The standard test procedure for the thermal evaluation and qualification of electrical insulation systems (EISs) for random-wound ac electric machinery, where thermal degradation is the dominating aging factor, is described. The relative thermal performance of a candidate EIS is compared to that of a reference EIS. Insulation systems for such machinery with input voltage of up to 600 V at 50/60 Hz are described in this standard. A statistical method for establishing a relative life-temperature relationship for an insulation system is also described. To have any significance, the reference insulation system must be supported with adequate field service data. Evaluation of insulation systems for use in air-cooled, random-wound ac electric machinery with “usual service conditions” is this procedure’s intent. This procedure, on its own, does not cover insulation systems such as exposure to conducting contaminates, radiation, inverter applications, or operation in oils, refrigerants, or other media that potentially degrades insulating materials.