In message <[email protected]>,
dated Tue, 13 May 2014, "ce-test, qualified testing bv - Gert Gremmen"
<[email protected]> writes:
I wanted to emphasize that chemical reactions do not suddenly happen on
the arrival of a electrical field, but need time to process in a
similar way a battery is not fully charged on the connection of a
voltage on its terminals.
Speed of reaction varies over about 30 orders of magnitude. If you put
two electrodes in a salt solution and apply a voltage, the evolution of
gases at the electrodes is almost instantaneous. That is not an
unreasonable model of salt pollution of a creepage distance, although
salt itself does no produce permanent degradation of most insulators.
Apparently the committee writing IEC 60664 found the application of RMS
more suitable then using an average (=voltage integrated over time).
I doubt that it was based on a deep insight into electrochemistry.
Rich wrote that the process of deterioration is of a dissipative nature
and power related. Of course, more volt, more power, so that relation
is clear.
Ah, well, now we see that there can be two effects. The initial
conductive path is established by electrolytic action in the pollutant.
This may be sufficient in itself to create a minor hazard (e.g. the
resistance changes from 100 Mohms to 0.5 Mohms), and the effect is
greater for greater *peak* voltages. The current is quite low, so
self-heating does not have a large effect, but the chemical reactions do
generally (not always) go faster at higher ambient temperatures.
A *second stage* can occur if the resulting current creates enough heat
to degrade the surface of the insulator itself. This is usually rapidly
progressive, because the degradation products are almost always
carbon-rich and cause the current to increase without limit, i.e. the
insulation collapses catastrophically. Because this is heat-related, it
is reasonable to consider RMS current, but the voltage is hardly
relevant and in any case the effect is transient.
I wonder if that relation (deterioration = dissipation) is as strict as
that (I mean dissipative) given the number of processes that seem to
happen at the boundary of (polluted) air and insulator. But as the
majority seem to be electrochemical I am not surprised that the effects
are better measured using RMS (or average) value than a peak value.
Maybe the explanation above will help.
--
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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