Why does the fuse in my plug keep blowing?

The UK is unusual in fusing at the plug as well as at the consumer unit, and there is a good reason for it. A ring final circuit is protected at 32A, but the flex on a table lamp might only safely carry three. Without the plug fuse, a fault in that flex would draw well over its rating and still not trouble the MCB.

Safety

Step by step

1. Check the fuse rating against the appliance first.The rule of thumb is 3A for anything up to about 700W and 13A above it. A 500W lamp on a 13A fuse is badly protected; a 2kW heater on a 3A fuse will blow every time on principle. Many appliances state the required fuse in the manual or on the rating plate.
2. Look at the flex along its whole length.Run it through your hand. Nicks, flattening under a chair or door, kinks near the plug, and the classic failure right where the flex leaves the appliance body. Damaged flex is the commonest cause after wrong fuse rating.
3. Check the cord grip in the plug.On a rewireable plug, the grip must clamp the outer sheath, not the individual cores. If the sheath has pulled through, the cores are taking the strain and one will eventually break or short. This is a repair for a competent person and often the honest answer is a new moulded lead.
4. Note when it blows.On switch-on suggests a motor or element short, or an inrush too big for an undersized fuse. After running for a while suggests an overload or a thermal fault. Only when the flex is moved suggests a broken conductor inside the insulation.
5. Test the appliance elsewhere.A different socket on a different circuit with a correctly rated fuse. If it blows there too, the appliance or its lead is at fault, and that is confirmation rather than coincidence.
6. Fit the correct fuse exactly once.Correct rating, from a reputable source, marked BS 1362 with a ceramic body. Cheap unmarked fuses are a real problem — they do not necessarily rupture when they should. If the correct fuse blows again, the diagnosis is complete: the appliance is faulty.
7. Stop using anything that blows a correct fuse twice.Two blown fuses on a correct rating is an appliance fault. Repeated replacement is how a fault that would have been a repair becomes a fire. Have it repaired or replace it.
8. Escalate anything with a smell, a scorch mark or a bang.A blackened fuse holder, a melted plug pin, a smell of burnt plastic or a flash on switch-on means the fault is not subtle. Stop, unplug, and check the socket as well — a plug that has been arcing will often have damaged the socket contacts too.

Tips

• Buy BS 1362 fuses in a mixed pack and keep them with the toolkit. A drawer of unmarked fuses of unknown rating is a trap.
• A moulded plug cannot be rewired but its fuse can still be changed — the flap or the slide-out carrier is on the pin face.
• If a lead is damaged near the plug on a moulded flex, the whole lead is finished. Cutting it back and fitting a rewireable plug is legitimate only if the remaining flex is undamaged and the appliance is not a class-critical item; replacing the lead is better.

Common mistakes

• Fitting a 13A fuse because the 3A one keeps blowing — The fuse rating matches the flex, not your patience. A 13A fuse on a lamp flex lets a fault draw four times what that flex can carry, and the flex heats up inside a room with no protection operating.
• Assuming a blown fuse is just what fuses do — A correctly rated fuse in a healthy appliance can go years without blowing. Repeated blowing is always information.
• Ignoring the socket after a plug has arced — Arcing at the pins pits and weakens the socket contacts, so the next plug in that socket runs hot too. A socket that has had an arcing plug in it needs an electrician to look at it.

If it doesn't work

Questions people ask