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INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL INSULATING MATERIALS –
THERMAL ENDURANCE PROPERTIES –
Part 8: Instructions for calculating thermal endurance characteristics using
simplified procedures
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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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
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This International Standard has been developed by a Joint Working Group of IEC TC 112 and
ISO TC 61 SC6.
The text of this standard is based on the following documents:
FDIS
Report on voting
XX/XX/FDIS
XX/XX/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.
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The committee has decided that the contents of this publication will remain unchanged until
the stability date 1 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withdrawn,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.
—————————
1 The National Committees are requested to note that for this publication the stability date is 2018
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1
Introduction
2
3
4
5
The term thermal endurance is used here to refer to tests made in air, excluding any other influence or
stress applied to the test specimens. Thermal endurance properties evaluated in different
environments and/or with different stresses applied to the test specimens require different test
procedures.
6
7
8
In this International Standard, the study of the thermal ageing of materials is based solely on the
change in certain properties resulting from a period of exposure to elevated temperature. The
properties studied are always measured after the temperature has returned to ambient.
9
10
11
Properties of materials change at various rates on thermal ageing. To enable comparisons to be made of the
thermal ageing of different materials, the criteria for judgment depend on the type of property to be
studied and its acceptable limiting value.
12
13
14
In the application of this standard it is assumed that a practically linear relationship exists between the
logarithm of the time required to cause the predetermined property change and the reciprocal of the
corresponding absolute temperature (Arrhenius Law).
15
16
For the materials tested, no transition, in particular a first-order transition, should occur in the
temperature range under study.
17
18
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CONTENTS
19
20
1
Scope ...............................................................................................................................7
21
2
Normative references .......................................................................................................7
22
3
Terms, definitions, symbols and abbreviated terms...........................................................8
4
3.1 Definitions ...............................................................................................................8
3.3 Symbols and abbreviated terms...............................................................................9
Testing procedure .......................................................................................................... 10
23
24
25
4.1
4.2
4.3
26
27
28
29
30
31
5
32
33
34
35
36
37
38
39
40
41
42
43
Outline description of procedures .......................................................................... 13
Simplified calculation procedures .......................................................................... 14
5.2.1 Times to end-point, thermal endurance graph ............................................ 14
5.2.2 Calculation of the regression line............................................................... 14
5.2.3 Calculation of deviation from linearity ........................................................ 15
5.2.4 Temperature index and halving interval ..................................................... 15
5.2.5 Validity of simplified calculations ............................................................... 16
5.3 Use of correlation coefficient ................................................................................. 16
5.4 Determination of RTI ............................................................................................. 16
5.5 Evaluation of results .............................................................................................. 17
5.6 Test report ............................................................................................................ 20
Bibliography.......................................................................................................................... 21
44
Annex A (informative) Example: simplified statistical analysis ............................................... 22
45
46
47
48
49
50
51
52
53
54
General ................................................................................................................. 10
Test specimens ..................................................................................................... 10
Preparation and number of test specimens ............................................................ 11
4.3.1 Preparation................................................................................................ 11
4.4 Exposure temperatures and times ......................................................................... 11
Simplified numerical and graphical evaluation procedures .............................................. 13
5.1
5.2
A.1
A.2
A.3
A.4
A.5
Simplified statistical analysis of TI data ................................................................. 22
A.1.1 Temperature index and halving interval ..................................................... 22
2
Comparation between linearity evaluation through s y and r .................................. 22
Effect of number of temperatures .......................................................................... 22
Conclusions .......................................................................................................... 22
Examples Test Reports ......................................................................................... 23
A.5.1 Example 1 ................................................................................................. 23
A.5.2 Example 2 ................................................................................................. 23
A.5.3 Example 3 ................................................................................................. 24
A.5.4 Dependence of cut-off value for correlation coefficient............................... 25
55
56
57
Figure 1 - Determination of the time to reach the end-point at each temperature Property variation (according to IEC 60216-1) ........................................................................... 18
58
59
Figure 2 - Thermal endurance graph - Temperature index - Halving interval (according to
IEC 60216-1) ......................................................................................................................... 19
60
61
Figure 3 - Thermal endurance graph - Relative temperature index - Halving interval
(according to IEC 60216-1)..................................................................................................... 20
62
63
64
Table 1 – Suggested exposure temperatures and times ........................................................ 13
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THERMAL ENDURANCE PROPERTIES –
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Part 8: Instructions for calculating thermal endurance characteristics using simplified
procedures
71
1
Scope
72
73
This part of IEC 60216 specifies the general ageing conditions and simplified procedures to
be used for deriving thermal endurance characteristics.
74
75
The procedures specify the principles for evaluating the thermal endurance properties of materials
exposed to elevated temperature for long periods.
76
77
78
In the application of this standard, it is assumed that a practically linear relationship exists
between the logarithm of the time required to cause the predetermined property change and
the reciprocal of the corresponding absolute temperature (Arrhenius relationship).
