Temperature classes for dry-type transformers

INSULATING MATERIALS currently are classified into classes O, A, B, C, and H (AIEE Standard Number 1). Although this classification may be adequate as a rough indication of the nature of a material, it does not appear to be adequate for establishing standard operating temperature limits either for materials, or, particularly, for a complete apparatus. The inherent difficulties in establishing these limits are 1. The organic resins, which are used as a bonding agent in composite insulation, have a wide range of thermal stability. 2. The nature of the inorganic component (whether split mica or glass fiber) in composite insulation largely determines the aging properties of insulation, used as a dielectric barrier. 3. The function of the composite organic-inorganic material in the complete apparatus influences its life, whether a dielectric barrier, or just a spacer. 4. The mechanical arrangement of insulation influences its life, confined and supported, self-supporting, or acting as a mechanical support.