Air blast breakers are mechanically similar to vacuum breakers, but rather than using the vacuum to extinguish an arc it uses, as the name suggests, a blast of compressed air is used instead. I suppose the advantage of the vacuum type is that it doesn't require any outside power source or air supply when falling into it's failsafe (open) state making it more reliable.
As I tried to demonstrate in
my post above the point of the vacuum is that there is
no[1]
air between the contacts to ionise. Any 'arc' which is drawn as the contacts open under load consists of metal vapour from the contact surfaces which condenses rapidly on nearby cold surfaces so no 'arc' is supported.
Pedant mode! After I wrote my first post I remembered that, strictly, the vacuum chamber and its contacts is called the 'vacuum interrupter'; the whole kaboodle of springs and operating gubbins is strictly the 'circuit breaker'.
At the low pressures present in the vacuum interupter the 'mean free path', that is the average distance a gas molecule or metal atom will travel before hitting another gas molecule is in the order of 10mm to 100mm at normal temperatures. This means that a particle emitted from one of the contact surfaces is very likely to hit the contact surface opposite or the walls of the vessel and be adsorbed there before it hits and ionises any of the residual air molecules in the chamber.
The advantage of the vacuum interrupter is that it is smaller than the switch needed for breaker opening in the air, which at these power levels also needs a blast of air to blow the arc out and is often accompanied by arc shutes to limit the arc's spread. It is also smaller than all the gubbins needed to surround the opening switch with sulphur hexafluoride which has also been used. The contact surfaces also last a lot longer as they are not eroded by hot arcs or exposed to the elements.
[1] The pressures in a vacuum interrupter are typically 10^-4 to 10^-5 torr (in old money), 0.00013 millibar to 0.000013 millibar in new(ish) money. 1 torr (named after Torricelli) is the pressure exerted by a column of mercury 1mm high; 1 bar is atmospheric pressure.