Functional evaluation of transformer insulation

IN THE PAST few years a strong trend has developed toward the evaluation of electrical insulation by functional testing. It seems generally agreed that (a) the present AIEE, A, B, and H temperature classifications are no longer adequate, and (b) functional tests rather than chemical structure alone should provide the basis for classification. For example, several recent AIEE papers have proposed various accelerated life test procedures for deciding whether a given material or system will have a satisfactory life expectancy at a continuous hot-spot temperature of 105, 130, or 180 C and, therefore, can be designated properly as Class A, B, or H.