This International Standard 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. This standard is for use by all IEC technical committees responsible for equipment having an EIS.