prEN IEC 61857-33:2023

• Status: Not Published
• Publication Date: 25-Nov-2024
• Withdrawal Date: 02-Jun-2019
• ICS: 29.080.30 - Insulation systems
• Technical Committee: CLC/SR 112 - Evaluation and qualification of electrical insulating materials and systems (to be defined)
• Drafting Committee: IEC/TC 112 - IEC_TC_112
• Current Stage: 4060 - Enquiry results established and sent to TC, SR, BTTF - Enquiry
• Start Date: 28-Jul-2023
• Completion Date: 28-Jul-2023
• Ref Project: oSIST prEN IEC 61857-33:2023 - Electrical insulation systems - Procedures for thermal evaluation - Part 33: Multifactor evaluation with increased ageing factors at elevated temperature

Overview

The prEN IEC 61857-33:2023 standard, issued by the CLC, focuses on the thermal evaluation procedures for electrical insulation systems (EIS). Specifically, this part 33 introduces a multifactor evaluation method incorporating increased ageing factors at elevated temperatures. It is designed to assess the durability and performance of electrical insulation materials when subjected to multiple stresses simultaneously, including thermal, electrical, ambient/environmental, and mechanical factors.

This advanced standard is part of the IEC 61857 series that standardizes procedures for thermal endurance testing and evaluation of electrical insulation systems. The methodology aims to offer a structured framework for determining insulation reliability under multifactor ageing conditions that better simulate real-world operating environments.

Key Topics

• Multifactor Ageing Stresses: The standard identifies four key ageing stresses affecting electrical insulation longevity:

  • Thermal (T)
  • Electrical (E)
  • Ambient/Environmental (A)
  • Mechanical (M)

  The notation distinguishes ambient (A) from electrical (E) to avoid confusion prevalent in previous references.

• Two-step Evaluation Process:

  1. Establish a baseline thermal classification by subjecting both reference and candidate EIS samples to thermal ageing at multiple elevated temperatures.
  2. Perform comparative testing where additional stresses (E, A, M) are applied alongside thermal stress continuously throughout the ageing process to evaluate their combined influence on insulation performance.

• Test Methodology:

  • Accelerated ageing at three or more elevated temperatures (e.g., high, medium, low).
  • Three sets of test objects each for reference and candidate materials.
  • Utilizes diagnostic testing and evaluation based on International Electrotechnical Commission technical reports.

• Data Analysis and Uncertainty Management:

  • Comparative analysis using temperature index values.
  • Evaluation through a 5-degree Celsius uncertainty criterion to ensure robust interpretation.
  • Incorporates graphical representations such as thermal endurance graphs for clarity.

• Applicability:

  • Designed to cover a broad range of electrical applications where insulation systems experience complex operational stress scenarios.
  • Complements and extends the multifactor procedures outlined in IEC 61857-32 by applying continuous multifactor stress during the entire ageing process.

Applications

The application of prEN IEC 61857-33:2023 is essential for:

• Manufacturers of Electrical Insulation Systems looking to validate product longevity and safety under complex environmental conditions.
• Quality Assurance Teams within electrical equipment industries who require rigorous testing protocols to certify insulation performance and compliance.
• Research and Development sectors aiming to design new insulation materials or improve existing solutions, using realistic ageing simulations that factor in multiple simultaneous stresses.
• Regulatory Bodies and Certification Entities ensuring products meet international standards for thermal endurance and multifactor ageing impacts.
• Electrical Equipment Designers who must select appropriate insulation materials able to maintain performance across mechanical, electrical, and environmental stresses combined with elevated temperatures.

Related Standards

For comprehensive understanding and implementation, prEN IEC 61857-33:2023 should be used together with:

• IEC 61857 Series: Covers procedures for thermal evaluation of electrical insulation systems.
• IEC TR 61857-2: Provides detailed test methods for accelerated ageing of EIS.
• IEC 61857-32: Addresses multifactor evaluation by diagnostic procedures but with different stress application methodology.
• IEC 60505: Defines ageing stress categories and fundamental electrical insulation qualification.
• ISO/IEC Directives Part 2: Governs the development process for international standards ensuring consistency.

---

Keywords: Electrical insulation systems, thermal evaluation, multifactor ageing, accelerated ageing, electrical stress, thermal endurance, insulation test methods, IEC 61857-33, insulation performance, elevated temperature testing, insulation mechanical stress, ambient/environmental ageing.

