Abstract: In a 500 kV substation, the outermost copper foil of soft connection of the horizontal knuckle-arm disconnector that was put into operation for only 1 year was broken and failed. Through macroscopic observation, chemical composition analysis, mechanical performance test and fracture analysis, the reasons for copper foil fracture were analyzed. The results showed that fracturing occurred at transition position of copper foil and copper plate, copper foil had high hardness, high tensile strength, low elongation and poor toughness. Soft connection needed to be repeatedly bent during disconnector work, which eventually led to copper foil fracture, and the soft connection was failed.