Car Electrics: Wiring, Fuses And Resistance

A circuit that keeps blowing fuses has a fault. Fitting a bigger fuse is how wiring fires start.

Aside from the electrical systems already described in starting the modern day motor vehicle, cars are now fitted with an ever increasing number of electrical components. These split further into even more electrically controlled devices, which seem to increase in complexity with every single model change. As the number of electrical systems is so large, this article is restricted to general principles only.

Using a low voltage system

One of the main advantages of using a low voltage system is that there is virtually no chance of a person being on the receiving end of an electric shock. With this in mind, all the system designer has to do is put together a collection of wires — the loom — from a power supply to the components concerned, via the correct switching and circuit protection devices. The return circuit does not need to be wired, as the vehicle body can serve as the return path to the negative terminal on the battery.

With so many wires in the vehicle, some method had to be devised to identify different circuits from each other, hence the advent of colour coding incorporated into the PVC insulation covering each wire.

Wire size is set by current

The size, or more correctly the diameter, of each wire is dictated by the current it is expected to carry in normal use. Note that current is the deciding factor, not voltage. Compare two lengths of wire, one feeding the starter motor and the other feeding the stereo. Both carry 12 V DC, but the starter feed may carry hundreds of amps and the stereo supply little more than one amp.

For the technically interested, wire sizes are denoted by the number of strands and their individual diameter contained within the insulation. So 9/0.30 means 9 strands of 0.30 mm diameter, and 37/0.90 means 37 strands of 0.90 mm diameter. A typical application for the 9/0.30 would be a sidelight circuit drawing 5.75 amps; the 37/0.90 a starter feed drawing 350 amps.

Fuses and their ratings

Already mentioned is the requirement for circuit protection. This is normally provided by a fuse, a metal alloy wire of a size that has limited current carrying capacity. Should the current flow exceed the design level, the fuse element will melt away, breaking the circuit and saving the wiring. For various current loads several fuse ratings are produced, usually colour coded for ease of selection.

It is important that the fuse rating specified by the vehicle manufacturer is not exceeded. A circuit which is continually blowing fuses has a fault, pure and simple. The risk of serious wiring damage or even fire caused by an electrical fault in a circuit fitted with a fuse of too high a rating is very real.

Supply and resistance

With the voltage fixed in the majority of cases at 12 V DC, what other factor does an electrical circuit have that can vary between two extremes? The answer is resistance. If you were to take two pieces of material of the same dimensions, one plastic and the other copper, and place them in turn into an electrical circuit, you would quickly find that the plastic component breaks the circuit and the copper one makes little or no difference to the flow of current. The plastic component has such a high resistance that it becomes an isolator; copper on the other hand has an extremely low resistance, therefore offering a clear path through the circuit.

When powering devices around the vehicle, the electrical path must be of low resistance so that there is little drop in power between supply and load, hence the widespread use of copper wiring. There are, however, situations where a certain level of high resistance is required, such as heating elements and filament bulbs. Both of these applications rely on a carefully calculated degree of resistance. With the heated rear screen, the passing of current causes a small heat effect, impossible to see unless the glass is misted or covered in ice; the filament bulb on the other hand reacts to the passing of current by glowing white hot.