IEC 60071-1:2019

IEC 60071-1 ®
Edition 9.0 2019-08
REDLINE VERSION
INTERNATIONAL
STANDARD
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inside
HORIZONTAL STANDARD
Insulation co-ordination –
Part 1: Definitions, principles and rules

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IEC 60071-1 ®
Edition 9.0 2019-08
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
HORIZONTAL STANDARD
Insulation co-ordination –
Part 1: Definitions, principles and rules

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.080.30 ISBN 978-2-8322-7295-4

– 2 – IEC 60071-1:2019 RLV  IEC 2019
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Abbreviated terms and symbols . 10
4.1 General . 15
4.2 Subscripts . 15
4.3 Letter symbols . 15
4.4 Abbreviations . 16
5 Procedure for insulation co-ordination . 16
5.1 General outline of the procedure . 16
5.2 Determination of the representative voltages and overvoltages (U ) . 17
rp
5.3 Determination of the co-ordination withstand voltages (U ) . 19
cw
5.4 Determination of the required withstand voltage (U ) . 19
rw
5.5 Selection of the rated insulation level . 20
5.6 List of standard rated short-duration power frequency withstand voltages . 21
5.7 List of standard rated impulse withstand voltages . 21
5.8 Ranges for highest voltage for equipment . 21
5.9 Environmental conditions . 21
5.9.1 Normal environmental conditions . 21
5.9.2 Standard reference atmospheric conditions . 21
5.10 Selection of the standard insulation level . 22
5.11 Background of the standard insulation level . 26
5.11.1 General . 26
5.11.2 Standard rated switching impulse withstand voltage . 27
5.11.3 Standard rated lightning impulse withstand voltage . 27
6 Requirements for standard withstand voltage tests . 27
6.1 General requirements . 27
6.2 Standard short-duration power-frequency withstand voltage tests . 28
6.3 Standard impulse withstand voltage tests. 28
6.4 Alternative test situation . 29
6.5 Phase-to-phase and longitudinal insulation standard withstand voltage tests
for equipment in range I . 29
6.5.1 Power-frequency tests . 29
6.5.2 Phase-to-phase (or longitudinal) insulation lightning impulse tests . 30
6.6 Phase-to-phase and longitudinal insulation standard withstand voltage tests
for equipment in range II . 30
Annex A (normative) Clearances in air to assure a specified impulse withstand voltage
installation . 31
A.1 General . 31
A.2 Range I Lightning impulse . 32
A.3 Range II Switching impulse . 34
Annex B (informative) Values of Rated insulation levels for 1kV < U ≤ 245 kV for
m
highest voltages for of equipment, U , not standardized by IEC based on current
m
practice in some countries . 36
Bibliography . 37

Figure 1 – Flow chart for the determination of rated or standard insulation level . 17

Table 1 – Classes and shapes of overvoltages, Standard voltage shapes and Standard
withstand voltage tests . 18
Table 2 – Standard insulation levels for range I (1 kV < U ≤ 245 kV) . 24
m
Table 3 – Standard insulation levels for range II (U > 245 kV) . 25
m
Table A.1 – Correlation between standard rated lightning impulse withstand voltages
and minimum air clearances . 33
Table A.2 – Correlation between standard rated switching impulse withstand voltages
and minimum phase-to-earth air clearances . 34
Table A.3 – Correlation between standard rated switching impulse withstand voltages
and minimum phase-to-phase air clearances . 35
Table B.1- Values of rated insulation levels for 1kV < U ≤ 245 kV for highest voltages
m
for equipment U not standardized by IEC based on current practice in some countries .
m
Table B.1 – Rated insulation levels for highest voltages of equipment U not
m
standardized by IEC. 36

– 4 – IEC 60071-1:2019 RLV  IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
___________
INSULATION CO-ORDINATION –
Part 1: Definitions, principles and rules

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 standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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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) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.

