Electrical fault finding

A trip is a protective device doing its job. The question is what it is protecting against.

• A circuit breaker (MCB) trips on overload, more current than the cable is meant to carry, or on a short circuit between conductors.
• An RCD trips when current leaks to earth: a faulty appliance, water in an outside light or socket, damp in a junction box, a nail through a cable, or insulation that has broken down.
• An RCBO does both, for a single circuit.

On older boards one RCD often covers half the house, so a fault on one circuit switches off several. Faults that come and go, after rain or when the heating starts, are the hardest to pin down, so it helps to note when it happens.

1. Look at the board. Find which device has tripped: a breaker or RCBO for one circuit, an RCD for a group of them.
2. Unplug everything on the affected circuits, and switch fixed appliances off at their own switches.
3. Reset once. If it stays on, plug things back in one at a time. If it trips when one goes in, that appliance is the likely culprit, not the wiring.
4. If it trips with nothing plugged in, leave it off and call. The fault is in the fixed wiring or a fitted appliance.

Do not keep resetting a device that trips straight away, do not open up sockets or switches, and never fit a bigger fuse. If the whole house is off and nothing on the board has tripped, check whether the neighbours have power. A power cut is for the network operator: call 105.

A circuit that is dead with its breaker on, or a single socket that has stopped working, usually means a connection has failed: a loose terminal at a socket, a burnt-out joint in a junction box, or a conductor that has broken.

A ring circuit can lose one leg and carry on working, which hides the fault while the cable that is left carries more than it should. Loose connections also get hot, so a dead socket is worth finding rather than living with.

One light flickering is usually the lamp, or an LED lamp or driver that does not suit the dimmer it is on. Several lights flickering together, or lights dipping when something heavy switches on, points to a loose connection they share: a junction box, a switch, the board, or occasionally the supply itself.

Flickering with buzzing or a smell is not a lamp problem. Switch that circuit off at the board and call.

These are signs of a connection overheating, and they should not be left:

• A smell of hot plastic, or a fishy burning smell, near a socket, a switch or the board.
• Scorch marks on a socket, plug or switch.
• Sockets or plugs that are warm to the touch.
• Buzzing or crackling from an accessory or the board.

Switch the circuit off at the board, or the main switch if you are not sure which, stop using it and call. If there is smoke or fire, get everyone out and call 999.

A fitted appliance can fail in two places: inside the appliance, or in the circuit that feeds it. Electric showers and cookers draw heavy currents, so a slightly loose terminal in the isolator or cooker switch gets hot, discolours and eventually fails.

Testing tells the two apart. The circuit, the isolator or cooker switch, the connection unit and the terminals at the appliance are all checked, so you know whether the fix is in the wiring or the appliance before anything is replaced.

An oven that trips the RCD as it warms up, a shower that cuts out, or an extractor fan that runs on or will not start are all worth a call.

Fault finding is methodical, and it starts with making the circuit safe.

1. Isolate. The circuit is switched off, locked off so nobody can turn it back on, and proved dead with a tester that is itself checked before and after.
2. Test. An insulation resistance tester puts a test voltage between the conductors and to earth. A low reading means current has somewhere to leak.
3. Divide. The circuit is split at a socket or junction partway along and each part tested. The bad part is split again, until the fault is down to one length of cable or one accessory.
4. Repair, then retest the whole circuit before the power goes back on.

Computers and other sensitive equipment are unplugged first, because insulation testing uses a higher voltage than the mains.

The repair fits the fault: a new length of cable joined in an accessible or maintenance-free junction box, a connection remade, a damaged socket or switch replaced, or the supply to an appliance put right.

Afterwards the circuit is tested again, and you are told what was found and what was done. New work comes with a certificate: a Minor Electrical Installation Works Certificate for small changes to an existing circuit, or an Electrical Installation Certificate where a circuit is new.

If testing shows a wider problem, such as insulation breaking down across several circuits, we say so plainly. The next step may be an EICR or a rewire, and you decide with the readings in front of you.