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believe the answer you're looking for has to do with the pressure-altitude-voltage derating related to corona discharge, which increases (gets worse) with higher altitude and peaks at around 80-100,000' Typically this is considered to occur at votlages above 300 V peak, but be sure you are accounting for all transients and internal voltages.
The effects of volcanic ash from Puyehue-Caulle Range eruption on the ... by Fernandez-Arhex, V.; Buteler, M.; Amadio, M.E.; Enriquez, A.; Pietrantuono, A.L.; Stadlee, T.; Becke / Florida Entomologist. Aircraft frequently encounter. when airborne.