SIST EN 61251:2016

SLOVENSKI STANDARD
SIST EN 61251:2016
01-maj-2016
(OHNWURL]RODFLMVNLPDWHULDOL2FHQMHYDQMHY]GUåOMLYRVWLSULL]PHQLþQLQDSHWRVWL,(&

Electrical insulating materials - A.C. voltage endurance evaluation (IEC 61251:2015)
Matériaux isolants électriques - Evaluation de l'endurance à la tension alternative
Ta slovenski standard je istoveten z: EN 61251:2016
ICS:
17.220.99 Drugi standardi v zvezi z Other standards related to
elektriko in magnetizmom electricity and magnetism
29.035.01 Izolacijski materiali na Insulating materials in
splošno general
SIST EN 61251:2016 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN 61251:2016

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SIST EN 61251:2016


EUROPEAN STANDARD EN 61251

NORME EUROPÉENNE

EUROPÄISCHE NORM
February 2016
ICS 17.220.99; 29.035.01

English Version
Electrical insulating materials and systems - A.C. voltage
endurance evaluation
(IEC 61251:2015)
Systèmes et matériaux isolants électriques - Évaluation de Elektrische Isolierstoffe und -systeme - Ermittlung der
l'endurance a la tension alternative Wechselspannungsbeständigkei
(IEC 61251:2015) (IEC 61251:2015)
This European Standard was approved by CENELEC on 2015-12-23. 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.


European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels
© 2016 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
 Ref. No. EN 61251:2016 E

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SIST EN 61251:2016
EN 61251:2016
European foreword
The text of document 112/338/FDIS, future edition 1 of IEC 61251, prepared by IEC/TC 112
"Evaluation and qualification of electrical insulating materials and systems" was submitted to the
IEC-CENELEC parallel vote and approved by CENELEC as EN 61251:2016.

The following dates are fixed:
(dop) 2016-09-23
• latest date by which the document has to be
implemented at national level by
publication of an identical national
standard or by endorsement
• latest date by which the national (dow) 2018-12-23
standards conflicting with the
document have to be withdrawn

Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC [and/or CEN] shall not be held responsible for identifying any or all such
patent rights.

Endorsement notice
The text of the International Standard IEC 61251:2015 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 standards indicated:

IEC 60243-1 NOTE Harmonized as EN 60243-1.
IEC 60243-2 NOTE Harmonized as EN 60243-2.
IEC 60243-3 NOTE Harmonized as EN 60243-3.
IEC 60343 NOTE Harmonized as EN 60343.
IEC 61649 NOTE Harmonized as EN 61649.

2

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SIST EN 61251:2016
EN 61251:2016


Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
NOTE 1 When 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.cenelec.eu.
Publication Year Title EN/HD Year
IEC 62539 -  Guide for the statistical analysis of - -
electrical insulation breakdown data

3

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SIST EN 61251:2016

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SIST EN 61251:2016



IEC 61251

®


Edition 1.0 2015-11




INTERNATIONAL



STANDARD




NORME



INTERNATIONALE











Electrical insulating materials and systems – AC voltage endurance evaluation



Systèmes et matériaux isolants électriques – Évaluation de l'endurance à la


tension alternative

















INTERNATIONAL

ELECTROTECHNICAL

COMMISSION


COMMISSION

ELECTROTECHNIQUE


INTERNATIONALE




ICS 17.220.99; 29.035.01 ISBN 978-2-8322-2990-3



Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN 61251:2016
– 2 – IEC 61251:2015 © IEC 2015
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and symbols. 6
3.1 Terms and definitions . 6
3.2 Symbols . 7
4 Voltage endurance . 7
4.1 Voltage endurance testing . 7
4.2 Electrical stress . 7
4.3 Voltage endurance (VE) graph . 8
4.4 Short-time electric strength . 8
4.5 Voltage endurance coefficient (VEC) . 9
4.6 Differential VEC (n ) . 9
d
4.7 Electrical threshold stress (E ) . 9
t
4.8 Voltage endurance relationship . 10
5 Test methods . 11
5.1 Introductory remarks . 11
5.2 Tests at constant stress . 11
5.2.1 Conventional VE test . 11
5.2.2 Diagnostic measurements . 12
5.2.3 Detection of an electrical threshold . 12
5.3 Tests at higher frequency. 12
5.4 Progressive stress tests . 13
5.5 Preliminary tests to determine the initial part of the VE line . 15
5.6 Recommended test procedure . 15
6 Evaluation of voltage endurance . 15
6.1 Significance of the VEC . 15
6.2 Significance of the electrical threshold stress . 16
6.3 Dispersion of data and precision requirements . 16
6.4 Presentation of the results . 16
Annex A (informative) The Weibull distribution . 18
A.1 Weibull distribution times to dielectric breakdown . 18
A.2 Weibull distribution dielectric breakdown stresses . 18
A.3 Generalized Weibull distribution of the dielectric breakdown stresses . 18
A.4 Inverse power model for the time to dielectric breakdown . 19
Bibliography . 20

