Faults and tripping

An RCD trips when it detects current leaking to earth — typically from a faulty appliance, damp reaching an outdoor or bathroom circuit, a damaged cable, or water in a light fitting. Several appliances each leaking a small amount can also add up and trip it.

A quick test: unplug everything on the affected circuits, reset, then plug items back one at a time. If it holds until a particular appliance goes on, that appliance is the likely cause. If it trips with nothing plugged in, the fault is in the fixed wiring and needs testing.

A device that trips again the instant it is reset is telling you there is a live fault on that circuit. Do not keep forcing it back on and do not tape it up — repeated resetting on a real fault is how damage and fires start.

Leave that circuit off, unplug anything connected to it, and arrange fault finding. Everything else in the house can usually stay in use in the meantime.

Night-time tripping usually points at something that only runs at certain times — an immersion heater or storage heater on a night rate, a fridge or freezer cycling, a boiler, or an outdoor circuit that gets damp as the temperature drops.

Note the time it happens and what is on a timer. That pattern is genuinely useful diagnostic information and is worth including in an enquiry.

That usually means one circuit or one RCD has tripped rather than a supply failure. Look at the consumer unit for a switch that is down or halfway when the others are up, and check whether the lost rooms share a circuit.

If resetting it works but it trips again later, there is an intermittent fault. If nothing on the board looks tripped and rooms are still dead, the fault is likely in the fixed wiring.

First check the breaker for that circuit and any fused spur or switched connection unit feeding the room — a blown 13A fuse in a spur will kill everything after it. Sockets are usually wired as a ring or radial shared across rooms, so a single dead room often means a broken connection at one accessory.

If the breaker is on and the room is still dead, the joint needs locating with test equipment rather than by guesswork.

Flickering has three common causes: an LED lamp that does not suit the dimmer or transformer fitted, a loose connection somewhere on the lighting circuit, or a voltage dip when a large load such as a shower or kettle starts.

If a single fitting flickers, it is usually the lamp or that fitting. If lights across the whole house flicker or dim together, that points at a loose connection at the board or at the supply, and it should be checked promptly.

Repeated failures in one fitting normally mean cheap lamps, heat build-up in an enclosed fitting, an unsuitable dimmer, or vibration. Repeated failures across several fittings can indicate a loose neutral or a supply voltage issue, which is a different and more serious matter.

Note whether it is one fitting or many, and whether the lamps are dimmable and rated for the fitting. That distinction decides whether it is a lamp problem or a wiring problem.

An appliance that trips the board is usually leaking to earth internally — heating elements in washing machines, dishwashers, tumble dryers and kettles are the classic culprits as they age. Test it by running the appliance on a different circuit if you safely can.

If the same appliance trips wherever it is plugged in, it is the appliance. If any appliance trips that one socket, the circuit needs testing.

Check the lamp first in another fitting, then whether the lighting breaker is on. A switch that has physically failed, or a connection behind it that has worked loose, is common — especially on two-way switching where one of the pair stops responding.

The switch plate should not be removed while the circuit is live. Switch the lighting circuit off at the board and arrange for it to be checked.

Yes, but intermittent faults need a record. Note the date, time, what was running, the weather if an outdoor circuit is involved, and which rooms or fittings were affected each time.

That log genuinely shortens diagnosis, because testing can be targeted at the conditions that reproduce the fault instead of the whole installation.

An MCB (breaker) protects the circuit against overload and short circuit — it trips when too much current flows. An RCD protects people, tripping when current leaks to earth. An RCBO is both in one device, covering a single circuit.

An old rewireable fuse does the MCB's job by melting a wire, and gives no earth-leakage protection at all. Which device tripped tells you a lot about the type of fault.