79
80
For the valid application of the standard, no transition, in particular no first-order transition
should occur in the temperature range under study.
81
82
Throughout the rest of this standard the term "insulating materials" is always taken to mean
"insulating materials and simple combinations of such materials".
83
2 Normative references
84
85
86
87
88
89
90
The following normative documents contain provisions which, through reference in this text,
constitute provisions of this part of IEC 60216. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to
agreements based on this part of IEC 60216 are encouraged to investigate the possibility of
applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of IEC
and ISO maintain registers of currently valid International Standards.
91
IEC 60085: Electrical insulation - Thermal evaluation and designation
92
93
IEC 60216-1: Electrical insulating materials –Properties of thermal endurance – Part 1:
Ageing procedures and evaluation of test results
94
95
IEC 60216-2: Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 2: Choice of test criteria
96
97
IEC 60216-3: Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 3: Instructions for calculating thermal endurance characteristics
98
99
IEC 60216-4-1: Guide for the determination of thermal endurance properties of electrical
insulating materials – Part 4: Ageing ovens – Section 1: Single-chamber ovens
100
101
i)
IEC 60216 - 5: Determination of Relative Thermal Endurance Index (RTE) of an
Insulating Material
102
ISO 291:1997, Plastics – Standard atmospheres for conditioning and testing
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104
ISO 11346:1997, Rubber, vulcanized or thermoplastic – Estimation of life-time and maximum
temperature of use from an Arrhenius plot
105
3 Terms, definitions, symbols and abbreviated terms
106
107
For the purposes of this document, the following terms, definitions, symbols and abbreviations
apply.
108
3.1 Definitions
109
110
111
112
113
114
3.1.1
temperature index
TI
numerical value of the temperature in degrees Celsius derived from the thermal endurance
relationship at a time of 20 000 h (or other specified time)
[IEC 60050-212-12-11, modified]
115
116
117
118
119
120
3.1.2
halving interval
HIC
numerical value of the temperature interval in kelvins which expresses the halving of the time
to end-point taken at the temperature equal to TI
[IEC 60050-212-12-13, modified]
121
122
123
124
125
3.1.3
thermal endurance graph
graph in which the logarithm of the time to reach a specified end-point in a thermal endurance
test is plotted against the reciprocal thermodynamic test temperature
[IEC 60050-212-12-10]
126
127
128
129
130
131
132
3.1.4
thermal endurance graph paper
graph paper having a logarithmic time scale as the ordinate, graduated in powers of ten
(from 10 h to 100 000 h is often a convenient range). Values of the abscissa are proportional
to the reciprocal of the thermodynamic (absolute) temperature. The abscissa is usually
graduated in a non-linear (Celsius) temperature scale oriented with temperature increasing
from left to right.
133
134
135
3.1.5
degrees of freedom
number of data values minus the number of parameter values
136
137
138
139
3.1.6
correlation coefficient
number expressing the completeness of the relation between members of two data sets, equal
to the covariance divided by the square root of the product of the variances of the sets
140
NOTE
141
142
143
3.1.7
end-point
a point at which a definable event in a study takes place
144
145
146
3.1.8
time to end-point (failure time)
time measure until a failure occurs.
The value of its square is between 0 (no correlation) and 1 (complete correlation).
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150
3.1.9
square root of mean square deviation of points from regression line
Sy
mean deviation of data points from the regression line
151
152
153
154
155
3.1.10
square of the correlation coefficient
2
r
represents the fraction of the variation in one variable that may be explained by the other
variable.
156
157
158
159
3.1.11
destructive test
diagnostic property test, where the test specimen is irreversibly changed by the property
measurement, in a way which precludes a repeated measurement on the same specimen
160
161
162
163
164
3.1.12
non-destructive test
diagnostic property test, where the properties of the test specimen are not permanently
changed by the measurement, so that a further measurement on the same specimen may be
made after appropriate treatment
165
166
167
168
169
3.2
proof test
diagnostic property test, where each test specimen is, at the end of each ageing cycle,
subjected to a specified stress, further ageing cycles being conducted until the specimen fails
on testing
170
171
172
173
3.2.1
temperature group (of specimens)
number of specimens being exposed together to the same temperature ageing in the same
oven
174
175
NOTE Where there is no risk of ambiguity, either temperature groups or test groups may be referred to simply as
groups.
176
177
178
179
3.2.2
test group (of specimens)
number of specimens removed together from a temperature group (as above) for destructive
testing
180
3.3 Symbols and abbreviated terms
a,b
Regression coefficients
a,b,c,d
Numbers of specimens for destructive tests
n
Number of y-values
N
Total number of test specimens
r
Correlation coefficient
F
Fisher distributed stochastic variable
sy
Square root of mean square deviation of points from regression line
x
Reciprocal thermodynamic temperature (1/Θ)
y
Logarithm of time to end-point
ϑ
Temperature, °C
Θ
Temperature, thermodynamic (Kelvins)
Θ0
Value in Kelvins of 0 °C (273,15 K)
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τ
Time (to end-point)
χ2
χ 2 -distributed stochastic variable
µ2
Central second moment of group of values
TI
Temperature Index
TC
Lower 95 % confidence limit of TI
HIC
Halving interval at temperature equal to TI
RTI
Relative temperature index
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4 Testing procedure
182
4.1 General
183
184
Simplified procedures, which do not test the data dispersion but only deviations from linear
behaviour, are described.
