SIST EN IEC 60076-25:2023

Power transformers - Part 25: Neutral grounding resistors (IEC 60076-25:2023)

This part of IEC 60076 applies to dry type natural air-cooled resistors, for neutral grounding of
transformers and generators, in order to limit the earth fault current in power systems by means
of metallic resistive elements.
For the purposes of this document, the resistor can be:
• used alone or in combination with other electrotechnical products not covered by this
document, such as (but not limited to): a step-down single-phase transformer, an open
triangle or zig-zag transformer (where the neutral point is not available) and a Petersen coil
reactor (in order to increase active power contribution to the fault or reduce time constant
for proper protection operation or both);
• designed, manufactured and verified on a one-off basis or fully standardized and
manufactured in quantity.
Both terms "neutral grounding resistor" (NGR) and "neutral earthing resistor" (NER) can be
used. However, for the purposes of this document and in order to avoid any confusion with
"neutral earthing reactor" (NER), the term "neutral grounding resistor" (NGR) is used.
This document specifies:
• the characteristics of the NGR;
• the service conditions requirements for NGRs;
• the tests and test methods for confirming that these conditions have been met;
• the requirements relating to marking for NGRs.
Annex A provides guidance on how to consider the effect of resistance variation with
temperature.

Leistungstransformatoren – Teil 25: Neutrale Erdungswiderstände (IEC 60076-25:2023)

Transformateurs de puissance - Partie 25: Résistances de mise à la terre du neutre (IEC 60076-25:2023)

L'IEC 60076-25:2023 s'applique aux résistances de type sec refroidies par air naturel pour la mise à la terre (MALT) du neutre des transformateurs et des générateurs dans le but de limiter le courant de défaut à la terre dans les réseaux d'énergie au moyen d'éléments résistifs métalliques.
Pour les besoins du présent document, la résistance peut être:

   
• utilisée seule ou avec d'autres produits électrotechniques non couverts par le présent document, notamment: un transformateur abaisseur monophasé, un transformateur en triangle ouvert ou en zigzag (où le point neutre n'est pas disponible) et une bobine Petersen (afin d'augmenter la contribution de la puissance active au défaut et/ou de réduire la constante de temps pour améliorer la protection);

   
• conçue, fabriquée et vérifiée à l'unité ou entièrement normalisée et fabriquée en quantité.

En anglais, les deux termes "neutral grounding resistor" (NGR) et "neutral earthing resistor" (NER) peuvent être utilisés pour désigner la résistance de mise à la terre du neutre (MALT). Toutefois, pour les besoins du présent document et afin d'éviter toute confusion avec la "réactance de mise à la terre du neutre" (NER, Neutral Earthing Reactor), le terme "neutral grounding resistor" (NGR) est utilisé.
Le présent document spécifie:

   
• les caractéristiques de la MALT;

   
• les exigences de conditions de service des MALT;

   
• les essais et les méthodes d'essai pour confirmer que ces conditions ont été remplies;

   
• les exigences relatives au marquage des MALT.

L'Annexe A fournit des recommandations pour prendre en compte l'impact de la température sur la variation de la résistance.

Močnostni transformatorji - 25. del: Upori za ozemljitev ničlišča (IEC 60076-25:2023)

Ta del standarda IEC 60076 se uporablja za naravno zračno hlajene suhe upore za ozemljitev ničlišča
transformatorjev in generatorjev za omejevanje toka zemeljskega stika v napajalnih sistemih z uporabo
kovinsko uporovnih elementov.
V tem dokumentu se lahko upor:
• uporablja samostojno ali v kombinaciji z drugimi elektrotehničnimi izdelki, ki niso zajeti v tem
dokumentu, kot so med drugim: enofazni transformator s transformacijo navzdol, odprti
transformator s trikotno ali cikcak vezavo (kjer nevtralna točka ni na voljo) in reaktor s
Petersenovo dušilko (za povečanje oddajanja delovne moči na stiku ali zmanjšanje časovne konstante
za ustrezno zaščito ali oboje);
• je lahko oblikovan, izdelan in preverjan enkratno ali je v celoti standardiziran in
proizveden v velikih količinah.
Uporabljata se lahko oba izraza »upor za ozemljitev ničlišča« (NGR) in »nevtralni ozemljitveni upor« (NER)
. Vseeno pa se v tem dokumentu zato, da ne bi prišlo do zamenjav z
»nevtralnim ozemljitvenim reaktorjem« (NER), uporablja izraz »upor za ozemljitev ničlišča« (NGR).
Ta dokument določa:
• značilnosti upora za ozemljitev ničlišča;
• zahteve za pogoje delovanja uporov za ozemljitev ničlišča;
• preskuse in preskusne metode za določanje, ali so ti pogoji izpolnjeni;
• zahteve v zvezi z označevanjem uporov za ozemljitev ničlišča.
Dodatek A vsebuje smernice o tem, kako upoštevati učinek spremembe upornosti s
temperaturo.

