IEC 60505:2011

IEC 60505 ®
Edition 4.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Evaluation and qualification of electrical insulation systems

Évaluation et qualification des systèmes d'isolation électrique
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IEC 60505 ®
Edition 4.0 2011-07
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Evaluation and qualification of electrical insulation systems

Évaluation et qualification des systèmes d'isolation électrique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XC
ICS 29.080.30 ISBN 978-2-88912-539-5

– 2 – 60505 © IEC:2011
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 9
3.1 General terms . 10
3.2 Terms related to service stresses and ageing . 10
3.3 Terms related to testing . 11
4 Ageing . 12
4.1 Ageing mechanism . 12
4.2 Assessment of ageing mechanisms . 14
4.3 Electrical ageing . 15
4.4 Thermal ageing . 17
4.5 Mechanical ageing . 19
4.6 Environmental ageing . 21
4.7 Accelerated ageing . 22
4.8 Multifactor ageing . 23
5 Basic evaluation considerations . 23
5.1 Elements for preparing an evaluation method . 23
5.1.1 Object . 23
5.1.2 Service conditions . 23
5.1.3 Life values . 24
5.2 Types of evaluation procedures . 24
5.3 Choice of the test object . 26
5.4 Experimental test procedures . 26
5.5 Conclusions for standardization practices . 27
6 Functional ageing tests. 27
6.1 Test objects. 27
6.1.1 Construction of test objects . 27
6.1.2 Number of test objects . 28
6.1.3 Quality assurance tests . 28
6.1.4 Preconditioning subcycle . 28
6.1.5 Initial diagnostic tests . 28
6.1.6 Reference EIS . 28
6.2 Test conditions . 28
6.2.1 Continuous and cyclic testing . 28
6.2.2 Levels of test stresses, ageing factors and diagnostic factors . 29
6.3 Determination of EIS service life . 29
6.3.1 Extrapolation of life test results . 29
6.3.2 Comparison of life test data . 29
6.4 Diagnostics . 30
6.4.1 Diagnostic tests – End point criteria. 30
6.4.2 Additional specific tests . 31
6.5 Analysing the data . 31
6.6 Test report . 31
Annex A (informative) Glossary . 32

60505 © IEC:2011 – 3 –
Bibliography . 71

Figure 1 – Ageing of an EIS . 13
Figure 2 – Intrinsic/extrinsic electrical ageing of practical EIS . 15
Figure 3 – Intrinsic/extrinsic thermal ageing of practical EIS . 17
Figure 4 – Intrinsic/extrinsic mechanical ageing of practical EIS . 20
Figure 5 – Intrinsic/extrinsic environmental ageing of practical EIS . 22
Figure 6 – Elements of evaluation methods . 23
Figure 7 – Type of evaluation procedure . 25
Figure 8 – Selection of test object . 26
Figure 9 – Establishing the test method . 27
Figure A.1 – Surface abrasion damage . 32
Figure A.2 – Surface enamel peeling like string . 32
Figure A.3 – Scheme of the measurement set-up for the charging/discharging current . 33
Figure A.4 – Example of sample preparation . 33
Figure A.5 – Charging/discharging current on HDPE film . 34
Figure A.6 – Property versus time behaviour, detection of threshold (end point, p )
L
and maintenance time . 35
Figure A.7 – Correspondence between the ageing plots of the property p (in red),
obtained at different stress levels, and the resulting life line . 35
Figure A.8 – Example of charge injection of positive carriers (holes) from the anode
and of negative charge carriers (electrons) from the cathode in a PE flat specimen,
detected by space charge measurement performed by PEA method . 36
Figure A.9 – Stress-strain curve for a typical material . 37
Figure A.10 – Scheme of measurement set- up for charging/discharging current . 38
Figure A.11 – Example of sample preparation . 38
Figure A.12 – Charging/discharging current on HDPE film . 38
Figure A.13 – Charging current at 135 °C and different values of DC electrical field . 39
Figure A.14 – Charging current at 120 °C and different values of DC electrical field . 39
Figure A.15 – Corona at post insulator head . 40
Figure A.16 – Corona on top and arcing to ground . 40
Figure A.17 – Stages of mechanical ductile fracture (cracking) (Source unknown) . 41
Figure A.18 – Photo showing orderings in epoxy structure and void . 42
Figure A.19 – Discharge between conductors through air. 44
Figure A.20 – Paper insulation degraded by electrical surface discharges . 44
Figure A.21 – Example of electric strength test on XLPE sample 0,2 mm thick . 45
Figure A.22 – Two parameters Weibull plot electric strength results performed on
seven XLPE specimens, 0,2 mm thick . 45
Figure A.23 – Loss angle of a dielectric . 47
Figure A.24 – Loss factor for pre-treated and thermally aged (at 110 °C and 130 °C)
XLPE cables measured at 90 °C plotted vs. frequency . 47
Figure A.25 – Field lines from a positive charge above a plane conductor . 48
Figure A.26 – Electrical tree. 49
Figure A.27 – EPDM ashing and erosion on fitting . 50
Figure A.28 – Failing external insulation . 51