185
186
187
It is possible, with some limitations, to evaluate the thermal endurance data graphically. In
this case, statistical assessment of data dispersion is not possible, but it is considered
important to evaluate any deviation of the data from the linear relationship.
188
189
190
Since the temperature is very often the dominant ageing factor affecting an electrical
insulating material (EIM) certain basic thermal classes are useful and have been recognized
as such internationally (see IEC 60085).
191
4.2 Test specimens
192
193
194
The accuracy of endurance test results depends largely on the number of specimens aged
at each temperature. Generally, the following instructions, which influence the testing
procedure, apply.
195
196
197
Where the test criterion for non-destructive or proof tests is based upon the initial value of the
property, this should be determined from a group of specimens of at least twice the number of
specimens in each temperature group.
198
199
The dimensions and method of preparation of the test specimens shall be in accordance with the
specifications given for the relevant test method.
200
201
For a criterion requiring a destructive test, the minimum total number (N) of test specimens
needed depends on
202
N=a×b×c+d
203
(1)
where
204
205
a
is the number of specimens in a test group undergoing identical treatment at one
temperature and discarded after determination of the property (usually five);
206
b
is the number of treatments, i.e. exposure lengths, at one temperature;
207
c
is the number of exposure temperature levels;
208
209
210
211
212
d
is the number of specimens in the group used to establish the initial value of the
property. Normal practice is to select d = 2a when the diagnostic criterion is a
percentage change of the property from its initial level. When the criterion is an
absolute property level, d is usually given the value of zero, unless reporting of the
initial value is required.
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214
a) For a criterion requiring non-destructive testing, in most cases a group of five test
specimens for each exposure temperature is adequate.
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b) For proof-test criteria, a group of at least 11 and possibly 21 specimens will be required
for each exposure temperature.
217
218
c) For a criterion requiring a destructive test, the minimum total number (N) of test
specimens needed is derived as follows:
219
220
221
222
223
224
NOTE 1
When there is a large number of specimens to be tested, it may be possible in certain cases
to deviate from the relevant test specifications and to reduce this number. However, it must be recognized that the
precision of the test result depends to a large extent on the number of specimens tested. In contrast, when the
individual results are too scattered, an increase in the number of specimens may be necessary in order to obtain
satisfactory precision. It is advisable to make an approximate assessment, by means of preliminary tests, of the
number and duration of the ageing tests required.
225
4.3 Preparation and number of test specimens
226
4.3.1
227
228
The specimens used for the ageing test should constitute a random sample from the
population investigated and are to be treated uniformly.
229
230
The material specifications or the test standards will contain all necessary instructions for the
preparation of specimens.
231
232
233
234
235
The thickness of specimens is in some cases specified in the list of property measurements
for the determination of thermal endurance (see IEC 60216-2); otherwise the thickness shall
be reported. Some physical properties are sensitive even to minor variations of specimen
thickness. In such cases, the thickness after each ageing period may need to be determined
and reported if required in the relevant specification.
236
237
238
239
The thickness is also important because the rate of ageing may vary with thickness. Ageing
data of materials with different thicknesses are not always comparable. Consequently, a
material may be assigned more than one thermal endurance characteristic derived from the
measurement of properties at different thicknesses.
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241
242
243
The tolerances of specimen dimensions should be the same as those normally used for
general testing; where specimen dimensions need smaller tolerances than those normally
used, these special tolerances should be given. Screening measurements ensure that
specimens are of uniform quality and typical of the material to be tested.
244
245
246
247
Since processing conditions may significantly affect the ageing characteristics of some
materials, it shall be ensured that, for example, sampling, cutting sheet from the supply roll,
cutting of anisotropic material in a given direction, moulding, curing, pre-conditioning, are
performed in the same manner for all specimens.
248
4.4 Exposure temperatures and times
249
250
251
For TI determinations, test specimens should be exposed to not less than three, preferably at
least four, temperatures covering a sufficient range to demonstrate a linear relationship
between time to end-point and reciprocal thermodynamic (absolute) temperature.
252
253
254
To reduce the uncertainties in calculating the appropriate thermal endurance characteristic,
the overall temperature range of thermal exposure needs to be carefully selected, observing
the following requirements:
255
256
257
a) the lowest exposure temperature shall be one which will result in a mean or median time
to end-point more than 1/4 the extrapolation time (which is generally 20000 hours) when
determining TI.
