*Polyimide for Electronic and Electrical Engineering Applications*

relatively short, and the oven temperature is well below the class temperature (that can be 180°C, 200°C, 220°C, or 240°C). This implies that the dielectric will not undergo a significant amount of oxidation or other forms of chemical degradation. Besides, the impact of hydrolysis will not be accounted for. As a result, it will not be easy to extrapolate the results to practical applications, as the combined electricalthermal-ambient stress will differ substantially from the tests experienced during the tests.

In order to show how these stresses can alter the endurance of PI insulation to PD activity, we shall discuss in the forthcoming sections experiments carried out on CR Kapton films subjected to thermal and ambient stress and tested at regular times for PD endurance. The results will highlight the most likely phenomenon leading to a significant reduction of the endurance properties.