– 4 – 60505 © IEC:2011
Figure A.29 – Failing external insulation . 51
Figure A.30 – Critical failure of solid cable insulation (XLPE) by electrical breakdown . 52
Figure A.31 – Example flashover . 53
Figure A.32 – Substation – Outdoor installation . 54
Figure A.34 – Internal interfaces in epoxy structure and void . 56
Figure A.35 – Example of craze and crack development in an inter-lamellar space
under mechanical tension T . 57
Figure A.36 – Water treeing . 58
Figure A.37 – After 11 years in service UV and moisture impact . 59
Figure A.38 – Random (amorphous) structure of a molecular chain . 59
Figure A.39 – Oriented structure (semi-crystalline) of a molecular chain . 59
Figure A.40 – Typical morphology of melt-grown polyethylene spherulites . 60
Figure A.41 – Areas in which PD generally occur . 61
Figure A.42 – Classes of defect – Internal, surface and corona PD . 61
Figure A.43 – Basic PD measurement circuit . 62
Figure A.44 – Examples of PD patterns relevant to internal, surface and corona PD . 62
Figure A.45 – GIS research – Metal conductor protrusion . 63
Figure A.46 – Internally strained epoxy – Frozen in strains in epoxy resin due to
thermal stress, measured by TMA curves . 64
Figure A.47 – Externally strained parts in an on-load tap changer (OLTC) . 64
Figure A.48 – A material being loaded in a) compression, b) tension, c) shear . 65
Figure A.49 – Effect of thermal-mechanical stresses leading to interfacial electrical
tracking . 66
Figure A.50 – Stress-strain curve for a typical material . 66
Figure A.51 – Over crimped rod; breaks during tensile test . 67
Figure A.52 – Typical installation fault . 68
Figure A.53 – Surface tracking on sheds and fitting end . 68
Figure A.54 – Vented trees – Initiate at interface . 69
Figure A.55 – Tape wrinkling . 70

Table 1 – Ageing temperatures . 19
Table 2 – Cyclical and continuous procedures . 30

60505 © IEC:2011 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
EVALUATION AND QUALIFICATION
OF ELECTRICAL INSULATION SYSTEMS