258
259
NOTE 1 The mean time corresponding to TI is generally 20000 hours, thus the lowest exposure temperature shall
correspond to a mean time >= 5000 hours;
260
b) the extrapolation necessary to establish TI shall not be more than 25 K;
Preparation
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c) the highest exposure temperature shall be one which will result in a mean or median time
to end-point of more than 100 h.
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264
265
NOTE 2
For some materials, it may not be possible to achieve a time to end-point of less than 500 h while
retaining satisfactory linearity. However, it is important that a smaller range of mean times to end-point will lead to
a larger confidence interval of the result for the same data dispersion.
266
Table 1 gives guidance in making initial selections.
267
268
A number of recommendations and suggestions,
temperatures, will be found in IEC 60216-1 Annex B.
269
270
271
When required, before the heat-ageing procedure is started, an initial test shall be made at
room temperature with the required number of specimens conditioned and tested in
accordance with the chosen test method.
272
273
274
Selection of adequate exposure temperatures requires previously determined information on the
material under test. If such information is not available, exploratory tests may help in selecting
exposure temperatures which are suitable for evaluating the thermal endurance characteristics.
275
276
For heat ageing, ovens shall be used that meet the requirements specified in IEC 60216-4, in
particular with respect to the temperature tolerances and ventilation rates of air exchange.
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278
279
280
281
282
283
It is good practice to keep an adequate number of test specimens separately as a reserve of the
original material batch from which such specimens may subsequently be prepared. In this
way, any required ageing of additional specimens in case of unforeseen complications will
introduce a minimum risk of producing systematic differences between groups of specimens.
Such complications may arise, for example, if the thermal endurance relationship turns out to
be non-linear, or if specimens are lost due to thermal runaway of an oven. Moreover they can
be used:
useful
in
establishing
times
284
- for cases in which the accuracy requires heat ageing at an additional temperature;
285
- as reference specimens.
and
286
They shall be stored in an appropriately controlled atmosphere (see ISO 291).
287
288
289
Before the heat-ageing procedure is started, an initial test shall be made at room temperature
with the required number of specimens conditioned and tested in accordance with the chosen
test method.
290
291
Thermosetting materials shall be conditioned for 48 h at the lowest exposure temperature of
the range selected.
292
293
NOTE 3 If necessary, thermoplastic materials should be annealed for 48 h at the lowest exposure temperature of the range
selected.
294
295
Place the required number of specimens in each of the ovens maintained at the selected
temperatures.
296
297
If there is a risk of cross-contamination between test specimens originating from different
materials, use separate ovens for each material.
298
299
300
301
At the end of each heat-ageing period, the required number of test specimens is removed from
the oven and conditioned, if necessary, under the appropriately controlled atmosphere (see
ISO 291). The test, in accordance with the selected test criterion, shall be carried out at room
temperature.
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NOTE 4 After the first week, during which the number of determinations of the characteristic under investigation can vary
depending on the material under test, the test durations are generally selected in accordance with annex C.
304
305
Continue this procedure until the numerical value of the characteristic under investigation
reaches the relevant threshold value.
306
Table 1 – Suggested exposure temperatures and times for TI corresponding to 20000 h
Estimated
value of TI
in range
°C
95-104
Exposure temperature
°C
Boxes: duration of exposure cycle in days
120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 360
28
105-114
115-124
125-134
135-144
145-154
155-164
165-174
175-184
14
28
7
14
28
3
7
14
28
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
3
7
14
28
185-194
195-204
205-214
215-224
225-234
235-244
245-254
1
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
1
3
7
14
28
3
7
14
28
1
1
3
7
14
1
3
7
1
3
307
308
309
NOTE 2 This table is intended primarily for cyclic proof testing and non-destructive tests, but may also be used as
a guide for selection of suitable time intervals for destructive tests. In this case, cycle times of 56 days, or even
more, may be required.
310
311
312
NOTE 3 When extending the test programme by submitting additional specimens to ageing at temperatures below
the lower of the originally planned ageing temperatures, a temperature interval of 10 K and cycle duration of 42
days for TI determination should be considered.
313
5 Simplified numerical and graphical evaluation procedures
314
5.1
315
316
317
At a chosen temperature, the variations in the numerical value of a chosen characteristic (for
example, a mechanical, optical or electrical property: see IEC 60216-2) are determined as a
function of time.
318
319
The procedure is continued until the specified end-point value of that characteristic has been
reached, resulting in the time to end-point at that particular temperature.
320
321
322
323
Further specimens are exposed at a minimum of two other temperatures and the variations in
the relevant characteristic determined. It is recommended that test specimens be heated-aged
at three or four temperatures, and the time to end-point for each of the temperatures
determined.
Outline description of procedures
1
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325
326
When the data at all temperatures are complete, a thermal endurance graph is drawn, and a
relatively simple statistical calculation made, to assess whether the linearity of the graph
justifies the calculation of thermal endurance characteristics.
