From the instant of installation and operation, power cables start deteriorating. Cable systems can be maintained not only by monitoring the insulation status of the insulation layer and oversheath, but also the insulation status of the terminal and junction in high-voltage power cables. When the cable system (the cable itself and cable junctions combined) deteriorates, fire accidents happen due to dielectric breakdowns. We have invented a device to monitor the deteriorating status of cables, and installed it at Korea Western Power Co. Ltd. located in Taean, Chungcheongnam-do Province. In this paper, we present the results obtained using our device, through analysing and calculating the standard deviation of leakage current from cable insulators attached to the cables. When the standard deviation of analysed leakage current falls below a critical value, a cable system is deemed to be operating safely. But when the standard deviation of analysed leakage current is larger than the critical value, the insulation status of the terminal and junction in the cable system is considered to have seriously deteriorated. The terminal and junction in the relevant system should then be replaced preemptively in order to prevent blackout accidents of cables caused by the suspension of power supply.