International Standard IEC 60071-1 has been prepared by IEC technical committee 99:
Insulation co-ordination and system engineering of high voltage electrical power installations
above 1,0 kV AC and 1,5 kV DC.
This ninth edition cancels and replaces the eighth edition published in 2006 and
Amendment 1:2010. This edition constitutes a technical revision.
It has the status of a horizontal standard in accordance with IEC Guide 108.
The main changes from the previous edition are as follows:
a) all references are updated to current IEC standards, and the bibliography is deleted;
b) some definitions are clarified in order to avoid overlapping and ensure clear understanding;
c) letter symbols are changed and corrected in order to keep the consistency with relevant
IEC standards;
d) some titles are changed to clarify understanding (see Clauses A.2, A.3 and Annex B).
The text of this International Standard is based on the following documents:
CDV Report on voting
99/199/CDV 99/227/RVC
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 60071 series, published under the general title Insulation co-
ordination, can be found on the IEC website.
The committee has decided that the contents of the base publication and its amendments will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
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 publication using a colour printer.

– 6 – IEC 60071-1:2019 RLV  IEC 2019
INSULATION CO-ORDINATION –
Part 1: Definitions, principles and rules

1 Scope
This part of IEC 60071 applies to three-phase AC systems having a highest voltage for
equipment above 1 kV. It specifies the procedure for the selection of the rated withstand
voltages for the phase-to-earth, phase-to-phase and longitudinal insulation of the equipment
and the installations of these systems. It also gives the lists of the standard withstand
voltages from which the rated withstand voltages should be are selected.
This document recommends describes that the selected withstand voltages should be are
associated with the highest voltage for equipment. This association is for insulation co-
ordination purposes only. The requirements for human safety are not covered by this
document.
Although the principles of this document also apply to transmission line insulation, the values
of their withstand voltages may can be different from the standard rated withstand voltages.
The apparatus committees are responsible for specifying the rated withstand voltages and the
test procedures suitable for the relevant equipment taking into consideration the
recommendations of this document.
NOTE In IEC 60071-2, Application Guide, all rules for insulation co-ordination given in this document are justified
in detail, in particular the association of the standard rated withstand voltages with the highest voltage for
equipment. When more than one set of standard rated withstand voltages is associated with the same highest
voltage for equipment, guidance is provided for the selection of the most suitable set.
This horizontal standard is primarily intended for use by technical committees in the
preparation of standards in accordance with the principles laid down in IEC Guide 108.
One of the responsibilities of a technical committee is, wherever applicable, to make use of
horizontal standards in the preparation of its publications. The contents of this horizontal
standard will not apply unless specifically referred to or included in the relevant publications.
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 60038:2002, IEC standard voltages
IEC 60060-1:1989, High-voltage test techniques – Part 1: General definitions and test
requirements
IEC 60071-2, Insulation co-ordination – Part 2: Application guidelines
IEC 60099-4, Surge arresters – Part 4: Metal-oxide surge arresters without gaps for a.c.
systems
IEC 60507, Artificial pollution tests on high-voltage insulators to be used on a.c. systems

IEC 60633, Terminology for high-voltage direct current (HVDC) transmission
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
insulation co-ordination
selection of the dielectric strength of equipment in relation to the operating voltages and
overvoltages which can appear on the system for which the equipment is intended, and taking
into account the service environment and the characteristics of the available preventing and
protective devices
[IEC 604-03-08:1987, modified]
Note 1 to entry: By "dielectric strength" of the equipment, is meant here its rated insulation level (3.36) or its
standard insulation level as defined in 3.35 and 3.36 respectively (3.37).
[SOURCE: IEC 60050-614:2016, 614-03-08, modified – Note 1 to entry has been added]
3.2
external insulation
distances in atmospheric air, and the surfaces in contact with atmospheric air of solid
insulation of the equipment which are subject to dielectric stresses and to the effects of
atmospheric and other environmental conditions from the site, such as pollution, humidity,
vermin, etc.
[IEC 604-03-02:1987, modified]
Note 1 to entry: External insulation is either weather protected or non-weather protected, designed to operate
outside or inside closed shelters, respectively.
[SOURCE: IEC 60050-614:2016, 614-03-02, modified – Note 1 to entry has been added]
3.3
internal insulation
internal distances of the solid, liquid, or gaseous insulation of equipment which are protected
from the effects of atmospheric and other external conditions
[IEC 604-03-03:1987]
[SOURCE: IEC 60050-614:2016, 614-03-03]
3.4
self-restoring insulation
insulation which, after a short time, completely recovers its insulating properties within a short
time interval after a disruptive discharge during test
[IEC 604-03-04:1987, modified]
Note 1 to entry: Insulation of this kind is generally, but not necessarily, external insulation.

