SIST EN IEC 60076-4:2026

SLOVENSKI STANDARD
01-november-2026
Nadomešča:
SIST EN 60076-4:2004
Močnostni transformatorji - 4. del: Testiranje atmosferske udarne napetosti in
preklopnega impulza za močnostne transformatorje in reaktorje (IEC 60076-4:2026)
Power transformers - Part 4: Lightning impulse and switching impulse tests of power
transformers and reactors (IEC 60076-4:2026)
Leistungstransformatoren - Teil 4: Leitfaden zur Blitz- und Schaltstoßspannungsprüfung
von Leistungstransformatoren und Drosselspulen (IEC 60076-4:2026)
Transformateurs de puissance - Partie 4: Essais au choc de foudre et au choc de
manœuvre des transformateurs de puissance et bobines d'inductance (IEC 60076-
4:2026)
Ta slovenski standard je istoveten z: EN IEC 60076-4:2026
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60076-4

NORME EUROPÉENNE
EUROPÄISCHE NORM September 2026
ICS 29.180 Supersedes EN 60076-4:2002
English Version
Power transformers - Part 4: Lightning impulse and switching
impulse tests of power transformers and reactors
(IEC 60076-4:2026)
Transformateurs de puissance - Partie 4: Essais au choc de Leistungstransformatoren - Teil 4: Leitfaden zur Blitz- und
foudre et au choc de manœuvre des transformateurs de Schaltstoßspannungsprüfung von Leistungstransformatoren
puissance et bobines d'inductance und Drosselspulen
(IEC 60076-4:2026) (IEC 60076-4:2026)
This European Standard was approved by CENELEC on 2026-08-13. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60076-4:2026 E

European foreword
The text of document 14/1204/FDIS, future edition 2 of IEC 60076-4, prepared by TC 14 "Power
transformers" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-09-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-09-30
document have to be withdrawn
This document supersedes EN 60076-4:2002 and all of its amendments and corrigenda (if any).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60076-4:2026 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 60076-1:2011 NOTE Approved as EN 60076-1:2011 (not modified)
IEC 61083-2:2013 NOTE Approved as EN 61083-2:2013 (not modified)
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60060-1 2010 High-voltage test techniques - Part 1: EN 60060-1 2010
General definitions and test requirements
IEC 60060-2 - High-voltage test techniques - Part 2: EN IEC 60060-2 -
Measuring systems
IEC 60076-3 2013 Power transformers - Part 3: Insulation EN 60076-3 2013
levels, dielectric tests and external
clearances in air
IEC 60076-6 - Power transformers - Part 6: Reactors EN 60076-6 -
IEC 61083-1 - Instruments and software used for - -
measurements in high-voltage and high-
current tests - Part 1: Requirements for
instruments for impulse tests
IEC 60076-4 ®
Edition 2.0 2026-07
INTERNATIONAL
STANDARD
Power transformers -
Part 4: Lightning impulse and switching impulse tests of power transformers
and reactors
ICS 29.180  ISBN 978-2-8327-1194-1

IEC 60076-4:2026-07(en)
IEC 60076-4:2026 © IEC 2026
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 General . 7
5 Specified waveforms. 8
6 Test circuit . 8
7 Verification of the impulse voltage measuring system before a test . 10
8 Lightning impulse tests . 10
8.1 Waveforms. 10
8.1.1 General . 10
8.1.2 Front time T . 10
8.1.3 Non-linear elements . 11
8.1.4 Time to half-value T . 11
8.2 Impulses chopped on the tail . 12
8.2.1 Time to chopping . 12
8.2.2 Test voltage function for tail-chopped lightning impulse tests . 12
8.2.3 Test voltage function and presentation of test results . 12
8.2.4 Rate of collapse and amplitude of reversed polarity of the chopped
impulse . 13
8.3 Terminal connections and applicable methods of failure detection . 13
8.3.1 Terminal connections . 13
8.3.2 Applicable methods of failure detection. 14
8.4 Test procedures . 14
8.5 Recording of tests . 15
8.5.1 General . 15
8.5.2 Digital recording systems . 15
8.5.3 Void . 15
8.5.4 Digital recording . 15
9 Switching impulse tests . 16
9.1 Special requirements . 16
