Re: Why is pressurized air a better electrical insulator than air at

Excellent question.  I'm not 100% sure on the answer, and none of my books 
or friends know the exact answer either.

I *think* I know what's happening, though.  You have to distinguish between 
low level conduction (picoamps) thru a mass of air and HV breakdown across 
two electrodes.  If you're talking about the picoamp situation (which I 
doubt) that's bad because I have no idea about what's going on.  HOWEVER, 
to prevent HV arcs or corona discharge around an electrode at HV, the 
reason that high pressure air (or most gases, especially SF6 which is used 
commercially) works as a better insulator than lp air is that the HP air 
moves in very quickly (shorter mean free path between molecules) to quench 
any arc that might be developing.  Contributing to this would be the fact 
that the hp air has almost certainly had most of its moisture removed 
during the compression process;  no moisture, no conductive material in 
which to strike an arc.

I hope this helps!