The mechanism of clearing in metalized film capacitors

Self clearing in metallized film capacitors results in gradual failure from capacitance loss rather than sudden failure after the first breakdown. During clearing, the arc normally extinguishes with little change in the potential across the capacitor, i.e., the source potential (and impedance) for the clearing discharge is approximately constant. Usually an arc driven by constant source potential and impedance does not extinguish, and the reasons that this is not the case in self-clearing remain obscure. Applied voltage, interfacial pressure, and metallization resistance are generally considered the most important factors which influence the clearing process. We analyze the clearing process in the context of energy loss from the arc and physical processes related to gaseous products created by the arc. Increase of arc resistance and cooling of the arc as it lengthens contribute to clearing.