IEC TS 61251:2008

IEC/TS 61251
Edition 2.0 2008-05
TECHNICAL
SPECIFICATION
Electrical insulating materials – A.C. voltage endurance evaluation –
Introduction
IEC/TS 61251:2008(E)
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IEC/TS 61251
Edition 2.0 2008-05
TECHNICAL
SPECIFICATION
Electrical insulating materials – A.C. voltage endurance evaluation –
Introduction
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
R
ICS 29.035.01; 17.220.99 ISBN 2-8318-9777-7

– 2 – TS 61251 © IEC:2008(E)
CONTENTS
FOREWORD.3
1 Scope and object.5
2 Normative references .5
3 Terms and definitions .5
4 Voltage endurance .7
4.1 Voltage endurance testing.7
4.2 Electrical stress.7
4.3 Voltage endurance (VE) graph .7
4.4 Short-time electric strength .8
4.5 Voltage endurance coefficient (VEC) – n .8
4.6 Differential VEC (n ) .8
d
4.7 Electrical threshold stress (E ).9
t
4.8 Voltage endurance relationship .9
5 Test methods .10
5.1 Introductory remark .10
5.2 Tests at constant stress .11
5.2.1 Conventional VE test .11
5.2.2 Diagnostic measurements.11
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 .14
5.6 Suggested test method.14
6 Evaluation of voltage endurance.15
6.1 Significance of the VEC.15
6.2 Significance of the electrical threshold stress .15
6.3 Dispersion of data and precision requirements .15
6.4 Presentation of the results.16
Annex A (informative) The Weibull distribution.17
Bibliography.19

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 with progressive stress.13

TS 61251 © IEC:2008(E) – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL INSULATING MATERIALS –
A.C. VOLTAGE ENDURANCE EVALUATION –
INTRODUCTION
FOREWORD
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The main task of IEC technical committees is to prepare International Standards. In
exceptional circumstances, a technical committee may propose the publication of a technical
specification when
• the required support cannot be obtained for the publication of an International Standard,
despite repeated efforts, or
• The subject is still under technical development or where, for any other reason, there is
the future but no immediate possibility of an agreement on an International Standard.
Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC/TS 61251, which is a technical specification, has been prepared by IEC technical
committee 112: Evaluation and qualification of electrical insulating materials and systems.
This second edition cancels and replaces the first edition which was issued in 1993. It
constitutes a technical revision.

– 4 – TS 61251 © IEC:2008(E)
The main changes with respect to the previous edition are listed below:
• extension of the scope to cover electrical insulating material and insulation systems;
• removal of references to short time dielectric breakdown strength measurements as an
indicator of voltage endurance;
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
112/88/DTS 112/95/RVC
Full information on the voting for the approval of this technical specification 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 maintenance result 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
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

TS 61251 © IEC:2008(E) – 5 –
ELECTRICAL INSULATING MATERIALS –
A.C. VOLTAGE ENDURANCE EVALUATION –
INTRODUCTION
1 Scope
This technical specification explains 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 a.c. voltage
endurance of insulating materials and systems from the results of the tests.
The terminology to be used in voltage endurance is defined and explained. It should be
emphasized that where this technical specification is concerned with materials, the results
may not be directly applicable to the performance of insulating systems.
Voltage endurance tests are used to compare and evaluate insulating materials with regard to
their various applications in electrical systems. Determining the ability of electrical insulating
materials and systems to endure a.c. voltage stress is complex. The results of voltage
endurance tests are influenced by many factors so this technical specification should only be
considered as an attempt to present a unified view of voltage endurance for simplified
planning and analysis. Some documents for the various practical cases exist and others are
being developed.
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 60243 (all parts), Electric strength of insulating materials – Test methods
IEC 62539, Guide for the statistical analysis of electrical insulation dielectric breakdown data
(IEEE Standard 930:2004)
3 Terms, definitions and symbols
For the purposes of this document, the following terms, definitions and symbols apply.
3.1 Terms and definitions
3.1.1
voltage endurance
VE
measure of the ability of solid insulating materials to endure voltage
NOTE In this technical specification, only a.c. voltage is considered.
3.1.2
life
time of any technical system until its failure or loss of serviceability

