Circuit breaker is an important electrical control on and off in the electrical line, which refers to the switch device that can close, load and break the current under normal loop conditions, and can close, load and break the current under abnormal loop conditions in the specified time. Circuit breakers are divided into high voltage circuit breakers and low voltage circuit breakers according to their range of use. Low voltage circuit breakers are common to molded case circuit breakers. Play a protective role in the safety of electrical lines and electrical appliances. When a fire occurs, the action of the circuit breaker plays a vital role in the occurrence and development of the fire. Therefore, the circuit breaker is an important material evidence in the investigation of the cause of electrical fire, and it will play an irreplaceable role in the investigation of the cause. In particular, the plastic case circuit breaker is the most commonly used type in life. The plastic case circuit breaker extracted from the fire scene, according to the appearance characteristics or internal structure characteristics, identify its working state in a period of time before and after the fire, and can judge the relationship between the physical evidence and the cause of fire.