Auto Electronics Systems
The electrical system of a car is a closed circuit with an independent power source the battery. It operates on a small fraction of the power of a household circuit.
Current flows along a single cable from the battery to the component being powered, and back to the battery through the car's metal body. The body is connected to the earth terminal of the battery by a thick cable.
This type of circuit is called an earth-return system any part of it connected to the car body is said to be earthed.
Short Circuits and Fuses
If the wrong-sized wire is used, or if a wire becomes broken or disconnected, this can cause an accidental short circuit which bypasses the resistance of the component. The current in the wire may become dangerously high and melt the wire or cause a fire.