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This guide is primarily concerned with outdoor AC substatons, either air-insulated or gas-insulated. Distribution, transmission and power plant substations are covered by this guide. The intent of the guide is to provide guidance and information pertinent to the basic design of a substation ground grid, safe limits of potential differences that can exist in a substation under fault conditions between points that can be contacted by a human body. This guide is primarily concerned with safe grounding practices for power frequencies in the 50-60 Hz range.