The application of composite cross-arm tower which saves the transmission corridor will be of a great significance. The 500 k V composite cross-arm comprises of two horizontal large size composite insulators which are arranged in "V" structure and two tensile composite insulators which are arranged in inclination position. In order to obtain the pollution characteristics of 500 k V composite cross-arm

we performed the pollution tests of full-scale AC 500 k V unilateral composite cross-arm with several pollution degree under the high altitude condition. Synchronously

the partial arc current and the flashover development process of pollution test were also studied. And the pollution outdoor insulation configuration margin of 500 k V composite cross-arm was verified. The results indicate that the 50% flashover voltage of unilateral composite cross-arm presents a negative power relationship with pollution degree. The biggest partial arc current increases approximately in linearity form with the pollution degree at the 50% flashover voltage. The sporadically discharge and partial arc discharge appear at the bottom side of the large diameter insulator more easily. The 50% flashover voltage of bilateral composite cross-arm is lower than that of unilateral by approximately 3%

and the partial arc alternately appears at the bilateral composite cross-arm. The 500 k V composite cross-arm has a relatively high pollution insulation configuration.