Coupling of Mine Grounds to Surface Grounds

The mining industry in the United States is unique in utilizing a separate mine ``safety ground'' for the working area of the mine which is kept isolated from other ground systems used to ground lightning arrestors, static lines, or equipment frames in surface substations. It is kept isolated from the other grounds to prevent potentials which appear on these grounds as the result of lightning activity or faults outside the mine from energizing the frames of mining equipment, thereby posing a shock hazard to workers using that equipment. Although this separation of grounds is desirable and is required by MSHA regulations for many types of mining operations, the goal of preventing the coupling of potentials from one ground to the other can be difficult or almost impossible to achieve in some rather typical mine environments. Several ways in which coupling has been observed to occur are documented, and the extent of such coupling is estimated. The difficulties inherent in reducing this coupling are reviewed, along with some specific new hazards which may arise in so doing. Finally, estimates are presented which indicate that the absolute potentials of large structures are not as hazardous as similar potentials on much smaller structures.