Critical examination of the life models for electrical insulation under combined stresses in the light of the boundary conditions

The combined life models for insulating materials and the boundary conditions that must be satisfied for their mathematical correctness are examined. In the case of materials with threshold, these considerations lead to the introduction of a lower limit gradient of electrical aging, Eo, the existence of which is widely acceptable from a physical point of view. In considering models for materials with threshold, the existence of a thermal threshold, which allows a new general definition of such materials to be forward, is pointed out. Models for materials with threshold in which the limit gradient Eo has been introduced are examined and their mathematical correctness verified. The existence of a threshold curve, and thus of infinite electrothermal threshold couples, is emphasized.