IEC TS 61857-42:2025

Electrical insulation systems - Procedures for thermal evaluation - Part 42: Specific requirements for evaluation of an electrical insulation system (EIS) used for road transportation applications

IEC TS 61857-42:2025 provides a procedure to evaluate the lifetime of an electrical insulation system (EIS) in a drivetrain unit within road transportation (automotive) applications. Typical applications include motors and generators in hybrid and full electric passenger vehicles, light-duty and heavy-duty commercial vehicles, as well as buses.
In general, the IEC 61857 series is applicable to EIS used in electrotechnical products with an input voltage of up to 1 000 V where the predominant ageing factor is thermal. In the context of this document the limit of 1 000 V is understood to be the application-specific battery DC voltage.
The EIS evaluated by this procedure will operate free from partial discharges over its whole lifetime.
Evaluation of EIS in the following applications is outside the scope:
- motors and generators within the scope of IEC TC 2, Rotating machinery;
- rail traction machines in the scope of IEC TC 9, Electrical equipment and systems for railways;
- motors and generators for road vehicles that are not intended for the traction of them.

General Information

• Status: Published
• Publication Date: 14-Aug-2025
• ICS: 29.080.30 - Insulation systems
• Technical Committee: TC 112 - Evaluation and qualification of electrical insulating materials and systems
• Drafting Committee: WG 6 - TC 112/WG 6
• Current Stage: PPUB - Publication issued
• Start Date: 15-Aug-2025
• Completion Date: 01-Aug-2025

Overview

IEC TS 61857-42:2025 is a Technical Specification from IEC that defines a procedure to thermally evaluate the lifetime of an electrical insulation system (EIS) used in drivetrain units for road transportation (automotive) applications. Typical targets are motors and generators in hybrid and full electric passenger vehicles, light- and heavy-duty commercial vehicles, and buses. The series applies to EIS in electrotechnical products with an application-specific battery DC voltage up to 1 000 V where thermal ageing is the predominant degradation mechanism. The EIS assessed by this procedure is intended to operate free from partial discharges (PD) over its whole lifetime.

Key topics and technical requirements

• Scope & exclusions
  • Applies to traction motors/generators for road vehicles; excludes rotating machinery under IEC TC 2, rail traction under IEC TC 9, and non-traction road-vehicle motors.
• Test program structure
  • Initial diagnostic subcycle (impulse PD test, mechanical, cold, moisture exposures, dielectric diagnostics).
  • Controlled thermal ageing stages (specified ageing temperatures, periods, heating methods).
  • Repeated diagnostic subcycles to detect degradation.
• Diagnostics & acceptance criteria
  • Impulse partial discharge testing and derived peak-to-peak test voltages (Annex C).
  • Dielectric diagnostics, mechanical and environmental stress exposures.
  • End-point criteria, life determination methods, and reporting rules.
• Data analysis & extrapolation
  • Use of Arrhenius-based extrapolation, projected life with confidence limits, and methods to evaluate candidate vs. reference EIS (qualification cases A–D).
• Multifactor evaluation
  • Guidance for combining thermal with mechanical, electrical or chemical stresses.
• Informative annexes
  • Chemical compatibility with cooling fluids (Annex A), hairpin formette test construction (Annex B), PD voltage derivation (Annex C) and data analysis guidance (Annex D).

Applications and who uses it

• Primary users
  • Automotive OEMs, traction motor/generator designers, insulation material and varnish suppliers, and test laboratories.
• Use cases
  • Design qualification of EIS for EV/hybrid drivelines, comparison of candidate vs. reference insulation systems, lifetime projection for warranty and safety assessments, and integration into component validation programs.
• Practical benefits
  • Consistent thermal-lifetime assessment methodology, PD-safe qualification, and data-driven service-life projections to support robust drivetrain electrification.

Related standards

• IEC 61857 series (general procedures for thermal evaluation)
• IEC 62332 series (referred to in Annex and multifactor guidance)
• Other IEC standards on rotating machinery and railway equipment as applicable for excluded domains

Keywords: IEC TS 61857-42:2025, electrical insulation system, EIS, thermal evaluation, drivetrain, electric vehicle, insulation lifetime, partial discharge, Arrhenius extrapolation.

