allanhurst:
I've used RIFA or WIMA paper X2 caps in series with mains eg to get a high dc voltage in a cockroft-walton multiplier.
Well, as I said, I am pretty sure that X caps are fine for that, regulationwise. Y is mandatory for line-to-ground and that sort of stuff. The question it, what OTHER property except for the failure behavior that differentiates between X and Y determines if a cap is suitable for in-series use.