SIST EN IEC 61109:2025

SLOVENSKI STANDARD
01-september-2025
Izolatorji za nadzemne vode - Sestavljeni obesni nosilni in strižni zatezni izolatorji
za izmenične sisteme z nazivno napetostjo nad 1 000 V - Definicije, preskusne
metode in prevzemna merila (IEC 61109:2025)
Insulators for overhead lines - Composite suspension and tension insulators for a.c.
systems with a nominal voltage greater than 1 000 V - Definitions, test methods and
acceptance criteria (IEC 61109:2025)
Isolatoren für Freileitungen - Verbund-Hänge- und -Abspannisolatoren für
Wechselstromsysteme mit einer Nennspannung über 1 000 V - Begriffe, Prüfverfahren
und Annahmekriterien (IEC 61109:2025)
Isolateurs pour lignes aériennes - Isolateurs composites de suspension et d'ancrage
destinés aux systèmes à courant alternatif de tension nominale supérieure à 1 000 v -
Définitions, méthodes d'essai et critères d'acceptation (IEC 61109:2025)
Ta slovenski standard je istoveten z: EN IEC 61109:2025
ICS:
29.080.10 Izolatorji Insulators
29.240.20 Daljnovodi Power transmission and
distribution lines
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61109

NORME EUROPÉENNE
EUROPÄISCHE NORM April 2025
ICS 29.080.10 Supersedes EN 61109:2008
English Version
Insulators for overhead lines - Composite suspension and
tension insulators for a.c. systems with a nominal voltage greater
than 1 000 V - Definitions, test methods and acceptance criteria
(IEC 61109:2025)
Isolateurs pour lignes aériennes - Isolateurs composites de Isolatoren für Freileitungen - Verbund-Hänge- und -
suspension et d'ancrage de tension supérieure à 1 000 V Abspannisolatoren mit einer Wechselspannung über 1 000
en courant alternatif et à 1 500 V en courant continu - V und einer Gleichspannung über 1500 V - Begriffe,
Définitions, méthodes d'essai et critères d'acceptation Prüfverfahren und Annahmekriterien
(IEC 61109:2025) (IEC 61109:2025)
This European Standard was approved by CENELEC on 2025-04-02. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 61109:2025 E
European foreword
The text of document 36/609/FDIS, future edition 3 of IEC 61109, prepared by TC 36 "Insulators" was
submitted to the IEC-CENELEC parallel vote and approved by CENELEC as EN IEC 61109:2025.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-04-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-04-30
document have to be withdrawn
This document supersedes EN 61109:2008 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 61109:2025 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60721-1 NOTE Approved as EN 60721-1
IEC 60721-1 NOTE Approved as EN 60721-1
IEC 60587 NOTE Approved as EN IEC 60587
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60060-1 - High-voltage test techniques - Part 1: General EN 60060-1 -
definitions and test requirements
IEC 60383-1 - Insulators for overhead lines with a nominal EN IEC 60383-1 -
voltage above 1000 V - Part 1: Ceramic or
glass insulator units for a.c. systems -
Definitions, test methods and acceptance
criteria
IEC 60383-2 - Insulators for overhead lines with a nominal EN 60383-2 -
voltage above 1000 V - Part 2: Insulator
strings and insulator sets for a.c. systems -
Definitions, test methods and acceptance
criteria
IEC 60437 - Radio interference test on high-voltage EN IEC 60437 -
insulators
IEC 61284 - Overhead lines - Requirements and tests for EN 61284 -
fittings
IEC 61466-1 - Composite string insulator units for overhead EN 61466-1 -
lines with a nominal voltage greater than 1
000 V - Part 1: Standard strength and end
fittings
IEC 61467 - Insulators for overhead lines - Insulator strings EN 61467 -
and sets for lines with a nominal voltage
greater than 1 000 V - AC power arc tests
1 2
IEC 62217 — Polymeric HV insulators for indoor and EN IEC 62217 —
outdoor use - General definitions, test
methods and acceptance criteria

Under preparation. Stage at the time of publication: IEC/FDIS 62217:2025.
Under preparation. Stage at the time of publication: FprEN IEC 62217:2025
Publication Year Title EN/HD Year
IEC 62231 - Composite station post insulators for EN 62231 -
substations with a.c. voltages greater than 1
000 V up to 245 kV - Definitions, test methods
and acceptance criteria
ISO 3452 series Non-destructive testing – Penetrant testing EN ISO 3452 series

IEC 61109 ®
Edition 3.0 2025-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
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 –

