Hi Richard:
> What is a good source that explains the rational for the values for creepage
> distances?
There was a good deal of research done in the late
'70s and early '80s on creepage distances and the
deterioration due to deposition of foreign materials
and the effects of humidity.
As I recall, the results of this research was
published as working papers of the IEC TC that did
the research (TC 64?). I suppose you could ask the
TC for copies of this research.
In a nutshell, the voltage across the contaminants
causes micro-arcing. The heat from these microarcs
causes tracking on the surface of the insulator.
Eventually, the micro-arcs develop into trees, and
the leakage current becomes significant.
The phenomenon is a long-term one, so the effective
voltage across the insulator is the significant
parameter.
Best regards,
Rich
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