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

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 . 3
...