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
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60505 has been prepared by IEC technical committee 112:
Evaluation and qualification of electrical insulating materials and systems.
This fourth edition cancels and replaces the third edition, published in 2004, and constitutes a
technical revision.
The main change with respect to the previous edition is the addition of a Glossary in the form
of Annex A to this standard.
The text of this standard is based on the following documents:
FDIS Report on voting
112/174/FDIS 112/184/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 6 – 60505 © IEC:2011
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
The contents of the corrigendum of March 2017 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
60505 © IEC:2011 – 7 –
INTRODUCTION
The life of an electrical insulation system (EIS) or systems frequently determines the life of
electrical equipment which can be affected by electrical, thermal, mechanical or
environmental stresses acting either individually or in combination.
Intended, estimated or proven service life times are essential parameters for describing the
life of electrical insulation systems. In the early days of electrotechnical engineering, life
figures were rather vague. The limitation of the life of the insulation under thermal stress was
one of the first indicators of the effect of ageing in some equipment in service. As experience
in using EIS increased, it was appreciated that there was a need to select specific materials
having satisfactory life time at a given temperature, to enable the required service life to be
achieved and to allow for the calculation of the thermal capability of equipment.
The user of this standard may evaluate existing test methods and provide correlation with his
equipment. Therefore, the user of this standard is responsible for demonstrating the validity of
the existing test method in accordance with the principles of this standard.
The determination of the prospective life is a fundamental task when developing and
designing an EIS. Estimated service life of an EIS needs to be established for several
reasons:
– for type testing when introducing a new EIS into production;
– for quality assurance of production;
– for estimating the life expectancy of new equipment;
– for estimating the remaining life for maintenance purposes.
“Ageing” focuses on the mechanisms affecting the EIS performance. “Evaluation” links these
potential mechanisms by “Analysis” and “Diagnostics” to the design of a specific kind of
evaluation test procedure.
The keyword structure below meets such requirements and allows an easier choice of the
parts of interest.
– 8 –                60505 © IEC:2011

60505 © IEC:2011 – 9 –
EVALUATION AND QUALIFICATION
OF ELECTRICAL INSULATION SYSTEMS

1 Scope
This International Standard establishes the basis for estimating the ageing of electrical
insulation systems (EIS) under conditions of either electrical, thermal, mechanical,
environmental stresses or combinations of these (multifactor stresses).
It specifies the principles and procedures that shall be followed, during the development of
EIS functional test and evaluation procedures, to establish the estimated service life for a
specific EIS.
This standard should be used by all IEC technical committees responsible for equipment
having an EIS.
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 60216-2, Electrical insulating materials – Thermal endurance properties – Part 2:
Determination of thermal endurance properties of electrical insulating materials – Choice of
test criteria
IEC 60216-3, Electrical insulating materials – Thermal endurance properties – Part 3:
Instructions for calculating thermal endurance characteristics
IEC 60216-5, Electrical insulating materials – Thermal endurance properties – Part 5:
Determination of relative thermal endurance index (RTE) of an insulating material
IEC 60493-1, Guide for the statistical analysis of ageing test data – Part 1: Methods based on
mean values of normally distributed test results
IEC 60544-1, Electrical insulating materials – Determination of the effects of ionizing radiation
– Part 1: Radiation interaction and dosimetry
IEC/TS 61251, Electrical insulating materials – AC voltage endurance evaluation –
Introduction
IEC 62539, Guide for the statistical analysis of electrical insulation breakdown data
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

