The 40-/spl Omega/ ground-fault phenomenon

Coordination of protective devices on electrical distribution systems always involves the calculation of fault currents. These calculations almost always include a minimum fault current value. This minimum fault current value is not a bolted fault calculation; it includes an assumed impedance. Many utilities have assumed this impedance to be 40 /spl Omega/. This paper examines that assumption and provides some guidance on this issue. This paper is based on a study sponsored by the Cooperative Research Network, a division of the National Rural Electric Cooperative Association, based in Arlington, VA. This original study was completed in 1997.