9.2 Transformers – Switching impulse tests . 17
9.2.1 Waveforms . 17
9.2.2 Terminal connections and applicable methods of failure detection . 17
9.2.3 Test procedures. 18
9.2.4 Recording of tests . 19
9.3 Reactors – Switching impulse tests . 20
9.3.1 Waveforms . 20
9.3.2 Terminal connections and applicable methods of failure detection . 20
9.3.3 Test procedures. 21
9.3.4 Digital recording of impulse voltage waveform and impulse current . 21
10 Interpretation of oscillograms. 21
10.1 General . 21
10.2 Lightning impulse – Oscillogram interpretations . 21
10.2.1 General . 21
10.2.2 Voltage recordings – Full wave tests . 22
IEC 60076-4:2026 © IEC 2026
10.2.3 Current recordings – Full wave tests . 23
10.2.4 Voltage and current recordings – Chopped-wave tests . 23
10.3 Switching impulse – Oscillogram interpretations. 24
10.3.1 Voltage recordings . 24
10.3.2 Recordings of the impulse current . 24
11 Digital recording – Transfer function analysis . 25
12 Impulse testing report . 27
13 Impulse test circuits and terminal connections . 28
Annex A (informative) Principles of waveform control . 34
A.1 General . 34
A.2 High-impedance windings (L > 100 mH) . 34
t
A.3 Low-impedance windings (L < 20 mH) . 35
t
Annex B (informative) Typical oscillograms and digital recordings . 42
Annex C (informative) Examples of oscillograms with overshoots . 64
Bibliography . 71

Figure 1 – Typical impulse test circuit . 28
Figure 2 – Lightning impulse test terminal connections and applicable methods of
failure detection . 29
Figure 3 – Transformer and reactor switching impulse waveforms. 30
Figure 4 – Switching impulse test terminal connections and methods of failure
detection . 31
Figure 5 – Three-phase transformer test connections (three-limb core) for switching
impulse . 32
Figure 6 – Three-phase transformer test connections (five-limb core) for switching
impulse . 33
Figure A.1 – Waveform control for high-impedance windings . 34
Figure A.2 – Wavetail control for low impedance windings . 36
Figure A.3 – Damped oscillation . 37
Figure A.4 – Effects due to short length of wavetail . 39
Figure A.5 – Winding earthed through a resistor . 40
Figure A.6 – Glaninger circuit . 41
Figure B.1 – Lightning impulse, full wave failure – Line-to-neutral breakdown across
high-voltage winding of 400 kV generator transformer . 43
Figure B.2 – Lightning impulse, full wave failure – Breakdown between discs at
entrance to high-voltage winding of 115 kV transformer . 44
Figure B.3 – Lightning impulse, interlayer breakdown in coarse-step tapping winding of
a 400/220 kV transformer . 45
Figure B.4 – Lightning impulse, full wave failure – Breakdown between leads of two
1,1 % sections of outside tapping winding of 400 kV generator transformer . 46
Figure B.5 – Lightning impulse, full wave failure – Breakdown short-circuiting one
section of the fine-step tapping winding of a 220 kV transformer . 47
Figure B.6 – Lightning impulse, full wave failure – Breakdown between parallel
conductors of a main high-voltage winding of a 220/110 kV transformer . 47
Figure B.7 – Lightning impulse, full wave failure – Breakdown between foils of 66 kV
bushing on tested winding. 48
Figure B.8 – Lightning impulse, full wave failure – Bushing flange grounding . 49
IEC 60076-4:2026 © IEC 2026
Figure B.9 – Lightning impulse, chopped-wave failure – Breakdown between turns in
the main high-voltage winding of a 115 kV transformer . 50
Figure B.10 – Lightning impulse, chopped-wave failure – Breakdown between turns in
a fine-step tapping winding of a 220 kV transformer . 51
Figure B.11 – Chopped lightning impulse – Impulses at different voltage levels with
identical times to chopping when testing a 115 kV transformer. 52
Figure B.12 – Chopped lightning impulse – Effects of differences in times to chopping
when testing a 220 kV transformer . 53
Figure B.13 – Full lightning impulse – Effect of non-linear resistors embodied in
neutral end on-load tap-changer of a transformer with separate windings . 54
