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Use of the functional testing approach to predict the thermal capability of insulation systems by comparison with service- proved systems under identical simulated operating conditions more severe than normal is reported. As an example, the development of an effective functional test model and test procedure for the evaluation of armature coil insulation systems for large dc machines is described. Direct current form-wound motorette investigations are reviewed, with the evolutionary procedural changes and test results over the past ten years. Particular attention is given the criteria used to determine the point at which the thermal aging process impairs the dielectric properties of the coil insulation. Information is provided for appraising test codes for the evaluation and classification of insulation systems for dc machines being considered by IEEE Technical Committees.