What Fuse to use in a UK 13A Plug (probably not 13A)

I was just chatting to someone who has changed her first electrical plug fuse today. It took me a while to remember how to work out what size fuse to put in, which is embarrassing as I took a university degree that was half physics (obviously, it must have been the half that didn’t include electricity. Take that, Unified Theory!) If I’ve forgotten after 30 years, then perhaps you have too or maybe no-one ever told you. If you are a Mr/Ms know-it-all, skip straight to the bold at the bottom for the exciting equation bit.
(Don’t worry, it’s hardly even an equation.)

A normal household electrical system in the UK consists of a number of circuits running off a central ‘consumer unit’, often known as a ‘fuse-box’. This is very unlikely to actually have any fuses in it because they have probably been replaced by circuit-breakers. If something goes wrong with the equipment in your house e.g. a light bulb blows, there may be a surge of electricity which will be sensed by the circuit-breaker, causing it to ‘trip’, probably plunging part of your house into darkness. Once any fault causing the problem has been fixed or removed, simply reset the breaker switch that tripped.

Similarly, the fuse in a UK 13A plug is there to protect you and your electrical equipment from over-loading. It is called a 13A plug because that is the maximum current it is designed to carry. This does NOT mean it should be fitted with a 13A fuse. Instead of just ‘tripping’, a fuse ‘blows’ and has to be replaced. There can be an easy-access plastic flap that you lever off with a screw-driver or you may have to unscrew the plug case and go inside. Some plugs like to scatter their components across the floor at this point, so watch out for that unless you like puzzles. Plug designs vary but find yourself a table top to work on and you’ll be fine.

Having removed the blown fuse, you need a new one. But how many Amps? They usually have a value (in Amps) written on them and they’re colour coded but the colours often fade or change. The fuse may also have been fitted by someone who didn’t know how to select the correct fuse. Don’t trust them.

A fuse is there so that if there is a power-surge, the fuse blows before anything else does. It needs to be the ‘weak point’ in the circuit. You want to fit the lowest-powered fuse there is which doesn’t quite blow when everything is operating normally. This is how you calculate the value:

The ‘current drawn’ by your device (in Amps) = the ‘power of your device’ (in Watts) divided by ‘mains Voltage’ (in Volts)

Mains Voltage is 230V Alternating Current in the UK, though many people still think it is 240V. We changed in 2002 so that new devices can be sold to be used across Europe.

Find the power rating of your device, in Watts. It may be written on the device or you may have to look at the instruction manual or look it up on the Internet. If it is given in kiloWatts then multiply that number by 1000 i.e. a 2.5 kW kettle is 2,500 Watts.

Divide the Power value of your device by 230. You will get a number that will not be more than 13 (Amps.) Pick the fuse that is the next up from this number. You can normally buy 3A, 5A and 13A fuses.

[ The PHYSICS: this is all so simple because Power (in Watts) = Current (in Amps) x Voltage (in Volts). Aren’t SI units clever? ]

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