Figure 1 – General voltage endurance line . 8
Figure 2 – Determination of the differential VEC n at a generic point P of the VE line . 9
d
Figure 3 – Plotting the VE line in a progressive stress test using different rates of
stress rise . 14

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SIST EN 61251:2016
IEC 61251:2015 © IEC 2015 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

ELECTRICAL INSULATING MATERIALS AND SYSTEMS –
AC VOLTAGE ENDURANCE EVALUATION

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, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their 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) 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 61251 has been prepared by IEC technical committee 112:
Evaluation and qualification of electrical insulating materials and systems.
This first edition of IEC 61251 cancels and replaces the second edition of IEC TS 61251,
published in 2008. This edition constitutes a technical revision.
This edition includes the following significant technical changes with respect to the second
edition of IEC TS 61251:
a) upgrade from Technical Specification to an International Standard;
b) clarification of issues raised since publication of IEC TS 61251.

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SIST EN 61251:2016
– 4 – IEC 61251:2015 © IEC 2015
The text of this standard is based on the following documents:
FDIS Report on voting
112/338/FDIS 112/347/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.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC website 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.

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SIST EN 61251:2016
IEC 61251:2015 © IEC 2015 – 5 –
INTRODUCTION
This International Standard covers insulating materials and systems. Voltage endurance tests
are used to compare and evaluate insulating materials and systems. It is complex to
determine the capability of electrical insulating materials and systems to endure a.c. voltage
stress. The results of voltage endurance tests are influenced by many factors. Therefore this
International Standard can be considered as an attempt to present a unified view of voltage
endurance for simplified planning and analysis.

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SIST EN 61251:2016
– 6 – IEC 61251:2015 © IEC 2015
ELECTRICAL INSULATING MATERIALS AND SYSTEMS –
AC VOLTAGE ENDURANCE EVALUATION



1 Scope
This International Standard describes many of the factors involved in voltage endurance tests
on electrical insulating materials and systems. It describes the voltage endurance graph, lists
test methods illustrating their limitations and gives guidance for evaluating the sinusoidal a.c.
voltage endurance of insulating materials and systems from the results of the tests. This
International Standard is applicable over the voltage frequency range 20 Hz to 1 000 Hz. The
general principles can also be applicable to other voltage shapes, including impulse voltages.
The terminology to be used in voltage endurance is defined and explained.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 62539, Guide for the statistical analysis of electrical insulation dielectric breakdown data
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
voltage endurance
VE
measures of the capability of a solid insulating material to endure voltage
Note 1 to entry: In this International Standard, only a.c. voltage is considered.
Note 2 to entry: This note only applies to the French language.
3.1.2
life
time to dielectric breakdown
3.1.3
voltage endurance coefficient
VEC
numerical value of the reciprocal of the slope of a straight line log-log VE plot
Note 1 to entry: This note only applies to the French language.
3.1.4
specimen
representative test object for assessing the value of one or more physical properties