Standards Content (Sample)

IEC TS 61857-42 ®
Edition 1.0 2025-08
TECHNICAL
SPECIFICATION
Electrical insulation systems - Procedures for thermal evaluation -
Part 42: Specific requirements for evaluation of an electrical insulation system
(EIS) used for road transportation applications
ICS 29.080.30  ISBN 978-2-8327-0586-5

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CONTENTS
FOREWORD. 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
3.14 Terms related to Annex C . 9
4 General considerations . 11
4.1 Approach to qualification of EIS for road transportation applications . 11
4.2 Overview of test procedure . 11
4.3 Thermal endurance considerations for electrical insulating materials . 13
4.4 Chemical compatibility considerations for electrical insulating materials . 13
5 Test objects . 14
5.1 Test object selection . 14
5.2 General purpose models . 15
5.3 Prototype parts for design qualification . 15
6 Initial diagnostic subcycle . 17
6.1 Overview . 17
6.2 Initial impulse partial discharge test . 17
6.3 Mechanical stress exposure . 17
6.4 Cold exposure . 18
6.5 Moisture exposure . 18
6.6 Initial dielectric diagnostic test . 18
7 Thermal ageing . 19
7.1 General . 19
7.2 Ageing periods and temperatures . 19
7.3 Methods of heating . 20
7.4 Ageing procedure . 20
7.5 Confidence in a projected thermal rating . 21
8 Diagnostic subcycle . 21
8.1 Overview . 21
8.2 Impulse partial discharge test and alternative test procedures . 21
8.3 Mechanical stress exposure and alternative test procedures . 22
8.4 Cold exposure and alternative test procedures . 22
8.5 Moisture exposure and alternative test procedures . 22
8.6 Dielectric diagnostic test . 22
8.7 Other diagnostic tests . 23
9 Analysing, reporting and classification . 23
9.1 End-point criterion . 23
9.2 Method of determining life . 24
9.2.1 End of life . 24
9.2.2 Average life . 24
9.3 Extrapolation of data . 25
9.3.1 Projected life and confidence limits . 25
9.3.2 Extrapolation of data - Example . 25
9.4 Utilization of ageing data for different life requirements . 27
9.5 Report of results . 27
10 Evaluation of data from candidate EIS and reference EIS . 28
10.1 General . 28
10.2 Determining qualification . 28
10.3 Qualification cases . 28
10.3.1 Case A: Qualification for the same class temperature and same
expected service life . 28
10.3.2 Case B: Qualification for the same class temperature and a different
expected service life . 29
10.3.3 Case C: Qualification for a different class temperature and same
expected service life . 30
10.3.4 Case D: Qualification for a different class temperature and different
expected service life . 31
11 Evaluation of ageing factors in addition to thermal . 32
11.1 General . 32
11.2 Multifactor evaluation examples . 33
11.2.1 Combined thermal and mechanical stress evaluation (EIS ) . 33
TM
11.2.2 Combined thermal and electrical stress evaluation (EIS ) . 33
TE
11.2.3 Thermal evaluation of an EIS when combined with a liquid (EIS ) . 34
TA
11.3 Cases of multifactor candidate EIS qualification . 34
11.4 Single or mutliple temperature multifactor evaluation . 34
11.5 Analysis of results from multifactor evaluation . 35
Annex A (informative) Chemical compatibility of electrical insulating materials with
cooling fluids . 38
Annex B (informative) GPM construction: Hairpin formette . 39
B.1 Arrangement of hairpin formette . 39
B.2 Components of hairpin formette . 39
B.3 Assembly of hairpin formette . 41
B.4 Test positions in hairpin formette . 41
Annex C (normative) Derivation of peak-to-peak test voltages for a diagnostic impulse
PD test . 42
C.1 General . 42
C.2 Derivation of maximum allowable peak-to-peak voltages in service . 42
C.3 Example of calculation of maximum allowable voltages in operation . 45
C.4 Enhancement factors. 46
C.4.1 PD safety factor . 46
C.4.2 Temperature enhancement factor . 46
C.4.3 Ageing factor . 46
C.5 Derivation of peak-to-peak test voltages . 47
Annex D (informative) Additional discussion on analysis of test data . 48
Bibliography . 50

