IEC TR 61857-2:2015

Overview

IEC TR 61857-2:2015 is an International Electrotechnical Commission (IEC) technical report that provides comprehensive guidelines for selecting the appropriate test methods for the evaluation and classification of electrical insulation systems (EIS). This standard is relevant for both existing and proposed EIS used in electrotechnical products across various voltage ranges. Its primary objective is to ensure that EIS used in equipment such as motors, transformers, and generators are properly evaluated to meet safety, reliability, and operational requirements under specified operating conditions.

Selecting optimal test procedures is crucial for the effective thermal performance evaluation, long-term reliability, and suitability of electrical insulation systems. This standard assists manufacturers and designers in choosing the most appropriate evaluation strategy based on EIS construction and intended application environments.

Key Topics

• Purpose of Evaluation
  • Identifying whether thermal decomposition or multiple stress factors (thermal, electrical, environmental, mechanical) need to be assessed.
  • Distinguishing between single-factor (thermal only) and multi-factor test requirements.
• Test Object Types
  • Defining test objects: completed products (such as motors or transformers) and laboratory models.
  • Guidelines on test objects for various voltage classes (low-voltage up to 600 VAC, and medium/high voltage above 600 VAC).
• Test Methods Selection
  • Explains how to align EIS testing with product design, construction, and expected operating conditions.
  • Provides reference to various IEC test standards for different scenarios, including new and established EIS, as well as modifications.
• Stresses Considered in EIS Evaluation
  • Focus on four main stress categories: thermal, electrical, environmental, and mechanical.
  • Emphasis on thermal stresses as the dominant concern but acknowledges applications where multi-factor evaluation is needed.

Applications

IEC TR 61857-2:2015 is relevant across a broad spectrum of electrotechnical applications, including:

• Rotating Machinery
  • Evaluation of electrical insulation systems in motors and generators, covering both random-wound and form-wound coil designs.
• Transformers and Coils
  • Testing of insulation performance for wire-wound and encapsulated coils used in transformers and solenoids.
• Low-, Medium-, and High-Voltage Equipment
  • Applies to EIS used in devices operating below and above 600 VAC, with test methods tailored for each class.
• Modification and Qualification
  • Guidance on evaluating changes to established EIS designs, ensuring continued compliance and performance.
• Environmental and Combined Stresses
  • Recommendations for environmental compatibility, such as oil-immersed transformers or equipment exposed to repetitive voltage impulses.

By providing structured procedures and referencing applicable IEC standards, the document ensures that products meet international safety and performance benchmarks, supporting product certification and market entry.

Related Standards

IEC TR 61857-2:2015 refers to and supports the use of several related IEC standards for EIS evaluation, including:

• IEC 61857-1: General requirements for thermal evaluation - low-voltage
• IEC 60034-18-21: Thermal evaluation of random-wound winding insulation in rotating machines
• IEC 60034-18-31: Thermal evaluation of form-wound winding insulation in rotating machines
• IEC 61857-21/22: Specific requirements for wire-wound and encapsulated coil models
• IEC 61858-1/2: Evaluation of modifications to established EIS for wire-wound and form-wound windings
• IEC TS 62101: Combined thermal and electrical stress evaluation
• IEC TS 62332-1/2: Thermal assessment for EIS with combined liquid and solid components
• IEC 60505: General guidance on EIS evaluation and qualification
• IEC 62068, IEC 61934: Electrical endurance and measurement of partial discharges under voltage impulses

A thorough understanding and application of IEC TR 61857-2:2015 help ensure that electrical insulation systems are rigorously tested, appropriately classified, and compliant with global electrotechnical standards, supporting reliability and operational safety in demanding environments.