– 10 – 60505 © IEC:2011
3.1 General terms
3.1.1
electrical insulation system
EIS
insulating structure containing one or more electrical insulating materials (EIM) together with
associated conducting parts employed in an electrotechnical device
3.1.2
electrical insulating material
EIM
material with negligibly low electric conductivity, used to separate conducting parts at different
electrical potentials
[IEC 60050-212:2010, 212-11-01, modified]
3.1.3
reference EIS
established EIS evaluated on the basis of either a known service experience record or a
known comparative functional evaluation
3.1.4
candidate EIS
EIS under evaluation to determine its service capability (with regard to electrical, thermal,
mechanical, environmental or multifactor stresses)
3.1.5
intended life
design life of an EIS under service conditions
3.1.6
estimated life
expected service life derived from either service experience or the results of tests performed
in accordance with appropriate evaluation procedures, or both, as established by the
responsible organization or technical committee
3.1.7
evaluation
establishment of relationships between service requirements and life data obtained from
service experience analysis or from the results of functional tests
3.2 Terms related to service stresses and ageing
3.2.1
ageing stress
electrical, thermal, mechanical or environmental stress whose action on an EIS causes
irreversible property changes
3.2.2
potentially destructive stress
stress in service which can cause the failure of the aged EIS, alone or in combination with
other stresses
3.2.3
service conditions
combination of stresses and duty that are to be expected in a specific application of an
electrical device
60505 © IEC:2011 – 11 –
3.2.4
reference operating conditions
service conditions of the equipment to which the test conditions of the functional test
procedure are related
3.2.5
service requirements
specified stresses, intended performance and duty of an electrical device
3.2.6
service experience
the quantitative and/or qualitative record during service, with or without failure of an EIS
3.2.7
ageing
irreversible changes of the properties of an EIS due to action by one or more stresses
NOTE 1 Some changes (e.g. hydrolytic changes) can be partly reversible if the ambient conditions change.
NOTE 2 Ageing leads to degradation of the EIS.
3.2.8
intrinsic ageing
irreversible changes of fundamental properties of an EIS caused by the action of ageing
factors on the EIS
3.2.9
extrinsic ageing
irreversible changes of properties of an EIS caused by action of ageing factors on
unintentionally introduced imperfections in the EIS
3.2.10
interaction
modifications of the type or degree of ageing produced by the combination of two or more
stresses relative to their ageing effect if acting individually on separate objects
3.2.11
direct interaction
interaction between simultaneously applied stresses that differs from that occurring with
sequentially applied stresses
3.2.12
indirect interaction
interaction which occurs between simultaneously applied stresses, which remains unchanged
when the factors are applied sequentially
3.3 Terms related to testing
3.3.1
functional test
procedure to obtain information about the suitability of an EIS under specified conditions
3.3.2
test object
sample of original equipment or part thereof, or model representing the equipment completely
or partially, including the EIS, to be used in a functional test

– 12 – 60505 © IEC:2011
3.3.3
accelerated ageing
ageing resulting of an increase in the level and/or frequency of application of the stress
beyond normal service conditions
3.3.4
accelerated test
functional test applying accelerated ageing to shorten testing time
3.3.5
conditioning
subjecting a specimen to an atmosphere of a specified relative humidity or complete
immersion in water or other liquid, at a specified temperature for a specified period of time
3.3.6
prediagnostic conditioning
variable or fixed stresses, which can be applied continuously or periodically to an EIS to
enhance the ability of a functional test to detect the degree of ageing
NOTE Prediagnostic conditioning may cause additional ageing.
3.3.7
diagnostic factor
variable or fixed stress which is applied to an EIS to establish the degree of ageing
3.3.8
diagnostic test
periodic or continuous application of a specified level of a diagnostic factor to a test object to
determine whether or when the end-point criterion has been reached
3.3.9
end-point criterion
moment when a system is no longer able to fulfil its service purposes
3.3.10
life
time for a property to reach the end-point criterion for objects in functional tests
3.3.11
test cycle
in a test, repetitive period of application of one or more stresses, either sequentially or
simultaneously, and of diagnostic factors
3.3.12
subcycle
defined period within test cycle
NOTE The subcycle may be, for instance, a period of application of high temperature and humidity for influencing
the system properties, or application of high voltage for diagnostic purposes
4 Ageing
4.1 Ageing mechanism
Ageing is defined as the irreversible changes of the properties of an EIS due to action by one
or more stresses. Ageing stresses may cause either intrinsic or extrinsic ageing. A schematic
representation of the basic process is shown in Figure 1.

60505 © IEC:2011 – 13 –
Ageing stresses
Electrical Thermal Mechanical Environmental

