Ongoing problems with X2 mains capacitors in triac switch application

Cheaper X2 and Y2 capactiors are a bit deficient when it comes to the methods used to attach a conductive layer to the ends of the wound (for X2) and plate (for Y2) capacitors. High quality X2 makers use a zinc sputtering method in a nitrogen and partial vacuum environment to give a good layer of metal that really adheres to the cap winding film/foil and which serves as a good base for bonding the leadout wires.

It is the failure of the metalisation spray to the film/foil (usually a few microns of Al sputtered onto a polypropylene film) that causes the issues mentioned in this thread, pulse currents (often not that high for cheap X2 caps) cause failure at the nearest film/foil to the leadout wire and this continues until the entire X2 cap becomes open circuit, similar mechanisms cause failure of the Y2 disc type caps (the cheap ones) although this is usually rarer).

This is why good makers of these caps specify max dv/dt for their caps, sometimes esr and esl, and how many volts at various frequencies a designer can apply for a given temp rise.

Cheaply made caps simply do not have good contact to the film/foil and reduce C over time under modest operating conditions.

Hope this answers your questions, Regards, Orson Cart