Sensitive DC Ground Fault Relays for Coal Mines

Recent US Bureau of Mines sponsored research aimed at virtually eliminating electrocutions on dc utilization circuits has resulted in an effective dc circuit protection device, called a sensitive ground fault relay (GFR). This relay can detect and act to interrupt small deadly ground currents that can trigger ventricular fibrillation in humans while ignoring spurious power system signals. Initially relay performance criteria were established to ensure reliability for mine duty as well as effectiveness for shock prevention. Mineworthy prototypes, consisting of a saturable-transformer current sensor and an electronic relay, were constructed. They complied with the performance standards in laboratory tests.