A short circuit fault in a new 220 kV gas insulated switchgear（GIS） happened when the GIS was starting

leading to surface burn of insulator.In this study

the possible causes of the fault are analyzed and verified.The result shows that excessive silicone grease on the seal ring results in this fault

where some grease sticks to the basin-type insulator spacer to form an grease film while mating two flanges

and the moisture in SF6 gas condenses on the film and the insulator to reduce the dielectric strength greatly.Besides

insufficient delivery test and acceptance test are also one of the causes of the fault.Some precautionary measures are proposed to insure the GIS against this kind of faults.