Abstract: The good operation condition of electric power equipment is the foundation of the healthy and stable operation of the electric network, because the huge scale of the electric network is composed of thousands of equipment, the equipment runs with electricity for a long time, the need for“Grid guards” to diagnose, when equipment failure tripping, must be timely analysis and judgment by professional and technical personnel, especially when the main equipment of the grid tripping, need a number of professionals at the same time joint analysis, fault equipment for all-round no dead-angle inspection, analysis of fault causes, summarized preventive measures. Based on a transformer trip accident in a substation, the Accident Investigation Group is composed of many professionals to investigate and analyze.