In message <[email protected]>,
dated Tue, 13 May 2014, "ce-test, qualified testing bv - Gert Gremmen"
<[email protected]> writes:
The theory assumes that most damage is done by small local discharges
caused by a discontinuous conductive path (imagine condensation drops
or small wet particles)
called scintillations.
This is not really an assumption; it's one of at least two mechanisms
that are known to occur. Another is the electrolytic decomposition of
conducting salts (no necessarily NaCl) on the surface. This can liberate
gases in an activated form that attacks the insulating material.
These are micro sparks (visible with the right equipment) that create
local temperatures well above 1000 degrees,
Because they are sparks, the peak voltage is relevant, at least
initially.
and contribute to a local burning disintegration of the (carbon
containing) base material in areas of about 0.2 mm. Once enough areas
have been affected a full continuous (carbon) leakage path is created
on the surface that may lead to further damage. This is a more or less
statistical process.
The size and frequency of these scintillations and affected areas seem
dependent of the voltage and total damage is therefore a time
integration of voltage over the creepage path.
Not really. The carbon track resistance progressively reduces as more
degradation occurs. This is probably a mixture of thermal and
electrochemical degradation, thermal dominating when/if the current
becomes high enough. The time-integral of the current is relevant but
because the resistance decreases with time, that is not proportional to
the integrated voltage.
In my opinion this leads to the conclusion that an average voltage
value should be choosen, Instead of rms, to determine creepage path,
but I may overlook certain aspects. Correct me where I am wrong.
I think there is avoidable risk if one ignores peak voltage.
Stimper states that leakage currents due to fully continuous conductive
paths
can be neglected,
That's a VERY bold statement. There is a third mechanism that can catch
one out if unlucky. This is electromigration of metallic film across
insulation, and it can occur at quite low voltages. Silver is
particularly prone to electromigration, as I found long ago when trying
to explain why some reverse-biased Ge 'power' diodes failed
short-circuit. Attacking the glass insulator with metal polish restored
diode operation!
and only in the last phase of drying, sufficient discontinuities exist
for scintillation. Alternating humidity conditions such as day/night
rhythms, in relatively cold (need for condensation droplets) designs
are most liable to creepage problems
Those effects occur but they are not all of the story.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
Nondum ex silvis sumus
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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