Circuit breaker (English Name: circuit breaker, circuit breaker) refers to the switch device that can close, carry break the current under normal circuit conditions, can close, carry break the current under abnormal circuit conditions within a specified time. Circuit breakers are divided into high-voltage circuit breakers low-voltage circuit breakers according to their application scope. The division of high-voltage low-voltage boundaries is relatively vague. Generally speaking, those above 3KV are called high-voltage electrical appliances.

Circuit breakers can be used to distribute electric energy, start asynchronous motors infrequently, protect power lines motors. When they have serious overload, short circuit undervoltage, they can cut off the circuit automatically. Its function is equivalent to the combination of fuse type switch over / under heat relay. Moreover, it is necessary to change the parts after breaking the fault current. At present, it has been widely used.

In the generation, transmission use of electricity, power distribution is an extremely important link. The distribution system includes transformers various high low voltage electrical equipment, the low voltage circuit breaker is a kind of widely used electrical appliances.

The auxiliary switch is a part of the main switch, which is configured in high-voltage medium voltage circuit breaker, disconnector other electrical equipment as the opening, closing, signal control interlocking protection functions of the secondary control circuit, can also be used as combination switch transfer switch. The word "auxiliary" in the name of auxiliary switch is because it is an independent switch. It is a carrier of auxiliary breaking, connecting interlocking functions in the control system.

High voltage circuit breaker usually has several auxiliary connection points, whose on-off on-off are the same ( opposite) with the main contact of the circuit breaker. Therefore, in the secondary circuit (Dao control, protection, etc.), the opening closing position of the circuit breaker can be reflected through the auxiliary contact to realize automatic control (continuous tripping, BZT, mutual locking, start-stop protection, etc.).