327
328
329
330
331
The test chosen shall relate to a characteristic which is likely to be of significance in practice
and, wherever possible, use shall be made of test methods specified in international
standards (see, for example, IEC 60216-2). If the dimensions and/or form of the test
specimens are altered by the heat treatment, then only test methods which are independent of
these effects may be used.
332
For the selection of the end-point, two factors shall be considered:
333
334
a) the period of time for which a temperature index shall be estimated. For general purposes,
a period of 20 000 h is recommended; for special purposes, other times may be specified;
335
336
b) the acceptable change in value of the chosen characteristic. This value depends on the
conditions of use foreseen.
337
5.2 Simplified calculation procedures
338
5.2.1
339
340
341
342
343
For destructive tests for each exposure temperature and for the group removed from the oven
after each heat-ageing period, the mean value of the chosen property is plotted as a function
of the logarithm of the time of ageing (see figure 1). The point at which this graph intersects
the horizontal line representing the end-point criterion is taken as the time to end-point of the
temperature group.
344
345
346
347
For non-destructive tests, the value of property measured on each specimen after each
ageing period is plotted and the point at which this graph intersects the horizontal line
representing the end-point criterion is taken as the time to end-point of the specimen. The
time to end-point of the temperature group is the mean of the specimen times.
348
349
350
351
When applying a proof test, each ageing time shall be calculated as the mean of the times at
the beginning and end of the ageing period. The ageing time of the temperature group shall
be taken as the time of the ageing period in which the median failure on proof test takes
place.
352
353
354
The mean times to end point are plotted versus the reciprocal values of the exposure
temperatures. The intersection of this curve with the chosen time limit (in general 20 000 h)
gives the temperature index sought.
355
5.2.2
356
357
358
The ageing function assumed for the purposes of this standard is the equation relating
the absolute (Kelvin) temperature Θ to the time needed for a fixed change in the value of
property, τ :
359
τ = Ae B/Θ
Times to end-point, thermal endurance graph
Calculation of the regression line
360
where
361
A and B are constants dependent on material and diagnostic test;
362
Θ
is the absolute temperature, equal to ϑ + Θ 0 ;
363
ϑ
is the temperature in degrees Celsius (°C);
364
Θ 0 = 273,15 K.
365
This may be expressed as a linear equation:
(2)
60216-8 © IEC:201X
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y = a + bx
366
367
(3)
where
368
y = ln τ
369
x = 1/ Θ
370
a = ln A
371
b = B.
372
373
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Given a group of paired x, y values, the values of a and b giving the best fit linear relationship
are determined from the x, y values:
b=
374
(∑ xy − ∑ x∑ y / k )
⎡ x − ( x) / k ⎤
∑
⎢⎣∑
⎥⎦
2
a=
375
2
(∑ y − b∑ x )
k
(4)
(5)
376
where k is the number of x , y values.
377
378
379
NOTE 1
Since most "scientific" calculators with "statistics" functions have regression analysis
facilities, the calculations implied by equations (2) to (4) above are executed by the calculator. It is important in this
case that x is entered as the independent variable and y as the dependent.
380
381
NOTE 2
It is usually possible with such calculators to enter the time and temperature values and
convert them to x and y before the summation is executed.
382
383
NOTE 3
used.
384
5.2.3
385
386
Calculate the coefficient of determination (square of correlation coefficient). Again, this can be
done in the regression facility of the calculator.
Logarithms to another base (for example, 10) may be used but will affect the value to be
Calculation of deviation from linearity
r
387
2
(
xy − ∑ x ∑ y / k ) 2
∑
=
2
⎡ x 2 − ( x )2 / k ⎤ ⎡ y 2 − (
∑
∑ y ) / k ⎤⎥⎦
⎥⎦ ⎢⎣∑
⎢⎣∑
(6)
388
389
390
If the value of r>0,985 then the values of TI and HIC may be determined. If this condition is
not satisfied, then the deviations from the fundamental assumption are too great to allow the
calculation.
391
392
Note This is not a Fisher linearity test, but an expression of the mean deviation of data from the regression line, as
a fraction of the range of data values
393
5.2.4
394
Temperature index and halving interval
ϑ=
b
(ln τ − a )
− Θ0
(7)
395
396
Using equation (7), calculate the temperatures corresponding to values of τ (time in hours) of
20 000, 10 000 and 2 000, designated ϑ 20 000 , ϑ 10 000 , and ϑ 2 000 respectively.
397
398
Using the data pairs ( ϑ 20 000 , 20 000) and ( ϑ 2 000 , 2 000) draw the regression line on thermal
endurance paper to obtain the thermal endurance graph.
60216-8 © IEC:201X
399
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Calculate the values of TI and HIC:
400
TI = ϑ 20 000 , HIC = ϑ 10 000 – ϑ 2 000
401
402
NOTE If a value τ , different from 20 000 is used for the calculation of TI, replace 10 000 and 2 000 in the above by
τ /2 and τ /10.
