This guide provides information of special relevance to the planning@ design@ testing@ installation@ operation@ and maintenance of gas-insulated substations (GIS) and equipment. This guide is intended to supplement IEEE Std C37.122-1993 .1 In general@ this guide is applicable to all ac GIS from 72.5?C800 kV However@ the importance of the topics covered varies with circumstances. For example@ issues related to advanced field test techniques and very fast transients (VFT) are of particular interest for extra-high-voltage (EHV) GIS (345 kV and above)@ and are of lesser importance at lower voltage levels.