Automation

When a circuit breaker opens under load, an arc is drawn across the opening contact elements until the dielectric strength of the environment medium surrounding the contacts is high enough to resist the flow of electricity and extinguish it. In low voltage systems (under 1000V), the arc is no big deal and we can just contain it in an insulated case, called Molded Case Circuit Breakers (MCCBs), or Insulated Case (Power) Breakers if they are over 1200 or 1600A. But above 1000V it's another story. Sometimes people mistakenly refer to MCCBs or ICBs as "Air Breakers", but that is technically incorrect.

If the medium is just the air, the contact separation has to be very large as the voltage goes up above about 1000V because air will ionize and provide a plasma path for the arc to continue. It can still be done, and we call those Air Circuit Breakers, because they just relay on large separation distances of free air in order to extinguish the arc. An alternative would be Vacuum Circuit Breakers, which enclose the contacts inside of a vacuum bottle so there is no plasma-inducing medium. This means the vacuum bottle can be much much smaller. Another alternative, especially as the voltage level continues higher, is to house the contacts in a tube containing sulfur hexafluoride (SF6), an inert non-explosive gas that tends to extinguish higher voltage arcs even better than vacuum. These are called SF6 Breakers, Gas Blast Breakers, or Puffer Breakers because they blast or puff a little of the gas across the contacts as they open. In the old days they would also submerge the contacts in a tank of silicon oil, called Oil Immersed Circuit Breakers. Oil Immersed are still used, but SF6 are becoming the new standard because of the oil being a messy thing to deal with.