SLOVENSKI STANDARD
01-julij-2023
Sistemi električne izolacije - Postopki za toplotno vrednotenje - 33. del:
Ocenjevanje po več faktorjih s povečanimi dejavniki staranja pri povišani
temperaturi
Electrical insulation systems - Procedures for thermal evaluation - Part 33: Multifactor
evaluation with increased ageing factors at elevated temperature
Ta slovenski standard je istoveten z: prEN IEC 61857-33:2023
ICS:
29.080.30 Izolacijski sistemi Insulation systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

112/605/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 61857-33 ED1
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2023-05-05 2023-07-28
SUPERSEDES DOCUMENTS:
112/573/CD, 112/600/CC
IEC TC 112 : EVALUATION AND QUALIFICATION OF ELECTRICAL INSULATING MATERIALS AND SYSTEMS
SECRETARIAT: SECRETARY:
Germany Mr Bernd Komanschek
OF INTEREST TO THE FOLLOWING COMMITTEES: PROPOSED HORIZONTAL STANDARD:

TC 2,TC 14,TC 15,TC 23,TC 42,TC 55,TC 96
Other TC/SCs are requested to indicate their interest, if any,
in this CDV to the secretary.
FUNCTIONS CONCERNED:
EMC ENVIRONMENT QUALITY ASSURANCE SAFETY
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
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The attention of IEC National Committees, members of
CENELEC, is drawn to the fact that this Committee Draft for
Vote (CDV) is submitted for parallel voting.
The CENELEC members are invited to vote through the
CENELEC online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of
• any relevant patent rights of which they are aware and to provide supporting documentation,
• any relevant “in some countries” clauses to be included should this proposal proceed. Recipients are reminded that
the enquiry stage is the final stage for submitting "in some countries" clauses. See AC/22/2007.

TITLE:
Electrical insulation systems - Procedures for thermal evaluation - Part 33: Multifactor evaluation with
increased ageing factors at elevated temperature

PROPOSED STABILITY DATE: 2027
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.

IEC CDV 61857-33 © IEC 2023 2 112/605/CDV
1 CONTENTS
3 FOREWORD . 4
4 INTRODUCTION . 6
5 1 Scope . 7
6 2 Normative references . 7
7 3 Terms and definitions . 7
8 4 Procedure . 8
9 5 Test objects . 9
10 6 EIS evaluation . 9
11 6.1 Selection of the appropriate EIS test method . 9
12 6.2 Comparison of the reference and candidate EIS . 10
13 6.3 Evaluation of the additional stresses E, A and/or M in combination with T . 10
14 6.4 Variation . 10
15 7 Illustration of the structure – Thermal evaluation – Step 1 . 10
16 7.1 Establishing the baseline thermal classification. 10
17 7.2 Illustration of the thermal evaluation . 11
18 8 Evaluation of the influence of multifactor stresses – Step 2 . 12
19 8.1 General . 12
20 8.2 Selection of the ageing temperature for the one-temperature comparison . 12
21 8.3 Selection of the ageing temperatures for the two-temperature comparison . 13
22 8.4 Diagnostic tests and end-point criterion applied at elevated temperature . 13
23 8.5 Diagnostics . 13
24 8.6 Using ageing cycles . 13
25 8.7 Diagnostics applied at elevated temperature without ageing cycles. 14
26 8.8 Examples of different ambient/environmental stress during operation . 14
27 8.9 Examples of mechanical stress during operation . 14
28 9 Analysis of data . 15
29 9.1 General . 15
30 9.2 Evaluation of the other stresses of influence . 16
31 9.3 Use of the 5-degree uncertainty criterion . 16
32 The procedure for analysis of the comparison of results between the baseline EIS evaluated
33 with thermal stress as the only dominant stress is compared and the performance
34 when one or more of the other stress factors are simultaneously applied is; . 16
35 9.4 Comparison of the result between the baseline EIS and any of the sets of results for
T
36 other factors of influence . 16
37 10 Report. 16
38 Annex A (informative) Example of a test data sheet report . 18
39 Annex B (informative) Example of a thermal endurance graph – Reference EIS . 19
40 Annex C (informative) Example of a test data sheet for a baseline thermal classification when
41 the reference thermal index value is known . 20
42 Annex D (informative) Establishing the thermal endurance of the baseline using the reference
43 correlation time . 21
44 Annex E (informative) Analysis of data using the 5-degree uncertainty criterion . 23
45 Annex F (informative) . 27

