Transformer heating under short-circuit conditions

THE TEMPERATURE LIMIT under short-circuit conditions has been 250 degrees centigrade for power transformers for many years. This temperature has been calculated assuming that all of the heat during the short circuit has been stored in the copper. There is good reason to believe that a temperature of 250 degrees centigrade is seldom reached under short circuit; first, because some of the heat is conducted to the turn insulation; second, full voltage is not maintained unless the kilovolt-ampere capacity of the transformer is small in comparison with the source of power; and third, the protective relay usually comes into action before the time limit is reached.