Abstract: The secondary cable is the important bridge connecting the primary equipment and the secondary equipment,and the insulation reliability will affect the operation reliability of the secondary equipment. Trough field sampling and dissecting the accident cable,it is found that the causes of insulation defects of secondary cable are cable head damage, moisture intrusion,meterial and structure defects and operation inspection management defects, and the technical solution is given.

Key words: secondary cable, delay protective devices, operation reliability, insulation defect