Figure 1 – Test procedure flowchart . 12
Figure 2 – Example for a segmented stator . 15
Figure 3 – Example for a prototype stator . 16
Figure 4 – Arrhenius plot – for example ageing . 26
Figure 5 – Case A: Candidate EIS qualified for the same thermal class and the same
expected service life . 29
Figure 6 – Case B: Candidate EIS qualified for the same thermal class and different
expected service life . 30
Figure 7 – Case C: Candidate EIS qualified for a different class temperature and the
same expected service life . 31
Figure 8 – Case D: Candidate EIS qualified for a different service life and different
thermal class from the reference . 32
Figure 9 – Multifactor Case A : Multifactor candidate EIS EIS qualified for the same
TX
class temperature and the same expected service life . 36
Figure 10 – Multifactor Case C - Multifactor candidate EIS EIS qualified for a
TX
different (lower) class temperature and the same expected service life . 37
Figure B.1 – A three-dimensional view of a suitable hairpin formette made with steel
frame and pairs of bolted L-shaped slot plates . 39
Figure B.2 – Three-dimensional view of hairpin formette frame . 40
Figure B.3 – Drawing of hairpin formette frame . 40
Figure C.1 – Voltage impulse waveshape parameters . 43
Figure C.2 – Jump voltage (U ) associated with a converter drive . 43
j
Figure C.3 – Comparison of phase-to-phase (left), phase-to-ground (centre), and turn-
to-turn (right) voltages for a two-level converter . 44
Figure C.4 – Worst-case voltage stressing the turn-to-turn insulation in a variety of
random wound stators as a function of the impulse rise time . 45
Figure D.1 – Arrhenius plot – using low average passing times . 48
Figure D.2 – Arrhenius plot – using minimum passing times . 49

Table 1 – Guidance for test object selection . 14
Table 2 – Initial dielectric diagnostic test settings . 18
Table 3 – Suggested ageing temperatures and ageing periods . 20
Table 4 – Test voltages . 23
Table 5 – Example of calculated failure times for a 24-h ageing cycle at 235 °C . 24
Table 6 – Log average life of set of test objects – 24-h ageing cycle at 235 °C . 24
Table 7 – Log average life of set of test objects – 96-h ageing cycle at 215 °C . 25
Table 8 – Log average life of set of test objects – 288-h ageing cycle at 195 °C . 25
Table 9 – Data for Arrhenius plot . 26
Table 10 – Conditions for qualification of candidate EIS . 28
Table 11 – Overview on IEC 62332 series [20] . 34
Table C.1 – Summary of enhancement factors to be applied to the operating voltages . 46

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Electrical insulation systems - Procedures for thermal evaluation -
Part 42: Specific requirements for evaluation of an electrical insulation
system (EIS) used for road transportation applications

