Failures on power cable system lead to severe damage to power grid and tremendous repairing cost. Therefore, various techniques for monitoring and diagnosing cable system have been actively utilized. Most of diagnostic techniques are off-line systems, since they are limited to cables which are disconnected from power sources. In this paper, a method for online diagnosis of power cables that are connected to power grid is proposed using a state of art cable diagnostic technique, time-frequency domain reflectometry (TFDR) and inductive couplers. The performance of the proposed diagnostic technique for live cables is verified with two different live conditions: DC and AC. It is expected that the proposed diagnostic system will become an effective solution for ensuring reliability and safety of power grid in the future.