it removes the protection that the installation is required to have.

It is also not solved by resetting it, and a switch that is reset repeatedly through a genuine fault is being asked to interrupt a fault current over and over.

Where it joins up with the rest of the building

Because the leading cause is water, the repair is frequently not an electrical repair.

A roof leak above a light fitting, as in [roof tiles, ridge capping and the leaks they cause](/blog/roof-tiles-ridge-capping-and-the-leaks-they-cause). A damp sub floor, as in [sub floor ventilation and timber rot](/blog/sub-floor-ventilation-and-timber-rot). A bathroom with no working exhaust, as in [bathroom condensation, mould and the exhaust fan](/blog/bathroom-condensation-mould-and-the-exhaust-fan). An external outlet with a failed cover and a downpipe discharging beside it.

Which means the electrician finds the leakage and the repair belongs to another trade, and the job is only finished when both are done. One team handling the diagnosis and the water source means the circuit stays on rather than tripping again next time it rains.

On an older installation with a lot of accumulated leakage and not enough circuits, the real answer is often a switchboard upgrade with the load split across more circuits, which is [switchboard upgrades in older buildings](/blog/switchboard-upgrades-in-older-buildings), and the faults that turn up most often are in [the electrical faults that turn up most in occupied apartments](/blog/the-electrical-faults-that-turn-up-most-in-occupied-apartments).

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