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Figure 12.2 - Air insulated circuit breakers

It’s free and the cost is certainly a big advantage of air.
However, air has the worst dielectric strength of all the insulators
in this listing, so distances will need to be the greatest for a given
geometry.
A second consideration is that surfaces need remain clean and
free of dust over long periods, so that there is no arcing along a
surface.

2. Sulphur Hexafluoride (SF6)

This is a great insulator in that parts remain clean, and
assemblies are easy to work on once the housing is opened and the
gas is replaced by air. One difficulty with using SF6 is that the
housing must be hermetically sealed. Two safety issues exist.
Poisonous gasses are formed whenever there are arcs.
Also, it is extremely hazardous if someone breathes in any SF6.
It is denser than air, and will settle in the bottom of one’s lungs,
and suffocation can result. Since it is a colourless, odourless gas, it
goes undetected. Extreme care must be taken to avoid inhalation.

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