Electrical systems and circuits

Electrical systems and circuits

What is a system?

The system is a word used to describe a collection of related components, which interact as a whole. A motorway system, the education system or computer systems, are three varied examples.

Using the systems approach helps to split extremely complex technical entities into more manageable parts. It is important to note, however, that the links between the smaller parts and the boundaries around them are also very important. System boundaries will also overlap in many cases.

The modern motor vehicle is a very complex system and, in itself, forms just a small part of a larger transport system. It is the ability for the motor vehicle to be split into systems on many levels that aids both in its design and construction.

Vehicle systems

Splitting the vehicle into systems is not an easy task because it can be done in many different ways. From the viewpoint of this book, a split between mechanical systems and electrical systems would seem a good start. This division though can cause as many problems as it solves. For example, in which half do we put anti-lock brakes, mechanical or electrical? The answer is of course both! However, even with this problem it still makes it easier to be able to consider just one area of the vehicle and not have to try to comprehend the whole. Most of the chapters in this book are major sub-systems of the vehicle and, indeed, the sub-headings are further divisions.

Electrical wiring, terminals and switching


Cables used for motor vehicle applications are almost always copper strands insulated with PVC. Copper, besides its very low resistivity of about 1.7 108m, has ideal properties such as ductility and malleability. This makes it the natural choice for most electrical conductors PVC as the insulation is again ideal, as it not only has very high resistance, the order of 1015m, but is also very resistant to petrol, oil, water and other contaminants.

Harness design

The vehicle wiring harness has developed over the years from a loom containing just a few wires to the looms used at present on top range vehicles containing well over 1000 separate wires. Modern vehicles tend to have wiring harnesses constructed in a number of ways. The most popular is still for the bundle of cables to be spirally wrapped in non-adhesive PVC tape.

Printed circuits

The printed circuit is used almost universally on the rear of the instrument pack and other similar places. This allows these components to be supplied as complete units and also reduces the amount and complexity of the wiring in what are usually cramped areas. The printed circuits are constructed using a thin copper layer that is bonded to a plastic sheet – on both sides in some cases. The required circuit is then printed onto the copper using a material similar to wax. The unwanted copper can then be etched away with an acid wash. A further layer of thin plastic sheet can insulate the copper strips if required.

Fuses and circuit breakers

Some form of circuit protection is required to protect the electrical wiring of a vehicle and also to protect the electrical and electronic components. It is now common practice to protect almost all electrical circuits with a fuse. The simple definition of a fuse is that it is a deliberate weak link in the circuit. If an overload of current occurs then the fuse will melt and disconnect the circuit before any serious damage is caused. Automobile fuses are available in three types, glass cartridge, and ceramic and blade type. The blade type is the most popular choice owing to its simple construction and reliability against premature failure due to vibration.


Bluetooth in the automobile is here and it is here to stay. The possibilities are endless and if used correctly will be advantageous for manufacturers and consumers alike. The convergence of different technologies seems inevitable and the Bluetooth-enabled vehicle clearly encourages this. Communications and voice activation systems are already in use; diagnostic systems are coming soon.

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