I feel that those curves support the hypothesis that the variation is
due to non-uniformity in the material. I first suggested voids (because
I recall a spectacular failure of a line output transformer design in
the late 1950s whose HV winding was encapsulated in polythene. Air in
the voids ionized and the ions gobbled up the polythene. Attempt to
eliminate the voids in viscous molten polythene under vacuum were partly
successful, but did not survive the moulding process. Voids are only one
possibility; simple variations in density may be sufficient to
concentrate the electric field just where it will do the most damage.
Can some tests be done on a solid material that has been certified to be
highly uniform? What happens with liquids, which should be orders of
magnitude more uniform than the average solid?
On 2024-06-25 20:35, Richard Nute wrote:
Thanks to Adam for all the references. They address very thin solid
insulations. But they confirm that dielectric strength is not a
constant for very small distances, and they do not have an answer as
to why.
My concern is verifying clearances in safety standards. I’ve attached
curves of three standards clearance requirements (logarithmic scale
for volts per millimeter). The solid curves represent the clearances
in standards and are close to power curves (dotted lines). The
equations are for a best-fit power curve.
The solid green curve is from an old standard and depicts actual
withstand measurements.
I suspect the electric strength curves are related to the reason for
Paschen’s finding that gases do not break down at low voltages.
My objective is to predict clearance dimensions without tables.
Best regards,
Rich
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Keep trying
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