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46 Annex G (informative) . 29
47 Annex H (informative) Calculations for the 5-degree uncertainty criterion . 36
48 Bibliography . 37
50 Figure 1 – Overview of the two-step evaluation procedure . 9
51 The thermal classification of the baseline is established when the thermal ageing of both the
52 reference EIS and the candidate EIS have completed as determined in clause 6.1. The
53 analysis of the candidate test results is compared to the reference EIS; refer to . 10
54 Figure 2 – Illustration of the establishment of the thermal classification of the baseline EIS . 11
55 Figure 3 – Comparison of the influence of any of the multifactor exposures with the baseline
56 performance . 12
57 Figure 4 – Comparison of the influence of a different electrical stress applied during thermal
58 exposure with the baseline thermal design . 14
59 Figure 5 – Comparison of the influence of a different ambient/environmental stress applied
60 during thermal exposure with the baseline thermal design . 14
61 Figure 6 – Comparison of the influence of a different mechanical stress applied during thermal
62 exposure with the baseline thermal design . 15
63 Figure B.1 – Time coordinate established at 48 400 h with a known temperature of 185 °C . 19
64 Figure D.1 – Temperature index at 160 °C with a correlation time of 48 400 h . 21
65 Figure E.1 – Plotting the data in Annex C showing confidence ranges . 25
67 Table 1 – Example of thermal test results for reference EIS based on data sheets given in
68 Annex A . 11
69 Table A.1 – Example of a test data sheet report . 18
70 Table C.1 – Example of a test data sheet for a baseline thermal classification when the
71 reference thermal index value is known . 20
72 Table E.1 – Example of test results . 24
73 Table E.2 – Limits of the range of ± 5-degrees . 24
74 Table E.3 – Examples of multifactor exposure . 25
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79 INTERNATIONAL ELECTROTECHNICAL COMMISSION
80 ____________
82 ELECTRICAL INSULATION SYSTEMS –
83 PROCEDURES FOR THERMAL EVALUATION –
85 Part 33: Multifactor evaluation with increased ageing factors
86 at elevated temperature
88 FOREWORD
89 1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising all national
90 electrotechnical committees (IEC National Committees). The object of IEC is to promote international co-operation on all
91 questions concerning standardization in the electrical and electronic fields. To this end and in addition to other activities,
92 IEC publishes International Standards, Technical Specifications, Technical Reports, Publicly Available Specifications (PAS)
93 and Guides (hereafter referred to as “IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC
94 National Committee interested in the subject dealt with may participate in this preparatory work. International, governmental
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96 the International Organization for Standardization (ISO) in accordance with conditions determined by agreement between
97 the two organizations.
98 2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international consensus
99 of opinion on the relevant subjects since each technical committee has representation from all interested IEC National
100 Committees.
101 3) IEC Publications have the form of recommendations for international use and are accepted by IEC National Committees in
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103 cannot be held responsible for the way in which they are used or for any misinterpretation by any end user.
104 4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications transparently to
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109 independent certification bodies.
110 6) All users should ensure that they have the latest edition of this publication.
111 7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and members of
112 its technical committees and IEC National Committees for any personal injury, property damage or other damage of any
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114 use of, or reliance upon, this IEC Publication or any other IEC Publications.
115 8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is indispensable
116 for the correct application of this publication.
117 9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent rights.
118 IEC shall not be held responsible for identifying any or all such patent rights.
119 International Standard IEC 61857-33 has been prepared by IEC technical committee 112: Evaluation
120 and qualification of electrical insulating materials and systems.
121 The text of this International Standard is based on the following documents:
FDIS Report on voting
112/XXX/FDIS 112/XXX/RVD
123 Full information on the voting for the approval of this International Standard can be found in the report
124 on voting indicated in the above table.
125 This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
126 A list of all parts in the IEC 61857 series, published under the general title Electrical insulation systems
127 – Procedures for thermal evaluation, can be found on the IEC website.

