DGUV Information 213-013e - SF6-Switchgear and Equipment

3.2.1
Sulphur hexafluoride (SF₆) is a colourless and odourless gas. It has a density of ₆.07 g/l at 20 °C and 1013 hPa. SF₆ is approximately five times denser (heavier) than air and tends to sink and collect at lower levels. Once SF₆ mixes with the ambient air, it can no longer be segregated out. SF₆ liquefies when compressed, making it possible to store and transport it in pressurized gas containers as a liquefied gas.

Pure SF₆ is a chemically stable, inactive (inert) gas, is nearly insoluble in water and is non-flammable.

Due to its high dielectric strength and outstanding arc-extinguishing properties, SF₆ is ideally suited as an insulating and extinguishing agent in electrical switchgear and equipment.

3.2.2
SF₆ tends to decompose when subjected to temperatures associated with electrical discharge or arcing exceeding 500 °C. Subsequent cooling largely effects a recombination process, yet it is also possible for secondary reactions to occur with ambient air and steam as well as with structural elements, such as vaporizing molten material from the switching contacts. This can produce gaseous sulphur fluoride oxides and carbon fluorides, such as thionyl fluoride (SOF₂) and tetrafluoromethane (CF₄), solid metal fluorides, sulphides and oxides as well as harmful hydrogen fluoride (HF) and sulphur dioxide (SO₂) if moisture is present.

Such chemical reactions can, for example, take place inside SF₆ circuit breaker switching chambers under normal switching operations or fault shutdowns, or inside SF₆ gas compartments in conjunction with electric arcing due to an internal fault.

Gaseous SF₆decomposition products have an unpleasant, pungent odour similar to rotten eggs. Solid SF₆decomposition products form dust deposits (e.g. metal salts) in SF₆gas compartments, such as so-called switching dust or caking.