IEC TR 61857-2:2025

Overview

IEC TR 61857-2:2025 is a Technical Report from the IEC that provides clear guidance for selecting the most appropriate test method to perform the thermal evaluation and classification of electrical insulation systems (EIS). The report applies to existing or proposed EIS across a wide range of electrotechnical products and operating voltages and is intended as a selection guide rather than a prescriptive test procedure.

Key practical points:

• Thermal evaluation is the starting point for assessing EIS decomposition at the molecular level.
• The document recognizes four ageing stress factors: thermal (T), electrical (E), ambient/environmental (A) and mechanical (M).
• Selection of standards should match the construction and intended operating application of the EIS.

Key Topics

• Purpose and scope: Guidance on choosing the most relevant IEC standard for an application - whether single-factor thermal tests, conditioned thermal tests for air, or tests for applications other than in air.
• Stress factors: Introduces and reinforces the concept that thermal stress is one of four principal factors (T, E, A, M) influencing EIS performance, and highlights when multifactor evaluation may be required.
• Organized standards: The report organizes established EIS-related standards into five practical tables (Tables 1–5) to simplify selection:
  • Table 1 - Overview and structure of the EIS evaluation series
  • Table 2 - Screening and pre-ageing tests to evaluate materials or processes before long-term thermal ageing
  • Table 3 - Test standards for thermal stresses in air (expanded list)
  • Table 4 - Standards for special environmental or non‑air applications (e.g., liquid-immersed)
  • Table 5 - Standards for modification of an established EIS

Applications

This Technical Report is useful for:

• Engineers and test planners choosing which IEC test standard to apply when qualifying a new or modified EIS.
• Manufacturers of electrotechnical products (transformers, rotating machines, and other equipment) needing to map application conditions to the right test method.
• Technical committees and design teams determining whether to apply single-factor thermal tests or multifactor accelerated ageing based on IEC 60505 guidance.

Practical benefits:

• Faster, more accurate identification of the correct standard for the intended service environment (air, non‑air, immersed).
• Clearer decision-making on whether to use screening tests before long-term thermal ageing.
• Alignment with relevant TC work (IEC TC 112 general EIS standards and IEC TC 2 rotating machinery standards).

Related Standards

• IEC 60505 - Base standard describing the four ageing stress categories and general EIS evaluation principles.
• IEC 61857 series - Core series for thermal evaluation and classification of EIS.
• IEC 61858 series - Procedures for modification of established EIS (does not assign thermal class ratings).
• IEC 60034-18 series - Rotating machinery-specific guidance and tests for insulation systems.

For performance or safety requirements tied to a specific application, refer to the technical committee requirements for that application and the individual test standards listed in Tables 1–5 of IEC TR 61857-2:2025.