pnBAS IEC TR 61857-2:2026

IEC TR 61857-2:2025 gives guidelines to identify the appropriate test method for the evaluation of a proposed electrical insulation system (EIS). Some of the standards evaluate the EIS using one stress factor which is thermal. Other standards evaluate the thermal stress factor with adjustments to the conditioning applied prior to the diagnostic testing to simulate applications in air. A third group of these standards when the application is other than in air. This technical report is applicable to existing or proposed EIS used in electrotechnical products across a wide range of operating voltages of IEC Standards. The report takes care to identify the appropriate standard based on construction and intended operating application. For all performance and safety requirements related to an application, refer to the technical committee requirements for the application. This edition includes the following significant technical changes with respect to the previous edition a) The Introduction is expanded to convey that both TC 112 and TC 2 have established EIS test standards. b) Clause 4 introduces the fact that there are four stress factors, not only thermal. This essential concept carries over into part of Clause 5. c) The large number of established standards are organized into common concept groups in Tables 1 to 5: 1) Table 1 covers an overview of EIS evaluations for background and structure. 2) Table 2 covers evaluation of test objects prior to the start of long-term thermal ageing using screening tests. 3) Table 3 covers test standards for the evaluation of thermal stresses in air and has an increased number of standards. 4) Table 4 covers standards for special environmental applications other than in air. 5) Table 5 covers standards for the modification of an established EIS.