A faulty trip of No. 2 bus in a substation was analyzed. Humidity test and analysis of SF6 decomposition products in the gas cabinet of switch at the bus side were conducted

and the test data indicated that C-phase of the 50532 switch was abnormal. The trace of arc discharge between the internal catheter and the casing flange was observed via visual examination. Disassembly inspection was conducted in the manufacturer to discover the cause of the fault via fault tree and component analysis. It is deducted that a foreign metallic body with a size of around7 mm adhered to the catheter during the site assembly stage

and fell from the lower electric field region to the higher electric field region after 3 months due to vibration

resulting in the flashover.