Air Gap - Vertical Bus for Switchgear

Starting the ball rolling:

• Given that the insulation breakdown voltage in dry air at ambient temperatures and pressures is about 30kV/cm, the absolute minimum it could ever be for uninsulated conductors is 5mm, and will in practice be considerably more.

It depends upon the air gap and electrode geometry (rod or sphere) and the application altitude above sea level. Standard 1/2" rod and sphere gap spacing for alternating and impulse voltages, as well as the appropriate correction factors, are included in IEEE Std 4 (ANSI C68.1).

When I worked in medium voltage switchgear, we actually used a combination of air gap and epoxy coating to insulate the phases from each other and from ground. Of course the thickness of the epoxy affected the amount of air gap required, and visa versa.