The article considers an electrical device designed for diagnostic work in distribution networks of 6-10 kV, namely, the determination of damage zones in case of failure of insulating materials of cable lines during high-voltage diagnostics with a rectified voltage of negative polarity. The novelty of the developed device is its ability to connect directly to a high-voltage cable line without additional matching devices. The device is capable of detecting damage zones in lines up to 10 km remotely with an error of up to 0.2% of the cable length. The prefix allows you to fix any variants of damage to single-phase and three-phase cable lines with high transient resistance at the site of damage without the operation of “burning” insulation, which significantly reduces the time of repair work at the controlled facility.