Contacts with energized conductors are a major cause of electrocutions in underground coal mines. Sensitive ground fault interrupters (gfi's) installed on in-mine three-phase ac utilization circuits would probably prevent the majority of these deaths. A sensitive gfi is a protective device that detects and interrupts small deadly ground currents in the milliampere range before those currents can cause ventricular fibrillation in humans. Commercially available three-phase sensitive gfi's have not been specifically designed for application in coal mines. The Bureau of Mines therefore tested three commercial gfi models to determine their worthiness for mine power systems. Gfi design and construction, transient immunity, reliability, and time-current characteristics were evaluated in laboratory tests. No commercial device was found suitable for mine use without design modifications. The tests results will serve as a basis for the development of a mineworthy sensitive gfi in ongoing Bureau research.