Last month we discussed ground fault protection of personnel, using the Ground Fault Circuit Interrupter (GFCI) to detect and interrupt extremely low levels of ground leakage current that present an electrocution hazard. In this column, we'll cover ground fault protection of equipment, intended to address another hazard of low-level ground leakage currents.
In the early years of 480/277 V system use, a number of service burn-downs were attributed to sustained arcing ground faults which were not interrupted quickly because they involved current levels below normal load current and thus did not activate fuses or circuit breakers. These current levels, while low in comparison to faults involving two or more phases, are still high enough to dissipate substantial energy at the point of the fault, resulting in melting of metal, burning of insulation, and even explosive arcing. Detecting and quickly interrupting these low levels of ground fault current is the job of equipment-level Ground Fault Protection (GFP).