Establishes the basis for estimating the ageing of electrical insulation systems (EIS) under conditions of either electrical, thermal, mechanical, environmental stresses or combinations of these (multifactor stresses). It specifies the principles and procedures that should be followed, during the development of EIS functional test and evaluation procedures, to establish the estimated service life for a specific EIS. The main changes with respect to the previous edition concern the amalgamation of the following standards, which, with the exception of IEC 60727-1, will be withdrawn when this third edition is published: IEC 60791:1984, Performance evaluation of insulation systems based on experience and functional tests IEC 60792-1:1985, The multi-factor functional testing of electrical insulation systems - Part 1: Test procedures IEC 60941:1988, Mechanical endurance functional tests for electrical insulation systems IEC 61356:1995, Functional evaluation of electrical systems - Principles for test procedures when comparative testing is not feasible IEC 61359:1995, Evaluation and identification of electric insulation systems - Environment evaluation IEC 60727-1: 1982, Evaluation of electrical endurance of electrical insulation systems - Part 1: General considerations and evaluation procedures based on normal distributions