– 8 – IEC 60071-1:2019 RLV  IEC 2019
Note 2 to entry: This definition applies only when the discharge is caused by the application of a test voltage
during a dielectric test. However, discharges occurring in service may cause a self-restoring insulation to lose
partially or completely its original insulating properties.
[SOURCE: IEC 60050-614:2016, 614-03-04]
3.5
non-self-restoring insulation
insulation which loses its insulating properties, or does not recover them completely, after a
disruptive discharge during test
[IEC 604-03-05:1987, modified]
Note 1 to entry: This definition applies only when the discharge is caused by the application of a test voltage
during a dielectric test. However, discharges occurring in service may cause a self-restoring insulation to lose
partially or completely its original insulating properties.
[SOURCE: IEC 60050-614:2016, 614-03-05]
3.6
insulation configuration terminal
any of the terminals between any two of which a voltage that stresses the insulation can be
applied
Note 1 to entry: The types of terminal are:
a) phase terminal, between which and the neutral is applied in service the phase-to-neutral voltage of the system;
b) neutral terminal, representing, or connected to, the neutral point of the system (neutral terminal of
transformers, etc.);
c) earth terminal, always solidly connected to earth in service (tank of transformers, base of disconnectors,
structures of towers, ground plane, etc.).
3.7
insulation configuration
complete geometric configuration of the insulation in service, consisting of the insulation and
of all terminals and including all elements (insulating and conducting) which influence its
dielectric behaviour. The following insulation configurations are identified:
Note 1 to entry: The insulation configurations defined in 3.7.1 to 3.7.4 are identified.
3.7.1
three-phase insulation configuration
insulation configuration having three phase terminals, one neutral terminal and one earth
terminal
3.7.2
phase-to-earth (p-e) insulation configuration
three-phase insulation configuration where two phase terminals are disregarded and, except
in particular cases, the neutral terminal is earthed
3.7.3
phase-to-phase(p-p) insulation configuration
three-phase insulation configuration where one phase terminal is disregarded. In particular
cases, the neutral and the earth terminals are also disregarded
3.7.4
longitudinal(t-t) insulation configuration
insulation configuration having two phase terminals and one earth terminal, the phase
terminals belonging to the same phase of a three-phase system temporarily separated into
two independently energized parts (e.g. open switching devices)

