The purpose of this test procedure is to classify insulation systems for direct-current machines in accordance with their limiting temperatures as determined by test rather than by chemical composition, in accordance with IEEE Std 1-1969, General Principles for Temperature Limits in the Rating of Electric Equipment. The intention is to classify according to the recognized A, B, F, and H categories by determining thermal capability in accordance with machine temperature-rise standards. However, no standard hot-spot allowances have yet been established for direct-current machines, at least for integral-horsepower motors. These would, of course, vary with the ventilation, enclosure, etc. Insulation systems may be shown by experience to have suitable thermal endurance when operated in a machine at the limiting temperature rise specified in the standard for the machine under consideration or by accepted test wherein the thermal, electric, and other stresses occurring in the operation of a machine are simulated and the deleterious effects accelerated in a manner affording correlation with service conditions. Other insulation systems may then be compared to service-proven systems under similar test conditions. If a comparison is made on a system of a given insulation class, the new system must have equal or longer thermal life under the same test conditions to be considered in that temperature class. Data from such tests may be used to establish the temperature classification of new insulation systems before they are service proven. The concepts implemented herein are based on IEEE Std 1-1969. A wide and increasing variety of synthetic electrical insulating materials is available for application in electric machinery and apparatus. Included in these materials are wire enamels, films, papers, felts, elastomers, resins, and varnishes. As there is a growing tendency either to rely solely on these materials as electrical insulation or to employ them with the old familiar materials in novel combinations, there is a corresponding increase in the problems associated with the selection and evaluation of insulations. An insulation system or combination of insulation materials must be evaluated rather than one insulating material. Many of the specifications regulating the use of insulation materials were written before the advent of the newer synthetics and were based upon experience gained with the old materials over a long period of time. Difficulties arise, therefore, when an effort is made to classify these new insulation materials or combinations under the various standards as Class A, Class B, Class F, or Class H insulations. This test procedure has been prepared to indicate accepted tests for the evaluation and classification of insulation systems for direct-current electric machines. It is expected that the various insulating materials or components making up the insulation system to be tested will first have been screened in accordance with specific material test procedures, such as ASTM D2307 for magnet wire, to establish their probable success. This procedure is intended to evaluate insulation systems for use in usual service conditions. It does not cover special requirements such as for machines in gas atmospheres being subjected to strong chemicals, to metal dusts, or to submersion in liquids.