Gary, all,
The following is from Doug McKean.
By the way, at room temperature (20 degrees C, 293 degrees Kelvin)
and 1 atmosphere at sea level (760 mm mercury,) this formula
simplifies to:
V(kV)= 24.2h +6.1(h)^0.5
where
V is the breakdown voltage in KV
h is electrode spacing in cm.
Re-reading Gary's email, he mentioned this, too:
"At STP, S = 1 so 30KV/cm is pretty good"
Scroll down for the complete formula.
Also, is anyone else surprised that humidity does not
play a factor in this empirical formula?
Regards,
Dan
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Date: Thu, 04 Mar 1999 12:34:46 -0800
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From: Douglas McKean <dmck...@corp.auspex.com>
Subject: Re: UL 1950 Requirements for Operational Insulation
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At 09:30 AM 3/3/99 -0800, Allen Tudor wrote:
>Clause 5.4.4 of UL 1950, Third Edition specifies that in lieu of creepage
and clearance distance requirements for operational insulation, the
electric strength test for operational insulation (Table 18) can be used.
>
>Does anyone know how to calculate minimum distances between traces on a
printed circuit board that would allow one to pass the dielectric test at a
given voltage? Or has anyone taken any empirical data that would shed
light on this topic? Also, would trace width have an impact on the
dielectric test results?
This is highly empirical and best done with a
hi-pot tester and some bare boards.
You're talking about Paschen's Law.
Paschen's Law: Breakdown Voltage (BV) for uniform gaps
V(kV)= 24.2Sh +6.1(Sh)^0.5
where
V is the breakdown voltage in KV
S=(293p)/760T
h is electrode spacing in cm.
p is pressure mm of mercury
T is temperature in degrees Kelvin
At STP, S = 1 so 30KV/cm is pretty good
This relationship does not work for all values of pressure.
This is all greatly dependent upon the geometry of the
two test points of concern. I believe Paschen came up with
this empirical equation with point probes. I like to remember
that 3M volts for 1 meter at STP (roughly). That still works
with the above equation.
But remember that the field about two *point charges* varies
inversely with the square of the distance. The field about
two *line charges* varies inversely only with distance. So be
careful fudging distances with traces.
If we have 3MV @ 1m, then 3KV @ 1 mm which is roughly 40 mils.
Double it for a x2 safety factor to 2 mm or 80 mils. For
reinforced insulation, it's a x2 yet again so that now you
have a 4 mm or 160 mil separation. Pick the safety factor
that you want. I'm just suggesting this. I seem to remember
something about wire mfrs build in a safety factor of x7 into
the insulation, but I'm not sure about that.
Now, take a look at UL-1950, Table 3, "Minimum Clearances for
insulation i primary circuits, and between primary and secondary
circuits ..."
In the column for >150V, <= 300V with a transient of 2.5KV,
you get a rough idea (agreed very rough idea) of how these
numbers work out ...
" Vrms = 300V, Op = 1.7, B/S = 2.0, R = 4.0 "
Some of those clearance numbers look familiar when compared to
working it out long hand in my paragraph above?
You can bet they do.
The trace width does not have an impact on dielectric testing if
you're talking about two traces horizontally adjacent on the same
layer of the board. Traces vertically adjacent to each other will
be a different story since the old standby FR-4 with a Dk = 4.7
will increase the BV by a factor of roughly 4.7.
But, again, do your own empirical evaluations on your own boards.
It will prove to be invaluable information. Hi-pot testing is
one of the most common areas of safety testing failure.
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> From: Gary McInturff <gmcintu...@telect.com>
> To: "'emc-p...@ieee.org'" <emc-p...@ieee.org>
> Subject: Paschen's law.
> Date: Tue, 5 Oct 1999 15:40:30 -0700
> MIME-Version: 1.0
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>
>
> A small blurb on this came across the list-server awhile ago, unfortunately,
> I moved companies and apparently didn't get this out of my electronic filing
> cabinet. Did anyone else save this tidbit? And would they forward it to me
> again.
> It was the basic formula and a brief discussion of the effects.
> Anyone having trouble with the expanding foam packaging getting into Europe.
> There's a web site somewhere for banned substances but it is electronically
> right next to my Pachen's law folder I'm thinking.
> Thanks
> Gary
>
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>
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