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128 The committee has decided that the contents of this document will remain unchanged until the stability
129 date indicated on the IEC website under "http://webstore.iec.ch" in the data related to the specific
130 document. At this date, the document will be
131 • reconfirmed,
132 • withdrawn,
133 • replaced by a revised edition, or
134 • amended.
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.
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140 INTRODUCTION
141 Accelerated thermal ageing of an electrical insulation system (EIS) is intended to evaluate the thermal
142 classification of the EIS. Many applications need to include the evaluation of other ageing stresses in
143 addition to the thermal stress.
144 IEC 60505 provides four categories of ageing stresses which influence the performance of products in
145 use under a wide range of operating conditions. In IEC 60505 the stresses are presented as thermal (T),
146 electrical (E), environmental (E) and mechanical (M). In this part of IEC 61857, environmental (E) is
147 replaced with ambient (A) to avoid the confusion of having two stresses represented by the same letter.
148 In this part of IEC 61857-33 the stresses are therefore presented as thermal (T), electrical (E),
149 ambient/environmental (A) and mechanical (M).
150 This document follows the structure presented in the Scope of IEC 60505, the evaluation of either
151 thermal, electrical, ambient/environmental, mechanical or combination of these as multifactor stresses.
152 In order to determine the thermal endurance of an EIS, ageing tests are carried out in accordance with
153 the selected test method of IEC TR 61857-2. To obtain the results within reasonable time these tests
154 are performed at several temperatures considerably higher than the normal service temperature. This
155 document deals with testing where the test objects are exposed to other possible ageing stresses, such
156 as E, A, and/or M in combination with T.
157 Both the reference EIS and the candidate EIS undergo thermal ageing at three or more elevated
158 temperatures. In this document the examples use three ageing temperatures of high, middle, and low.
159 A total of three sets of test objects for the reference EIS plus a total of three sets of test objects for the
160 candidate EIS are used in comparisons. The preferred number of test objects per set is provided in the
161 selected EIS test method from IEC TR 61857-2.
162 This document provides the structure for the evaluation of one or more of the three additional stresses
163 E, A and M in combination with T by direct comparison to the baseline established by T. Without the
164 baseline, analysis of the influence of the additional factors is limited.
165 A test is performed with thermal stress as the only ageing factor. Similar tests are then performed with
166 one or more of the other ageing factors E, A and/or M simultaneously added. Analyses of the results
167 are made to reveal the influence of the E, A and/or M factors.
168 While similar in their conception, IEC 61857-32 (Multifactor evaluation by diagnostic procedures) and
169 IEC 61857-33 (Multifactor evaluation with increased factors at elevated temperature) have different
170 structure and evaluation conditions. IEC 61857-32 uses thermal exposure as the only intended ageing
171 stress and applies additional stress(es) only during the diagnostic part of each test cycle. IEC 61857-
172 33 combines two or more ageing stresses and the combination of the stresses are applied continuously
173 throughout the ageing process. In other words, IEC 61857-32 uses additional diagnostic procedure(s)
174 and IEC 61857-33 uses additional ageing or stress exposure procedure(s).
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176 ELECTRICAL INSULATION SYSTEMS –
177 PROCEDURES FOR THERMAL EVALUATION –
179 Part 33: Multifactor evaluation with increased ageing factors
180 at elevated temperature
183 1 Scope
184 This part of IEC 61857 series is applicable to the evaluation of an EIS for applications where the
185 stresses of the application are a combination of the multifactor ageing stresses identified in IEC 60505.
186 The increased stress factors are expected to occur during operation at elevated temperatures.
187 This document establishes the procedure to evaluate the influence of stresses on the performance
188 established following the thermal classification of the EIS. The thermal classification is established in
189 Step 1 where the only ageing stress is thermal. The candidate EIS is first evaluated based on thermal
190 stress only. This evaluation is defined as the baseline of the candidate EIS. In Step 2, the evaluation of
191 the additional stresses applied at elevated temperatures provides the measurement needed to establish
192 the influence of the additional stress factors on the thermal performance of the baseline EIS established
193 in Step 1.
194 This document is about thermal endurance testing with one or more stresses added during the thermal
195 ageing process. It provides guidance regarding interpretation of the test results.
196 For performance requirements of any product designed and constructed using the EIS established in
197 accordance with this document refer to the appropriate IEC Technical Committee applicable to the
198 application.
199 This document is applicable to a range of established EIS test standards. IEC TR 61857-2 provides a list of
200 many established EIS test standards which cover low-voltage [up to 1kV a.c.] and high-voltage [above 1kV a.c.].
201 The use of 1kV as the transition voltage point between low-voltage and high-voltage can be found in standards
202 such as IEC TC 17 High-voltage switchgear and control gear.
203 2 Normative references
204 The following documents are referred to in the text in such a way that some or all of their content
205 constitutes requirements of this document. For dated references, only the edition cited applies. For
206 undated references, the latest edition of the referenced document (including any amendments) applies.
207 IEC 60505, Evaluation and qualification of electrical insulation systems
208 IEC TR 61857-2, Electrical insulation systems – Procedures for thermal evaluation – Part 2: Selection
209 of the appropriate test method for evaluation and classification of electrical insulation systems
210 IEC 61858-1, Electrical insulation systems – Thermal evaluation of modifications to an established
211 electrical insulation system (EIS) – Part 1: Wire-wound winding EIS
213 3 Terms and definitions
214 For the purposes of this document, the terms and definitions given in IEC 60505 and the following apply.
215 ISO and IEC maintain terminological databases for use in standardization at the following addresses:
216 • IEC Electropedia: available at http://www.electropedia.org
217 • ISO Online browsing platform: available at http://www.iso.org/obp

