An Air Circuit Breaker has open contacts in open air, SF6 circuit breaker uses sulfur hexafluoride to surrounded the contacts to quench the arc.

Both breakers operate on the same principle, but the SF6 breaker is filled with compressed SF_6 (sulfur hexafluoride), whereas an air circuit breaker is filled with air. The advantage of the SF6 is that it has about 3 times the breakdown strength of air, which is very significant since the whole idea of these types of breakers is to minimize the effects of arcing.