SF6 gas is used as an insulating gas in interlocking plants, as an insulating and cooling medium in transformers, and as an insulating and switch medium in high and medium-voltage switches. These are all closed systems with tight seals that do not release any gasses.
High and medium/voltage switches are required in the electric mains system to e.g. enable power switch-off in case of faults for safe protection of humans and connected equipment. When power is switched off, an electric arc forms between the switching contacts. Switches containing SF6 gas are electrically insulated and can efficiently put out such electric arcs.
SF6 equipment requires only very little maintenance. The few times a gas-filled unit needs to be opened, the gas is pumped into a suitable container. Following filtration, the gas can be reused and transferred back to the unit.