IEC TR 62662:2010

IEC/TR 62662 ®
Edition 1.0 2010-08
TECHNICAL
REPORT
colour
inside
Guidance for production, testing and diagnostics of polymer insulators with
respect to brittle fracture of core materials

IEC/TR 62662:2010(E)
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IEC/TR 62662 ®
Edition 1.0 2010-08
TECHNICAL
REPORT
colour
inside
Guidance for production, testing and diagnostics of polymer insulators with
respect to brittle fracture of core materials

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
Q
ICS 29.080.10 ISBN 978-2-88912-172-4
– 2 – TR 62662 © IEC:2010(E)
CONTENTS
FOREWORD.3
INTRODUCTION.5
1 Scope.6
2 Normative references .6
3 Terms and definitions .6
4 Description of brittle fracture .7
5 Identification of brittle fracture .8
6 Failure mechanisms .8
6.1 Description of identified failure mechanisms.8
6.2 Mechanism M1 – Acid generated by electrical activity .8
6.3 Mechanism M2 – Acid ingress from environment .9
6.4 Mechanism M3 – Acid generated inside the core .10
7 Risk assessment .10
7.1 FMEA approach .10
7.2 Results.11
7.3 Discussions.13
8 Guidance for material selection and production .13
8.1 Rod materials and production process.13
8.1.1 Fibreglass .13
8.1.2 Resins .14
8.1.3 Rod production .14
8.2 Housing and sealing.14
9 Preventive methods.15
9.1 Sealing.15
9.2 Corona rings .15
9.3 Material selection .16
9.4 Housing quality .16
9.5 Handling, transportation and storage.16
Bibliography.17

Figure 1 – Typical brittle fracture .

Table 1 – Risk potential number (RPN) and standardized risk potential of brittle
fracture failure modes .11
Table 2 – Risk assessment of the identified brittle fracture failure modes.11

TR 62662 © IEC:2010(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GUIDANCE FOR PRODUCTION, TESTING
AND DIAGNOSTICS OF POLYMER INSULATORS
WITH RESPECT TO BRITTLE FRACTURE OF CORE MATERIALS

FOREWORD
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all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
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The main task of IEC technical committees is to prepare International Standards. However, a
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data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 62662, which is a technical report, has been prepared by IEC technical committee 36:
Insulators.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
36/294/DTR 36/297/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.

– 4 – TR 62662 © IEC:2010(E)
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TR 62662 © IEC:2010(E) – 5 –
INTRODUCTION
There is an urgent need within utilities and industry for material standards, which define the
physical properties of the polymers applied for outdoor insulation. As a first step, a state-of-
the-art report was issued by CIGRE which led to the publication of IEC 62039. This IEC
technical report presents – as a conclusion of the CIGRE-report – the important material
properties for polymeric materials used in outdoor insulation and, where applicable, lists the
standardized test methods including the minimum requirements. The acid (brittle fracture)
resistance of FRP core materials (see 3.7) was recognized as an important property for
suspension/tension composite insulators. This technical report presents more detailed
guidance on this subject taking into account different insulator designs and production
techniques. The risk of occurrence and the influencing parameters were evaluated by failure
mode effect analysis (FMEA). Brittle fracture is not the only failure mechanism for insulators
in service and is generally less frequently observed than other modes, such as failure due to
tracking and erosion. However, this subject is not yet covered by any IEC test procedures
specifically designed to detect or prevent brittle fracture.

– 6 – TR 62662 © IEC:2010(E)
GUIDANCE FOR PRODUCTION, TESTING
AND DIAGNOSTICS OF POLYMER INSULATORS
WITH RESPECT TO BRITTLE FRACTURE OF CORE MATERIALS

1 Scope
This technical report presents an analysis of the risk of influencing factors for brittle fracture
of composite insulators that are mostly loaded in the tensile mode (suspension and tension
insulators). Guidance is given to reduce the risk of in-service brittle fractures.
This phenomenon is limited to tension and suspension insulators. However, the general
information given concerning the importance of various parameters can be used as a
guideline for the design and production of any kind of composite insulator.
2 Normative references
The following referenced documents are indispensable for the application 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 61109, 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/TR 62039, Selection guide for polymeric materials for outdoor use under HV stress
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
fibre reinforced plastic material
FRP
composite material consisting of reinforcing components e.g. glass or synthetic fibres that are
embedded in a polymer matrix e.g. epoxy or polyester. The FRP core is the integral load-
carrying part of a composite insulator
3.2
stress corrosion cracking
SCC
failure of material subjected to a constant tensile stress in a corrosive environment
3.3
brittle fracture
abnormal and sudden breakage of FRP core materials with well-defined characteristic fracture
patterns
NOTE Before brittle fracture, no apparent plastic deformation takes place. In the case of FRP core materials,
brittle fracture is caused by SCC.
3.4
failure mechanism
principal and fundamental process that leads to a characteristic failure, e.g. brittle fracture

TR 62662 © IEC:2010(E) – 7 –
NOTE A failure mechanism may have several modes of final failure.
3.5
failure mode
specific failure scenario or optional path of a failure mechanism
3.6
sealing system
technical arrangement to prevent the ingress of moisture, gases, etc., at a material transition
point exposed to the environment
3.7
failure mode effect analysis
FMEA
standardized risk assessment tool generally used for failure prevention
3.8
damage
degradation of a component leading to penetration by acid or moisture
4 Description of brittle fracture
Brittle fracture is the commonly used term for stress corrosion-induced failure of insulator core
rods manufactured from resin bonded glass fibre material (RBGF, commonly known as fibre
reinforced plastic FRP). This failure mechanism results in a complete mechanical separation
of the core (normally near the energized end fitting), and can occur at tensile loads well below
the rated mechanical strength of the insulators. In addition to a (minimum) tensile stress of
approximately 50 MPa, the brittle fracture mechanism requires the presence of acid from
either external or internal sources. The chemical process of stress corrosion is an ion
exchange mechanism whereby ions in the glass fibres are replaced by hydrogen ions from the
acid (see IEC/TR 62039).
IEC  2042/10
Figure 1 – Typical brittle fracture

