The U.S. Bureau of Mines designed and constructed sensitive and coordination-free ground-fault relays (gfr's) for use on mine power systems. First, a list of gfr attributes for mine ac utilization applications was compiled. These practical guidelines specified design, construction, transient immunity, reliability, and operating criteria. The time-current characteristics of the ac and DC units, subsequently fabricated, were designed to be below the human electrocution threshold. The significant and highly variable capacitance of high-voltage distribution cables was found to preclude the sensing of ground currents in the milliampere range. However, the coordination-free system developed for high-voltage distribution should enhance safety by significantly decreasing response time to ground faults. Implementation of the sensitive gfr technology in the mining industry has the potential to eliminate the majority of injuries and nearly all the deaths resulting from contact with energized components.