EIS
Ageing mechanisms
Intrinsic/extrinsic
Electrical Thermal Mechanical Environmental
Direct/indirect interactions
Failure
IEC  1231/11
Figure 1 – Ageing of an EIS
The type and level of contamination and/or the extent of imperfections in an EIS will, in many
types of electrical apparatus, significantly affect the service performance. In general, the
fewer and less severe the contaminant and/or defects in the EIS, the better is its
performance. To avoid obtaining misleading results from functional tests, a candidate EIS
should contain, as far as practicable, the full range of contaminants and/or defects expected
when the actual system is used in service.
The ageing stresses produce electrical, thermal, mechanical or environmental ageing
mechanisms that eventually lead to failure. During ageing, applied stresses, which initially do
not affect the EIS, can cause additional ageing and, as a result, modify the rate of
degradation.
When ageing is dominated by one ageing factor, this is referred to as single-factor ageing.
Multifactor ageing occurs when more than one ageing factor substantially affects the ageing
of the EIS. Ageing factors can act synergistically, i.e. there can be direct interactions between
the stresses. Interactions may be either positive or negative.
The ageing of a practical EIS may be complex and failure is usually caused by a combination
of ageing mechanisms, even if there is only one dominant ageing factor as, for example, in
single-factor ageing.
Where experience or existing knowledge of how a specific EIS will perform in service is
limited, the user of this standard shall decide whether single or multifactor test procedures are
appropriate for his specific equipment or apparatus.
NOTE The classification of the operational environments of electrical equipment is dealt with in IEC publications
prepared by IEC technical committee 75, and methods for environmental endurance testing of electrical equipment
are described in IEC publications prepared by IEC technical committee 50 (notably IEC sub-committee 50B), see
bibliography.
When speaking of environmental effects, this is understood to comprise environments other
than the normal standard laboratory atmospheres specified in IEC 60212.
A number of other standards that provide methods of exposure or characterization of
insulation are listed in the bibliography.

– 14 – 60505 © IEC:2011
4.2 Assessment of ageing mechanisms
Figures 2, 3, 4 and 5, present four flow charts which describe respectively in some detail
intrinsic and extrinsic electrical, thermal, mechanical and environmental ageing of an EIS.
Each chart is based on the service experience of different types of EIS and shows possible
mechanisms of deterioration and failure that can occur for the different types of ageing and
the interactions between ageing factors. Although several failure mechanisms are shown, the
charts are not intended to be exhaustive of mechanisms that might be found in actual service
conditions of all equipment. It is important to note that ageing that leads to possible failure is
usually caused by more than one mechanism.
These charts should be used as follows:
a) as a checklist to determine the ageing mechanisms of equipment and apparatus. The
mechanisms can occur sequentially or simultaneously;
b) to develop functional and accelerated ageing tests or test cycles. The magnitudes and
types of applied stresses and their duration will depend upon how they affect the ageing
mechanisms;
c) to develop suitable diagnostic tests or test cycles to assess the condition of the EIS.
Based on knowledge of service experience, operating conditions and the properties of the
components of the EIS under consideration, the user of this standard should select one or
more charts that show the main ageing factor or factors. The various ageing mechanisms that
lead to failure should be carefully examined, taking into account the levels of contaminants
and defects in the EIS. A revised chart, which only includes the relevant ageing mechanisms,
should then be produced as an aid in the development of the functional ageing and diagnostic
test cycles.
If there is insufficient information available concerning service experience and/or the possible
ageing mechanism, then the ageing conditions should be based upon the most severe levels
of stresses expected in service for which the EIS has been designed.

60505 © IEC:2011 – 15 –
4.3 Electrical ageing
CONTAMINANTS PRODUCED BY: DEFECTS
• Impurities • Processing • Cavities
• Particles • Manufacture • Protrusions
• By-products • Transport • Interfaces, cracks
• Moisture • Installation • Missing components
• Material • Thermal, mechanical • Wrinkles
incompatibilities and environmental
• Processing errors
etc. stress
• Discontinuities
• Abuse, accident
etc.
• Mis-application
etc.
ELECTRICAL STRESS (a.c., d.c., f, transients)
ELECTRICAL INSULATION SYSTEM
Intrinsic Surface, partial Absorption, Charge Water Cavity formation
breakdown discharge, corona conduction injection treeing and expansion, or
current mechanical rupture
Chemical changes Cavity
Breakdown
formation
Corrosive
Tracking Erosion Temperature
Partial
by-products
rise
discharges
Flashover
Loss of Thermal
Electrical Partial
conductor ageing
treeing discharge
Electrical continuity
treeing
FAILURE
(Different mechanisms of failure occur)
IEC  1232/11
NOTE Other stresses may contribute to failure.
Figure 2 – Intrinsic/extrinsic electrical ageing of practical EIS
Electric ageing (either a.c., d.c. or impulse) involves:
a) the effects of partial discharges when the local field strength exceeds the breakdown
strength in the liquid or gaseous dielectric adjacent to, or included in, the EIS;
b) the effects of tracking;
c) the effects of treeing;
d) the effects of electrolysis;