Note 1 to entry: The four terminals belonging to the other two phases are disregarded or earthed. In particular
cases one of the two phase terminals considered is earthed.
3.8
nominal voltage of a system
U
n
suitable approximate value of voltage used to designate or identify a system
[SOURCE: IEC 60050-601:1985, 601-01-21, modified – A symbol has been added.]
3.9
highest voltage of a system
U
s
highest value of the phase-to-phase operating voltage (RMS value) which occurs under
normal operating conditions at any time and at any point in the system
[SOURCE: IEC 60050-601:1985, 601-01-23, modified – Clear meaning on the voltage has
been added.]
3.10
highest voltage for equipment
U
m
highest value of phase-to-phase voltage (RMS value) for which the equipment is designed in
respect of its insulation as well as other characteristics which relate to this voltage in the
relevant equipment standards
Note 1 to entry: Under normal service conditions specified by the relevant apparatus committee, this voltage can
be applied continuously to the equipment.
[IEC 604-03-01:1987, modified]
[SOURCE: IEC 60050-614:2016, 614-03-01]
3.11
isolated neutral system
system where the neutral point is not intentionally connected to earth, except for high
impedance connections for protection or measurement purposes
[SOURCE: IEC 60050-601:1985, 601-02-24:1985]
3.12
solidly earthed neutral system
system whose neutral point(s) is(are) earthed directly
[SOURCE: IEC 60050-601:1985, 601-02-25:1985]
3.13
impedance earthed (neutral) system
system whose neutral point(s) is(are) earthed through impedances to limit earth fault currents
[SOURCE: IEC 60050-601:1985, 601-02-26:1985]
3.14
resonant earthed (neutral) system
system in which one or more neutral points are connected to earth through reactances which
approximately compensate the capacitive component of a single-phase-to-earth fault current
[IEC 601-02-27:1985]
– 10 – IEC 60071-1:2019 RLV  IEC 2019
Note 1 to entry: With resonant earthing of a system, the residual current in the fault is limited to such an extent
that an arcing fault in air is usually self-extinguishing.
[SOURCE: IEC 60050-601:1985, 601-02-27]
3.15
earth fault factor
k
at a given location of a three-phase system, and for a given system configuration, the ratio of
the highest RMS phase-to-earth power-frequency voltage on a healthy phase during a fault to
earth affecting one or more phases at any point on the system to the RMS phase-to-earth
power-frequency voltage which would be obtained at the given location in the absence of any
such fault
[IEC 604-03-06:1987]
[SOURCE: IEC 60050-614:2016, 614-03-06, modified – A symbol has been added and
description on voltage has been modified.
3.16
continuous (power frequency) voltage
power-frequency voltage, considered having constant RMS value, continuously applied to any
pair of terminals of an insulation configuration
3.17
classification of voltages and overvoltages
according to their shape and duration, voltages and overvoltages are divided in the following
classes
NOTE More details on the following six first voltages and overvoltages are also given in Table 1.
3.17
overvoltage
any voltage:
– between one phase conductor and earth or across a longitudinal insulation having a peak
value exceeding the peak of the highest voltage of the system divided by ;
[IEC 604-03-09, modified] or
– between phase conductors having a peak value exceeding the amplitude of the highest
voltage of the system
[IEC 604-03-09:1987, modified]
Note 1 to entry: Unless otherwise clearly indicated, such as for surge arresters, overvoltage values expressed in
p.u. refer to U × 2 3
s
[SOURCE: IEC 60050-614: 2016, 614-03-10]
3.17.1
temporary overvoltage
TOV
power-frequency overvoltage of relatively long duration
[IEC 604-03-12:1987, modified]
Note 1 to entry: The overvoltage may be undamped or weakly damped. In some cases, its frequency may be
several times smaller or higher than power frequency.
[SOURCE: IEC 60050-614:2016, 614-03-13]

3.17.2
transient overvoltage
short-duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory, usually
highly damped
[IEC 604-03-13:1987]
Note 1 to entry: Transient overvoltages may be immediately followed by temporary overvoltages. In such cases
the two overvoltages are considered as separate events.
[SOURCE: IEC 60050-614:2016, 614-03-14]
Transient overvoltages are divided into:
3.17.2.1
slow-front overvoltage
SFO
transient overvoltage, usually unidirectional, with time to peak 20 µs < T ≤ 5 000 µs, and tail
p
duration T ≤ 20 ms
3.17.2.2
fast-front overvoltage
FFO
transient overvoltage, usually unidirectional, with time to peak 0,1 µs < T ≤ 20 µs, and tail
duration T < 300 µs
3.17.2.3
very-fast-front overvoltage
VFFO
transient overvoltage, usually unidirectional with time to peak T ≤ 0,1 µs, and with or without
f
superimposed oscillations at frequency 30 kHz < f < 100 MHz
3.17.3
combined overvoltage
overvoltage consisting of two voltage components simultaneously applied between each of
the two phase terminals of a phase-to-phase (or longitudinal) insulation and earth
Note 1 to entry: It is classified by the component of higher peak value (temporary, slow-front, fast-front or very-
fast-front).
3.18
standard voltage shapes for test
the following voltage and the overvoltage shapes for test that are standardized determined in
amplitude, wave front, wave tail and duration
Note 1 to entry: More details on the following three first standard voltage shapes are given in IEC 60060-1 and
also in Table 1.
3.18.1
standard short-duration power-frequency voltage
sinusoidal voltage with frequency between 48 Hz and 62 Hz, and duration of 60 s
3.18.2
standard switching impulse
impulse voltage having a time to peak of 250 µs and a time to half-value of 2 500 µs
3.18.3
standard lightning impulse
impulse voltage having a front time of 1,2 µs and a time to half-value of 50 µs

