Metallized film capacitors have self-healing properties that are not available in film/foil configurations. When a sufficient voltageapplied, point defect shorts between the metallized electrodes can evaporate due to the high arc temperature because thedielectric plastic material at the breakdown point and the metallized electrodes around the breakdown point are both very thin(about 0.02 to 0.O5 microns), The point defect cause of the short circuit is burned, and the resulting steam pressure also blows tharc away. This process can be completed in less than 10 us, usually without interrupting the useful operation of the affectedcapacitor, This self-healing property allows the use of single-layer windings of metallized film without any additional defectprotection,thereby reducing the amount of physical space required to achieve a given performance specification. In other wordsthe so-called volumetric efficiency of the capacitor is improved. The self-healing ability of the metallized film is used many timesduring the manufacture of metallized film capacitors. Generally, after the metallized film has been cut to a desired width, anyresulting defects can be burned (repaired) by applying a suitable voltage before winding. The same method is also used aftercontact surface metallization (schopage) to remove any defects in the capacitor caused by the second metallization processMetallized pinholes caused by self-healing arcs can very slightly reduce the capacitance of the capacitor, However, the magnitudof this reduction is small; even if there are thousands of defects to be burned out, this reduction is usually much less than 1% ofthe total capacitance of the capacitor. For large film capacitors with very high stability and long life standards, such as buffercapacitors, special fault isolation patterns can be used for metallization. In the picture on the right, this metallization forming a Tshaped pattern is shown. Each of these T-shaped patterns creates an intentionally narrowed cross-section in the conductive

metallization layer. These limits work like a miniature fuse, so if a point defect short circuit occurs between the electrodes, the hic

current of the short circuit will only burn the fuse around the fault. Thus, the affected part is disconnected and isolated in acontrolled manner without any explosion around the larger short-circuit arc. Therefore, the affected area is limited, the fault ismitigated and controlled, and the internal damage to the capacitor is signitficantly reduced. In the field installation of power

distribution equipment,the fault tolerance of the capacitor bank is usually improved by connecting multiple capacitors in paralleEach capacitor is protected by an internal or external fuse. If a single capacitor is internally short-circuited, the resulting faultcurrent (enhanced by the capacitive discharge of adijacent capacitors) will blow the fuse,thereby isolating the faulty capacitor frothe rest of the equipment. This technique is similar to the T-metallization technique described above, but operates on a muchlarger physical scale. More complex series and parallel arrangements of capacitor banks are also used to allow continuity ofservice, albeit in the event of a single capacitor failure of this larger scale.