The object of this test procedure is to establish a uniform method for investigating the effect of operating temperature on the life expectancy of oil-immersed distribution transformers. It is intended to give a direct evaluation of the composite insulation system of an oil-immersed distribution transformer, tested as an integral unit. It is to be noted that the oil in which the transformer is immersed is to be considered a part of the insulation system. It is the intent of the procedure to provide that each component of the insulation structure operate during the test under conditions that are, as nearly as possible, the same as those that it would encounter in service. Thus, it is intended that each component be evaluated in accordance with its actual function.