Figure B.14 – Full lightning impulse – Effect of generator firing differences at different
voltage levels when testing a 400 kV transformer . 55
Figure B.15 – Switching impulse – Satisfactory test on a 400 kV three-phase generator
transformer . 56
Figure B.16 – Switching impulse – Breakdown by axial flashover of the main high-
voltage winding of a 525 kV single-phase, generator transformer . 57
Figure B.17 – Switching impulse – Satisfactory test on a 33 MVAr, 525 kV single-phase
reactor . 58
Figure B.18 – Full lightning impulse – Evaluation of a non-standard waveform –
Influence of in-built smoothing algorithms in digitizers . 59
Figure B.19 – Full lightning impulse – Non-standard waveform, superimposed
oscillations with >50 % amplitude and frequency <500 kHz . 59
Figure B.20 – Chopped lightning impulse – Non-standard chopped wave on a layer
type winding . 60
Figure B.21 – Lightning impulse – Comparison of the transfer function of a full wave
and a chopped wave . 61
Figure B.22 – Full lightning impulse – Test-circuit problem caused by a sparkover to
earth from a measuring cable. 62
Figure B.23 – Full lightning impulse – Failure digital recordings of a flashover between
tap leads of a tap changer and of a flashover between coarse and fine tapping
windings . 63
Figure C.1 – Lightning impulse oscillogram with an overshoot having a frequency
higher than 500 kHz (beta_k = ß′ = 7,35 %) – 3 phase transformer, 27,6 kV/208 V,
150 kVA, YNyn0 . 65
Figure C.2 – Lightning impulse oscillogram with an overshoot having a frequency
higher than 500 kHz (beta_k = ß′ = 10,3 %) – Transformer, 138 kV/13,8 kV, 33,3 MVA,
Dyn1 . 66
Figure C.3 – Lightning impulse oscillogram with an overshoot having a frequency
higher than 500 kHz (beta_k = ß′ = 14,2 %) – Transformer, 14,4 kV/120 V – 240 V,
50 kVA, 1 phase . 67
Figure C.4 – Lightning impulse oscillogram with an overshoot having a frequency less
than 500 kHz (beta_k = ß′ = 30,2 %) – Transformer, 34,5 kV/560 V, 3 600 kVA, 3 phase . 67
Figure C.5 – Lightning impulse oscillogram with an overshoot − ß′ = 6,80 % Single-
phase auto-transformer 267 MVA − 420 kV/247 kV/28 kV – LV terminal 247 kV . 68
Figure C.6 – Lightning impulse oscillogram with an overshoot − ß′ = 13,40 % Three-

phase auto-transformer 600 MVA − 345 kV/141,5 kV/13,8 kV – LV terminal 141,5 kV . 68
Figure C.7 – Lightning impulse oscillogram with an overshoot – ß′= 23,22 % Single-
phase auto-transformer 267 MVA – 420 kV/247 kV/28 kV – TV terminal 28 kV . 69
Figure C.8 – Example of lightning impulse oscillogram on bushing (capacitance test
object) with an overshoot ß′ = 3,92 %, without complex oscillations. 70

IEC 60076-4:2026 © IEC 2026
Table B.1 – Summary of examples illustrated in oscillograms and digital recordings . 42
Table C.1 – Summary of lightning impulse test oscillograms with overshoot . 64

IEC 60076-4:2026 © IEC 2026
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Power transformers -
Part 4: Lightning impulse and switching impulse tests
of power transformers and reactors

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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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6) All users should ensure that they have the latest edition of this publication.
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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 can 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 can
be required to implement this document. However, implementers are cautioned that this cannot represent the
latest information, which can 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 60076-4 has been prepared by IEC technical committee 14: Power transformers. It is an
International Standard.
This second edition cancels and replaces the first edition published in 2002. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) lightning impulse tests in the presence of a relative overshoot value of 5 % or more (8.1.2).
