Arcing fault current and the criteria for setting ground fault relays in solidly-grounded low voltage systems

In the 1960s numerous damages were documented on 480 V, 3-phase, 4-wire power systems due to arcing ground faults, To alleviate this problem, the National Electrical Code (NEC) now requires ground fault protection in low voltage switchgear with continuous current ratings above 1,000 A and operating at more than 150 V phase-to-ground on solidly-grounded systems. However, burn down damage and personal injury continues to occur. This paper provides a simplified technique to accurately estimate this arcing fault current, and discusses its effects on the settings of relays and interrupting devices utilized in low voltage systems. It suggests that the criteria for setting the ground fault sensing devices should be changed to accommodate the maximum allowable exposure of personnel to arc flashes that may occur in the system. This is a much stricter guideline than that currently required by the NEC.