Gas-Insulated Switchgear (GIS) — Medium Voltage

A type of switchgear (IEC 62271-203) in which the circuit breakers, disconnectors, earthing switches and busbar connections are housed inside a hermetically sealed metal enclosure filled with SF6 gas (or an SF6-free alternative such as dry air or a fluoronitrile mixture), as opposed to air-insulated switchgear (AIS) where the surrounding air itself provides the insulation. Because the insulating gas has a much higher dielectric strength than air, the equipment can be built with greatly reduced phase clearances, cutting the required footprint to a fraction of a comparable AIS installation — a significant advantage in inner-city substations or underground spaces where space is scarce. Drawbacks include higher acquisition cost, the need for gas-tightness checks and monitoring throughout the service life, and, for SF6-filled equipment, the gas's strong greenhouse effect if it leaks, which is driving the industry towards SF6-free alternatives.