Dielectric Properties of Fibrous Insulation As Affected by Repeated Voltage Application

It is believed by many that the most plausible explanation of fibrous insulation failure is the pyro-electric theory. Under this theory as elaborated by Steinmetz, Wagner and others, insulation under stress is heated by the transformation of electric energy to thermal energy. Insulations are considered as of the nature of poor conductors and subject to the same characteristics. The transformation of electric to thermal emergy is therefore dependent upon the inherent electrical and thermal conducting properties of the material. This transformation proceeds at a rate proportional to the stress applied, until such a voltage value is reached where the heat is generated in the insulation at a rate faster than it is dissipated to the surrounding medium. Further increase in voltage leads to a rapidly mounting temperature with ultimate insulation failure. A strict interpretation of this theory would indicate that insulation failure is a matter of the insulation resistance-temperature relation. Therefore, as the heat stored in a dielectric during stress is allowed to dissipate, care being taken to prevent injury from the testing electrodes, etc., the orignial properties of the material should be restored. This has been found to be true only to a limited extent. The present paper deals largely with the effect of relatively high-voltage applications on sheet insulation tested between parallel plate electrodes. It is shown that the question of the mechanism of insulation failure can be separated into two parts.