In VCB or vacuum circuit breakers vacuum is used for arc quenching. The contacts of the breaker are kept in vacuum inside an airtight chamber. There is no air to sustain the arc. During the opening of the contacts the metal is vaporized which helps to sustain the arc for the first cycle but when the current reaches zero the vapour turns back into solid and the arc cannot sustain.
On the other hand in SF6 circuit breakers SF6 gas is used as the insulating medium. SF6 gas has very high dielectric strength, high electro negativity (which helps to absorb the electrons from the arc), high thermal stability. The SF6 gas is made to run around the contacts axially when the arc is formed and hence quenching the arc rapidly.
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