Electrical and mechanical properties are the key factors for long service time of composite insulator.Eleven 220 kV composite insulators that had worked on the northeast grid of China for different time were selected and tested in order to investigate their electrical and mechanical properties.Power frequency transformer

impulse voltage generator and tensile machine were utilized to test the natural pollution flashover voltage

artificial pollution flashover voltage

50% lightning flashover voltage

50% specified mechanical load and maximum mechanical load.The test results show that the electrical property of the composite insulators are satisfactory

and the pollution flashover voltage and lightning flashover voltage increase with the height of composite insulator structure but are independent on working time.The mechanical property of the composite insulators represent dispersivity

where the mechanical property of early stage composite insulator with wedge structure even descends below the nominal value.It is therefore suggested that attention should be paid to mechanical property of composite insulator

and periodical detection be performed.