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218 3.1
219 reference EIS
220 an established EIS evaluated on the base of either a known service experience record or a known
221 comparative functional evaluation
222 Note to entry: Refer to IEC 60505
223 3.2
224 thermal classification of the reference EIS
RT
225 established thermal classification of the reference EIS where the thermal index value of the reference
226 EIS is used to establish the correlation time needed to calculate the thermal index value of the candidate
227 EIS
228 3.3
229 thermal candidate EIS
CT
230 EIS under evaluation to determine its service capability with regard to thermal stress
231 3.4
232 thermal classification of the candidate EIS
CT
233 assigned thermal classification of the candidate EIS based on the comparison to the reference EIS
234 represented as the temperature coordinate of the time/temperature graph when the time coordinate of
235 the intercept is the value of the correlation time
236 3.5
237 Baseline EIS
T
238 the designated candidate EIS after completion of the thermal classification and used to evaluate the
239 influence of additional stresses
240 3.6
241 Multifactor candidate EIS
TX
242 EIS under evaluation to determine its service capability with regard to thermal stress in combination
243 with electrical, ambient/environmental, or mechanical stress(es), where “X” is replaced with E for
244 electrical, A for environmental/ambient or M for mechanical)
245 3.7
246 5-degree uncertainty criterion
247 When two sets of ageing test results are within ±5 K of each other the analysis of the test results
248 cannot distinguish a significant difference in long-term endurance of the EIS.
250 3.8
251 EIS assessed thermal endurance index EIS ATE
252 numerical value of temperature in degrees Celsius for the reference EIS as derived from known
253 service experience or a known comparative functional evaluation
256 3.9
257 EIS relative thermal endurance index EIS RTE
258 numerical value of the temperature in degrees Celsius for the candidate EIS which is relative to
259 the known EIS ATE of a reference EIS, when both EIS are subjected to the same ageing and
260 diagnostic procedures in a comparative test
261 4 Procedure
262 Figure 1 shows an overview of the two steps of the evaluation procedure starting with the established
263 thermal classification of the reference system EIS and including the additional ageing influences (E,
RT
264 A, M) at elevated temperature of the candidate system EIS .
CT
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266 Figure 1 – Overview of the two-step evaluation procedure
267 The thermal classification of the baseline EIS is established in Step 1. It is derived from the reference
268 EIS for which test data and experience exist.
269 Step 2 evaluates only the influence of one or more other stresses (E, A, and/or M) in combination with
270 T but cannot alter or change the thermal classification of the baseline candidate EIS ; this procedure
CT
271 only evaluates the influence of the multifactor stress combination.
272 Further detail is provided in IEC TR 61857-2 with examples provided in 6.1 of the standard and Annex
273 E of this document.
274 5 Test objects
275 Both the reference EIS and the candidate EIS undergo thermal ageing at three or more elevated
276 temperatures. The examples in this document use three ageing temperatures of high, middle, and low.
277 A total of three sets of test objects for the reference EIS plus a total of three sets of test objects for the
278 candidate EIS are used in the examples. Test objects shall be in accordance with the test method
279 selected from IEC TR 61857-2.
280 The design of the sets of test objects shall be in accordance with the test method selected. When part
281 of the evaluation is to compare processing or alternate designs of the same functioning product, the
282 modifications of the design or processing shall be part of the multifactor test object construction with all
283 modifications documented in the report.
284 The preferred number of test objects is contained in individual test methods in IEC TR 61857-2.
285 6 EIS evaluation
286 6.1 Selection of the appropriate EIS test method
287 The EIS being evaluated is referred to as the candidate EIS until completion of the thermal classification.
288 Once the thermal classification has been established the candidate is described as the baseline EIS.
289 IEC TR 61857-2 provides a list of EIS test methods covering a range of applications. Selection of the
290 EIS test method should be based on the most appropriate match to the application.
291 EXAMPLE 1 The application is for form-wound motors with an operating voltage of 4,2 kV. An appropriate EIS test method to
292 establish the thermal classification is IEC 60034-18-31.
293 EXAMPLE 2 The application is for an encapsulated low-voltage motor. An appropriate EIS test method to establish the thermal
294 classification is IEC 61857-22.
295 EXAMPLE 3 The application is for oil immersed transformers. An appropriate EIS test method is either IEC TS 62332-1 or
296 IEC TS 62332-2 depending on the selection of time and test object.
297 EXAMPLE 4 The application is for form-wound generators when the influence of mechanical (M) stress in the endurance of the
298 EIS is of interest. An appropriate EIS test method is IEC 60034-18-34, a thermomechanical test.

