Abstract: This paper analyzes an event of 500 kV line and bus trip. Through the inspection and analysis of primary and secondary equipment, it is determined that the tripping event is caused by switch fault, and the whole tripping development process is analyzed. During the further inspection after returning to the factory, it was found that the fault switch had the problems of inadequate assembly of the guide ring of the moving contact base of the closing resistance and poor assembly of the closing resistance, resulting in the fragmentation of the guide ring and the closing resistance, resulting in electric field distortion, air gap discharge and tripping. The incident exposed the serious deficiencies of the manufacturer in product quality control, and reminded us to be rigorous and meticulous in all links of equipment management to avoid such incidents again.