The loop in the automobile circuit refers to the part of the circuit that returns to the negative terminal of the battery after passing through the electrical appliance. There are three concepts in the DC loop in automobile circuits: main power supply loop, structural loop and internal loop. For example, in a light bulb circuit, the current starts from the positive terminal of the battery, enters the light bulb through the control of the switch, and returns to the negative terminal of the battery after passing through the light bulb. Then, the line connecting the light bulb to the negative terminal of the battery is the circuit of the light bulb.

Three concepts: the connection between the negative ground wire of the battery and the metal part of the car body; the connection between the wiring harness ground wire of each part and the metal part of the car body; the connection between the internal circuit of each controller and the shell or terminal