This guide is primarily concerned with outdoor ac substations@ either conventional or gas-insulated. Distribution@ transmission@ and generating plant substations are included. With proper caution@ the methods described herein are also applicable to indoor portions of such substations@ or to substations that are wholly indoors.1No attempt is made to cover the grounding problems peculiar to dc substations. A quantitative analysis of the effects of lightning surges is also beyond the scope of this guide.