General Information

• Status: Published
• Publication Date: 10-Apr-2023
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: ETR - Power transformers
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 05-Apr-2023
• Due Date: 10-Jun-2023
• Completion Date: 11-Apr-2023
• Ref Project: EN IEC 60076-25:2023 - Power transformers - Part 25: Neutral grounding resistors

Standards Content (Sample)

SLOVENSKI STANDARD
SIST EN IEC 60076-25:2023
01-junij-2023
Močnostni transformatorji - 25. del: Upori za ozemljitev ničlišča (IEC 60076-
25:2023)
Power transformers - Part 25: Neutral grounding resistors (IEC 60076-25:2023)
Leistungstransformatoren – Teil 25: Neutrale Erdungswiderstände (IEC 60076-25:2023)
Transformateurs de puissance - Partie 25: Résistances de mise à la terre du neutre (IEC
60076-25:2023)
Ta slovenski standard je istoveten z: EN IEC 60076-25:2023
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
SIST EN IEC 60076-25:2023 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN IEC 60076-25:2023

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SIST EN IEC 60076-25:2023


EUROPEAN STANDARD EN IEC 60076-25

NORME EUROPÉENNE

EUROPÄISCHE NORM March 2023
ICS 29.180

English Version
Power transformers - Part 25: Neutral grounding resistors
(IEC 60076-25:2023)
Transformateurs de puissance - Partie 25: Résistances de Leistungstransformatoren - Teil 25: Neutrale
mise à la terre du neutre Erdungswiderstände
(IEC 60076-25:2023) (IEC 60076-25:2023)
This European Standard was approved by CENELEC on 2023-03-30. 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
© 2023 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
 Ref. No. EN IEC 60076-25:2023 E

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SIST EN IEC 60076-25:2023
EN IEC 60076-25:2023 (E)
European foreword
The text of document 14/1097/FDIS, future edition 1 of IEC 60076-25, prepared by IEC/TC 14 "Power
transformers" was submitted to the IEC-CENELEC parallel vote and approved by CENELEC as
EN IEC 60076-25:2023.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2023-12-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2026-03-30
document have to be withdrawn
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 60076-25:2023 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 60068-3-3 NOTE Approved as EN IEC 60068-3-3
IEC 60071-1 NOTE Approved as EN IEC 60071-1
IEC 60076-1 NOTE Approved as EN 60076-1
IEC 60664-1 NOTE Approved as EN IEC 60664-1
IEC 62271-1 NOTE Approved as EN 62271-1
2

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SIST EN IEC 60076-25:2023
EN IEC 60076-25:2023 (E)
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: General EN 60060-1 2010
definitions and test requirements
IEC 60071-2 - Insulation co-ordination - Part 2: Application EN IEC 60071-2 -
guidelines
IEC 60076-3 2013 Power transformers - Part 3: Insulation levels, EN 60076-3 2013
dielectric tests and external clearances in air
+ A1 2018 + A1 2018
IEC 60529 1989 Degrees of protection provided by enclosures EN 60529 1991
(IP Code)
- - + corrigendum May 1993
+ A1 1999 + A1 2000
+ A2 2013 + A2 2013

