MT 18 - TC 36/MT 18

IEC 61109:2025 applies to composite insulators for overhead lines consisting of a load-bearing cylindrical insulating solid core consisting of fibres - usually glass - in a resin-based matrix, a housing (surrounding the insulating core) made of polymeric material and metal end fittings permanently attached to the insulating core. Composite insulators covered by this document are intended for use as suspension/tension line insulators, but these insulators could occasionally be subjected to compression or bending, for example when used as interphase-spacers. Guidance on such loads is outlined in Annex C.
The object of this document is to:
- define the terms used,
- specify test methods,
- specify acceptance criteria.
This third edition cancels and replaces the second edition published in 2008. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous edition:
- extension of this document to apply both to AC and DC systems;
- modifications of Clause 3, Terms, definitions and abbreviations;
- removal of Clause 7, Hybrid insulators, from this document;
- modifications of tests procedures recently included in IEC 62217 (hydrophobicity transfer test, stress corrosion, water diffusion test on the core with housing);
- modifications on environmental conditions;
- modifications on classification of tests and include the relevance of the interfaces;
- clarification and modification of the parameters determining the need to repeat design and type tests;
- revision of Table 1;
- revision of electrical type tests;
- revision of re-testing procedure of sample test;
- addition of a new Annex D on electric field control for AC;
- addition of a new Annex E on typical sketch for composite insulators assembly;
- addition of a new Annex F on mechanical evaluation of the adhesion between core and housing;
- addition of a new Annex G on applicability of design- and type tests for DC applications.

IEC 61466-1:2016 is applicable to composite string insulator units for a.c. overhead lines with a nominal voltage greater than 1 000 V and a frequency not greater than 100 Hz. It also applies to insulators of similar design used in substations or on electric traction lines. This standard applies to string insulator units of composite type with ball, socket, tongue, clevis, Y-clevis or eye couplings, or a combination thereof. The object of this standard is to prescribe specified values for the mechanical characteristics of the composite string insulator units and define the main dimensions of the couplings to be used on the composite string insulator units in order to permit the assembly of insulators or fittings supplied by different manufacturers and to allow, whenever practical, interchangeability with existing installations. It also defines a standard designation system for composite string insulator units. This edition includes the following significant technical changes with respect to the previous edition:
a) Addition of strength classes reflecting UHV practice;
b) Inclusion of Corrigendum 1:2008 for Y fitting hole dimensions.