The purpose of this recommended practice is to provide a general form for the preparation of test procedures and to suggest the points to be considered by technical committees in the preparation of specific instructions for the thermal evaluation of insulation systems for equipment. The thermal evaluation of an insulation system involves the thermal (T) factor of influence as described in IEC 505 (1975), Guide for the Evaluation and Identification of Insulation Systems of Electrical Equipment. These test procedures involve accelerated thermal aging of insulation systems and specify tests which the committees deem pertinent, based on conditions of use. The objective of these test procedures is to provide for the functional evaluation, by test, of insulation systems for electric equipment. The evaluation of insulating materials and simple combinations thereof is provided for by a separate material test guide, IEEE Std 98-1972, Guide for the Preparation of Test Procedures for the Thermal Evaluation and Establishment of Temperature Indexes of Solid Electrical Insulating Materials. For a detailed discussion of the principles on which IEEE Std 98-1972 and IEEE Std 99-1980 are based, see IEEE Std 1969, General Principles for Temperature Limits in the Rating of Electrical Equipment. Tests based on IEEE Std 98-1972 may be considered as preliminary tests for insulation system evaluation, but should not be used as accepted tests for the classification of systems.