New methods for detecting DC current and ground fault current in a three-phase power line system are described. Two detectors both utilize the skin effect of conducting current in a ferromagnetic pipe. The detectors have no windings. The DC current detector outputs almost sinusoidal voltage whose amplitude corresponds well to the DC current. The detector has a simplified circuit configuration and short response time. There are no contacts between the DC line and the sense circuit. The ground fault detector breaks down the three-phase power line within three drive-frequency cycles, This paper summarizes those results.