– 8 – TR 62662 © IEC:2010(E)
5 Identification of brittle fracture
The macroscopic features associated with the brittle fracture of an FRP core rod have been
described by CIGRE [2] . A typical brittle fracture is shown in Figure 1. The fracture surfaces
typically have the following characteristics:
• a smooth, clean, planar surface perpendicular to the core axis, comprising a portion of the
rod cross-section. Multiple failure planes separated by axial delamination may be present;
• normal tensile fracture (fibrous) in the remaining rod cross-section.
In addition to these macroscopic features, confirmation of the brittle fracture mechanism is
possible through the identification of several distinctive features of the fracture surface of the
individual glass fibres using scanning electron microscopy [3].
• The mirror zone is a smooth region perpendicular to the fibre axis that includes the stress
corrosion initiation site for the individual fibre and may cover from <10% to >90% of the
fibre cross-section.
• The hackle zone is a rough region on the fibre fracture surface that failed mainly due to
mechanical stresses.
• The mist zone is a transition zone between the mirror and hackle zones and is
intermediate in roughness.
Chemical analysis techniques to show the change in the glass chemistry due to the ion
exchange mechanism may also be used as confirmation of the brittle fracture mechanism, see
[2], [4], [5], and [6].
6 Failure mechanisms
6.1 Description of identified failure mechanisms
For the time being, three failure mechanisms have been identified and are well described in
recent literature [7], [8], and [9]. The final physical-chemical mechanism is stress corrosion
cracking (SCC) which leads to “brittle fracture” of the FRP core. The initiation of SCC requires
the presence of acid in direct contact with the FRP core material for all mechanisms. The way
in which this acid appears on or inside the FRP core material can be differentiated into the
following three mechanisms.
NOTE Other mechanisms are currently under study, for example, crevice corrosion. The mechanisms are not yet
sufficiently investigated to be included in this technical report. However, all require the presence of water
(moisture).
6.2 Mechanism M1 – Acid generated by electrical activity
This mechanism is characterized by acid generation by internal and/or external electrical
discharges with the acid being finally the root cause for stress corrosion of the FRP core
material and the resulting brittle fracture of the FRP core.
Acid (nitrogenous acids) is generated by electrical discharges (corona) on the insulating
material (external), at the metal fittings (external) or by internal partial discharges within
internal voids. Electrical discharges in air generate radicals, ozone and nitrogen oxides which
form acids when combined with water (e.g. moisture from the air). Internal partial discharges
within voids of the FRP core material or at the interface between FRP core and housing may
lead to acid generation. In some cases moisture vapour transmission in a hot wet environment
followed by a cold cycle will cause condensation inside voids. The acid is then in direct
contact with the FRP core material. Moisture is required for all modes of this mechanism to
generate acid. Therefore the penetration path to the FRP core is a very critical criterion for
___________
Figures in square brackets refer to the Bibliography.

TR 62662 © IEC:2010(E) – 9 –
this mechanism. Moisture penetration may be possible if the sealing system is defective or
insufficient. Other paths for moisture penetration to the FRP core may be housing damage or
housing porosity. The moisture penetration can occur in the form of molecular diffusion due to
the moisture permeability of the housing. Acids generated by electrical discharges are in
general strong inorganic acids. The main chemical structures are HNO and HNO . Polymeric
2 3
compounds show in general a high resistance to HNO and are often resistant to other acids.
The same applies to the sealing systems at the end fitting (triple) point realized with such
material compounds. Two main grades of FRP core materials exist:
• acid resistant core materials that pass the chemical resistance test in accordance with
IEC 62039;
• non-acid resistant core materials that do not pass the chemical resistance test.
Acid resistant rods can resist acid for a much longer time period than non-resistant materials
(see Clause 8).
The critical design features of composite insulators regarding this mechanism are:
• chemical (acid) resistance of the FRP core material;
• quality of the FRP core material regarding internal voids that may lead to internal partial
discharges;
• quality of the macroscopic interface between housing and FRP core material regarding
voids that may lead to internal partial discharges;
• long-term tightness of the sealing system;
• corona protection and field grading to reduce the appearance of acid-producing corona or
partial discharges;
• resistance of the housing and sealing system regarding transportation and handling
damage.
6.3 Mechanism M2 – Acid ingress from environment
This mechanism is characterized by acid ingress from the environment (e.g. pollution) with the
acid being finally the root cause for SCC of the FRP core material and the resulting brittle
fracture of the FRP core.
Acid or its anhydrides are present in the environment of the insulator installation. To obtain an
acidic solution moisture is also necessary. Therefore the penetration path to the core is also a
very critical criterion for this mechanism. Acid penetration may be possible if the sealing
system is defective or insufficient. Other paths for acid penetration to the FRP core may be
housing damages or housing porosity. Acids in the environment may be weak to strong acids
depending on the kind of pollution. Inorganic polymeric compounds show in general a high
resistance against such acids. Silicone or EPDM housing materials are sufficiently resistant to
most acids but also may exhibit porosity. The same applies to the sealing systems realised
with such material compounds. Two grades of FRP core materials exist:
• acid resistant core materials that pass the chemical resistance test in accordance with
IEC 62039;
• non-acid resistant core materials that do not pass the chemical resistance test.
Acid resistant rods can resist acid for a
...