Abstract: This paper introduces the fault tripping process of No.2 bus bar when the disconnector of a 500 kV GIS substation in Inner Mongolia power grid is opened from hot standby to cold standby. Based on the field fault oscillogram, gas detection and disassembly inspection of the fault equipment, qualitative and quantitative calculations are carried out with the help of component analysis and very fast transient overvoltage(VFTO). It is concluded that the fault is caused by the design defects of the switch contacts. There is a risk that the arc cannot be extinguished reliably when operating the disconnector. In addition, during the transient process, the maximum amplitude of VFTO reaches 2.183(p.u.), the arc cannot be extinguished and is prone to drift outward. The transient earth voltage(TEV) transmitted between the shell and the ground through traveling wave coupling rises to 0.47(p.u.). Under the action of VFTO, there are metal foreign matters near the disconnector, which cause electric field distortion, increase of oscillation frequency and electric field nonuniformity, and finally lead to the occurrence of breakdown fault. Finally, some suggestions and measures are proposed to improve the isolation contact, which has a positive guidance and reference for improving the operation reliability of GIS.