NEC requirements of isolated safety ground for medium voltage mobile or portable equipment in industrial environment

The National Electrical Code specifies that an isolated safety ground, separated by at least 20 feet from the system ground, should be created for grounding of mobile or portable medium voltage equipment. This paper demonstrates that isolation of the system ground and construction of a separate safety ground may not be practical in industrial, nonmining environments. Coupling between these grounds occurs through soil resistivity even if it is possible to eliminate all metallic bonds between the two grounding systems. Worker safety is enhanced if all grounds are solidly bonded together. This is particularly true if an equipotential ground plane can be established throughout the work area in which medium voltage portable or mobile equipment operate.