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299 6.2 Comparison of the reference and candidate EIS
300 The reference EIS and candidate EIS shall be evaluated using the same test method, ageing cycle,
301 conditioning, and diagnostic testing. The end-point criterion of the reference and candidate shall be the
302 same. This direct comparison is essential for analysis of the results. All sets of test objects of the
303 reference and candidate EIS shall be of the same design and construction unless the purpose of the
304 project is to evaluate design changes.
305 There is no requirement for the reference and candidate EIS to be expected to have the same thermal
306 classification since the thermal classification of the candidate EIS cannot be known until completion of
307 the thermal ageing. The thermal ageing shall continue in accordance with the guidelines of the selected
308 EIS test method until both the reference and candidate EIS have completed the thermal ageing.
309 In a situation where no established reference EIS can be identified, a preselected time coordinate, or
310 correlation time, is usable as the means to establish the thermal classification of the candidate EIS. The
311 preselected time coordinate, or correlation time, is based on the application in agreement with the users.
312 This process establishes only the thermal classification of the candidate.
313 With the completion of the thermal ageing, the candidate EIS becomes the baseline EIS needed to
314 evaluate the influence of the additional stresses.
315 6.3 Evaluation of the additional stresses E, A and/or M in combination with T
316 After the thermal classification is completed, the candidate EIS (EIS ) is defined as being the baseline
CT
317 EIS (EIST). The influence of the additional factors is determined by the comparison of the endurance of
318 the combined stresses to the endurance of the baseline EIS (EIS ). The evaluation as to an exposure
T
319 of any combined stresses uses the two levels of analysis presented in clause 9 that consists of the
320 analysis of the pattern in the mode(s) of insulation failure and the 5-degree uncertainty criterion in Annex
321 H.
322 6.4 Variation
323 To minimize possible variations between equipment, all sets of test objects are expected to be in the
324 same type of ageing chambers and conditioned using the same apparatus. When different sets of test
325 objects cannot be placed into the same ageing chambers or conditioned using the same apparatus, all
326 variations such as differences in ageing ovens uniformity, oven temperature stability, air change rate
327 differences between ageing ovens at the same ageing temperature, different vibration tables – if used,
328 different condensation chambers – if used, different electrical insulation apparatus, or any other
329 equipment or procedure used which is not the same shall be described and included in the test report.
330 7 Illustration of the structure – Thermal evaluation – Step 1
331 7.1 Establishing the baseline thermal classification
332 The thermal classification of the baseline is established when the thermal ageing of both the reference
333 EIS and the candidate EIS have completed as determined in clause 6.1. The analysis of the candidate
334 test results is compared to the reference EIS; refer to
335 Figure 2. The reference EIS is used only to establish the thermal classification of what becomes the
336 baseline EIS.
While under test
339      Re  fer  e nc  e
designated as
EIS
340 T
Candidate EISCT
Completion of thermal
evaluation of
Candidate EISCT
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Thermally Classified Baseline
Candidate EISCT EIST
357 Figure 2 – Illustration of the establishment of the thermal classification of the baseline EIS
358 Both the reference EIS and candidate EIS undergo thermal ageing at three or more elevated
359 temperatures. This example uses three ageing temperatures of high, middle, and low. The total number
360 of sets of test objects needed is six, i.e. three for the reference and three for the candidate. The preferred
361 number of test objects per set is provided in the selected EIS test method.
362 Based on 10 test objects for each of the three ageing temperatures, Annex A (test sheet) and Annex B
363 (diagram) give an example for the reference EIS . The same test sheet and thermal ageing diagram
RT
364 for the thermal classification of candidate system EIS are given in Annex C and Annex D.
CT
365 For the example in the annexes, the reference EIS has an established 180 °C thermal class with the
366 assessed thermal endurance (ATE) of 185 °C. The ATE value is needed to make the analysis of the
367 candidate EIS for it to become a baseline EIS.
368 7.2 Illustration of the thermal evaluation
369 To establish the thermal endurance of the candidate EIS, the candidate EIS data is compared to a
370 reference EIS; refer to Figure 2.
371 Table 1 provides a sample of test data. Table 1 and Annex A and Annex B provide one illustration of
372 test results. The sample assumes ten test objects in each set. Annex A presents the data in a test report
373 format. Annex B presents a diagram of the same sample data. In this example, the thermal index rating
374 of the reference EIS is 185 °C.
375 Table 1 – Example of thermal test results for reference EIS based on data sheets given in
376 Annex A
Ageing temperature Average hours (h) of the set of test objects of the
reference EIS
High
H = 359
R
220 °C
Middle
M = 1 329
R
210 °C
Low
L = 5 391
R
200 °C
Thermal rating Assessed thermal endurance = 185 °C
Correlation time calculated to be 48 400 h
377 Note to Table 1: Refer to annex D, the calculated values for the number of hours in Annex D is 48 393 which is rounded to 48
378 400 for significant figures in the test values.
379 The ATE of the reference EIS is 185°C. Finding the coordinate on the best-fit line where the temperature
380 coordinate is 185°C gives the time correlation at 48 400 h. The 48 400 h correlation time value is needed
381 to analyse the test results of the candidate EIS; refer to annexes B, C, and D for further explanation.

