Working from a data sheet reference design a 47nF/X 250V capacitor is recommended.

What makes a capacitor an X capacitor? From some googling i've found that X capacitors are meant to go across mains. From some vague references they are metalized film capacitors because they are self healing.

Would any metalized film capacitor be an X capacitor as long as the voltage rating is met or exceeded, or is there some other specific rating that officially makes it an X capacitor?

Would a data sheet specifically say "X" capacitor or is there some other subtle way that it would be listed?


The datasheet has to say "X capacitor" in pretty large letters on the first page. You shouldn't be inferring the X-rating from electrical specs. (Same for Y-rating, while we're at it.)

Some additional background on X- and Y-capacitors can be found here (presentation slides as PDF) and here.


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