– 12 – IEC 60071-1:2019 RLV  IEC 2019
3.18.4
standard combined switching impulse
for phase-to-phase insulation, combined impulse voltage having two components of equal
peak value and opposite polarity
Note 1 to entry: The positive component is a standard switching impulse and the negative one is a switching
impulse whose times to peak and half-value should not be less than those of the positive impulse. Both impulses
should reach their peak value at the same instant. The peak value of the combined voltage is, therefore, the sum of
the peak values of the components.
3.18.5
standard combined voltage
for longitudinal insulation, combined voltage having a standard impulse on one terminal and a
power-frequency voltage on the other terminal
Note 1 to entry: The impulse component is applied at the peak of the power-frequency voltage of opposite polarity.
3.19
representative overvoltage
U
rp
overvoltage assumed to produce the same dielectric effect on the insulation as the
overvoltage of a given class occurring in service due to various origins
Note 1 to entry: Representative overvoltages consist of voltages with the standard shape of the class, and may be
defined by one value or a set of values or a frequency distribution of values that characterize the service conditions.
Note 2 to entry: This definition also applies to the continuous power-frequency voltage representing the effect of
the service voltage on the insulation.
3.20
overvoltage limiting device
device which limits the peak values of the overvoltages or their durations or both
Note 1 to entry: They are classified as preventing devices (e.g. a preinsertion resistor) or as protective devices
(e.g. a surge arrester).
3.21
lightning [or switching] impulse protective level
U [or U ]
pl ps
maximum permissible peak voltage value on the terminals of a protective device subjected to
lightning [or switching] impulses under specific conditions
[IEC 604-03-56:1987 and IEC 604-03-57:1987]
[SOURCE: IEC 60050-614:2016, 614-03-56]
3.22
switching impulse protective level
U
ps
maximum permissible peak voltage value on the terminals of a protective device subjected to
switching impulses under specific conditions
[SOURCE: IEC 60050-614:2016, IEC 614-03-57]
3.23
performance criterion
basis on which the insulation is selected so as to reduce to an economically and operationally
acceptable level the probability that the resulting voltage stresses imposed on the equipment
will cause damage to equipment insulation or affect continuity of service
Note 1 to entry: The performance criterion is usually expressed in terms of an acceptable failure rate (number of
failures per year, years between failures, risk of failure, etc.) of the insulation configuration.