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for
standardization comprising all national electrotechnical committees (IEC National Committees).
The object of IEC is to promote international co-operation on all questions concerning
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced
publications is indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve
the use of (a) patent(s). IEC takes no position concerning the evidence, validity or applicability
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IEC had not received notice of (a) patent(s), which may be required to implement this document.
However, implementers are cautioned that this may not represent the latest information, which
may be obtained from the patent database available at https://patents.iec.ch. IEC shall not be
held responsible for identifying any or all such patent rights.
IEC TS 61857-42 was prepared by IEC technical committee 112: Evaluation and qualification
of electrical insulating materials and systems.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
112/681/DTS 112/687/RVDTS
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 61857 series, published under the general title Electrical insulation
systems – Procedures for thermal evaluation, can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
INTRODUCTION
As per today all standards to evaluate the lifetime of electrical insulation system (EIS) are linked
to the needs of industrial motors (e.g. IEC 60034-18-21 [1] developed in IEC TC 2 and IEC
61857-21 [2] developed in IEC TC 112). Drivetrain units for road transportation applications can
have a similar technical concept to industrial motors but are different from them in terms of their
operational demands. They mostly operate at variable loads and speeds, at increased
mechanical stresses, at variable climatic conditions , and are powered from battery voltage
levels such that they can pose a greater risk of partial discharges.
The aim of this document is to close this gap and to provide users with a suitable test procedure
to evaluate the EIS in drivetrain units for road transportation.
As a key example, one parameter is the estimated lifetime of the unit. While industrial motor
EIS is typically qualified based on a thermal evaluation of 20 000 h lifetime, drivetrain units for
passenger cars are designed for a typical lifetime of 8 000 h. This document gives guidance on
how to adjust the test procedure for the thermal evaluation to the particular and unique need of
the individual application.
Other influences on the EIS, like compatibility with cooling fluids (oils), different mechanical
load profiles are possible to screen by using a multifactor evaluation and an adjusted lifetime
can be calculated.
In the IEC 61857 series, thermal ageing is the dominant ageing stress for the evaluation and
qualification of EIS. The test is established for general purpose models (GPMs) or simple
models (such as partial segments of a motor stator), all the way to full stator designs and takes
into account specific winding configurations such as round wire (random windings) and
rectangular wire (e.g. hairpin).
Due to the new content and a lack of test results based on the new test geometry, this document
is published as a Technical Specification.
1 Scope
This document provides a procedure to evaluate the lifetime of an electrical insulation system
(EIS) in a drivetrain unit within road transportation (automotive) applications. Typical
applications include motors and generators in hybrid and full electric passenger vehicles, light-
duty and heavy-duty commercial vehicles, as well as buses.
In general, the IEC 61857 series is applicable to EIS used in electrotechnical products with an
input voltage of up to 1 000 V where the predominant ageing factor is thermal. In the context of
this document the limit of 1 000 V is understood to be the application-specific battery DC
voltage.
The EIS evaluated by this procedure will operate free from partial discharges over its whole
lifetime.
Evaluation of EIS in the following applications is outside the scope:
– motors and generators within the scope of IEC TC 2, Rotating machinery;
– rail traction machines in the scope of IEC TC 9, Electrical equipment and systems for
railways;
– motors and generators for road vehicles that are not intended for the traction of them.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60216-5, Electrical insulating materials - Thermal endurance properties - Part 5:
Determination of relative temperature index (RTI) of an insulating material
IEC 61857-1, Electrical insulation systems - Procedures for thermal evaluation - Part 1: General
requirements - Low-voltage
IEC TS 61934, Electrical insulating materials and systems - Electrical measurement of partial
discharges (PD) under short rise time and repetitive voltage impulses
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 61857-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
3.1
electrical insulation system
EIS
insulating structure containing one or more electrical insulating materials together with
associated conducting parts employed in an electrotechnical device
3.2
electrical insulating material
EIM
material with negligibly low electric conductivity, used to separate conducting parts at different
electrical potentials
3.3
reference EIS
established EIS evaluated on the basis of either a known service experience record or a known
comparative functional evaluation
3.4
candidate EIS
EIS under evaluation to determine its service capability
Note 1 to entry: Service capability can be with regard to electrical, thermal, mechanical, environmental or multi-
factor stresses.
3.5
thermal endurance
TE
ability of an electrical insulation system (EIS) to withstand the action of temperature determined
by a test on its own
Note 1 to entry: This rating is based on end-of-life criteria and lifetime required in the specific EIS test. A choice of
EIS test methods is listed in IEC TR 61857-2 [3]. The method identifies how the lifetime is determined, and how the
rating is listed in the report.
3.6
relative thermal endurance
RTE
numerical rating expressed in degrees Celsius assigned by comparing the results of a thermal