403
5.2.5
404
405
406
407
408
409
The calculation procedure given above is only valid when the numbers of data contributing to
the mean time to end-point for all temperature groups are approximately equal (if the value of
s y is less than 0,30 and/or r>0). In addition, the procedure does not test the scatter of the test
data for acceptability. For these reasons, the result cannot be given the status of full
statistical acceptance, and the procedure should only be used when there is already
satisfactory experience of the behaviour of the material in thermal endurance testing.
410
411
In all cases of doubt, the fuller analysis described in IEC 60216-3 should be carried out,
especially if there is doubt about the acceptability of scatter of test data.
412
5.3
413
414
415
The reliability of the extrapolation of -the graph depends on obtaining an acceptable Arrhenius
plot, which may not be possible with materials showing behaviour related to a transition
phenomenon in the chosen temperature range.
416
417
418
For this purpose the correlation coefficient r is calculated in accordance with 5.2.3. If this
calculation results in a value smaller than 0,985 (for three test temperatures; see also IEC
60216) an additional test at a different test temperature may improve the linearity of data.
419
420
421
422
423
Note It should be taken care of misinterpretation of the value of the correlation coefficient. A coefficient greater
than 0,95 gives not automatically satisfactory calculations. The correlation coefficient cannot be used in this way. If
the correlation coefficient is greater than 0,95, this indicates that the regression is responsible for a significant part
of the variation of the result. It does not indicate that the deviations from regression linearity are small enough for
the regression estimated to be valid.
424
5.4
425
426
For determination of RTI, the chosen reference material, its thermal endurance and the
method of determination are of central importance.
427
428
429
430
431
The reference material shall be of the same type as the tested material, and have a history of
satisfactory service. It shall have a known temperature index for the property and a threshold
value which are the same, or at least reasonably similar to, those to be employed in the RTI
test. The TI and HIC of the reference material should also be approximately the same as the
values expected for the tested material.
432
433
434
435
Since processing conditions may significantly affect the ageing characteristics of some
materials, the sampling, cutting of sheet from the supply roll, cutting of anisotropic material in
the same direction, moulding, curing, preconditioning, etc. shall be performed in the same
manner for both materials, and the specimens shall be tested in the same thickness.
436
437
438
439
The relative temperature index (RTI) is a thermal endurance characteristic which is derived
from the two thermal endurance relationships or curves resulting from the comparative testing
of the test material and the reference material. The RTI is specifically related to the time
corresponding to the TI originally determined for the reference material.
440
441
442
The RTI consists of two numbers; one representing the temperature derived and the other
the associated HIC. The derivation may be numerical or graphical (more details are given in IEC
60216-5).
Validity of simplified calculations
Use of correlation coefficient
Determination of RTI
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443
444
445
446
The test chosen shall relate to a characteristic which is likely to be of significance in practice and,
wherever possible, use shall be made of test methods specified in International Standards. If the
dimensions and/or form of the test specimens are altered by the heat treatment, then only test
methods which are independent of these effects may be used.
447
For the selection of the end-point, two factors shall be agreed upon:
448
449
a) the period of time for which a time-temperature limit shall be estimated; for general purposes, a
period of 20 000 h is recommended;
450
NOTE 3 Other times (shorter or longer than 20 000 h) may be chosen if necessary.
451
452
b) the acceptable threshold value of the chosen characteristic; this threshold value depends on
the conditions of use foreseen.
453
5.5 Evaluation of results
454
455
456
457
For both destructive and non-destructive tests, for each exposure temperature and for each
heat ageing period, the value of the chosen property is plotted as a function of the logarithm of
the time (see figure 1). The point at which this graph intersects the horizontal line representing
the end-point criterion is taken as the time to failure.
458
When applying a proof test, the times to failure shall be calculated as the mean values.
459
460
461
The calculation of the thermal endurance curve is based on these times to failures and the respective
exposure temperatures. If mean values are used, the logarithmic mean represents the time to
failure.
462
Calculate a first-order regression line.
463
464
465
466
467
Plot the times to failure versus exposure temperatures on graph paper having a logarithmic time scale
as the ordinate and an abscissa based on the reciprocal of the absolute temperature but showing
the correlated values in degrees Celsius. The first order regression line is drawn through the
points plotted on the graph, which thus represents the thermal endurance of the material under
test. An example is given in figure 2.
468
469
Calculate the temperature index (TI) corresponding to the time limit chosen (generally 20 000
h) and the halving interval (HIC); calculate the correlation coefficient
60216-8 © IEC:201X
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Property value in
relative units %
100
End-point criterion
50
Exposure temperatures
190 °C
171 °C
149 °C
130 °C
0
0
20
50
100
200
500
1 000
2 000
5 000
10 000
Ageing time h
IEC 1197/01
470
471
472
Figure 1 - Determination of the time to reach the end-point at each temperature - Property
variation (according to IEC 60216-1)
473
474
475
476
NOTE When the temperature scale is chosen in such a way that equal intervals correspond to equal intervals of
reciprocal Kelvins, then the various points obtained will be found to lie on a straight line if a linear dependence
exists. When the temperature range used is comparatively small, a curve can be prepared using an abscissa scale
proportional to the temperature; in this case the curve can be fit to a straight line only with great circumspection.