3.24
withstand voltage
value of the test voltage to be applied under specified conditions in a withstand voltage test,
during which a specified number of disruptive discharges is tolerated
Note 1 to entry: The withstand voltage is designated as:
a) conventional assumed withstand voltage, when the number of disruptive discharges tolerated is zero. It is
deemed to correspond to a withstand probability P = 100 %;
w
b) statistical withstand voltage, when the number of disruptive discharges tolerated is related to a specified
withstand probability. In this document, the specified probability is P = 90 %.
w
Note 2 to entry: In this standard, for non-self-restoring insulation are specified conventional assumed withstand
voltages, and for self-restoring insulation are specified statistical withstand voltages. In this document, the
conventional assumed withstand voltages are specified for non-self-restoring insulation. The statistical withstand
voltages are specified for self-restoring insulation.
3.25
co-ordination withstand voltage
U
cw
for each class of voltage, the value of the withstand voltage of the insulation configuration in
actual service conditions, that meets the performance criterion
3.26
co-ordination factor
K
c
factor by which the value of the representative overvoltage must be multiplied in order to
obtain the value of the co-ordination withstand voltage
3.27
standard reference atmospheric conditions
atmospheric conditions to which the standardized withstand voltages apply (see 5.9)
Note 1 to entry: See 5.9.2.
3.28
required withstand voltage
U
rw
test voltage that the insulation must withstand in a standard withstand voltage test to ensure
that the insulation will meet the performance criterion when subjected to a given class of
overvoltages in actual service conditions and for the whole service duration
Note 1 to entry: The required withstand voltage has the shape of the co-ordination withstand voltage, and is
specified with reference to all the conditions of the standard withstand voltage test selected to verify it.
3.29
atmospheric correction factor
K
t
factor to be applied to the co-ordination withstand voltage to account for the difference in
dielectric strength between the average atmospheric conditions in service and the standard
reference atmospheric conditions
Note 1 to entry: It applies to external insulation only, for all altitudes.
Note 2 to entry: For the atmospheric correction factor, the atmospheric conditions taken into account are air
pressure, temperature and humidity. For insulation co-ordination purposes, usually only the air pressure correction
needs to be taken into account.
NOTE 1 The factor K allows the correction of test voltages taking into account the difference between the actual
t
atmospheric conditions during test and the standard reference atmospheric conditions. For the factor K , the
t
atmospheric conditions taken into account are air pressure, temperature and humidity.
NOTE 2 For insulation co-ordination purposes usually only the air pressure correction needs to be taken into
account.
– 14 – IEC 60071-1:2019 RLV  IEC 2019
3.30
altitude correction factor
K
a
factor to be applied to the co-ordination withstand voltage to account for the difference in
dielectric strength between the average pressure corresponding to the altitude in service and
the standard reference pressure
Note 1 to entry: The altitude correction factor K is part of the atmospheric correction factor K .
a t
3.31
safety factor
K
s
overall factor to be applied to the co-ordination withstand voltage, after the application of the
atmospheric correction factor (if required), to obtain the required withstand voltage,
accounting for all other differences in dielectric strength between the conditions in service
during life time and those in the standard withstand voltage test
3.32
actual withstand voltage of an equipment or insulation configuration
U
aw
highest possible value of the test voltage that can be applied to an equipment or insulation
configuration in a standard withstand voltage test
3.33
test conversion factor
K
tc
for a given equipment or insulation configuration, the factor to be applied to the required
withstand voltage of a given overvoltage class, in the case where the standard withstand
shape of the selected withstand voltage test is that of a different overvoltage class
Note 1 to entry: For a given equipment or insulation configuration: the test conversion factor of the standard
voltage shape (a) to the standard voltage shape (b) must be higher than or equal to the ratio between the actual
withstand voltage for the standard voltage shape (a) and the actual withstand voltage of the standard voltage
shape (b).
3.34
rated withstand voltage
value of the test voltage, applied in a standard withstand voltage test that proves that the
insulation complies with one or more required withstand voltages
Note 1 to entry: It is a rated value of the insulation of an equipment.
3.35
standard rated withstand voltage
U
w
standard value of the rated withstand voltage as specified in this document
Note 1 to entry: See 5.6 and 5.7.
3.36
rated insulation level
set of rated withstand voltages which characterize the dielectric strength of the insulation
3.37
standard insulation level
set of standard rated withstand voltages which are associated to U as specified in this
m
document
Note 1 to entry: See Table 2 and Table 3.

3.38
standard withstand voltage test
dielectric test performed in specified conditions to prove that the insulation complies with a
standard rated withstand voltage
Note 1 to entry: This document covers:
– – short-duration power-frequency voltage tests;
– – switching impulse tests;
– – lightning impulse tests;
– – combined switching impulse tests;
– – combined voltage tests.
Note 2 to entry: More detailed information on the standard withstand voltage tests is given in IEC 60060-1 (see
also Table 1 for the test voltage shapes).
Note 3 to entry: The very-fast-front impulse standard withstand voltage tests should be specified by the relevant
apparatus committees, if required.
4 Abbreviated terms and symbols
4.1 General
The lists provided below cover only the most frequently used symbols and abbreviations
which are useful for insulation co-ordination.
4.2 Subscripts
p-e related to phase to earth
t-t related to longitudinal
max maximum (IEC 60633)
p-p related to phase to phase
4.3 Letter symbols
f frequency
k earth fault factor
K atmospheric correction factor