ageing evaluation of an electrical insulation system (EIS) to that of a reference EIS
3.7
temperature index
TI
numerical value of the temperature expressed in degrees Celsius characterizing the thermal
capability of an electrical insulating material which is obtained by extrapolating the Arrhenius
plot of life versus temperature to a specified time
Note 1 to entry: The specified time is usually 20 000 h.
Note 2 to entry: A selection of end-of-life criteria is listed in IEC 60216-2 [4].
3.8
designed life
τ
EIS
expected or required minimum lifetime of an EIS
Note 1 to entry: Passenger cars typically have a service life of 8 000 h.
3.9
partial discharge
PD
electric discharge that only partially bridges the insulation between electrical conductors
Note 1 to entry: It can occur inside the insulation or adjacent to an electrical conductor.
[SOURCE: IEC 60034-18-41:2014 [5], 3.1]
3.10
partial discharge inception voltage
PDIV
lowest voltage at which partial discharges are initiated in the test arrangement when the voltage
applied to the test object is gradually increased from a lower value at which no such discharges
are observed
Note 1 to entry: The PDIV is defined as the peak-to-peak voltage. With sinusoidal applied voltages, the PDIV is
normally measured as the RMS value of the voltage. Converting to the peak-to-peak voltage requires the knowledge
of the form factor.
[SOURCE: IEC 60034-18-41:2014 [5], 3.2, modified – Note 1 to entry has been redrafted.]
3.11
partial discharge extinction voltage
PDEV
voltage at which partial discharges are extinguished in the test arrangement when the voltage
applied to the test object is gradually decreased from a higher value at which such discharges
are observed
Note 1 to entry: The PDEV is defined as the peak-to-peak voltage. With sinusoidal applied voltages, the PDEV is
normally measured as the RMS value of the voltage. Converting to the peak-to-peak voltage requires the knowledge
of the form factor.
[SOURCE: IEC 60034-18-41:2014 [5], 3.3, modified – Note 1 to entry has been redrafted.]
3.12
repetitive partial discharge inception voltage
RPDIV
minimum peak-to-peak impulse voltage at which more than five partial discharge pulses occur
on ten voltage impulses of the same polarity when the voltage applied to the test object is
increased with step-by-step method from a lower value at which no discharges are observed
Note 1 to entry: This is a mean value for the specified test time and a test arrangement where the voltage applied
to the test object is gradually increased from a value at which no partial discharges can be detected for the measured
partial discharge sensitivity.
3.13
repetitive partial discharge extinction voltage
RPDEV
maximum peak-to-peak impulse voltage at which less than five partial discharge pulses occur
on ten voltage impulses of the same peak-to-peak values when the voltage applied to the test
object is decreased with step-by-step method from a higher value at which such discharges are
observed
3.14 Terms related to Annex C
3.14.1
unipolar impulse
voltage impulse, the polarity of which is either positive or negative
Note 1 to entry: The term impulse is used to describe the transient stressing voltage applied to the test object and
the term pulse is used to describe the partial discharge signal.
[SOURCE: IEC 60034-18-41:2014 [5], 3.10]
3.14.2
bipolar impulse
voltage impulse, the polarity of which changes periodically from positive to negative or vice
versa
[SOURCE: IEC 60034-18-41:2014 [5], 3.11]
3.14.3
DC bus voltage
U
dc
voltage of the intermediate circuit of the voltage converter (dc-link-circuit)
Note 1 to entry: For a two-level converter U is equal to U in Figure C.1.
dc a
[SOURCE: IEC 60034-18-41:2014 [5], 3.23, modified – Note 2 to entry has been deleted.]
3.14.4
initial impulse voltage magnitude
U
initial magnitude of the voltage impulse
3.14.5
steady state impulse voltage magnitude
U
a
final magnitude of the voltage impulse
[SOURCE: IEC 60034-18-41:2014 [5], 3.5]
3.14.6
voltage overshoot
U
b
magnitude of the peak voltage in excess of the steady state impulse voltage magnitude
3.14.7
peak-to-peak voltage
U
p
difference between the initial voltage value and the maximum voltage reached during a voltage
impulse
3.14.8
peak-to-peak fundamental frequency voltage
U
pk/pk
peak-to-peak voltage at the fundamental frequency
3.14.9
jump voltage
U
j
change in phase-to-ground voltage at the terminals of the machine occurring at the start of each
impulse when fed from a converter
[SOURCE: IEC 60034-18-41:2014 [5], 3.22, modified – In the definition, "phase-to-ground" has
been added".]
3.14.10
overshoot factor
OF
ratio of the voltage appearing at the machine terminals and the voltage at the converter for each
converter level
[SOURCE: IEC 60034-18-41:2014 [5], 3.24]
3.14.11
impulse rise time
t
r
time between 10 % and 90 % of the voltage transient peak
3.14.12
diagnostic test
periodic or continuous application of a specified level of a diagnostic factor to a test object to
determine whether or when the end-point criterion has been reached
4 General considerations
4.1 Approach to qualification of EIS for road transportation applications
This document provides a methodology for the design qualification of an electrical insulation
system (3.1) (EIS) for road transportation applications.
While each automotive supplier has their own expectations for this type of testing (different life
requirements as well as differences in the additional factors to consider), there is a benefit in a
standardized approach to this testing for comparison purposes. This document provides a
recommended set of test conditions, as well as provides guidance on how to modify the test
protocol for requirements that deviate from this recommendation.
This document provides specific examples for multi-factor influences related to road
transportation examples such as oil resistance or higher vibration levels beyond the original
evaluation techniques specified in IEC 61857-21 [2]
Either only a candidate EIS (3.4) is tested to determine an absolute thermal endurance (3.5)
index, or the relative thermal endurance (3.6) index of a candidate EIS can be determined based
