The Basics Of The Car Electrical System

Modern vehicles are now fitted with an ever increasing number of electrical components. These can further split into even more electrically controlled devices which seem to increase in complexity with every single model change

Belowe we explore the basics of the electrical system.

The 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. Please note that this low risk factor does NOT apply to parts of the ignition system nor parts of the latest gas discharge type headlamps.

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.

Electrical Current Dictated By Size Of Wire

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 and not voltage. Compare two lengths of wire, one feeding the starter motor and the other feeding the CD player, both carry 12vDC but the starter feed may carry hundreds of amps and the CD 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, e.g.

  1. 9/0.30  means 9 strands of 0.30mm diameter
  2. 37/0.90 means 37 strands of 0.90mm diameter

Typical application for the 9/0.30 would be a sidelight circuit drawing 5.75amps, the 37/0.90 a starter feed drawing 350amps.

Protection Dictated By Fuse and Its Rating

Already mentioned is the requirement for circuit protection, this is normally provided by a fuse, this fuse is 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 thus saving the circuit.

For various current loads several fuse ratings are produced usually being colour coded for ease of selection.

It is important to note 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 higher rating is very real.

Voltage Supply and Resistance

With the voltage fixed, in the majority of cases, at 12vDC and a brief explanation of the applications for high and low current 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 will break the circuit and the copper one would make 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, e.g. 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 will cause 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.