Fuses
How many pieces of electrical equipment have you used today? You may have used a kettle and toaster, listened to the radio, cassette or CD player, watched television and switched on this computer. All the equipment you used had a plastic or a metal cover to protect you from the electricity flowing inside. Although you may think that when you flick a switch, a current flows around a simple circuit, in reality something rather different often happens. Some pieces of electrical equipment contain many complicated circuits with many different kinds of components. Whether a piece of electrical equipment has a simple electrical system or a set of complicated circuits, it could develop a fault, which could cause disaster.

When you switch on some electrical equipment a current flows through its wires and components. A current of electricity is made up of tiny particles called electrons. You can think of the electrons moving in a wire like tiny snooker balls moving along inside a tube. As the electrons move along they push their way through the metal and this creates a little heat. It is produced in a similar way to the heat you make when you rub your hands together. Electrical circuits, both simple and complicated, are designed to let the currents flow through them in a way which stops them producing much heat but sometimes things go wrong.

There can be hundreds of components and wires in a piece of electrical equipment and millions of electrical items are produced and used each year. The chances are that some of them are going to develop faults and when they do there is a device to make them safe. This device is a fuse.

To understand how useful a fuse is, think about some electrical equipment without a fuse. The equipment is switched on and the current flows. At the place where the fault has developed the current surges forwards. This rush of electrons increases the heat in the wire. The heat is so strong that it melts the plastic insulation. The heat may pass to other parts of the equipment and to materials around it. If a hot wire comes in contact with a carpet or a curtain it may make the fabric smoulder then burst into flames. A few moments later the heat around the equipment becomes so intense that wood catches fire and many items in the room start to burn. It is possible that the fire could spread from one room to another until the whole home is alight.

Now think about the equipment with a fuse in place. When the equipment is switched on the current flows as before. At the place where the fault occurs a surge in the current is produced. As the current flows on it passes through a fuse. This is a piece of wire in a glass cylinder. The wire is different from the other wires in the circuit. It cannot stand too much heat before it melts. As the current flows through the fuse wire the wire becomes so hot that it melts. This makes the wire snap and a gap develops in the circuit. As electrons cannot flow across the gap between the wires the current stops flowing. No more heat is produced and the electrical equipment cools down. A dangerous fire is averted.

Fuses are not just found in electrical equipment. They are found in circuits in the home. The fuses for all the circuits are located in the same place - a box called the consumer unit. Inside the consumer unit are fuses for the lighting circuits, the circuits linked to the plug sockets and fuses for items such as an electrical cooker or a shower. If a fault develops in any of the circuits the fuse blows and stops the current flowing.

You must never tamper with electrical equipment to look at its fuses. Just be content knowing that they are there. They may never be used but if a fault does occur anywhere they are there to protect you.

Are all fuses in glass cylinders?
No, some fuses consist of just a piece of wire called fuse wire, which is stretched across contacts in a rewirable fuse carrier. These kinds of fuse are also found in a consumer unit.

Why does fuse wire melt when other wires do not?
Fuse wire is made of a different metal from the wire used in other parts of the circuit. Metals are solids. You may have learned about solids being made of tiny particles like balls all locked together. When normal wire gets hot, the particles in the metal hold firm. When fuse wire gets hot, the particles in the metal lose their firm grip of each other and slide. When the particles begin to slide the metal turns into a liquid or becomes molten. It is not strong enough to hold its shape so it breaks.

Why can't the electrons move across the gap in the fuse wire?
There is air in the gap and air is an insulator. This means that it cannot conduct electricity so the electrons cannot move through it.

There are different thicknesses of fuse wire. Why is this?
Each thickness of wire is designed to be used in a certain circuit. The thinnest fuse wire is used in lighting circuits. They have small currents flowing through them so only a thin wire is needed. If there is a surge in current the wire melts and snaps. The thickest wires are designed for circuits which carry large currents, like the circuit for a cooker. A much larger current can flow through this wire than the fuse wire for the lighting circuit. However if there is a fault in the cooker and a very large current occurs this thick fuse wire will melt and snap too.

Are the fuses in cylinders all the same?
No. The fuses for lighting circuits have a thinner wire than the fuses for circuits such as water heaters, showers and cookers. Fuses which are held in glass cylinders are called cartridge fuses.

Are cartridge fuses found in consumer units?
Yes, they are. There are also cartridge units, which are used in plugs.

On a fuse it may say 5A or 13A. What does A stand for?
A stands for amps. This is the unit which is used to measure the size of a current. A 13 amp current is larger than a 5 amp current. The unit is named after André Ampere who performed many experiments on electricity in the nineteenth century. A 5A fuse will allow a current up to 5 amps to flow through it but if the current rises above this value the fuse melts.

If a current has one amp how many electrons are flowing?
Six million, million, million electrons flow past any point in the wire every second.