Fuses are used to protect the various electrical circuits on a car from excessively high currents and consequent overheating of wires and components. The principle is much the same as in our homes where circuit breakers, commonly known as "trip switches", are installed on the switchboard to do the same job.
A fuse is a beautifully simple and ingenious component. It relies on the fact that any wire through which an electric current is flowing will get warm. If it's a small current flowing through a wire that offers little resistance to it, almost no heat will be generated. But if it's a heavy current flowing through a thin wire, the wire will become very hot, as can be seen by the thin filament wire in a light bulb getting white hot.
And if the wire is made of a material with a fairly low melting point, almost like solder, instead of tungsten as in a light bulb, the wire will simply melt, thus breaking the circuit and cutting off the current. This is what happens when a fuse "blows".
The maximum current that a fuse will allow to go through depends on the material used for the piece of fuse wire and the thickness of the wire - the thicker the wire, the stronger the current needed to bring it to its melting point. Each fuse therefore comes with a specific current rating which will be marked on the fuse - a 5 Amp fuse, for instance, will blow as soon as the current exceeds 5 ampères.
Fuses have evolved through different designs over the years. At one stage the glass fuse, consisting of a thin wire surrounded by a clear glass tube with metal end caps, was popular.
Another type consisted of a short, flat metal strip stretched lengthwise over a small, torpedo-shaped ceramic support.
Today the most commonly used type is the so-called blade fuse, consisting of a short, curved metal strip encased in plastic and ending in two spade terminals.
The fuses on your car will be grouped together in a fuse box, which will possibly also contain some relays. As electricity has taken over more and more of the functions previously performed by mechanical means, the number of electrical circuits, and hence the number of fuses needed to stand guard over them, has increased.
You are unlikely to have fuses blowing during the first few years of a vehicle's life. But on an older vehicle, the chances of chafing wires and dodgy connectors increase. The consequences of a blown fuse range from annoying to catastrophic, such as the petrol pump cutting out.
The fuse box will often contain a selection of spare fuses. Replenish the ones that have been used. When replacing a fuse, always remember these two golden rules :
l Never replace a fuse without knowing what caused it to blow.
l Never replace a fuse with one having a higher current rating.