Definitions, test methods and acceptance criteria

Isolateurs pour lignes aériennes – Isolateurs composites de suspension et

d'ancrage de tension supérieure à 1 000 V en courant alternatif et à 1 500 V en

courant continu – Définitions, méthodes d'essai et critères d'acceptation

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.080.10  ISBN 978-2-8327-0210-9

– 2 – IEC 61109:2025 © IEC 2025
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms, definitions and abbreviated terms . 9
3.1 Terms and definitions . 9
3.2 Abbreviated terms . 12
4 Identification . 12
5 Environmental conditions . 12
6 Transport, storage and installation . 13
7 Tolerances . 13
8 Classification of tests. 14
8.1 Design tests . 14
8.2 Type tests . 15
8.3 Sample tests . 15
8.4 Routine tests. 15
9 Design tests . 18
9.1 General . 18
9.2 Test specimens . 18
9.2.1 Tests on interfaces and connections of end fittings . 18
9.2.2 Tracking and erosion test . 19
9.2.3 Tests on core material . 19
9.2.4 Tests on core with housing . 19
9.3 Product specific pre-stressing for tests on interfaces and connections of end
fittings . 19
9.3.1 General . 19
9.3.2 Sudden load release . 19
9.3.3 Thermal-mechanical pre-stress . 19
9.4 Assembled core load-time tests . 20
9.4.1 Test specimens . 20
9.4.2 Mechanical load test . 21
10 Type tests . 21
10.1 General . 21
10.2 Electrical tests on string insulator units . 21
10.2.1 General . 21
10.2.2 Test specimens . 22
10.2.3 Mounting arrangements for electrical tests . 22
10.2.4 Dry lightning impulse withstand voltage test. 22
10.2.5 Wet power-frequency voltage tests . 22
10.2.6 Wet switching impulse withstand voltage test . 22
10.2.7 Corona and radio interference voltage (RIV) tests. 22
10.2.8 Power arc test . 23
10.3 Damage limit proof test and test of the tightness of the interface between
end fittings and insulator housing . 23
10.3.1 Test specimens . 23
10.3.2 Performance of the test . 23

IEC 61109:2025 © IEC 2025 – 3 –
10.3.3 Evaluation of the test . 24
11 Sample tests . 25
11.1 General rules . 25
11.2 Verification of dimensions (E1 + E2) . 25
11.3 Verification of the end fittings (E2) . 26
11.4 Verification of tightness of the interface between end fittings and insulator
housing (E2) and of the specified mechanical load, SML (E1) . 26
11.5 Galvanizing test (E2) . 26
11.6 Minimum sheath thickness (E1) . 27
11.7 Re-testing procedure . 27
12 Routine tests . 28
12.1 Mechanical routine test . 28
12.2 Visual examination . 29
Annex A (informative) Principles of the damage limit, load coordination and testing for
composite suspension and tension insulators. 30
A.1 Introductory remark . 30
A.2 Load-time behaviour and the damage limit . 30
A.3 Service load coordination . 31
A.4 Verification tests . 33
Annex B (informative) Example of two possible devices for sudden release of load . 35
B.1 Device 1 (Figure B.1) . 35
B.2 Device 2 (Figure B.2) . 35
Annex C (informative) Guidance on non-standard mechanical stresses and dynamic
mechanical loading of composite insulators . 37
C.1 Introductory remark . 37
C.2 Torsion loads . 37
C.3 Compressive (buckling) loads . 37
C.4 Bending loads . 38
C.5 Dynamic mechanical loads . 38
C.6 Limits . 39
Annex D (informative) Electric field control for AC . 40
Annex E (informative) Typical sketches for composite insulator assemblies . 42
Annex F (informative) Mechanical evaluation of the adhesion between core and
housing . 43
F.1 General . 43
F.2 Method A: Pull-off test . 44
F.2.1 General . 44
F.2.2 Specimens . 44
F.2.3 Procedure . 44
F.3 Method B: Peel test . 46
F.3.1 General . 46
F.3.2 Specimens . 46
F.3.3 Procedure . 47
F.4 Method C: Shear test . 48
F.4.1 General . 48
F.4.2 Specimens . 48
F.4.3 Procedure . 48
Annex G (informative) Applicability of design and type tests for DC applications . 49
Bibliography . 51

