Theory of Equalization of Thermal Ageing Processes of Electrical Insulating Materials in Thermal Endurance Tests II: The Theory with Practical Approximations and Application Principles

An exact formulation is given of the theory of equalized ageing processes (EAP). It is shown in a very general form that the true thermal ageing process of a material proceeding at service conditions can be accurately reproduced only if all the relevant chemical and physical ageing reactions can be equally accelerated. For practical thermal endurance testing, suitable approximations are presented. Various ways in which behavior may deviate from the EAP principles are described and discussed. The practical EAP test method is divided in two parts: Firstly, reaction rate measurements where rates of all important ageing reactions are measured as a function of temperature and appropriate gas concentrations. Secondly, physicai ageing tests where deterioration of important properties like elongation and flexural strength are measured as a function of time, choosing gas concentrations in ageing test chambers such that all the important ageing reactions are identically accelerated. Results of such tests can be readily converted to describe the ageing behavior of the material at a lower temperature.