Hi Hal
High voltage arcing conducts between two sharp points in humid conditions at
sea level at about the rate of .250 inches/10,000 volts.  That can decrease
to as low as .180 per 10,000 volts in more humid conditions or at low
atmospheric pressures.
typicaly I like to design the board in FR4 at your 3mm/1000 volts, And the
air gap at the same fiqure if I can get it for safty and less corona
production. A smaller gap is allowed in air but why chance it. I will e-mail
you privetly re my high voltage information available on the net.
robert Tarzwell   megadawn.com

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