Failure of Metallized Film Capacitors

Question

I have the following : Metallized film capacitor. I read in some papers that MFC are protected against short-circuit with a self healing.
Self-healing is a phenomenon where in the event the electrodes are exposed to each other instead of the capacitor shorting, the capacitor repairs itself. This repairing of the capacitor is due to the thinness of the foils used.
In a film /foil capacitor when the foils are exposed to each other, the foils would touch and short together rendering the capacitor useless.
When a capacitor with metalized films has the foils exposed to each other, they also will touch each other, but here the combination of the foils thinness and the high energy density at the fault area causes the foils to vaporize and the capacitor stays in operation.
My question: In case of failure one foil is sucked to other foils the energy density is dissipated on large fault area which leads to not vaporize of the foils in contact and failure of the self-healing process. So short circuit of the MFC is possible, someone can confirm this through an experience with failure of MFC ?

Answer 1

Self healing caps are typically used for direct connection to mains supplies. Mains can often have 1500V or more spikes appearing on the conductors briefly, due to switching events or lightning strikes in the supply network.

In an MFC that is designed for self healing (not all are), the metal film is patterned into a mesh of squares with fine connections between them. These fine connections behave as weak links, or fuses. If an overvoltage punctures the insulator between two squares, the fuses to those squares vapourise. (How they chose manufacturing parameters so that the metal vapour doesn't condense to form a bridge between the films, I don't know, but I guess that's why they make capacitors, and I don't).

Now with the links gone, the punctured region is isolated from the rest of the capacitor. The capacitance will have dropped slightly, but with the electrodes made from hundreds of squares, it's a tiny fraction of its total capacitance. This can be repeated, and a capacitor exposed to many overvoltage incidents can lose a significant part of its capacitance evnetually.