Apparatus protected against ground faults for monitoring the ground connection between a grounded power source and a mobile machine provided with multi-phase power through a multi-conductor trailing cable has a star arrangement of impedances connected to the phase conductors at the power source and a like arrangement connected at the machine, the junctions of which are connected to the ground conductor at each end, through a silicon-controlled rectifier device which is paralleled by a monitoring impedance. At least one impedance of one of the star arrangement of impedances is adjusted to provide a low monitoring voltage which circulates a current through the ground conductor and monitoring impedance. The voltage across the latter actuates monitoring means. The silicon-controlled rectifier is adjusted to conduct when the voltage across it rises above the monitoring level. The monitoring current may be alternating current, or direct current obtained from a rectifier in circuit with the ground conductor.