Sulphur hexafluoride is a colourless, odourless and chemically very stable gas whose dielectric strength is about three times that of air at the same pressure. That property allows circuit breakers and switchgear to be built far more compactly than their air-insulated equivalents, and the gas also quenches the arc that forms when the contacts part.
SF6 degrades with every operation and with the moisture that seeps in through seals and bushings. The decomposition products (SO2, HF, H2S) are corrosive and toxic, and moisture lowers the withstand voltage of the insulation. The condition of the gas is therefore tracked through three measurements: purity, moisture expressed as dew point, and SO2 content.
It is also a very potent greenhouse gas, with a global warming potential in the order of 23,000 times that of CO2, which means it must be recovered rather than vented during any intervention. Hence field equipment combines gas analysis with recovery and treatment.