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Figure 1 – Thermal-mechanical pre-stressing . 20
Figure 2 – Examples for 1 min SML withstand test . 24
Figure 3 – Location for minimum sheath thickness measurement . 27
Figure 4 – Method of re-testing at different stages . 28
Figure A.1 – Load-time strength and damage limit of a core assembled with fittings . 31
Figure A.2 – Graphical representation of the relationship of the damage limit to the
mechanical characteristics and service loads of an insulator with a 16 mm diameter
core and an SML rating of 133 kN . 32
Figure A.3 – Applied specific force relationship, example 1 . 32
Figure A.4 – Applied specific force relationship, example 2 . 33
Figure A.5 – Test loads . 34
Figure B.1 – Example of possible device 1 for sudden release of load . 35
Figure B.2 – Example of possible device 2 for sudden release of load . 36
Figure C.1 – Example of compression loads in V-string assemblies . 38
Figure C.2 – Buckling of composite insulator in a phase-to-phase configuration . 38
Figure D.1 – Example for electrical field vectors on a composite insulator . 41
Figure E.1 – Interface description for insulator with housing made by modular
assembly and external sealant . 42
Figure E.2 – Interface description for insulator with housing made by injection molding
and overmolded end fitting . 42
Figure F.1 – Example for type of housing separation . 43
Figure F.2 – Example of specimen mounted in a tensile test machine . 45
Figure F.3 – Example of test object for pull-off test and application clamping and force . 45
Figure F.4 – Relevant dimensions for the calculation of the area of the pull-off section . 46
Figure F.5 – Example of test specimen for peel test . 47
Figure F.6 – Method of peel test and tested specimens after peel test . 47
Figure F.7 – Method of shear test and tested samples after shear test with cohesive
bonding, sample passed the test . 48

Table 1 – Normal environmental conditions . 13
Table 2 – Tests to be carried out after design changes . 16
Table 3 – Design tests . 18
Table 4 – Application and mounting arrangements for electrical tests . 23
Table 5 – Sample sizes . 25
Table G.1 – Design and type tests for DC applications . 49

IEC 61109:2025 © IEC 2025 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
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 –
DEFINITIONS, TEST METHODS AND ACCEPTANCE CRITERIA

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of (a) patent(s), which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 61109 has been prepared by subcommittee 36B: Insulators for overhead lines, of IEC
technical committee 36: Insulators. It is an International Standard.
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:
a) extension of this document to apply both to AC and DC systems;
b) modifications of Clause 3, Terms, definitions and abbreviations;
c) removal of Clause 7, Hybrid insulators, from this document;

– 6 – IEC 61109:2025 © IEC 2025
d) modifications of tests procedures recently included in IEC 62217 (hydrophobicity transfer
test, stress corrosion, water diffusion test on the core with housing);
e) modifications on environmental conditions;
f) modifications on classification of tests and include the relevance of the interfaces;
g) clarification and modification of the parameters determining the need to repeat design and
type tests;
h) revision of Table 1;
i) revision of electrical type tests;
j) revision of re-testing procedure of sample test;
k) addition of a new Annex D on electric field control for AC;
l) addition of a new Annex E on typical sketch for composite insulators assembly;
m) addition of a new Annex F on mechanical evaluation of the adhesion between core and
housing;
n) addition of a new Annex G on applicability of design- and type tests for DC applications.
The text of this International Standard is based on the following documents:
Draft Report on voting
36/609/FDIS 36/611/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
This International Standard is to be used in conjunction with IEC 62217:2012.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IEC 61109:2025 © IEC 2025 – 7 –
INTRODUCTION
Composite suspension and tension insulators (in the following the term "composite insulator" is
used) consist of fibreglass insulating core, bearing the mechanical load protected by a
polymeric housing, the load being transmitted to the core by metallic end fittings. Despite these
common features, the materials used and the design details and manufacturing process used
by different manufacturers may differ.
Some tests have been grouped together as "Design tests", to be performed only once on
insulators which satisfy the same design conditions. For all design tests of these composite
insulators, the appropriate common clauses defined in IEC 62217 are applied. As far as
practical, the influence of time on the electrical and mechanical properties of its components
(core, housing, interfaces etc.) and of the complete composite insulators has been considered
in specifying the design tests to ensure a satisfactory lifetime under normally known stress
conditions of transmission lines. Explanation of the principles of the damage limit, load
coordination and testing are presented in Annex A.
It has not been considered useful to specify a power arc test as a mandatory test. The test
parameters are manifold and can have very different values depending on the configurations of
the network and the supports and on the design of arc-protection devices. The heating effect of
power arcs need to be considered in the design of metal fittings. Critical damage to the metal
fittings resulting from the magnitude and duration of the short-circuit current can be avoided by
properly designed arc-protection devices. This document, however, does not exclude the
possibility of a power arc test by agreement between the manufacturer and customer.
IEC 61467 gives details on AC power arc testing of complete insulator sets, that match their
configuration with actual protective and string fittings, to recreate the real electromagnetic field
affecting the arc movement.
This document covers both AC and DC composite insulators. Before the appropriate standard
for DC applications is issued, the majority of tests listed in this document can also be applicable
for DC (Annex G). Due to the difference in AC and DC tracking performance, a specific tracking
and erosion test procedure for DC applications as a design test is planned to be developed.
The 1 000 h AC tracking and erosion test of IEC 62217 can be used only to establish a minimum
requirement for the tracking and erosion resistance. This 1 000 h salt fog tracking and erosion
test is considered as a screening test intended to reject materials in combination with the design
which are inadequate. Tracking and erosion tests are not intended to evaluate long term
performance of insulators. Such tests, e.g. the 5 000 h multiple stress test and wheel test in
IEC TR 62730 [1] , or other tests intended for research or sometimes used as a supplementary
design test, are not considered in this document.
Composite suspension and tension insulators are, in general, not intended for torsion or other
non-tensile loads. However, due to consideration to non-standard applications (interphase
spacers etc.) loads during handling and installation have to be considered in the design.
Guidance on non-standard loads is given in Annex C.
Wherever possible, IEC Guide 111 [2] has been followed for the drafting of this document.

