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A high rate of manufacturing rejects occur from time to time in production lots of instrument current transformers. This has initiated a systematic search for the causes. One cause is the aging of grain-oriented electrical steel strips. Presented are the effects of aging on magnetic ac permeability, over a wide range of inductions from 0.005 T to 1.4 T on laminated and wound cores exposed to temperatures of 150°C and 60°C for up to 10 000 hours. These cores were made from M5 grain-oriented electrical steel strips 0.30 mm thick from various manufacturers. It has been found that the steel manufacturers have generally solved the aging problem of the grain-oriented electrical steel strips, since strips of most manufacturers do not age in the as-supplied condition. However, certain quantities of the grain-oriented electrical steel strips of some steel manufacturers show a considerable tendency to aging. The instrument current transformer manufacturers, therefore, should by proper quality control limit the steel suppliers to those who manufacture a consistent product. Another cause of aging could also be an improper stress relief annealing performed by the instrument current transformer manufacturers.