Abstract: During the process of fire investigation of large-span steel structure factory, the investigator meets some mistakes when utilizing monitoring video to analyze and judge the mode of fire, in accordance with the position of operating handle of circuit breaker to analyse whether it powers up, using multimeter on and off gear to measure whether internal circuit breaker is turning on, and melted mark of aluminium cable whether electric cable is charged. This essay clarifies the process caused by corresponding error and analyses the reason of error from the angle of investigation and practice and put forward the countermeasure and suggestion of avoiding similar errors in the fire investigation.

Key words: fire investigation, electrical fire, surveillance video