Capacitors Self Healing

There are many different types of capacitors made of all different kinds of materials and configurations...There is a type called a vacuum capacitor(pictured below), that can be self-healing if conditions of over-arc current are not too extreme...Vacuum capacitors use highly evacuated glass or ceramic chamber with concentric cylindrical electrodes....They are extremely low loss and used for high voltage high power RF applications, such as transmitters and induction heating....

http://en.wikipedia.org/wiki/Vacuum_variable_capacitor

http://en.wikipedia.org/wiki/Types_of_capacitor

A self-healing metal film capacitor uses a thin, evaporated metal layer for one or both electrodes with polypropylene or polyamide (Mylar) as the dielectric film layer. If/when the dielectric fails, a small hole punctures the dielectric between the metalized layers, and the resulting arc vaporizes the metal near the defect, electrically isolating it. Many manufacturers also insert narrowed regions in the metallization that act as fuses to isolate larger regions of the capacitor in case the initial fault is not cleared properly. In practice, a self-healing capacitor may safely suffer hundreds of small failures without suffering a catastrophic failure. The fault is cleared by energy supplied from the rest of the internal capacitor elements with little, if any, additional external current being required. This technology has revolutionized the design and size of modern energy discharge and power factor correction capacitors. Each self-healing fault slightly reduces the total capacitance, and a 5% reduction in total capacitance is typically considered the point where the capacitor is approaching its end of life.

However, if your capacitor has caused an external fuse to blow, it apparently suffered from a catastrophic overvolting event that may have overwhelmed the self-healing mechanism, creating a permanent short circuit. If this is the case, your capacitor must be replaced.

My understanding of self-healing capacitors is related to the lower voltage 480V types.

They are a wound capacitor with a mylar or similiar plastic insulation between the plates. To keep the capacitors compact and low cost, the insulation is very thin and will deteriorate over time. The capacitor is designed that when a section of the plates short together they blow this area of the capacitor to "open" so that the capacitor, over time, still works but slowly loses capacitance. This makes for an interesting maintenance check--we have found capacitors happily functioning but only drawing 30% of their nameplate amperage.

If you have a self-healing capacitor that is blowing fuses that are correctly sized for the capacitor it is toast-replace it.

Jon.