The Contribution of Differential Thermal Analysis to the Estimation of Thermal Endurance of Insulation

Abstract

The equations governing the thermal degradation of electrical insulation and those governing the shape of a thermogram have been combined to give that of the required life line. The numerical solution has been shown to require an empirical correlation between the physical properties involved. In the case of thermogravimetric analysis (TGA), weight loss can be correlated with electric strength (the failure criterion) in an independent experiment. However, in differential thermal analysis (DTA) (at least with normal DTA technique) the heat evolved cannot be correlated with electric strength. By means of a numerical integration of a DTA thermogram, together with the determination of the cell cooling constant, an activation energy can nevertheless be obtained and the thermal rating estimated. DTA constitutes an ideal supporting technique, since if the activation energy agrees with that given by TGA this is good confirmation that the value is appropriate to life-line slope. A worked example of the method (using a formvar enamel) is described in detail.