IEC 61109:2025

Overview

IEC 61109:2025 - Insulators for overhead lines - Composite suspension and tension insulators with AC voltage greater than 1 000 V and DC voltage greater than 1 500 V - is the third edition (technical revision) of the international standard for composite insulators used on overhead lines. It defines terms, specifies test methods and acceptance criteria for composite insulators consisting of a fibre‑reinforced resin core, polymeric housing and permanently attached metal end fittings. The standard applies to suspension and tension insulators for both AC and DC systems and replaces the 2008 edition.

Key topics and technical requirements

• Scope and definitions: Updated Clause 3 expands terms, definitions and abbreviations relevant to composite insulators and test classification.
• Test classification: Clear differentiation of design tests, type tests, sample tests and routine tests with criteria for when tests must be repeated after design changes.
• Electrical testing: Revised electrical type tests including dry lightning impulse, wet power‑frequency, wet switching impulse, corona/RIV and power arc tests for insulator strings and assemblies.
• Mechanical testing: Assembled core load‑time tests, damage‑limit proof tests, verification of specified mechanical load (SML) and mechanical routine tests.
• Materials and interfaces: Modified procedures for hydrophobicity transfer, stress corrosion and water diffusion tests on the core with housing (harmonized with IEC 62217); added mechanical evaluation methods for core-to-housing adhesion (Annex F: pull‑off, peel, shear).
• Environmental and installation: Revised environmental condition classifications, identification, transport, storage and installation guidance.
• New informative annexes:
  • Annex D - Electric field control for AC;
  • Annex E - Typical assembly sketches;
  • Annex F - Mechanical evaluation of adhesion;
  • Annex G - Applicability of design and type tests for DC applications.
• Other changes: Removal of hybrid insulators clause; clarification of interfaces and retesting procedures.

Applications and who uses it

IEC 61109:2025 is used by:

• Manufacturers of composite suspension/tension insulators for product design validation and production control.
• Test laboratories for type and routine testing to internationally accepted methods and acceptance criteria.
• Transmission and distribution utilities, engineers and asset owners for procurement specifications, acceptance testing and lifecycle assessment of overhead‑line insulators.
• Specification writers and consultants preparing tender documents and conformity statements for AC and DC overhead line projects.

Practical uses include qualification of new insulator designs, verification of mechanical and electrical performance, assessment of housing‑core adhesion, and guidance for DC applicability and electric field control.

Related standards

• IEC 62217 - referenced for composite insulating material tests (hydrophobicity transfer, stress corrosion, water diffusion).