Diffusion Couples as Electrical Insulation Aging Monitors

Most aging phenomena are temperature dependent as indicated by the Arrhenius equation. Knowledge of probable aging rates is necessary for practical engineering design of equipment, and aging rates of many electrical insulation systems have been studied in detail. Service life expectancies can therefore be confidently estimated for particular insulation systems in specific thermal environments. However, it is not possible to assure that all electrical equipments will be operated within the design environments and ratings. Thus, variations in service life performance of electrical equipments often reflect variations in operating conditions. It has not been economically feasible to measure actual conditions of use and exposure of most electrical equipments. An inexpensive monitor of thermal exposures has been recently developed. The resistance changes of bimetallic elements are in accord with the Arrhenius model for normal diffusion. Selection of metals that interdiffuse with temperature sensitivity equivalent to that of an insulation system of known thermal aging behavior allows use of the bimetallic diffusion couple as an indirect monitor of the actual insulation thermal degradation. A preferred form of the bimetal is a composite wire, comprising alternate concentric layers of two pure metals. The diffusion couple must be placed in a location which exhibits a representative, if not identical, thermal exposure of the insulation system to be monitored. The monitors may be "built-in" and read in-situ or they may be inserted as replaceable probes which can be retrieved and returned to a central test facility for evaluation.