477
TI r
original TI of the reference material;
478
to
time corresponding to TIC;
479
480
A point on the test material's thermal endurance relationship or graph from the comparative
test, having coordinates 9A,to;
481
482
B
point on the reference material's thermal endurance relationship or graph, having coordinates
9B,to;
483
484
HICr HIC for the reference material at point TIC of its original thermal endurance relationship or
graph;
485
HIC(A) HIC for the test material at point A; HIC(B) HIC for the reference material at point B.
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100 000
Time h
10 000
1 000
100
TI
10
HIC
120
160
140
180
Temperature °C
2,6
2,5
2,4
2,3
Reciprocal thermodynamic temperature K
–3
2,2 × 10
–1
Temperature estimate, 20 000 h
Temperature estimate, 10 000 h
IEC 1200/01
486
487
488
Figure 2 - Thermal endurance graph - Temperature index - Halving interval
(according to IEC 60216-1)
489
490
The points A and B may be determined either graphically or numerically, and the RTI then
determined using equation (8):
491
RTI =TI t+9A-9B
.
(8)
492
493
494
When reporting the RTI, the usual information regarding the property, end-point and test
specimen data should be supplemented with corresponding information regarding the reference
material.
495
496
When required, the test material may be assigned to an insulation thermal class according IEC
60085.
60216-8 © IEC:201X
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100 000
B
A
Reference material, TI = TIr
RTI = Ti r + 1A – 1B
10 000
Time h
Test material
1 000
Reference material
100
1A
1B
10
120
160
140
180
Temperature °C
2,6
2,5
2,4
Reciprocal thermodynamic temperature K
2,3
–3
2,2 × 10
–1
IEC
1201/01
497
498
499
Figure 3 - Thermal endurance graph - Relative temperature index - Halving interval
(according to IEC 60216-1)
500
5.6 Test report
501
Make the test report, reporting in the format
TI s = xxx , HIC s = yy , y
502
503
The test report shall include
504
505
ƒ
all information necessary for complete identification of the material tested; description
of the tested material including dimensions and any conditioning of the specimens;
506
507
ƒ
the property investigated, the chosen end-point, and, if this is a percentage value, the
initial value of the property;
508
509
ƒ
the test method used for determination of the property (for example, by reference to
an IEC publication);
510
511
512
ƒ
any relevant information on the test procedure, for example, ageing environment;
details of the ageing conditions, if these are other than the exposure of unstressed
specimens to hot air
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513
514
ƒ
shape, dimensions and method of preparation of the test specimens, with reference to
the relevant standard;
515
ƒ
conditioning;
516
ƒ
type of oven, with details of rate and direction of airflow;
517
ƒ
times and temperatures of exposure in ovens;
518
ƒ
the individual test temperatures, with the appropriate data:
519
520
o
for non-destructive tests, the individual times to end-point, with the graphs of
variation of property with ageing time
521
522
o
for proof tests, the numbers and durations of the ageing cycles, with the
numbers of specimens reaching end-point during the cycles
523
524
o
for destructive tests, the ageing times and individual property values, with the
graphs of variation of property with ageing time;
525
ƒ
the thermal endurance graph;
526
ƒ
reference to this International Standard;
527
Bibliography
528
529
IEC 60050(212): International Electrotechnical Vocabulary (IEV) – Chapter 212: Insulating
solids, liquids and gases
530
531
IEC 60212: Standard conditions for use prior to and during the testing of solid electrical
insulating materials
532
533
IEC 60493-1, Guide for the statistical analysis of ageing test data – Part 1: Methods based on
mean values of normally distributed test results
534
535
ISO 2578:1993, Plastics – Determination of time-temperature limits after prolonged exposure
to heat
536
60216-8 © IEC:201X
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Annex A (informative)
Example: simplified statistical analysis
537
538
539
A.1
Simplified statistical analysis of TI data
540
Extract from “Simplified calculation procedures” in IEC 60216-1:
⎡
⎢
µ2 y = ⎣
()
∑ y 2 − ( ∑ y ) 2 / k ⎤⎥⎦
541
“
542
Calculate the mean deviation of data points from the regression line:
sy =
543
(10)
k
()
⎛⎜ 1 − r 2 ⎞⎟ µ y
⎝
⎠ 2
k −2
(
)
(11)
544
A.1.1
Temperature index and halving interval
545
546
547
If the value of s y obtained from equation (11) is less than 0,16 (or 0,0695 if using logarithms
to base 10) then the values of TI and HIC may be determined. If this condition is not satisfied,
then the deviations from the fundamental assumption are too great to allow the calculation.”
