I am sorry for the short reply and advertizment i will try and correct my
errors. It was late and I was taking the easy way out!

When dealing with high voltage there are many problems to solve other then
UL requirements, First you must solve the arc over and corona situation. At
1.5 kv there will be some corona at sea level and with  good board layout
and constuction the corona will not be a problem. however if you lower the
presure by puting the board in a airplane or space craft the corona will be
very much a problem. corona starts about 1 kv and becomes  a very corosive
plasma mix at 2 to 4 kvolts, at 20 to 30 kv its just a sizziling.  The arc
over is a normal formula based on voltage (ac is diffrent then DC) pressure,
and the dialectric medium its in. 10,000 volts will arc over with sharp
points at .25 inches at sea level in dry air.

The sharpness of the to upposing points greatly effects the ability to not
only arc over but generate corona. Sharp points concentrates the electrons
and allows them to leave creating streamers of inonized air. the streamers
are corona until they join and then they are a arc over.
UL specifies  for normal AC voltages (110-220).250 spacing  for high voltage
it has been my experiance although they have a formal distance, but rather
you have to demonstrate sufficent spacing and insulative protection to
prevent arcs and safty to personel working on the circuit. You should
contact UL early in the board design Phase and work with them to make sure
there are no surprizes
I hope this helps if you need more information please e-mail me
Robert Tarzwell  ( sorry steve)

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