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Calculating minimum air gap, part II

Calculating minimum air gap, part II

Calculating minimum air gap, part II

(OP)
Sorry for the dual post - this wasn't getting any responses in the other group so I'm reposting here...

I'm designing a circuit to meet the CE mark test of 8KV discharge.  I read through thread797-134233: calculating minimum air gap: calculating minimum air gap and saw a discrepancy that I'm looking to answer.  

The dielectric breakdown of air is 3KV/mm, or about 76KV/inch.  

I believe I have a copy of the 'Sparkgap breakdown voltages for needle gap' referenced in that thread (I'm not sure because I received it from a colleague in the mid-90s).  This table shows a breakdown voltage of about 26KV/in at 25C, 29.9 In Hg baromteric pressure.  

That's almost a 3:1 discrepancy between the two techniques.  While the chart is more real-world, that seems a rather large safety factor.  Are there some other facts that explain the difference between the two techniques?  For example, the chart shows that at -40C and 60 In Hg you need to apply a correction factor of 2.30 so that almost accounts for it; those aren't typical operating conditions though.  

What I really want to know is:  How far do I need to keep my traces from the aluminum enclosure (that is perpendicular to the PCB) to not have the an ESD event jump onto my PCB traces?  

RE: Calculating minimum air gap, part II

Two things.  Needle points arc at lower voltages than blunt things.

Is the CE arc discharge at the AC power cord or a spark to the operator panel?

RE: Calculating minimum air gap, part II

Paschen's law has a lot to say about this. You cannot use a single number to get the breakdown voltage for a certain distance. See http://adsabs.harvard.edu/abs/1955PhRv...97..255B

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Calculating minimum air gap, part II

(OP)
Thanks all!  That helps.  I have planes, not points.  

sreid -

CE mark requires ESD testing per EN 61000-4-2:1995.  The following tests are required:  

Surface    Test voltage    Contact method
Conductive surfaces    +/-4 KV    Direct
Non-conductive surfaces    +/-8 KV    Air discharge
Reference ground planes    +/-4 KV    indirect discharge


Z

RE: Calculating minimum air gap, part II

Was cleaning house and ran across a NASA Tech Brief titled, "Approximations for Predicting Electrostatic Discharges," dated April, 1988, from vol. 12, no. 4,  Item #122 or JPL Invention Report NPO-17065/6582.  A veritable truckload of equations.  Haven't read it through, but it looks interesting.

TTFN

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