High-Resistance Ground Fault Scheme in Distance Relays

By definition, distance (impedance) relays detect faults by the ratio of V/I (good old Ohm's Law in action). Normal, non-fault conditions are, also by definition, high-impedance. A fault that due to its impedance (distance from the sensing location, arcing, etc.) does not have high enough current and enough reduction in voltage to be distinguished from normal conditions will not be sensed as a fault. That's the main principle that allows step-distance relays to work for transmission line zone protection.