on testing alongside a reference EIS (3.3) with a known service life. A reference EIS shall be
tested using the same procedure as that used for the candidate EIS.
Once an EIS is qualified, there can arise needs for modifications or substitutions of components,
such as use of EIM from another supplier. The guidance on substitutions is provided by the IEC
61858 series [6].
4.2 Overview of test procedure
All test objects shall be subjected to initial screening tests followed by repeated thermal
endurance test cycles consisting of subcycles in the following order:
– thermal ageing subcycle;
– partial discharge (3.9) (PD) measurement;
– mechanical stress subcycle;
– cold exposure subcycle;
– moisture exposure subcycle;
– withstand voltage test, or other diagnostic test (3.14.12).
The complete procedure is visualized in Figure 1.
Key
EIM electrical insulating material (3.2)
EIS electrical insulation system
GPM general purpose model
PD partial discharge
RH relative humidity
RPDIV repetitive partial discharge inception voltage (3.12)
TI temperature index (3.7)
Figure 1 – Test procedure flowchart
The procedure begins with preselection and pre-qualification of EIMs, from the perspective of
suitability for intended thermal class, as well as compatibility with other materials or
environment. This is described in 4.3.
5.1 deals with selection of suitable test objects and guidance for their construction.
The initial diagnostic subcycle is described in Clause 6.
Clause 7 deals with the thermal endurance subcycle of the main cycling procedure. The thermal
endurance is done by parallel exposure of a minimum three sets of test objects to temperatures
raised above expected thermal class of the candidate EIS.
Clause 8 deals with diagnostic subcycle of the main cycling procedure. The diagnostic subcycle
consists of a partial discharge check, mechanical stress subcycle, cold exposure subcycle,
moisture exposure subcycle and withstand voltage test. Apart from the base set of subcycles'
procedure parameters, several possibilities to include alternative application related diagnostic
parameters applied on additional sets of test objects are described as examples in Clause 8.
Clause 9 is dedicated to the procedure of processing and evaluation of the test data and
reporting of results.
Clause 10 deals with qualification of a candidate EIS when test data are available from a service
proven reference EIS.
Several possibilities to include additional sets of test objects exposed to multi-factor stressing
by heat and additional types of stress, to be evaluated in comparison to sets of test objects
exposed to thermal ageing only, are described as examples in Clause 11.
It is recognized that, depending on the test facilities available, the type of equipment employed,
and other factors, slight variations in the methods of exposing the test objects can be agreed
between involved parties. When any two different EIS are compared, the test objects of each
shall be subjected to identical exposures and other conditions of test. Unless otherwise
specified, pre-diagnostic conditioning and diagnostic tests shall be carried out at room
temperature (25 ± 5) °C and (50 ± 10) % relative humidity.
4.3 Thermal endurance considerations for electrical insulating materials
Insulation systems can be made up of materials with different thermal indices. It is possible for
a material with a lower TI to work reliably as a component inside an insulation system of a
higher thermal class. Selection of materials should be however made with the consideration
that a polymeric material can age dramatically faster than other components if it is exposed to
a service temperature too far from its stand-alone capability. The associated weight loss or
increase in brittleness can give rise to undesired effects, such as rise in PD activity or negative
impact on heat transfer properties.
For long-established chemistry groups, thermal endurance capability of materials in their usual
forms is typically known; however, if information for a novel material is missing, guidance for
its evaluation can be taken from IEC 60216-5 [7].
4.4 Chemical compatibility considerations for electrical insulating materials
Apart from thermal endurance limitation of materials, the lifetime of an insulation system can
also be limited by the ability of materials to resist chemical attack associated with fluids (liquid
or gas) that can be in contact with the insulation system in service. The practical examples can
be fluids used for active cooling, for lubrication of other e-drive components, or accidental spills
of other service fluids, such as antifreeze solutions.
Guidance for approaches to evaluate how selected EIMs withstand the harsher environments
is given in Annex A.
5 Test objects
5.1 Test object selection
The following types of test object can be used to assess an electrical insulation system:
– general purpose models (GPMs) - motorettes or hairpin formettes;
– prototype motor parts - full or segmented stators;
– production motor parts.
As presented in IEC 61857-1 [8], the selection of the preferred test object should be based on
the purpose of the evaluation. The evaluation of an EIS can be for many reasons, from
engineering design information to establishing a baseline to evaluate the influence of stress
factors other than and also in combination with the thermal stress factor. The purpose of the
evaluation also depends on the stage of development of the motor design, see Table 1. The
GPM is the preferred test object when the project is in the stage when the motor design has not
been finalized. If the design has been finalized the preference is to have the actual completed
motor stator as the preferred test object, either as a prototype or, if available and more
convenient, even from a production line. The motor stator shall also be used when the purpose
is to evaluate the performance of variations of motor design.
Table 1 – Guidance for test object selection
Project stage Purpose of EIS evalulation Prototype or General purpose Individual
production stator model material
(motorette or
hairpin formette)
Motor design Evaluate the complete stator

