For worldwide use in electromagnetic interference (EMI) suppression in across-the-line applications requiring X2 safety classification. Intended for use in situations where capacitor failure would not result in exposure to electric shock. Not for use in "series with mains" type application
manufacturer is covering his ass
Well that cannot be the only thing to it. Why would a manufacturer drive away customers by ruling out certain uses so that no reputable device manufacturer would use it?
I don't think X caps were ever supposed to be used in a series configuration, that's the whole idea, if they short, you'd have a hazard. An X cap is only for cases where a short circuit failure would not result in a safety hazard. If my interpretation is correct, the manufacturer is just being extra careful to warn people not to use them in places where they need Y caps.
They are self-healing when connected across the mains.But not with a small current through them as in a capacitive supply.
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.
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.