Circuit breaker refers to the switch device that can turn on, carry and turn off the current under abnormal circuit conditions within a specified time. The circuit breaker can be used to distribute electric energy, start asynchronous motor infrequently, and protect power lines and motors. When they have serious overload, short circuit, undervoltage and other faults, it can cut off the circuit automatically. Its function is equivalent to the combination of fuse switch and over / under heat relay. It is not necessary to change the parts after breaking the fault current. At present, BACO circuit breaker has been widely used. The circuit breaker is divided into high-voltage circuit breaker and low-voltage circuit breaker according to its scope of use. The demarcation of high-voltage and low-voltage boundary is relatively vague. Generally speaking, the circuit breaker above 3KV is called high-voltage apparatus.