3

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SIST EN IEC 60076-25:2023

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SIST EN IEC 60076-25:2023



IEC 60076-25

®


Edition 1.0 2023-02




INTERNATIONAL



STANDARD




NORME


INTERNATIONALE











Power transformers –

Part 25: Neutral grounding resistors



Transformateurs de puissance –

Partie 25: Résistances de mise à la terre du neutre
















INTERNATIONAL

ELECTROTECHNICAL

COMMISSION


COMMISSION

ELECTROTECHNIQUE


INTERNATIONALE




ICS 29.180 ISBN 978-2-8322-6431-7




Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

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SIST EN IEC 60076-25:2023
– 2 – IEC 60076-25:2023 © IEC 2023
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
4 Service conditions . 9
5 Ratings . 9
5.1 Rated ambient temperature . 9
5.2 Rated resistance (R) . 9
5.3 Maximum resistance variation . 9
5.4 Rated time . 9
5.5 Rated voltage (U ) . 9
r
5.6 Rated insulation level . 10
5.7 Rated short time current . 10
5.8 Rated continuous current . 10
5.9 Rated frequency . 10
6 Rating plates (Nameplates) . 11
6.1 General . 11
6.2 Minimum information to be provided . 11
7 Design and construction . 11
7.1 General . 11
7.2 Resistive elements . 11
7.3 Resistive banks. 12
7.4 Electrical connections . 12
7.5 IN and OUT terminals . 12
7.5.1 General . 12
7.5.2 IN terminal . 12
7.5.3 OUT terminal . 12
7.6 Insulators . 13
7.6.1 General . 13
7.6.2 Insulators between resistive elements . 13
7.6.3 Insulators between resistor banks and enclosure . 13
7.6.4 Creepage distance . 13
7.7 Enclosure . 13
7.7.1 Protection against contact with live parts, ingress of solid foreign bodies
and water . 13
7.7.2 Design . 13
7.7.3 Protection against corrosion . 14
8 Tests . 14
8.1 General . 14
8.2 Test classifications . 14
8.2.1 Routine tests . 14
8.2.2 Type tests . 14
8.2.3 Special tests . 15
8.3 Test documentation . 15
8.4 Routine tests. 15
8.4.1 Visual inspection and dimensional verification . 15

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SIST EN IEC 60076-25:2023
IEC 60076-25:2023 © IEC 2023 – 3 –
8.4.2 Rated resistance measurement. 15
8.4.3 Power frequency withstand voltage test . 15
8.4.4 Insulation resistance . 16
8.5 Temperature rise tests . 17
8.5.1 General . 17
8.5.2 Continuous current duty . 17
8.5.3 Rated time duty . 17
8.5.4 Tolerances . 18
8.5.5 Acceptance criteria . 18
8.6 Special tests . 18
8.6.1 Inductance measurement . 18
8.6.2 Lightning impulse test . 18
8.6.3 Mechanical design verification . 18
8.6.4 Protection degree of enclosure . 19
Annex A (informative) Resistance variation . 20
A.1 General . 20
A.2 Material properties . 20
A.3 Effect of NGR resistance variation . 20
A.4 Rate of variation . 21
A.5 Consecutive faults . 22
A.6 Protection coordination . 22
A.7 System capacitive charging current . 22
A.8 Conclusion . 22
Bibliography . 23

Figure A.1 – Ground fault protection settings example . 21

Table 1 – Rated insulation levels . 10
Table 2 – Maximum temperature rises above ambient for resistive elements. 12
Table 3 – Tests classifications . 14
Table A.1 – Resistance and current change with temperature rise . 21

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SIST EN IEC 60076-25:2023
– 4 – IEC 60076-25:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

POWER TRANSFORMERS –

Part 25: Neutral grounding resistors

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
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence between
any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
IEC 60076-25 has been prepared by IEC technical committee 14: Power transformers. It is an
International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
14/1097/FDIS 14/1101/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/standardsdev/publications.

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SIST EN IEC 60076-25:2023
IEC 60076-25:2023 © IEC 2023 – 5 –
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,
• replaced by a revised edition, or
• amended.

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SIST EN IEC 60076-25:2023
– 6 – IEC 60076-25:2023 © IEC 2023
POWER TRANSFORMERS –

Part 25: Neutral grounding resistors



1 Scope
This part of IEC 60076 applies to dry type natural air-cooled resistors, for neutral grounding of
transformers and generators, in order to limit the earth fault current in power systems by means
of metallic resistive elements.
For the purposes of this document, the resistor can be:
• used alone or in combination with other electrotechnical products not covered by this
document, such as (but not limited to): a step-down single-phase transformer, an open
triangle or zig-zag transformer (where the neutral point is not available) and a Petersen coil
reactor (in order to increase active power contribution to the fault or reduce time constant
for proper protection operation or both);
• designed, manufactured and verified on a one-off basis or fully standardized and
manufactured in quantity.
Both terms "neutral grounding resistor" (NGR) and "neutral earthing resistor" (NER) can be
used. However, for the purposes of this document and in order to avoid any confusion with
"neutral earthing reactor" (NER), the term "neutral grounding resistor" (NGR) is used.
This document specifies:
• the characteristics of the NGR;
• the service conditions requirements for NGRs;
• the tests and test methods for confirming that these conditions have been met;
• the requirements relating to marking for NGRs.
Annex A provides guidance on how to consider the effect of resistance variation with
temperature.
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 60071-2, Insulation co-ordination – Part 2: Application guidelines
IEC 60076-3:2013, Power transformers – Part 3: Insulation levels, dielectric tests and external
clearances in air
IEC 60076-3:2013/AMD1:2018
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60529/AMD1:1999
IEC 60529/AMD2:2013