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382 8 Evaluation of the influence of multifactor stresses – Step 2
383 8.1 General
384 Step 2 uses the baseline EIS to evaluate the influence of one or more additional stresses when applied
385 to the multifactor candidate EIS. Each additional stress or combinations of stresses for evaluation is
386 applied in combination with the thermal stress and shall be identified for inclusion into the total test
387 programme. To establish the thermal evaluation of the baseline EIS, both the reference EIS and the
388 candidate EIS undergo the applicable thermal ageing test as specified in IEC TR 61857-2 at three or
389 more elevated temperatures. As explained in clause 6.1, 6.2 and 6.3, after completion of the thermal
390 ageing of the EIS originally described as the candidate EIS, the candidate EIS becomes the baseline
391 EIS for the evaluation of the influence of the combination of applied stresses of the multifactor candidate
392 EIS.
393 The additional stress evaluation is the comparison of the multifactor candidate EIS (EIS ) set to the
TX
394 baseline EIS where only thermal stress was applied EIS . Comparison related to the evaluation of the
T
395 combination of stresses is not made back to the reference EIS (EIS ). Refer to Figure 3.
RT
396 Each additional stress or combination of stresses applied simultaneously shall be evaluated by means
397 of an additional set of test objects.
Multifactor
candidate with
Baseline
EIS additional factors
T
EIS
TX
406 Figure 3 – Comparison of the influence of any of the multifactor exposures
407 with the baseline performance
408 In Figure 3, EIS , the “X” is replaced with one or more letters representing the added stress E, A and/or
TX
409 M.
410 Examples of multifactor evaluations for Figure 3:
411 • thermal with mechanical: EIS ;
TM
412 • thermal with electrical: EIS ;
TE
413 • thermal with electrical, ambient, and mechanical: EIS .
TEAM
414 The established thermal classification of the baseline EIS is not changed by the multifactor evaluations.
415 8.2 Selection of the ageing temperature for the one-temperature comparison
416 For the evaluation of the influence of the combination of stresses the construction of the test specimens
417 shall be the same for the baseline EIS and multifactor candidate EIS . The initial evaluation of
T TX
418 influence of any additional stress can be evaluated by starting with a one-temperature comparison. The
419 preferred ageing temperature for the multifactor evaluation is one of the ageing temperatures used in
420 the evaluation of the EIS with the expected duration of the ageing to be in the range of 1 000 h to 2
T
421 000 h whenever possible. It is essential to select the ageing temperature for the one-temperature
422 comparison with an expected log average life long enough to be able to identify the ageing influence on
423 the multifactor candidate EIS’s performance. The ageing temperature shall be selected from the full
424 thermal ageing programme of the baseline EIS to give an expected test life of between 1 000 h to 2 000
425 h. This is typically the middle test temperature. However, if none of the original ageing temperatures
426 had a log average life value in the range of 1 000 to 2 000 hours, the temperature selected for the one-

