IEC TR 61857-2:2025

IEC TR 61857-2 ®
Edition 2.0 2025-05
TECHNICAL
REPORT
Electrical insulation systems – Procedures for thermal evaluation –
Part 2: Selection of the appropriate test method for evaluation and classification
of electrical insulation systems
ICS 29.080.30  ISBN 978-2-8327-0358-8

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CONTENTS
FOREWORD . 2
INTRODUCTION . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 General . 6
4.1 Purpose of the evaluation . 6
4.2 Test stresses . 6
5 Test methods for the evaluation and classification of an EIS . 6
Bibliography . 10

Table 1 – Overview of EIS evaluations for background and structure of the series . 7
Table 2 – For evaluation of test objects or materials prior to the start of long-term
thermal ageing using screening tests . 8
Table 3 – Test standards for the evaluation of thermal stresses in air . 8
Table 4 – Standards for special environmental applications other than in air . 9
Table 5 – Standards for the modification of an established EIS . 9

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Electrical insulation systems – Procedures for thermal evaluation –
Part 2: Selection of the appropriate test method for evaluation and
classification of electrical insulation systems

FOREWORD
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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
of any claimed patent rights in respect thereof. As of the date of publication of this document,
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 TR 61857-2 has been prepared by IEC technical committee 112: Evaluation and
qualification of electrical insulating materials and systems. It is a Technical Report.
This second edition cancels and replaces the first edition published in 2015. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition.
a) The Introduction is expanded to convey that both TC 112 and TC 2 have established EIS
test standards.
b) Clause 4 introduces the fact that there are four stress factors, not only thermal. This
essential concept carries over into part of Clause 5.
c) The large number of established standards are organized into common concept groups in
Tables 1 to 5:
1) Table 1 covers an overview of EIS evaluations for background and structure.
2) Table 2 covers evaluation of test objects prior to the start of long-term thermal ageing
using screening tests.
3) Table 3 covers test standards for the evaluation of thermal stresses in air and has an
increased number of standards.
4) Table 4 covers standards for special environmental applications other than in air.
5) Table 5 covers standards for the modification of an established EIS.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
112/671/DTR 112/682/RVDTR
Full information on the voting for the approval of this Technical Report can be found in the
report on voting indicated in the above table.
The language used for the development of this Technical Report is English.
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 http://www.iec.ch/members_experts/refdocs. The main document types developed by IEC
are described in greater detail at http://www.iec.ch/publications.
A list of all parts in the IEC 61857 series, 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
This document is part of a series of IEC test standards for the thermal evaluation and
classification of an electrical insulation system (EIS). The total series of EIS test standards are
the responsibility of IEC TC 112 for general test standards and IEC TC 2, Rotating machinery,
for more specific information for motors and generators.
The following are TC 112 series standards.
– The IEC 61857 series [1] is for the thermal evaluation and classification of an EIS. These
standards are for the evaluation and classification of a new EIS.
– The IEC 61858 series [2] is for modification of an established EIS after the EIS has been
established using the IEC 61857 series [1]. The IEC 61858 series [2] does not result in the
thermal evaluation and classification of an EIS; it addresses the procedures for
modifications. It does not assign a thermal class rating to the modification. The evaluation
is to develop measurable information as to the influence of additional stresses on the
expected performance of the EIS caused by the proposed modification.
– IEC 63177 [3], IEC 60085 [4] and IEC 60505 [5] specify basic EIS evaluation information
and recommended prethermal ageing or screening tests.
The following are IEC TC 2 series standards.
– The IEC 60034-18 series [6], while similar to the IEC 61857 series [1], provides additional
information for an EIS to be used in rotating machinery.
IEC 60505 [5] is the base standard for both the IEC 60034-18 series [6] and the IEC 61857
series [1]. For both series of EIS test standards, the procedure is to use accelerated thermal
ageing. Accelerated thermal ageing of an EIS is intended to evaluate the thermal classification
of the EIS for projected long-term performance by generating measurable performance at
elevated ageing temperatures and projecting to expected performance at lower operating
temperatures.
IEC 60505 [5] provides four categories of ageing stresses which influence the performance of
products in use under a wide range of operating conditions. In IEC 60505 [5], the stresses are
presented as thermal (T), electrical (E), environmental (E) and mechanical (M). In this
document, environmental (E) is replaced with ambient (A) to avoid the confusion of having two
stresses represented by the same letter. In this document, the stresses are therefore presented
as thermal (T), electrical (E), ambient or environmental (A) and mechanical (M).
In many applications, it can be important to include the evaluation of other ageing stresses in
addition to or in combination with thermal stress; this is known as multifactor evaluation. The
relevant IEC TC specifies the requirements for a particular application.
This document is presented as a selection guide to provide information to help users identify
the most appropriate standard for the application.
IEC 60034-18-1:2022 [7] states:
IEC 60085 [4] deals with thermal evaluation of electrical insulation materials and in particular
insulation systems used in electrical equipment. In particular, thermal classes of insulation
systems are defined and designations are given, such as 130 (B), 155 (F) and 180 (H) for use
in rotating machines belonging to IEC 60034-1 [8]. In the past, materials for insulation systems
were often selected solely on the basis of thermal endurance of individual materials performed
according to the IEC 60216 series [9]. However, IEC 60085 [4] recognizes that such selection
may be used only for screening materials prior to further functional evaluation of
...