Some fundamentals of equipment-grounding circuit design

An effective equipment-grounding system should, under conditions of maximum ground-fault current flow, accomplish the following objectives: 1. maintain a low potential difference, perhaps 50 volts maximum, between machine frames, equipment enclosures, conductor enclosures, building metallic structure, and metallic components contained therein to avoid electric shock hazard and unwanted circulating current, and 2. incorporate adequate conductance to carry this maximum ground short-circuit current without thermal distress and the attendant fire hazard. There is good reason to believe that current flow in the equipment-grounding system in a-c power systems will not stray far from the power cable over which the outgoing current flows. It follows that the installation of conductive material in an equipment ground system unless properly located can be ineffective and wasteful, and can create a false sense of security. This paper presents the results of a special series of full-scale tests dealing with this specific problem and a general analysis of the circuit behavior.