– 6 – TS 61251 © IEC:2008(E)
3.1.3
voltage life
time for solid insulating materials to dielectric dielectric breakdown under constant voltage
stress
3.1.4
voltage endurance coefficient
VEC
numerical value of the reciprocal of the slope of a straight line log/log VE plot
3.1.5
specimen
representative test object for assessing the value of one or more physical properties
3.1.6
sample
group of nominally identical specimens from the same manufacturing batch
3.2 Symbols
c,c′ constants in the inverse-power model
E electric stress
short-time electric strength
E
o
E short-time electric strength of prestressed specimens
s
E electric threshold stress
t
f frequency
h, k constants in the exponential model
L time to dielectric dielectric breakdown
m scale parameter in the simple 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
n initial VEC
i
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
t o time to dielectric breakdown with progressive stress producing dielectric breakdown at
p
stress E
o
tan δ dissipation factor
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
ν number of dielectric breakdown stress values
ν′ number of dielectric breakdown times

TS 61251 © IEC:2008(E) – 7 –
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 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 has broken 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 (e.g. Gaussian, Weibull, lognormal, etc.).
4.2 Electrical stress
In general, it is advisable to make reference to electrical stress (voltage per unit thickness)
instead of voltage. For uniform field, electrical stress is given by the voltage (effective value)
divided by the thickness of specimens.
If the electric field is not uniform, the maximum value should be considered.
4.3 Voltage endurance (VE) graph
This is the graph of the time to dielectric breakdown 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,
usually 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 should be compared at equal thickness.
An accurate plotting of the line requires many tests at different voltages and some tests are
required at voltages which result in long times to dielectric breakdown.
The voltage endurance line may be 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 may 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). The shape of the VE graph may change greatly from one material or system to
another.
– 8 – TS 61251 © IEC:2008(E)
Log E
E
o
E
t
t Log time to breakdown
o
IEC  726/08
Figure 1 – General voltage endurance line
4.4 Short-time electric strength
The short-time electric strength is generally measured using a linearly increasing voltage. The
duration of such a test, as used in this specification, is of the order of some tens of seconds
up to some tens of minutes. These short-time electric strength measurements can be used to
indicate the degree of ageing of specimens subjected to voltage by comparing the values
after voltage exposure with the initial ones.
The results of electric strength tests (or, in general, of tests with increasing voltage) are not
reported 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, say 0,8 or 0,9E ), is made to determine the time
o o
to dielectric breakdown, t , with constant stress. The point (E , t ) is the origin of the VE line.
o o o
More details on this procedure are given in 5.5. However, when this procedure is used, the
following precautions should be taken:
i) The test should 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.
ii) The test specimens and the conditions of the specimen dielectric breakdown should 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) – n
The slope of the VE line is an indicator of the response of a material or system to electrical
stress. The parameter n is dimensionless. With a low slope of the VE line, 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 Equation (1). A large value of the
VEC does not necessarily accompany a high electric strength. It may happen that the
material with lower VEC has a longer time to dielectric breakdown at the same stress if its
short-time electric strength is so high that its poorer endurance is compensated for. The
value of n should 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 may be measured by means of the
tangent at any point. For any electrical stress, and thus for any point on the line, the
differential voltage endurance coefficient, n , can be defined as the numerical value of the
d
TS 61251 © IEC:2008(E) – 9 –
reciprocal of 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  727/08
Figure 2 – Determination of the differential VEC n
d
at a generic point P of the VE line
4.7 Electrical threshold stress (E )
t
The VE line may tend to become horizontal with decreasing stress, suggesting a limiting
stress, E , below which electrical ageing becomes negligible. This limit is called the electrical
t
threshold stress. The tendency of the line to become horizontal can sometimes be detected
by means of tests of suitable duration. However, the tests do not always succeed in revealing
such a trend. Some insulating materials or systems do not show any electrical threshold
stress even for very long test times.
4.8 Voltage endurance relationship
The VE relationship is the mathematical model of voltage life, i.e. the equation 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 equation is of the type:
–n
L = c E (1)
where
L is the time to dielectric breakdown;
E is the electrical stress;
c and n are constants dependent on temperature and other environmental conditions.
Equation (1) constitutes the so-called inverse-power model, which is the voltage-life model
often encountered with voltage endurance data on solid electrical insulation. When data are
available for time to dielectric breakdown at two voltage stresses, this model may be used to
estimate the value of n by using Equation (2):
−n
L ⎛ E ⎞
1 1
=⎜ ⎟ (2)
L E
⎝ ⎠
2 2
– 10 – TS 61251 © IEC:2008(E)
If the VE test data do not form a straight line on log-log paper, the use of the inverse-power
model is incorrect. If the line approaches an electrical threshold stress, E , other models have
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
t
data 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 simple exponential model, corresponding to an approximately
straight line in semilog coordinates for E > E but gives infinite time to dielectric
t
breakdown when E tends to E . In Equations (3) and (4), constants c′, n, k, h and E
t t
depend on temperature and other environmental conditions .
Equations (3) and (4) can be used to generate two new equations which define the trend of
the VE line between any two points, (L , E ) and (L , E ). The following
...