Abstract: For a sudden bus-bar trip of 220 kV GIS in operation, multi means such as on-site inspection, document check-up, equipment disintegration, simulation calculation, analysis and discussion are taken to make it clear that the factory assembly process issues caused the failure. Due to lack of process control, the important parts in the bus-bar shell, the conductive rod and shielding cover, are not completely aligned as designed. The small metal particles generated by friction between conductive rod and shielding cover cause the ground short circuit of phase A and phase B in the bus-bar shell, and finally result in three-phase trip of the bus-bar.