This recommended practice outlines a test procedure for comparing two or more sealed insulation systems in accordance with their expected life at rated temperature. The procedure is limited to insulation systems for ac electrical machines using form-wound preinsulated stator coils rated 6900 V and below. The intent of this test procedure is to evaluate insulation systems for use with air cooling under severe environmental conditions, where the insulation is exposed to conducting contaminants. This procedure does not cover special requirements, such as those for machines enclosed in gas atmospheres or machines subjected to strong chemicals or submersion in liquid, etc. The procedure includes instructions for testing candidate systems in comparison with known systems having a proven record of service experience and interpreting the results of these tests.