Hal,

I was pretty sure of the right answer, but not wanting to put my foot in my mouth (me?... more like change feet) I posed the question to our Engineering manager.  Here is his response.

Phil,

1kV/3mm is equivalent to 8.5kV/inch.  That is a safe number for tracking on a relatively clean and smooth surface.  The breakdown electric field stress of air at 1atm is 30kV/cm for a uniform electric field.  Uniform electric field means that you have two infinite parallel plates.  My guess is this is not the case.  Without knowing more about the geometries of the electrodes in question then the 1kV/3mm is a good place to start for an air gap.  There is a caveat to this answer.  If the electrodes are both spherical and of reasonable diameter, >10mm, then 1kV/3mm is likely to be acceptable.  However, if the electrodes are both needles with very small radii of curvature then 1kV/3mm will most definitely not work.  

An analogy would be if you took the palm of your hand and you tried to penetrate your skin with a marble.  Pretty difficult to without a huge amount of force behind the marble.  Now try the same experiment with sharp pin. It penetrates without much effort at all.  The reason is that the total force applied to the marble is distributed over a significantly larger area for the marble when compared to the pin. 

I know this is a bit lengthy but the question while simply stated has a very complicated answer.  If the answer was a simple as the question this high voltage stuff would be a breeze.     

Respectfully,

Stephen J. Quigley
Engineering Manager
Kaiser Systems Inc.

-----Original Message-----
From: Hal Winslow [mailto:[log in to unmask]]
Sent: Wednesday, June 04, 2003 3:29 PM
To: [log in to unmask]
Subject: [TN] High voltage clearance (air gap)


Hi,
I've tried a search of the archives, but can't seem to dig up any answers.
And I don't have a copy of IPC-222X handy to look up the answer...

So with those disclaimers out of the way, can someone tell me the air space
I need between high voltage nodes (like the leads of large T/hole caps) to
withstand 1kV?  I'm familiar with the 3mm/kV rule of thumb for surface
creepage, but not sure what to design for through an air gap.

Thanks,
Hal Winslow

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