PURPOSE: To drop the test voltage of a power-frequency AC voltage so as to remarkably reduce the capacity of testing equipment by detecting the defect of a power cable line by impressing damping oscillatory waves upon the power- frequency AC voltage in a superimposing state.

CONSTITUTION: The defect of a power cable is detected by generating damping oscillatory waves from a damping oscillatory wave generating circuit. Since the oscillatory waves have a very sharp waveform, an electrical tree can be relatively easily generated from the defective part of the cable line, when the oscillatory waves are impressed upon the power cable. In the case where the electrical tree is once generated in the power cable due to the impression of the oscillatory waves, the tree readily develops when a low power-frequency AC voltage is applied and destroys the power cable line. When the damping oscillatory waves are superimposed upon the AC voltage, however, the defect of the power cable line can be detected without stopping the application of the AC voltage even when the AC voltage is relatively low.