Did they just forget to clear out the ground plane in that section?

Looks like. I gave it some backlighting so the problem would be obvious.

They must have done some copper hold off clearance, but only for the vias and the various pins, but not for the trace itself.

MANY bad spacings here of course, but after a few years, only the adjacent planes have eaten through. Give it a few more years, see if the progressive HV rot will continue.

Usually I'm working on e-beam driver etc. boards with 5KV supplies. In that case the designers router out some actual 1mm slots between adjacent circuit sections, to interrupt any progressive growth of conductive material. "World's slowest lightning bolt," where a sharp carbon filament is converting epoxy at its tip into more charcoal.

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William J. Beaty Research Engineer
beaty a chem washington edu UW Chem Dept, Bagley Hall RM74
billb a eskimo com Box 351700, Seattle, WA 98195-1700
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