Abstract: A marine ground isolator for selectively completing the current path through a ground connection. The herein illustrated embodiment includes a switch circuit connected in series between two portions of the ground connection and arranged so as to complete the current path therethrough only in response to an applied control signal; and means for monitoring the potential between the two portions of the ground connection and for applying the control signal to the switch circuit only when the absolute magnitude of the dc potential exceeds a first value or when the ac potential exceeds a second value.