The 2002 edition did not specified how to proceed when the overshoot exceeds 5 %. In this
revision, the testing laboratory is permitted to carry out the tests provided that the voltage
function calculation, as defined in IEC 60060-1:2010, is applied;
b) newly introduced test voltage for tail-chopped lightning impulse tests (8.2.2);
IEC 60076-4:2026 © IEC 2026
c) switching impulse tests on 3 phase transformers, test connections (see Figure 5 and
Figure 6);
d) new Glaninger circuit in Clause A.3 about low-impedance windings (L < 20 mH);
t
e) new Annex C with examples of oscillograms with peak voltage overshoot.
The text of this International Standard is based on the following documents:
Draft Report on voting
14/1204/FDIS 14/1208/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard 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 www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts in the IEC 60076 series, published under the general title Power transformers,
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.
IEC 60076-4:2026 © IEC 2026
1 Scope
This part of IEC 60076 applies to lightning and switching impulse tests on power transformers
and reactors.
Information is given on waveforms, test circuits including test connections, earthing practices,
failure detection methods, test procedures, measuring techniques and interpretation of results.
Where applicable, the test techniques are as described in IEC 60060-1 and IEC 60060-2.
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 60060-1:2010, High-voltage test techniques - Part 1: General definitions and test
requirements
IEC 60060-2, High-voltage test techniques - Part 2: Measuring systems
IEC 60076-3:2013, Power transformers - Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-6, Power transformers - Part 6: Reactors
IEC 61083-1, Instruments and software used for measurements in high-voltage and high-
current tests - Part 1: Requirements for instruments for impulse tests
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology 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
4 General
This document is primarily based on the use of conventional impulse generators for both
lightning and switching impulse tests of transformers and reactors. The practice of switching
impulse generation with discharge of a separate capacitor into an intermediate or low-voltage
winding is also applicable. However, the method which employs an additional inductance in
series with the capacitor to provide slightly damped oscillations transferred into the high-voltage
winding is not applicable.
Alternative means of switching impulse generation or simulation such as DC current interruption
on an intermediate or low-voltage winding or the application of a part-period of power frequency
voltage are not discussed since these methods are not as generally applicable.
IEC 60076-4:2026 © IEC 2026
Different considerations in the choice of test circuits (terminal connections) for lightning and
switching impulse tests apply for transformers and reactors. On transformers, all terminals and
windings can be lightning impulse tested to specific and independent levels. In switching
impulse test, however, because of the induced voltage transferred, a specified test level can
only be obtained on one winding (see IEC 60076-3).
Whilst, on reactors, lightning impulse tests is similar to that on transformers, i.e., all terminals
can be tested separately, different considerations apply and different problems arise in
switching impulse tests. Hence, in this document, lightning impulse tests are covered by a
common text for both transformers and reactors whilst switching impulse test is dealt with
separately for the two types of equipment.
5 Specified waveforms
The voltage waveforms to be used normally during lightning and switching impulse tests of
transformers and reactors are given in IEC 60076-3, IEC 60076-6 and the methods for their
determination shall refer to IEC 60060-1.
6 Test circuit
The physical arrangement of test equipment, test object and measuring circuits can be divided
into three major circuits:
– the main circuit including the impulse generator, additional waveshaping components and
the test object;
– the voltage measuring circuit;
– the chopping circuit where applicable.
This basic arrangement is shown in Figure 1.
The following parameters influence the impulse waveform:
a) the effective capacitance C , and inductance of the test object, L ; C is constant for any
t t t
given design and any given waveform, L is also a constant for any given design.
t
The effective L , however, can be influenced by the terminal treatment. It varies between
t
the leakage inductance L for short-circuited terminals and L for open-circuited terminals.
s o
More details in this respect are given in 8.1 and 8.3 and in Annex A;
b) the generator capacitance C ;
g
c) waveshaping components, both internal and external to the generator, R , R , R , C (plus,
si se p L
where applicable, the impedance of a voltage divider Z );
d) the stray inductance and capacitance of the generator and the complete test circuit;
e) chopping equipment, where applicable;
f) non-linear elements in the transformer, which can cause differences between impulses at
different voltage levels.
The front time T is determined mainly by combination of the effective surge capacitance of the
test object, including C , and the generator internal and external series resistances.