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427 temperature comparison shall be at a temperature based on the results of the full thermal evaluation of
428 the baseline EIS that is expected to have a log average life between 1 000 h to 2 000 h.
429 Prior to the completion of the baseline thermal ageing the multifactor set can be under evaluation. For
430 projects designed in this manner, where the life at any ageing temperature is not yet known, the
431 preferred ageing temperature should be the middle ageing temperature used in the baseline thermal
432 ageing.
433 If the evaluation indicates that the added stress has a significant measurable influence and the decision
434 is made to expand the evaluation to a full multiple temperature ageing, one of the ageing temperatures
435 will be completed.
436 8.3 Selection of the ageing temperatures for the two-temperature comparison
437 Performing the additional evaluation at two of the ageing temperatures can provide a better insight into
438 the possible influence of the added factors on the expected long-term performance. In addition to the
439 added insight, if the evaluation indicates that the added stress has a significant measurable influence
440 and the decision is made to expand the evaluation to a full multiple temperature ageing, two of the
441 ageing temperatures will be completed. The two ageing temperatures are expected to be two of the
442 temperatures which would be selected during the evaluation of the EIS .
T
443 8.4 Diagnostic tests and end-point criterion applied at elevated temperature
444 Prior to the start of any ageing, the project shall be clearly defined in terms of the method of diagnostics
445 with the end-point criterion needed to evaluate the influence of the added stresses. The diagnostic
446 measurement(s) to be used for assessing the ageing condition shall be defined depending on the type
447 of multifactor ageing and following the diagnostics in the EIS test method selected. The end-point
448 criterion of the multifactor evaluation tests has to be defined before starting the ageing tests. The end-
449 point criterion shall be in accordance with the conditions of the test method selected from IEC TR 61857-
450 2.
451 8.5 Diagnostics
452 Depending on the stresses to be evaluated, some evaluations shall follow the standard ageing,
453 conditioning and the diagnostic cycling used in the establishment of the baseline EIS. This is the
454 expected project structure when the added stress is ambient and/or mechanical. Some evaluations
455 require continual diagnostic monitoring while ageing at the elevated temperature.
456 Examples of diagnostic measurement conditions applied after each ageing cycle:
457 • test objects conditioned at an elevated temperature immersed in oil or other liquid for electrical tests;
458 • test objects conditioned by mechanical vibration at elevated temperature.
459 When the added aging stress evaluation is to be evaluated under the expected operating conditions the
460 ageing temperature selected for such an evaluation cannot be at the thermal ageing temperatures as
461 the thermal ageing temperatures are much higher than the expected operating temperature range. The
462 reason and selection of the ageing temperature used for any ageing evaluation other than those used
463 for the thermal ageing and classification of the EIS shall be given in the report.
464 8.6 Using ageing cycles
465 All stresses are applied at an elevated temperature with all diagnostic measurements being performed
466 during the combined exposure or after each ageing cycle. When ageing cycles are used, following each
467 ageing cycle the set of test objects are conditioned and then evaluated.
468 Examples of diagnostic measurement conditions applied after each ageing cycle:
469 • test objects conditioned at elevated temperature immersed in oil or other liquid for electrical tests;
470 • test objects conditioned by mechanical vibration during ageing at elevated temperature.

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472 8.7 Diagnostics applied at elevated temperature without ageing cycles
473 For multifactor evaluation with the stresses applied at elevated temperature(s) the diagnostic
474 measurement can also be made on a continual basis. The endurance shall be monitored, measured and
475 recorded on a continual basis during the exposure at elevated temperatures. Separation into ageing
476 cycles is not part of the design for this type of multifactor evaluation process. Each measured property
477 shall have a preselected limit to the change of performance to define the end-of-test for the selected
478 property.
479 Examples of diagnostics applied at elevated temperature on a continual basis:
480 • application of voltage, sinusoidal or non-sinusoidal;
481 • application of voltage in an atmosphere other than the environment used during the evaluation of
482 the EIS;
483 • Example of a different electrical stress during the ageing is shown in Figure 4.
484 •
485 • Multifactor
Baseline
Candidate
EIST
486 •
EISTE
487 •
488 Figure 4 – Comparison of
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