This guide briefly describes aging mechanisms of insulation systems and methods for ascertaining correlation of aging during testing and aging during actual service. Diagnostic techniques for use in functional tests are also listed. When the performance of an insulation system is evaluated by accelerated functional tests@ misleading results may be obtained if the aging mechanisms under the conditions of the test differ from those experienced in actual service applications. The risk of such errors increases as the aging stresses are intensified in relation to their normal levels in service. The equivalence of the aging mechanisms in the test and in service must be verified before an effective functional evaluation test can be performed. The importance of such verification procedures increases with the degree of stress intensification. considerations regarding the methodology of verification are given in Section 3. Diagnostic methods@ which are especially sensitive in ascertaining changes in the test specimen's condition@ may have the advantage that tests at relatively less intensified stresses can be included to determine the changes of the system's characteristics or aging rate. Diagnostic methods are reviewed in Section 4. It must be recognized that functional test procedures may require the selection of an arbitrary end-point criterion that does not correspond to equipment failure. This end-point@ however@ provides a relative measure of life expectancy. Purpose The purpose of this guide is to present background information necessary for proper construction of aging mechanisms and selection of diagnostic procedures when designing tests for functional evaluation of insulation systems for electrical equipment. This guide is primarily intended to aid committees to standardize tests within their scope of responsibilities.