L
The time to half-value T is, for lightning impulses, primarily determined by the generator
capacitance, the inductance of the test object and the generator discharge resistance or any
other parallel resistance. However, there are cases, for example, windings of extremely low
inductance, where the series resistance will have a significant effect also on the wavetail.
For switching impulses, other parameters apply; these are dealt with in Clause 9.
IEC 60076-4:2026 © IEC 2026
The test equipment used in lightning and switching impulse applications is basically the same.
Differences are in details only, such as values of resistors and capacitors (and the terminal
connections of the test object).
To meet the distinct waveform requirements for lightning and switching impulses, due
consideration shall be given to the selection of the impulse generator parameters, including
capacitance, series resistance and discharge (parallel) resistance. When generating switching
impulses, large series resistors, load capacitors, or both can be required, which can significantly
reduce efficiency.
While the output voltage of the impulse generator is determined by the test levels of the
windings with respect to their highest voltage for equipment U for the test object, the required
m
energy storage capability is essentially dependent on the inherent impedances of the test
object.
A brief explanation of the principles of waveform control is given in Annex A.
The arrangement of the test plant, test object and the interconnecting cables, earthing strips,
and other equipment is limited by the space in the test room and, particularly, the proximity
effect of any structures. During impulse tests, zero potential cannot be assumed throughout the
earthing systems due to the high values and rates of change of impulse currents and voltages
and the finite impedances involved. Therefore, the selection of a proper reference earth is
important.
The current return path between the test object and the impulse generator should be of low
impedance. It is good practice to firmly connect this current return path to the general earth
system of the test room, preferably close to the test object. This point of connection should be
used as reference earth and to attain good earthing of the test object it should be connected to
the reference earth by one or several conductors of low impedance (see IEC 60060-2).
The voltage measuring circuit, which is a separate loop of the test object carrying only the
measuring current and not any major portion of the impulse current flowing through the windings
under test, should also be effectively connected to the same reference earth.
In switching impulse tests, since the rates of change of the impulse voltages and currents are
much reduced compared with those in a lightning impulse test and no chopping circuit is
involved, the problems of potential gradients around the test circuit and with respect to the
reference earth are less critical. Nevertheless, it is suggested that, as a precaution, the same
earthing practices should be followed as used for lightning impulse tests.
Electromagnetic interference:
– Power transformers are more and more fitted with control and protection devices, which are
sensitive in regard of overvoltage, caused by (fast) transients.
– Potential differences, caused by special groundings at lightning and switching impulse tests.
The different grounding of the control-and protection device (in regard of safety) can
damage electronic parts.
Examples of affected devices:
– Mainly large transformers fitted with (computerized) condition monitoring systems.
– Cooling equipment (fans, pumps) is driven in dependence of transformer load and
transformer noise and controlled by electronic devices, etc.
During impulse tests, it is recommended to disconnect all electric and electronic equipment
installed on the transformer.
IEC 60076-4:2026 © IEC 2026
7 Verification of the impulse voltage measuring system before a test
The impulse voltage measuring system shall be verified in accordance with IEC 60060 (all
parts). Before a test, an overall check of the test circuit and the measuring system can be
performed at a voltage lower than the reduced test voltage level. In this check, the voltage can
be determined by means of a sphere gap or by comparative measurement with another
approved device. When using a sphere gap, it should be recognized that this is only a check
and does not replace the periodically performed calibration of the approved measuring system.
After any check has been made, it is essential that neither the measuring nor the test circuit is
altered except for the removal of any devices for checking.
Information on types of voltage dividers, their applications, accuracy, calibration and checking
shall be as given in IEC 60060-2.
8 Lightning impulse tests
8.1 Waveforms
8.1.1 General
The values of waveform specified cannot always be obtainable. In the impulse tests of large
power transformers and reactors, of either low winding inductance or high surge capacitance
or windings with low ohmic resistance, or both, wider tolerances can have to be accepted
(Table B.1).
8.1.2 Front time T
The surge capacitance of the transformer under test being constant, the series resistance shall
have to be reduced in an attempt to obtain the correct front time T or rate of rise, but the
reduction should not be to the extent that oscillations on the crest of the voltage wave become
excessive. If achieving a short front time (preferably within the specified limits) is considered
desirable, oscillations, overshoots, or both can have to be accepted. In such an event, a
compromise between the extent of allowable oscillations and the obtainable front time is
necessary. In general, the test circuit should be arranged in such way that overshoot and
oscillation are minimal.
