Implications of the anisotropic thermal conductivity of capacitor windings

Metallized film capacitors have the advantage of being selfclearing, i.e., a breakdown of the film evaporates/oxidizes the film metallization surrounding the breakdown site so that after the clearing event, the capacitor continues to operate normally with only a miniscule reduction in capacitance [1], [2]. As a result, such capacitors are often said to fail gracefully, i.e., they fail as a result of gradual reduction in capacitance rather than catastrophic breakdown. This is not the only possible mode of failure. If a metallized film capacitor is misapplied under conditions that cause too frequent clearing, the combination of gas and heat generated in the winding by clearing events can precipitate melting of the dielectric, catastrophic failure, and, possibly, fire.