Suntan Self healing of a metallized electrode in a film capacitor
For all capacitor technologies that do not have the ability to recover after a partial breakdown, the current flow is continuous.
However metallized film capacitors have the property to recover after an instantaneous breakdown (partial breakdown) due to the fact that the metallized electrodes (capacitor plates) act as a fuse. For fusing a small current is needed, but is not continuous. This effect is defined as “self healing” and not as breakdown.
Therefore in testing these capacitors on a proof voltage, it is always possible that the capacitors have the self healing effect, taking temporarily a peak current, but are completely isolated again after this phenomenon is stopped. To take this in account for qualifying capacitors, in all IEC standards of metallized film capacitors a breakdown is defined only when it is “permanent”.
Therefore a note is added for explanation.
Requirement: There shall be no permanent breakdown or flashover during the test.
Note: The occurrence of self-healing breakdowns during the application of the test voltages is allowed.
To detect if the behavior is a self healing or permanent breakdown, it is recommended to measure the remaining charging current only after one minute of charging and to neglect the small current peaks (arcing) before that time. Modern “high pot” test equipments have the function to allow arcing currents before the final permanent breakdown current.