Examples of oscillograms having overshoot are summarized in Table C.1 and shown in
Figure C.1, Figure C.2, Figure C.3, Figure C.4, Figure C.5, Figure C.6, Figure C.7, and
Figure C.8.
If the relative overshoot, ß′, exceeds 5 %, IEC 60076-3:2013, 13.2.1 gives options:
Option 1 – Adjust T
T can be increased; however if it exceeds 1,56 µs, a chopped wave is required to ensure high-
frequency content:
• For transformers with U ≤ 800 kV, T limit is 2,5 µs,
m 1
• For transformers with U > 800 kV, T > 2,5 µs may be accepted, subject of agreement
m 1
between purchaser and manufacturer.
Option 2 – Accept the overshoot
If the relative overshoot (ß′) exceeds 5 %, testing can proceed provided that the lightning
impulse parameters are calculated with the test voltage function in accordance with
IEC 60060-1:2010, Annex B.
IEC 60076-4:2026 © IEC 2026
NOTE When ß′ is large and the overshoot contains high-frequency components (> 500 kHz), the test voltage
function can reduce the test voltage value (U ) significantly below the peak value U of the recorded curve. This
t e
discrepancy can impose higher electrical stress on the insulation and increase the risk of dielectric breakdown.

For transformer testing, manual evaluation of the lightning impulse test voltage is generally
unreliable. If the manufacturer does not have the software that implement the IEC 60060-1 test
voltage function, the purchaser shall be informed at the quotation stage.
8.1.3 Non-linear elements
In some power transformers, the active parts are protected by non-linear surge arresters. When
lightning-impulse tests are applied, these devices can modify the voltage waveform; the shape
varies with the amplitude of the impulse.
According to IEC 60076-3, the test sequence shall consist of:
– three reference impulses;
– three 100 % full wave impulses;
– three comparison impulses,
with at least one of the reference impulses being lower than the arrester’s operating (knee)
voltage.
NOTE Sequence of reference impulses (see IEC 60076-3):
1) between 50 % and 60 % of the full wave test voltage;
2) between 60 % and 75 % of the full wave test voltage;
3) between 75 % and 90 % of the full wave test voltage.
8.1.4 Time to half-value T
For large power transformers and particularly the intermediate and low-voltage windings
thereof, the virtual time to half-value T cannot be achievable within the value set by the
tolerance. The inductance of such windings can be so low that the resulting waveform is
oscillatory. This problem can be solved to some extent by the use of large capacitance within
the generator, by parallel stage operation, by adjustment of the series resistor or by specific
test connections of the terminals of windings not under test or, in addition, of the non-tested
terminals of windings under test.
If lightning impulse tests are carried out on phase-terminals of a delta-winding, the not-tested
terminals of that winding can be resistance earthed.
If the neutral of a star-connected winding is tested, the phase-terminals can be resistance
earthed.
If the phase terminals of a star-connected winding are tested, the neutral-terminal shall be
solidly grounded or grounded through a low-ohmic shunt.
When resistance earthing of any non-tested line terminal is employed, it is necessary to ensure
that the voltage to earth appearing on any non-tested terminal does not exceed
– 75 % of the rated lightning withstand voltage of that terminal for star-connected windings;
– 50 % of the rated lightning withstand voltage of that terminal for delta-connected windings
(because of the undershoot voltages to earth on the delta terminals – see also 8.4).
When the waveform is oscillatory due to extremely low inductance or small impulse generator
capacitance, or both, the amplitude of the undershoot should not exceed 50 % of the test
voltage. With this limitation, guidance for selecting impulse generator capacitance and adjusting
waveforms is given in Annex A.
IEC 60076-4:2026 © IEC 2026
8.2 Impulses chopped on the tail
8.2.1 Time to chopping
Different times to chopping T (as defined in IEC 60060-2), will result in different stresses
c
(voltage and duration) in different parts of the winding(s) depending on the winding construction
and arrangement employed. Hence, it is not possible to state a time to chopping which is the
most onerous either in general or
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