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How to identify electrical faults

Introduction

It's annoying if an electrical socket or appliance doesn't work. But with a bit of logical detective work, you should be able to find the cause of the problem - and, in many cases, correct the fault yourself. Before you begin any kind of electrical work, though, you'll need to know how to isolate a circuit and double-check that it's dead.



A plug-in appliance doesn't work 

 

  1. If a plug-in light doesn't work, try changing the bulb. For any other kind of appliance, first try plugging it into another socket. If it works, the original socket could be faulty and need replacing. If it doesn't, try again on a different power circuit (probably on another floor). If it works there, you might have a dead circuit.
  2. If the appliance doesn't work in a socket you know is functioning, check the flex connections in the plug and replace the fuse - making sure it has the correct rating.
  3. Still not working? The appliance may have an internal fault which needs some professional attention.

 

A circuit is dead

 

  1. Turn off all the lights or unplug all the appliances on the affected circuit. Turn off the main isolating switch at the consumer unit, and repair the circuit fuse or reset the circuit breaker or RCD. Then turn the main switch back on.
  2. Switch on each light or plug in each appliance in turn to find out which item on the circuit is causing the fuse to blow or the circuit breaker to trip. When you find it, isolate the circuit again and check the fuse, the connections and the flex (see above).
  3. If the fuse blows or the circuit breaker trips again, the fault may lie in the fixed wiring. Call a qualified electrician.

 

A wall or ceiling light doesn't work

 

  1. First, check if the other lights on the circuit are working. If not, follow the steps for a dead circuit below.
  2. If the other lights on the circuit are working, turn off the light at the switch and replace the bulb.
  3. If that doesn't help, turn off the power and check the cable/flex connections at the light. If necessary, strip back the cores and remake the connections - making sure the terminal screws are tight. Still with the power off, check the condition of the flex with a continuity tester, and replace it if necessary.
  4. If that doesn't work, turn off the power again, remove the switch cover and check the cable connections. If they're loose, remake them. If they're fine, try replacing the switch.
  5. Still not working? Call in a qualified electrician.

 

All circuits are dead

 

  1. If the circuits in your home are protected by an RCD (residual current device), check to see if it has tripped. If so, reset it. If it trips again, carry out the checks for faulty lights, appliances and a dead circuit. If the problem continues, call in a qualified electrician.
  2. Check with neighbours or your electricity supplier to find out if the power to the neighbourhood has been cut. If not - and you can't find a problem with your domestic circuits - get in touch with your electricity supplier. They'll be able to check the main supply cable and service fuse.

 


Each of the electrical circuits in your home is provided with a fuse or a circuit breaker. They can be either an MCB (miniature circuit breaker) or RCBO (residual current breaker with overload protection).

These devices protect the circuit against overloading, which could generate heat within the wiring that melts the insulation and causes a fire.

They also react to short circuits that are caused when the current-carrying cores of cables come into contact with each other. This can happen if the cores become loose inside an electrical accessory, or if the cable is pierced accidentally by a drill or nail.

Fuses contain a special wire that melts, separates and cuts off the flow of electricity if the circuit draws too much current or a short circuit occurs.

The wire might be exposed within the carrier, or it can be contained within a special cartridge. Circuit breakers are trip switches that turn themselves off under the same circumstances, and can be reset by pushing a button or operating the switch.

The demand placed on circuits varies (light fittings consume less electricity than most plug-in appliances, for example). So as well as having different sized cable, the circuits are protected by fuses or circuit breakers with different ratings.

Lighting circuits are protected by 5 or 6 amp fuses, socket circuits by 30 or 32 amp fuses, an immersion heater by a 15 or 16 amp fuse, and so on. It's very important that you use fuses of the correct rating. One with too low a rating will keep blowing, while one with a rating that's too high might not protect the circuit against overloading - with potentially fatal consequences.

 

Safety first - Nuisance RCD tripping

 

To prevent your RCD-protected systems from tripping, you should work with the whole power supply switched off. Switching off a circuit breaker or removing a circuit fuse only isolates the L (live) side of the circuit, while the N (neutral) stays connected to the mains. This is quite safe for working on the circuit, but it means that any contact with the N wire will cause the RCD to trip and switch off the entire house supply. This is not only irritating, but can also be dangerous when you're in the middle of a repair.


Cartridge fuses are easy to replace - but make sure you use the correct fuse rating for the circuit, as fuses vary in size and colour coding according to their rating.

How-to-identify-electrical-faults

Step 1

Switch off the power and remove the cartridge fuse. Some are simply held in spring clips and can be prised out, while on others you'll need to open the fuse carrier by releasing a screw.

How-to-identify-electrical-faults

Step 2

Press a new fuse of the correct rating into the spring clips, or insert it into the open ends of the carrier's pins. Then, if necessary, reassemble the fuse carrier. Check the main power switch is off and replace the fuse - usually, the pins are offset to one side so it'll only fit one way round. After that, you can restore the power.

Video

Watch our how to video below which shows you how to change a plug and fuse.


If a rewirable fuse has blown, you should be able to see that the fuse wire has melted. You should replace it with new fuse wire of the correct amperage.

How-to-identify-electrical-faults

Top tip - Connection unit fuses

Fixed appliances are permanently wired to a connection unit that has an on/off wall switch and a cartridge fuse. To replace a connection unit fuse, first switch off the connection unit switch. Then unscrew or prise out the fuse holder. Remove the fuse and insert a new one of the correct rating. Make sure you push or screw the holder back fully before you restore the power.

How-to-identify-electrical-faults

Step 1

Turn off the power, lift the fuse cover on the consumer unit and remove the blown fuse. Release the terminal screws and remove the fuse wire.

Step 2

Take a length of fuse wire of the correct rating and insert it in the carrier. Wind the ends of the wire around the terminals, but don't pull it taut. Cut off the excess, then tighten the screws. Check the main power switch is off, then refit your fuse. Replace the cover and turn the power back on.