548
A.2
549
550
551
552
Each of the following 3 pages details the calculation on a set of data for TI. The times and two
of the three temperatures in each case are the same. The third temperature is given three
different (related) values, leading to variation of the correlation coefficient, and consequently
of the value of s y .
553
The descriptive table of this analysis is:
Comparation between linearity evaluation through s y and r 2
Linearity property of group
Correlation coefficient
Value of s y
Satisfactory linearity
0,999118177223061
0,0825382511051849
Compensatable non-linearity
0,991834310219611
0,250708128634552
Excessive non-linearity
0,981145251025673
0,37993902904637
554
555
556
557
In each page report in A.5, paragraph a) gives the 60216-3 report on the full analysis of the
data file. Paragraph b) gives the report calculated as in 60216-1 (simplified). Paragraph c)
lists the values in the original data file, with the mean values for the log times, and paragraph
d) shows the graphic output from 60216-3 analysis, including data points for individual times.
558
A.3
559
560
The tables in clause A.5.4 show the dependence of cut-off value for correlation coefficient on
the number of temperatures and the relative independence of the cut-off value for s y .
561
A.4
562
563
–
A value of 0.95 is much too low to be used realistically (0.995 would be more appropriate
for 3-temperature data).
564
–
The value of 0.16 proposed for the cut-off point in B.1.1 above is realistic.
Effect of number of temperatures
Conclusions
60216-8 © IEC:201X
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565
A.5
566
A.5.1
567
a) IEC 60216-3 report on full analysis
568
TI (HIC): 169,7 (10,0)
569
F = 0,56606 (1/12: F95 = 4,747) p = 0,4663
570
Examples Test Reports
Example 1
Linear
572
Correlation coefficient
Sy = 0,0825382511051849
0.999118177223061
c) Data file for 60216-3 analysis
Table A.1 Data set for example 1
574
Temperature
575
TC = 166,3
b) IEC 60216-1 (simplified) calculations
571
573
Original Time Values
Log mean time
185
7410
6610
6170
5500
8910
8,82836233194414
200
3200
2620
2460
2540
3500
7,95003798439189
220
1100
740
720
620
910
6,68642618690896
d) 60216-3 analysis graph
576
577
Figure A.1 Analysis Graph for Example 1
578
A.5.2
579
a) IEC 60216-3 report on full analysis
580
TI (HIC) : 163,4 (11.4)
581
F = 5,2227 (1/12: F95 = 4,747) p = 0,0413
582
112/165/NP
Example 2
TC = 158,7
b) IEC 60216-1 (simplified) calculations
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583
Minor non-linearity, compensated Sy = 0,250708128634552
584
Correlation coefficient
585
0.991834310219611
c) Data file for 60216-3 analysis#
Table A.2 Data set for example 2
586
Temperature
587
Original Time Values
Log Mean Time
180
7410
6610
6170
5500
8910
8,82836233194414
200
3200
2620
2460
2540
3500
7,95003798439189
220
1100
740
720
620
910
6,68642618690896
d) 60216-3 analysis graph
588
589
Figure A.2 Analysis Graph for Example 2
590
591
A.5.3
Example 3
592
a) IEC 60216-3 report on full analysis
593
TI (HIC) cannot be reported:
594
TI = 155,8, HIC = 12.8,
595
F = 11,995 (1/12: F95 = 4,747) p = 4,6884e-3
596
b) IEC 60216-1 (simplified) calculations
597
Excessive non-linearity
598
Correlation coefficient
599
TC = 148,.2
c) Data file for 60216-3 analysis
Sy = 0,37993902904637
0,981145251025673
60216-8 © IEC:201X
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Table A.3 Data set for example 3
600
Temperature
601
Original Time Values
605
606
7410
6610
6170
5500
8910
8,82836233194414
200
3200
2620
2460
2540
3500
7,95003798439189
220
1100
740
720
620
910
6,68642618690896
d) 60216-3 analysis graph
Figure A.2 Analysis Graph for Example 2
A.5.4
Dependence of cut-off value for correlation coefficient
Table A.4
Dependence of cut-off value for correlation coefficient on the number of temperatures
θ1
175
180
185
θ2
200
200
200
θ3
220
220
220
r0
0,981145251025673
0,991834310219752
0,999118177223061
sy
0,37993902904637
0,250708128634552
0,0825382511051849
Rejected in 60216-3
Accepted after adjustment
Acceptable linearity
Table A.5 Relative independence of the cut-off value for s y .
607
608
609
610
Log Mean Time
175
602
603
604
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θ1
175
180
185
θ2
200
200
200
θ3
220
220
220
θ4
240
240
240
r0
0,990128557470239
0,993223870062064
0,992078909781926
sy
0,236370223011478
0,195988591044968
0,211839838691374
Rejected in 60216-3
Accepted after adjustment
Accepted after adjustment