SF6 gas gets converted to a very toxic gas when an arcing takes place in its presence in a Circuit breaker.
Whereas, a noble gas has properties much better than SF6 gas, viz. very low melting and boiling points (hence used as refrigerant), etc.
Then why are they not used in Circuit Breakers in lieu of SF6 gas ?
Maybe the main reason lies in that noble gases have weak electrical strength. SF6 has about 2.5 times higher strength than air, while noble gases have less strength than air. One has to use high pressure to get the same insulation level, and the mechanical strength requirement is high, making the breaker bulky and heavy.
Relative strength, (taken from NATO Advanced Study Institute on Electrical Breakdown and Discharges in Gases (1981: Les Arcs, France), p168)
SF6 1
N2 0.36
H2 0.18
Ne 0.006
Ar 0.07
I recommend you read the following article:
http://www.alstom.com/press-centre/2014/8/connecting-innovation-for-a-sustainable-world/
Electrical strength of noble gases is notable less than one of SF6. Their use as arc quenching medium in high-voltage circuit-breakers will require more forceful pressing of the gas (see Paschen law) that may be inexpedient by some reasons. It concerns also to nitrogen oxide HV circuit-breakers.
CIGRE claimed in 2009 that SF6 circuit-breakers will keep their importance for HV and EHV electrical installations at least for a few decades.
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