Grounding medium-voltage mobile or portable equipment

Examples of medium-voltage mobile equipment in pulp and paper mills include rail-mounted portal cranes for log handling, chip and bark stackers, and chip-reclaim screws. No examples of portable medium-voltage equipment can be cited. National Electrical Code (NEC) section 250-188 describes the requirements for system and equipment grounding of portable or mobile equipment rated 1 kV and higher. One of the requirements of NEC section 250-188 is: the grounding electrode to which portable or mobile equipment is connected must be isolated from, and separated in the ground by at least 20 ft. from any other system or equipment grounding electrode. There must be no direct connection between the electrodes such as a buried pipe, fence, etc. This article demonstrates that even if two separate ground systems are installed in the vicinity of each other, with a spacing of 20 ft and no direct metallic bonds between these two systems, a high portion of the system ground potential rate (GPR) will be transferred to the safety ground. A spacing of 20 ft is grossly inadequate to isolate the two grid systems from most faults encountered in utility substations serving industrial distribution systems. We also demonstrate that in an industrial environment, achieving isolation between system and safety grounds may not be practical due to the physical size of the property, existing installations, or geological formations. In addition, it may not be possible to isolate medium-voltage mobile equipment frames from the plant's ground grid.