The utility model relates to a sensor used for direct current grounding line selection and AC access line selection. The sensor comprises at least one magnetic core or iron core for inducting a direct current or an alternating current. Induction signals of the magnetic core or the iron core are transmitted to an external interface circuit through a current signal adjusting unit and a microprocessing unit. The sensor can detect fault branches timely and provides basis of fault checking for the maintainers. The sensor provides strong guarantee for the users to fast solve and process the fault and for the safe and reliable operation of the dc source system.