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SIST EN 61251:2016
IEC 61251:2015 © IEC 2015 – 7 –
3.1.5
sample
group of nominally identical specimens extracted randomly from the same manufacturing
batch
3.2 Symbols
c, c′ constants in the inverse-power model
E electric stress
E short-time electric strength
o
E electric threshold stress
t
f frequency
h, k constants in the exponential model
L life
m scale parameter in the Weibull distribution (one variable)
M scale parameter in the generalized Weibull distribution (two variables)
n exponent of stress in the inverse-power model coinciding with the VEC
n differential VEC
d
R dimensional ratio
t time
t time to dielectric breakdown at constant stress
c
t time to dielectric breakdown at constant stress E
o o
t time to dielectric breakdown with progressive stress
p
tan δ dissipation factor
α scale parameter (63,2 percentile) in the Weibull distribution of times to dielectric
breakdown at constant stress
β shape parameter in the Weibull distribution of times to dielectric breakdown at
constant stress
γ shape parameter of the Weibull distribution of the dielectric breakdown stresses from a
progressive stress test
4 Voltage endurance
4.1 Voltage endurance testing
To evaluate the voltage endurance of insulating materials or systems, a number of specimens
are subjected to a.c. voltage and their times to dielectric breakdown are measured. In practice,
several samples of many specimens are tested at different voltages to reveal the effect of the
applied voltage on the time to dielectric breakdown. The arithmetic mean time to dielectric
breakdown of each sample is the average time to dielectric breakdown of all specimens tested
at that voltage. The time at which a certain percentage of specimens break down is the
estimated time to dielectric breakdown with a probability equal to this percentage.
The statistical treatment of the data (either by analytical or graphical methods) allows the
extraction of additional data such as other failure percentiles or confidence bounds and,
possibly, determination of the distribution (Gaussian, Weibull, lognormal, etc.).
4.2 Electrical stress
In general, reference to electrical stress (voltage per unit thickness) instead of voltage is
required. For a uniform field, electrical stress is given by the voltage (effective value) divided
by the thickness of specimens.

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If the electric field is not uniform, the maximum value shall be considered by the relevant
equipment committees.
4.3 Voltage endurance (VE) graph
The VE graph represents the time to dielectric breakdown (life) versus the corresponding
value of electrical stress. In the VE graph, the electrical stress is plotted as the ordinate with
either a linear or logarithmic scale. The times to dielectric breakdown are plotted on the
abscissa with a logarithmic scale. The voltage endurance line on this graph gives the final
result of the VE tests as it allows clear and complete evaluation of voltage endurance of the
specimens under the specified test conditions. For maximum significance, materials or
systems shall be compared at equal thickness and using the same type of electrodes,
temperature, humidity and ambient gas, or as agreed by the relevant equipment committees.
An accurate plotting of the line requires more than three tests at different voltages and one or
more tests are required at voltages which result in times to failure longer than 1 000 h. In any
case, a minimum number of three tests is required to draw the VE graph.
The voltage endurance line is straight or curved. In the latter case, its trend can often be
approximated by a few straight regions: sometimes a first part for short times with a low slope,
a middle region (which can extend to long times) with a steeper slope and finally a further
trend of the line showing a tendency to become horizontal (see Figure 1, where a general VE
line is shown). It is likely that the shape of the VE graph changes significantly from one
material or system to another. With a curve as shown in Figure 1, the VEC is not constant,
and the VEC will be different at different times (see n in Figure 2).
d

Log E
E
o
E
t
t Log time to breakdown
o
IEC

Figure 1 – General voltage endurance line
4.4 Short-time electric strength
The short-time electric strength is measured using a linearly increasing voltage. The duration
of such a test, as used in this International Standard, is of the order of one minute up to some
tens of minutes. The arithmetic mean value of the breakdown field for the tested sample is E .
o
The results of electric strength tests (or, in general, of tests with increasing voltage) are not
represented directly in the VE graph. Instead, a constant voltage test at the same stress as
the mean electric strength, E (or very close to it, 0,9E or as agreed), is made to determine
o o
the time to dielectric breakdown, t , with constant stress. The point (E , t ) is the origin of the
o o o
VE line. More details on this procedure are given in 5.5. However, when this procedure is
used, the following precautions shall be taken.

