This guide provides information of special relevance to planning@ design@ testing@ installation@ operation and maintenance of gas-insulated substations (GISs) and equipment. This guide is intended to supplement IEEE Std C37.122? 1. This guide is applicable to all GISs above 52 kV. However@ the importance of topics covered varies with application category. For example@ the issues related to advanced field test techniques and very fast transients (VFT) are of particular interest for extra-high voltage (EHV) GISs. 1 Information on references can be found in Clause 2.