Abstract: As the main power carriers in distribution grids, the safe and reliable operation of underground power cables has become the key to the reliability of the entire system. Most existing researches have focused on the tripping of faulty cables to avoid cascading failures, which may still cause regional power outages and act in a "passive" protection manner. In this paper, a high-accuracy condition sensing method for power cables in distribution grids was proposed. Common-mode (CM) leakage current was selected as the cable insulation indicator, and the CM leakage current sensing method was proposed based on magnetic shielding theory. With the sensing techniques, CM-signal-injection-based monitoring methods and cable segmental ageing location methods were developed, realizing online and quantitative monitoring of cable ageing state and accurate location of various segmental ageing patterns. Sensor prototype tests and simulations were conducted to validate the proposed scheme. By applying the proposed method in distribution grid, targeted maintenance can be achieved and the maintenance pattern can be changed from "passive" to "proactive", thus reducing insulation faults and improving system reliability.