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IEC 61251:2015 © IEC 2015 – 9 –
a) The electric strength tests shall be carried out under the same conditions (humidity,
temperature, etc.), in the same test cell and with the same procedures as for the voltage
endurance tests.
b) The test specimens, the breakdown path and the conditions of the specimen after
dielectric breakdown shall be examined and recorded for future use in the analysis of the
results. The latter is to ensure that the mode of failure at high stress is the same as that of
the other specimens tested later at lower stress.
4.5 Voltage endurance coefficient (VEC)
The slope of the VE line, n, is an indicator of the response of a material or system to electrical
stress. The parameter n is dimensionless. With a small slope of the VE line (i.e. a large value
of VEC), even a small reduction of stress produces a great increase in life. The reciprocal of
the slope is taken to be consistent with the numerical value of the exponent n in Formula (1).
A large value of the VEC does not correspond necessarily to high electric strength. It can
happen that the material with lower VEC has a longer time to dielectric breakdown at a given
stress if its short-time electric strength is so high that its poorer endurance is compensated for.
The value of n shall be associated with a high mean electric strength before attributing a high
endurance to the material. What is most significant is the retention of usable electric
strength for long periods of time.
4.6 Differential VEC (n )
d
If the VE line is curved in log-log coordinates, its slope is measured by means of the tangent
at any point. For any electrical stress, and thus for any point on the line, the differential
, can be defined as the absolute value of the reciprocal of
voltage endurance coefficient, n
d
the slope of the curve at that point (Figure 2) according to the life model described in
Clause 5.

E
Log
VE line
E
o
Line for determining n
d
1,0
Log time to breakdown
t
o
IEC

Figure 2 – Determination of the differential VEC n at a generic point P of the VE line
d
4.7 Electrical threshold stress (E )
t
If the VE line tends to become horizontal with decreasing stress within the test stress-times,
this indicates the presence of a limiting stress, E , below which electrical ageing becomes
t
negligible. This limit is called the electrical threshold stress. The tendency of the line to
become horizontal is detected by means of tests of suitable duration. However, the tests do
not always succeed in revealing such a trend in a reasonable time. Some insulating materials
or systems do not show any electrical threshold stress even for very long test times.

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4.8 Voltage endurance relationship
The VE relationship is the mathematical model of life under electrical stress or voltage, i.e.
the formula relating electrical stress and time to dielectric breakdown, whose graphical
representation is given by the VE line. If this line is straight on log-log graph paper, the
formula is of the type:
−n
L = c E (1)
where
L is the time to dielectric breakdown or time to failure or life;
E is the electrical stress;
c and n are constants dependent on temperature and other environmental parameters.
Formula (1) constitutes the so-called inverse-power model, which is the voltage-life model
often encountered with voltage endurance data on solid electrical insulation. In this case the
VEC is n, and it is constant. When data are available for time to dielectric breakdown at two
constant-voltage stresses, this model shall be used to get a rough estimate of the value of n
by using Formula (2):
−n
L  E 
1 1
 
=  (2)
 
L E
2 2
 
If the VE test data do not form a straight line on log-log paper, the use of the inverse-power
, other models have
model is incorrect. If the line approaches an electrical threshold stress, E
t
been proposed, among them
­n
L = c′ (E – E ) , (3)
t
which becomes the inverse-power model if E tends to 0 and is preferably used when the data
t
for short and medium times fit a straight line on log-log coordinates. Alternatively, another
model is
k exp (− h E)
L = , (4)
E − E
t
which derives from the exponential model, corresponding to an approximately straight line in
semilog coordinates for E > E but gives infinite time to dielectric breakdown when E tends to
t
E . In Formulas (3) and (4), constants c′, n, k, h and E depend on temperature and other
t t
environmental conditions.
Formulas (3) and (4) generate two new formulas which define the trend of the VE line
, E ) and (L , E ). The following formulas are obtained:
between any two points, (L
1 1 2 2
−n
 E − E 
L
1 1 t
 
= , (5)
 
L E − E
2  2 t 
L exp {− h (E − E )}
1 1 2
= . (6)

L (E − E )/ (E − E )
2 1 t 2 t

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SIST EN 61251:2016
IEC 61251:2015 © IEC 2015 – 11 –
The formulas of the VE line for a straight line or a straight-line segment on log-log plot are
Formulas (1) and (2). When there is a tendency toward a threshold after an approximately
linear trend on log-log or semilog graph paper, Formulas (3), (4), (5) and (6) apply.
By taking the logarithms, the inverse-power model, Formula (1), becomes
ln (L) = ln (c) − n ln (E) . (7)
This is the formula of the straight VE line in log-log coordinates. Its slope is −1/n. As the
numerical value of the reciprocal of the slope is equal to n, the VEC can also be defined as
the exponent n in the inverse-power model.
5 Test methods
5.1 Introductory remarks
Different methods of carrying out the VE test can be used. The differences concern the way of
applying voltage (constant or increasing with time), the frequency (service or higher) and the
time
...