PREFACE For many years@ temperature limits for electric apparatus have been founded upon classification of insulating materials by broad general definitions based upon chemical composition1. Recently@ it has become apparent that this method no longer meets the needs of the industry and that a new approach is needed@ based upon functional tests.w?6'6 The latest revision of AIEE No. I1 recognizes this need by the following statement: The electrical insulation of equipment is made up of many different components@ selected to withstand the widely different electric@ mechanical and thermal stresses occurring in different parts of the structure. To assure satisfactory service life@ therefore@ insulation specifications need to be supported by service experience or life tests@ on the complete insulation system. Accelerated life tests are increasingly important as a means of evaluating the many new synthetic insulating materials that are available@ thus shortening the period of service experience required before they can be used with confidence. It is desirable that life tests on materials be supplemented by tests on complete insulation systems@ representative of the different types of equipment. It is contemplated that@ in the course of time@ Institute Test Codes will be developed@ giving procedures for such accelerated life tests@ and other methods of evaluating insulating materials and systems. Accordingly@ this test procedure is intended to establish methods for evaluating the insulating systems of ventilated dry-type power and distribution transformers. It is expected that test procedures covering other classes of transformers will be prepared from time to time as the lengthy but necessary study and trials are completed. During the preparation of this test procedure@ members of the subgroup built and tested more than fifty transformer models in accordance with the recommended methods. The results demonstrated that the methods are workable@ even though there were a few details on which further work will be necessary before final decisions can be made. The test procedure@ therefore@ is offered for trial use in order to obtain wider distribution and to encourage discussion@ criticism@ and trial by others.