___________
Numbers in square brackets refer to the bibliography.

– 8 – IEC 61109:2025 © IEC 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 –
DEFINITIONS, TEST METHODS AND ACCEPTANCE CRITERIA

1 Scope
This International Standard 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 document does not include requirements dealing with the choice of insulators for specific
operating conditions or environments beyond normal environmental conditions defined in
Table 1.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60060-1, High-voltage test techniques – Part 1: General definitions and test requirements
IEC 60383-1, Insulators for overhead lines with a nominal voltage above 1000 V – Part 1:
Ceramic or glass insulator units for AC systems – Definitions, test methods and acceptance
criteria
IEC 60383-2, Insulators for overhead lines with a nominal voltage above 1 000 V – Part 2:
Insulator strings and insulator sets for AC systems – Definitions, test methods and acceptance
criteria
IEC 60437, Radio interference test on high-voltage insulators
IEC 61284, Overhead lines – Requirements and tests for fittings
IEC 61466-1, Composite string insulator units for overhead lines with a nominal voltage greater
than 1 000 V – Part 1: Standard strength classes and end fittings

IEC 61109:2025 © IEC 2025 – 9 –
IEC 61467, Insulators for overhead lines – Insulator strings and sets for lines with a nominal
voltage greater than 1 000 V – AC power arc tests
IEC 62217:— , Polymeric HV insulators for indoor and outdoor use – General definitions, test
methods and acceptance criteria
IEC 62231, Composite station post insulators for substations with AC voltages greater than
1 000 V up to 245 kV – Definitions, test methods and acceptance criteria
ISO 3452 (all parts), Non-destructive testing – Penetrant testing
3 Terms, definitions and abbreviated terms
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
Note 1 to entry: Certain terms from IEC 62217:2012 are reproduced here for ease of reference. Additional
definitions applicable to insulators can be found in IEC 60050-471 [3].
3.1 Terms and definitions
3.1.1
polymeric insulator
insulator whose insulating body consists of at least one organic based material
Note 1 to entry: Polymeric insulators are also known as non-ceramic insulators.
Note 2 to entry: Coupling devices may be attached to the ends of the insulating body.
[SOURCE: IEC 60050-471:2007, 471-01-13]
3.1.2
composite insulator
insulator made of at least two insulating parts, namely a core and a housing equipped with end
fittings
Note 1 to entry: Composite insulators can consist either of individual sheds mounted on the core, with or without
an intermediate sheath, or alternatively, of a housing directly moulded or cast in one or several pieces on to the core.
[SOURCE: IEC 60050-471:2007, 471-01-02]
3.1.3
core (of an insulator)
central insulating part of an insulator which provides the mechanical characteristics
Note 1 to entry: The housing and sheds are not part of the core.
[SOURCE: IEC 60050-471:2007, 471-01-03]
___________
Under preparation. Stage at the time of publication: IEC/RFDIS 62217:2025.