X
finalized thermal performance
Generate performance
X
capabilities for design purposes
Multifactor stresses
X X
Motor design EIS thermal evaluation

X X
not finalized
Compatibility in environments
X X X
other than air
Light grey covers the evaluation purposes for finalized motor designs.
Dark grey covers the evaluation purposes for not finalized motor designs.
Wherever feasible, test objects including GPMs should closely represent the actual construction
of the insulation system to be used in a motor stator. Usually this requires coils of full cross-
section with actual clearances and creepage distances, mounted in a fixture that simulates the
arrangement of coils in the machine. The coils or hairpin conductors are the test specimens,
and the complete fixture with specimens in place is the test object.
Where the test specimens are coils or hairpin conductors, they should represent the full
insulation design, including actual conductor dimensions, insulation thickness, coil-to-coil (or
hairpin-to-hairpin) clearance, and including impregnation resin. The specimens tested shall
represent the design for the intended maximum rated voltage and equipment standards.
Testing on prototype or production parts is beneficial to assess insulation system applied to a
specific motor design, with influences of most manufacturing processes considered, including
impregnation process, potting process, or winding connection process. The transfer of such
qualification can be limited if the design of the motor or manufacturing processes change.
5.2 General purpose models
The GPM shall reflect the type of winding style used in the stator for which the insulation system
is intended. In most cases, one of the GPMs below will closely represent the winding
arrangement in the application:
– a motorette with round-wire random wound coils;
– a formette with slots housing formed conductors, such as hairpin conductors, in a regular
pattern.
For guidance on the construction of a motorette, see IEC 61857-21:2009 [9], Clause 4 . The
minimum number of motorettes in a group for each ageing temperature shall be ten.
For guidance on the construction of a hairpin formette, see Annex B. The minimum number of
assessed slots in a formette used for each ageing temperature shall be ten.

Figure 2 – Example for a segmented stator
Hairpin formettes can take various forms, for example by taking advantage of using cut-up
sections of actual production stators (segments) to hold groups of conductors for providing a
sufficient number of test positions, such as depicted in Figure 2.
5.3 Prototype parts for design qualification
Prototype motor parts can be fully or partially wound stators, or their sections representing
several slots. Alternative manufacturing techniques can be used to prepare prototype test
objects, however as close as possible to manufacturing processes intended for serial
production, preferably including any finishing operations introducing significant thermal,
mechanical, chemical or electrical stresses, such as conductor stretching and bendin
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