For exploring the cause of a fault in 500 kV HGIS

the energy dispersive X-ray detector（EDX） and TGA were used to analyze the physical and chemical properties of the basin-type insulator in faulty gas chamber

and a water pressure strength test was conducted to test the change of mechanical strength of the basin-type insulator.Thus the causes of the HGIS equipment fault are found to include two aspects:1)In the manufacturing process of the basin-type insulator

the theoretical formulation of the insulating materials（epoxy resin

curing agent

alumina）is not strictly followed

or tiny impurities exist in insulating materials

which affects the overall quality of the basin-type insulators; 2)the mechanical aging resistance of the basin-type insulator is insufficient

which leads to micro-cracks or other defects under continuous mechanical stress and finally causes the fault of the basintype insulator under further effect of high strength electric field. Moreover

some quality control measures are offered for newly made HGIS equipment

and some fault prevention approaches are proposed for the running HGIS equipment from same manufacturer and same batch.