Fuses

Last updated: October 6, 2008.

When a fuse goes, you can often hear it blowing out with a sharp CRACK! that plunges your home into sudden darkness. It's a real nuisance when this happens late at night—but the alternative is much worse. If we didn't have fuses, electrical faults could start fires in our homes and burn them to the ground. Thank heavens, then, for these tiny electric protectors that keep us safe. Let's find out what they are and how they work!

Photo: The fuse inside an electrical plug (wired for the UK electricity system). The fuse is the brown vertical cylinder on the right.

Why do we need fuses?

If you've read our long article on electricity, you'll know that an electric current is a flow of electrons (tiny particles inside atoms) carrying energy through a metal or another substance in a loop called a circuit. You'll also know that electricity can be extremely dangerous: it's not something to mess with if you value your life. The electricity that comes to our homes from power plants travels at incredibly high voltages because that helps to save energy. Transformers in substations near to buildings turn the high voltage power into lower voltages that the appliances in our homes can safely use.

Photo: Two glass cylinder fuses from a household fusebox. These are rated at 30 amps.

The electrical system is incredibly reliable but it does, occasionally, go wrong. Lightning can strike power lines causing a surge in voltage that rushes into our homes. Or a surge can happen if electrical equipment is switched on and off too rapidly. Sometimes faults occur when the appliances in our homes malfunction or when rodents chew through wires. For a whole variety of reasons, the cables running into our appliances may suddenly find themselves carrying much more voltage than they should. If we didn't have fuses, power surges could damage our televisions, radios, computers, and light bulbs, cause fires, and maybe even put our lives at risk.

How fuses work

You probably know that wires get hot when electricity travels through them. That's how ordinary, incandescent lamps work. Electricity flows through a very thin wire called a filament making it so hot that it gives off light. The same idea is at work in an electric toaster. Here, electricity flows through a series of thin metal ribbons, making them so hot that they produce enough heat to cook bread. A fuse is exactly the same. It's a thin piece of wire designed to carry a limited electrical current. If you try to pass a higher current through the wire, it'll heat up so much that it burns or melts. When it melts, it breaks the circuit it's fitted to and stops the current flowing.

We fit fuses in different places in our homes. In some countries, such as the UK, fuses are fitted into plugs on every appliance that connects to an electrical outlet. Different appliances draw different amounts of current, so an electric toaster will need a higher fuse (typically 13 amp) than an electric light (usually just 3 amp).

Types of fuseboxes

Photo: An old-fashioned fusebox. The fuses are inside the white box on the left.

There are also fuses fitted at the junction where the main electricity supply flows into your home. This is called the junction box or fusebox. It divides the incoming electricity into a number of separate circuits and feeds them to different parts of your home. A high-power circuit feeds large items like electric cookers, while lower-rated circuits feed lights and other appliances. Having different parts of your home on separate circuits means that a failure in one circuit doesn't stop the others from working.

Typically each circuit in your home is fitted with its own fuse. In older fuseboxes, the fuse is just a bare piece of wire connected between two terminals. More recent fuseboxes have replaceable cartridge fuses with the fuse wire built into a glass cylinder that you can easily snap in and out. The latest fuseboxes do away with fuses altogether and have trip switches instead. If a fault occurs, the fusebox detects the problem instantly and the trip switch automatically switches off whichever circuits are affected. Once you've identified and solved the problem, you can simply flip the switch back to get the power working again.

Photo: A modern fusebox like this one, made by Wylex, uses trip-switches instead of fuse wire or cartridges. The left photo shows the entire fusebox; the right photo shows a close-up of the trip switches. If there's too much current flowing in one of the circuits, the switch for that circuit flips over and cuts off the electricity. You can restore the power by flipping the switch back again (after correcting whatever caused the problem). Half the circuits in this fusebox are fitted with automatic RCD (residual-current device) protection, which greatly reduce the risk of electric shocks when you accidentally cut through power cables.

Another way of protecting your appliances is with a surge protector, which attaches between your appliances and the electrical outlets on your wall. It works in a slightly different way to a fuse, identifying power surges and channelling them harmlessly to the ground.