Abstract: Focusing on operation and maintenance issues of the fault indicator in ring main units (RMU), such as periodic battery replacement and a single power supply mode, this paper develops a novel external power conversion device for fault indicators in 10 kV RMU. The device integrates reverse input polarity protection, output short-circuit protection, output overcurrent protection, and output overvoltage protection. It converts the operating power supply of the RMU into the working power supply for the fault indicator, effectively eliminating the need for periodic battery replacement. In addition, the device is equipped with a dual-power switching module and adopts a main and backup power supply configuration. In the event of a power outage on the RMU power supply side, the fault indicator is powered by the DC battery pack of the RMU through the power conversion device. The proposed device greatly reduces the workload of operation and maintenance personnel and effectively improves power supply reliability.