t
K altitude correction factor
a
K co-ordination factor
c
K safety factor
s
K test conversion factor
tc
P withstand probability
w
T front time
T time to half-value of a decreasing voltage
T time to peak value
p
T total overvoltage duration
t
U actual withstand voltage of an equipment or insulation configuration
aw
U co-ordination withstand voltage
cw
U highest voltage for equipment
m
U nominal voltage of a system
n
U lightning impulse protective level of a surge arrester
pl
U switching impulse protective level of a surge arrester
ps
– 16 – IEC 60071-1:2019 RLV  IEC 2019
U representative overvoltage
rp
U required withstand voltage
rw
U highest voltage of a system
s
U standard rated withstand voltage
w
4.4 Abbreviations
FFO fast-front overvoltage
ACWV standard rated short-duration power frequency withstand voltage of an
equipment or insulation configuration
LIPL lightning impulse protective level of a surge arrester
SIPL switching impulse protective level of a surge arrester
LIWV standard rated lightning impulse withstand voltage of an equipment or
insulation configuration
SFO slow-front overvoltage
SIWV standard rated switching impulse withstand voltage of an equipment or
insulation configuration
TOV temporary overvoltage
VFFO very-fast-front overvoltage

5 Procedure for insulation co-ordination
5.1 General outline of the procedure
The procedure for insulation co-ordination consists of the selection of the highest voltage for
the equipment together with a corresponding set of standard rated withstand voltages which
characterize the insulation of the equipment needed for the application. This procedure is
outlined in Figure 1 and its steps are described in 5.1 to 5.5. The optimization of the selected
set of U may require reconsideration of some input data and repetition of part of the
w
procedure.
The rated withstand voltages shall be selected from the lists of standard rated withstand
voltages given in 5.6 and 5.7. The set of selected standard voltages constitutes a rated
insulation level. If the standard rated withstand voltages are also associated with the same
U according to 5.10, this set constitutes a standard insulation level.
m
Figure 1 – Flow chart for the determination of rated or standard insulation level
5.2 Determination of the representative voltages and overvoltages (U )
rp
The voltages and the overvoltages that stress the insulation shall be determined in amplitude,
shape and duration by means of a system analysis which includes the selection and location
of the overvoltage preventing and limiting devices.
For each class of voltages and overvoltages, this analysis shall then determine a
representative voltage and overvoltage, taking into account the characteristics of the
insulation with respect to the different behaviour at the voltage or overvoltage shapes in the
system and at the standard voltage shapes applied in a standard withstand voltage test as
outlined in Table 1.
– 18 – IEC 60071-1:2019 RLV  IEC 2019
Table 1 – Classes and shapes of overvoltages, standard voltage shapes
and standard withstand voltage tests
Low frequency Transient
Class
Continuous Temporary Slow-front Fast-front Very-fast-front
1/f
T
f
1/f
Voltage or
over-
Tp
voltage
T
T
shapes t
1/f
T
T 2 1/f
t T 1
T ≤ 100 ns
f
Range of 10 Hz < f <
f = 50 Hz or 20 μs < T ≤ 0,1 μs < T ≤
voltage or 500 Hz p 1 0,3 MHz < f <
60 Hz 5 000 μs
20 µs
over-
100 MHz
voltage
0,02 s ≤ T ≤
T ≥ 3 600s t T ≤ 20 ms T ≤ 300 μs
t 2 2 30 kHz < f <
shapes 3 600 s 2
300 kHz
1/f
1/f
Standard
a
voltage
shapes
T
t se)
T
p T
T
t
T
T
f = 50 Hz 48 Hz ≤ f ≤
T = 250 μs T = 1,2 μs
p 1
or 60 Hz 62 Hz
T = 2 500 μs T = 50 μs
a
2 2
T T = 60 s
t t
Standard Short-duration
Switching impulse Lightning impulse
a a
withstand power
test test
voltage test frequency test
a
To be specified by the relevant apparatus committees.

The representative voltages and overvoltages may be characterized either by:
– an assumed maximum, or
– a set of peak values, or
– a complete statistical distribution of peak values.
NOTE In the last case additional characteristics of the overvoltage shapes may could have to be considered.
When the adoption of an assumed maximum is considered adequate, the representative
overvoltage of the various classes shall be:
– For the continuous power
...