Not sure how far you're going back but... Mains filters from the 50's and 60's likely had paper/wax capacitors in them. At least in the US, there doesn't seem to have been a special rating system for these but most cap failures were benign (not purely luck... even though there was no special rating they were generally made to be on the robust end.) "Modern" ones (70's onwards) have specially rated X1, X2, Y2 capacitors. In small filters these are likely to be ceramics and in larger ones (especially snubber networks) they are more likely to be boxed film capacitors. They all have special fire protection and surge voltage ratings. They do not necessarily lead "Easy" lives - if some nearby inductive appliance is putting crap into the power line, it's the small devices on the same circuit or nearby branch that may end up eating it. Tim.