The 40-ohm 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 ohms. 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, Virginia. This original study was completed in 1997.