– 10 – IEC 61109:2025 © IEC 2025
3.1.4
insulator trunk
central insulating part of an insulator from which the sheds project
Note 1 to entry: Also known as shank on smaller insulators.
[SOURCE: IEC 60050-471:2007, 471-01-11]
3.1.5
housing
external insulating part of composite insulator providing the necessary creepage distance and
protects the core from the environment
Note 1 to entry: An intermediate sheath made of insulating material may be part of the housing.
[SOURCE: IEC 60050-471:2007, 471-01-09]
3.1.6
shed (of an insulator)
insulating part, projecting from the insulator trunk, intended to increase the creepage distance
Note 1 to entry: The shed can be with or without under-ribs.
[SOURCE: IEC 60050-471:2007, 471-01-15]
3.1.7
interface
contact surface between the different materials
Note 1 to entry: Various interfaces exist in composite insulators, e.g.:
– between housing and end fittings;
– between various parts of the housing; e.g. between separately manufactured sheds, or between sheath and
sheds;
– between core and housing;
– between sealant and core;
– between sealant and end fittings.
(Annex E: Typical sketches for composite insulator assemblies)
[SOURCE: IEC 62217:—, 3.11, modified – "contact" added in definition, Note 1 to entry
modified]
3.1.8
end fitting
integral component or formed part of an insulator intended to connect it to a supporting
structure, or to a conductor, or to an item of equipment, or to another insulator
Note 1 to entry: Where the end fitting is metallic, in general the term "metal fitting" is used.
Note 2 to entry: Standard end fittings are defined in IEC 61466-1.
[SOURCE: IEC 60050-471:2007, 471-01-06]
3.1.9
connection zone
zone where the mechanical load is transmitted between the core and the end fitting
[SOURCE: IEC 62217:2012, 3.13, modified – "insulating body and the fixing device" replaced
by "core and the end fitting"]

IEC 61109:2025 © IEC 2025 – 11 –
3.1.10
coupling
part of the end fitting which transmits the load to the accessories external to the insulator
[SOURCE: IEC 62217:2012, 3.14, modified – "fixing device" replaced by "end fitting",
"hardware" replaced by "accessories"]
3.1.11
creepage distance
shortest distance or the sum of the shortest distances along the surface on an insulator between
two conductive parts which normally have the operating voltage between them
[SOURCE: IEC 60050-471:2007, 471-01-04]
3.1.12
arcing distance
shortest distance in the air external to the insulator between the metallic parts which normally
have the operating voltage between them
Note 1 to entry: The term "dry arcing distance" is also used.
[SOURCE: IEC 60050-471:2007, 471-01-01]
3.1.13
specified mechanical load
SML
withstand load, specified by the manufacturer, which is used for mechanical tests in this
document
3.1.14
routine test load
RTL
load applied to all assembled composite insulators during a routine mechanical test
3.1.15
mechanical failing load
maximum load that is reached when the insulator is tested under the standard conditions
[SOURCE: IEC 60050-471:2007, 471-01-12, modified – "prescribed" replaced by "standard",
Note 1 to entry removed]
3.1.16
insulator set
assembly of one or more insulator strings suitably connected together, complete with end
fittings and protective devices as required in service
Note 1 to entry: The terms "arcing and field grading devices" is also used for protective devices.
[SOURCE: IEC 60050-471:2007, 471-03-02]
3.1.17
string insulator unit
cap and pin insulator or long rod insulator of which the end fittings are suitable for flexible
attachment to other similar string insulator units or to connecting accessories
Note 1 to entry: Cap and pin insulators are not composite insulators and are not part of this document.
[SOURCE: IEC 60050-471:2007, 471-03-08]

– 12 – IEC 61109:2025 © IEC 2025
3.1.18
sealing
method for providing the ability of a component to resist the ingress of contaminants
Note 1 to entry: Contaminants include pollution and moisture.
[SOURCE: IEC 60050-581:2008, 581-23-16]
3.1.19
sealant
additional material used for sealing
Note 1 to entry: Typically RTV-silicones are used for composite insulators.
Note 2 to entry: See sealant in Annex E: Typical principles sketch for composite insulators assembly.
3.1.20
grading/corona ring
protective devices made from metal attached to the composite insulator end fitting or
intermediate string fitting intended to keep the electric field anywhere along the surface of
composite insulator below the specified maximum value
3.2 Abbreviated terms
The following abbreviated terms are used in this document:
E1, E2 Sample sets for sample tests
M Average 1 min failing load of the core assembled with fittings
AV
RTL Routine test load
RTV Room-temperature-vulcanizing silicone
SML Specified mechanical load
4 Identification
In addition to the requirements of IEC 62217, each insulator shall be marked with the SML.
It is recommended that each insulator is marked or labelled by the manufacturer to show that it
has passed the routine mechanical test.
5 Environmental conditions
The normal environmental conditions to which insulators are submitted in service are de
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