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Fuses

Almost all of our fuses are miniature cartridge fuses for appliances and electronics. We stock them in 5 x 20mm and 6 x 32mm sizes, as quick-blow and slow-blow types. We also stock a few plug fuses for UK plugs. Whichever you need, the right replacement matches the old fuse in size, amp rating and speed, as we explain below.

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Plug fuse or cartridge fuse

Start by working out where the blown fuse came from, because plug fuses and miniature cartridge fuses are not interchangeable. A plug fuse is the cartridge fuse inside a UK 13A plug, and it protects the flex that runs from the plug to the appliance. A moulded plug still has a replaceable fuse, held in a small carrier between the pins. A miniature cartridge fuse fits the fuse holder in an appliance or device, often behind a cap on the back panel or on a circuit board inside. Neither fits the other's holder, so check the old fuse before you order.

Choosing the amp rating for a fuse

A plug fuse is rated 3A, 5A or 13A, and the right rating is whatever the appliance or its manual gives. As a rough guide, a low-power item such as a table lamp usually takes a 3A fuse, while kettles, heaters and washing machines take a 13A fuse, and some appliances call for 5A. Never fit a higher rating than specified, because a 13A fuse in place of a 3A fuse may let a fault run long enough for a thin flex to overheat.

We stock a few plug fuses, among them a mixed pack that is handy for a spares drawer. For a cartridge fuse, match the current rating on the old fuse exactly, whether it is given in amps or milliamps.

Slow-blow or quick-blow fuses

Speed matters as much as rating. A time-delay fuse, also called slow-blow, rides out the brief surge that motors and power supplies draw as they start, yet still breaks the circuit if an overload carries on. A fast-blow, or quick-blow, fuse breaks the circuit very quickly, which protects sensitive electronics that even a short surge could damage.

Replace like for like, with the same rating and the same speed. A quick-blow fuse in place of a slow-blow fuse can blow as soon as the device switches on, while the reverse can let a fault damage delicate parts. We stock quick-blow and slow-blow cartridge fuses in 5 x 20mm and 6 x 32mm.

Matching the size of a cartridge fuse

Cartridge fuse sizes are written as diameter by length in millimetres, so a 5 x 20mm fuse is shorter and slimmer than a 6 x 32mm fuse. A fuse holder takes only the size it was made for: a fuse that is too short will not touch the contacts, and a longer fuse will not sit in the holder. If you are unsure, measure the old fuse or check the manual.

Check the voltage rating too. It is the highest voltage the fuse can safely break, so the replacement needs a voltage rating no lower than the original, such as 250V for mains equipment.

Reading the markings on a fuse

The letters and figures on a fuse, or on the label beside its holder, tell you what to buy. A T before the current rating marks a time-delay fuse and an F marks a fast-acting fuse, so match the letter as well as the figure when you choose a replacement.

Ratings below an amp are usually written in milliamps, shown as mA, and we stock some fuses rated this way. Read the unit as carefully as the figure, because the same figure means far less current in milliamps than in amps, and a mix-up leaves the device either unprotected or blowing fuses straight away.

Glass fuses and spotting a blown fuse

We stock some glass fuses, and their glass tube lets you see the fuse wire. A wire that has snapped, or a tube that has turned black or silvery inside, tells you the fuse has blown. Heavy blackening usually points to a short circuit rather than a mild overload, so have the device checked before you fit a replacement.

You cannot see inside a plug fuse, and even in a glass fuse the wire can break out of sight under an end cap. A multimeter on its continuity setting gives a clear answer, or you can try a fuse you know is good in its place.

Changing a fuse safely

Always switch off and unplug the appliance before you change a fuse, whether it sits in the plug or in a holder on the device. With a plug fuse, open the carrier or plug cover, ease the old fuse out with a small screwdriver, clip in a replacement of the same rating and close the plug before plugging it back in. If the holder is on a circuit board inside the case, leave the job to a qualified person, as parts such as capacitors can hold a charge after unplugging.

If the replacement blows again, stop there. The fuse is doing its job, and repeated blowing usually means a fault in the appliance, its flex or the wiring. Ask a qualified electrician to find it, and never fit a higher rating, wrap a fuse in foil or bridge the holder to keep things running.

Frequently Asked Questions

Which amp fuse do I need for a UK plug?
Fit the rating the appliance or its manual gives, which for a plug fuse is usually 3A, 5A or 13A. In general, a lamp or other low-power item takes 3A, while a kettle, heater or other high-power appliance takes 13A.

Can I use a 13A fuse instead of a 3A fuse?
No. A 13A fuse in a plug meant for 3A may not blow before a thin flex overheats in a fault, so always fit the rating the maker specifies. A lower rating than specified is no better, as it is likely to blow in normal use.

Should I buy a slow-blow or a quick-blow fuse?
Buy the same speed as the fuse you are replacing. A T on the old fuse means time-delay, or slow-blow, while an F means fast-acting, or quick-blow. We stock slow-blow and quick-blow cartridge fuses, so you can replace like for like.

Can I fit a miniature cartridge fuse in a UK plug?
No. A UK plug takes a plug fuse, while miniature cartridge fuses fit the fuse holders inside appliances and devices, and they are not interchangeable. For UK plugs, we stock a few plug fuses.

How do I know which size of cartridge fuse I need?
Measure the old fuse or check the manual, as sizes are given as diameter by length in millimetres. We stock 5 x 20mm and 6 x 32mm; match the old fuse exactly, because a holder will not accept a different size.

How can I tell if a fuse has blown?
On a glass fuse, look for a broken wire or a darkened tube. A plug fuse gives no visible sign, so test it with a multimeter on its continuity setting or, with the appliance unplugged, swap in a fuse you know is good.

Why does my replacement fuse keep blowing?
A fuse that blows again usually signals a fault somewhere in the appliance or its wiring, though a quick-blow fuse fitted where a slow-blow fuse belongs can also blow at switch-on. Check that the rating and speed match the original, and if it still blows, unplug the appliance and ask a qualified electrician to find the fault.

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