– 16 – 60505 © IEC:2011
e) the effects, related to those above, on adjacent surfaces of two insulating materials where
tangential fields of relatively high value can occur;
f) the effects of increased temperatures produced by high dielectric losses;
g) the effect of space charges.
Figure 2 shows intrinsic/extrinsic electrical ageing where electrical stresses are considered to
be the main ageing factor. Consider the example of a simple EIS consisting of two parallel
plane conductors embedded in an insulating material. Protrusions are known to occur on the
surfaces of conductors, and impurities (e.g., dust particles, etc.) can be included within the
insulation. The accelerated ageing should, therefore, be carried out by using ageing factors
that increase charge injection, for example by high voltage, and the diagnostic tests should be
designed to enable measurement of the effect of the injected charge and/or the partial
discharge characteristics.
In many practical EIS, the electrical ageing process that leads to failure is complex. No rigid
mathematical models have yet been developed which predict fully how the ageing factors
affect the life of an EIS. However, one empirical relationship, the inverse power model, is
often used to relate a.c. and d.c. electrical stress with life. This states that:
−n
L ∝ V
where
L is the life (time to failure or time to end-point);
V is the voltage;
n is the voltage life exponent.
The inverse power law model predicts a linear relationship between life and voltage when
plotted on log-log graph paper. Other models may be used.
Electrical ageing may also be accelerated by testing at a higher frequency than that
experienced in normal service. The frequency increase shall have been shown to provide no
change of the ageing mechanism, in the stress range, for either the candidate or the
reference system.
In special cases it may be possible to perform electrical endurance tests with stepwise
increasing stress for each test object. It is also necessary to specify a definite mathematical
relationship between stress level and time to failure, as well as a method for reducing test
results to a common time or a common stress level value. Either fixed or increasing stress
levels may be used in cyclic tests.
For data processing and life line presentations, refer to IEC/TS 61251 and IEC 62539.

60505 © IEC:2011 – 17 –
4.4 Thermal ageing
CONTAMINANTS DEFECTS
PRODUCED BY:
• Impurities • Cavities
• Processing
• Particles • Protrusions
• Manufacture
• By-products • Interfaces, cracks
• Transport, storage
• Wrinkles
• Moisture
• Installation
• Material
• Missing components
• Electrical, mechanical
incompatibilities
• Processing errors
and environmental
etc.
• Discontinuities
stresses
etc.
• Abuse, accident
• Misapplication
etc.
TEMPERATURE – high, low, cyclic, gradient
ELECTRICAL INSULATION SYSTEM
Change in state Chemical reactions Modify electric
Thermomechanical
of material stress
• Localized or bulk
• Various rates
Localized at defect or
contaminants or bulk, dependant
e.g.:   on defects, Cracks,
contaminants
• Melting
delaminations
• Vaporization
• Crystallisation
• Re-crystallization
• Glass transition
Increase in temperature
• Morphological
changes
etc.
Material modifications Corrosive
by-products
Cracks,
Localized or bulk, e.g.:
delaminations
• Polymerization
Penetration of
• Depolymerization
external
• Reduced mechanical
properties
contaminants
Penetration of
• Formation of hydrophilic Loss, migration
Loss of external
compounds of insulants
mechanical
contaminants
• Formation of conducting
strength
Loss,
compounds
migration of
• Water absorption,
Loss of
insulants
adsorption
mechanical
etc.
strength
Electrical Electrical ageing
(partial discharge,
ageing
Loss of conductor
electrical treeing,
(partial discharge,
continuity
etc.)
electrical treeing,
etc.)
FAILURE
(Different mechanisms of failure occur)

IEC  1233/11
NOTE Other stresses may contribute to failure.
Figure 3 – Intrinsic/extrinsic thermal ageing of practical EIS

– 18 – 60505 © IEC:2011
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