Degree of correlation between tan δ and electric strength of insulation

Both tan δ and electric strength of high voltage insulation are used as measures of its quality. Tan δ measurements, being non-destructive, can be used to follow the quality of an insulation through the processing into the machine and through service. They are also used to evaluate the life of insulation under thermal aging or voltage endurance by measurements on the same samples throughout the tests. In contrast, measurements of the electric strength, being destructive, require new samples for each test. However, an electric strength measurement is functional; it indicates how much stress the insulation can withstand above the operating and test stresses. Tan δ values are, at best, only an indirect measure of anticipated insulation performance and then only in the belief that some correlation exists between tan δ and the electric strength. It would be highly desirable if such a correlation could be substantiated.