OIL IS the major electrical insulation component of large high-voltage transformers. It must withstand, free of corona, the severe short-time electrical stresses imposed during factory test or abnormal service conditions, as well as the less severe normal-service stresses of long duration. It follows that best insulation design demands precise knowledge not only of the stresses imposed on the oil during test and operation, but also a sound understanding of the oil's ability to withstand successfully the stresses under a wide variety of adverse conditions.