Abstract: High voltage cable system is an important part of the power grid in cities,and huge loss would be caused if fault occurs. The correct interpretation of the cause of the fault will greatly help the repair work of the defective cable circuit,getting rid of the safety risk and enduring the proper operation of the power cable system. In this paper,fault cases caused by insulating material failure,inadequate interface pressure,and improper installation are analyzed. By locating the main breakdown channel,testing the insulating materials,reviewing the dimensions and installation requirements of the accessories,and considering the laying of the cable circuit and fault records,the causes and the developing process of the faults are investigated. Based on this work,suggestions to help to improve safe operation of high voltage cable system and discovering weak point in advance are proposed.