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SIST EN IEC 60076-25:2023
IEC 60076-25:2023 © IEC 2023 – 7 –
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
IEC and ISO maintain terminology 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
NGR
neutral grounding resistor
neutral grounding device where the principal element is resistance
3.2
resistive element
unitary current carrying conductor, usually in the form of grids, plates, strips, ribbons or wires
and which can have intermediate tapings
3.3
resistive bank
sub-assembly consisting of one or multiple resistive elements supported by the same structure
Note 1 to entry: One or multiple resistive banks form a complete NGR.
3.4
IN terminal
higher voltage terminal of the NGR, connected to the grounded equipment's neutral terminal
3.5
OUT terminal
lower voltage terminal of the NGR, connected to earth
3.6
rated short time current
I
str
value of the initial RMS current upon the application of the rated voltage
3.7
continuous current
I
r
steady state RMS value of current
3.8
extended time
period of time that is greater than the time required for the temperature rise to become constant
but limited to 90 days of operation per year
3.9
highest voltage for equipment
U
m
highest RMS phase to neutral voltage for which the NGR is designed in respect of its insulation

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3.10
power frequency withstand voltage
U
d
RMS value of sinusoidal power frequency voltage that the insulation of the NGR can withstand
during tests made under specified conditions and for a specified duration
3.11
impulse withstand voltage
U
p
highest peak value of impulse voltage of specified form and polarity which does not cause
breakdown of insulation under specified conditions
3.12
protective earthing terminal
terminal connecting the NGR's enclosure to earth for protective purposes
3.13
working voltage

U
w
highest RMS value of the AC or DC voltage across any particular insulation which can occur
when the equipment is supplied at rated voltage
3.14
temperature coefficient of resistance
parameter describing the change in resistance relative to a given change in temperature
Note 1 to entry: The temperature coefficient of resistance is expressed by:
RR−
21
α=
(1)
R (θ −θ)
12 1

where
R and R are resistances, in Ω, at temperatures θ and θ , in °C, respectively, and
1 2 1 2
−1
α is the temperature coefficient of resistance, expressed in K , for the temperature variation between θ
2
and θ .
1
3.15
uniform insulation
insulation of an NGR where all the active material have the same insulation level towards the
enclosure
3.16
non-uniform insulation
insulation level throughout the NGR construction that is graded based on the rated voltage at
each part of the construction i.e. connections, banks and elements and where the OUT terminal
is intended for direct connection to earth
3.17
duty cycle
specified sequence of ratings, each of them having a different combination of rated time and
rated current or voltage

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SIST EN IEC 60076-25:2023
IEC 60076-25:2023 © IEC 2023 – 9 –
4 Service conditions
Unless otherwise specified, NGRs conforming to this document are intended for use under the
normal service conditions described in IEC 60076-1.
For installations at an altitude higher than 1 000 m, the required insulation level shall be
corrected using the correction factor provided in IEC 60071-2.
5 Ratings
5.1 Rated ambient temperature
Unless otherwise specified, the rated ambient temperature shall be 20 °C.
5.2 Rated resistance (R)
The rated resistance is the specified resistance between the IN terminal and OUT terminal at
the rated ambient temperature.
Unless otherwise stated, the tolerance of the rated resistance value is ±10 %.
5.3 Maximum resistance variation
The purchaser shall also specify the maximum allowable variation of resistance at the maximum
temperature rise in per cent of rated resistance.
NOTE Certain applications such as high impedance grounding or sensitive networks can require a maximum
variation as low as 10 % whereas others can accept a variation as high as 100 %. See Annex A for guidance.
5.4 Rated time
The rated time is the time the NGR shall withstand the rated voltage and the specified
combination of rated short time current and rated continuous current.
The rated time shall be specified by the purchaser.
Typical rated times are 3 s, 5 s,10 s, 30 s and 60 s, extended time or continuous.
If a duty cycle is to be considered it shall be detailed in the technical specification.
5.5 Rated voltage (U )
r
When the NGR is connected directly between the neutral point and the earth the rated voltage
across the NGR is the phase to neutral voltage and is equal to the nominal system voltage for
which the equipment is designed, divided by the square root of three.
U
U =
 (2)
r
3

In some specific cases, the rated voltage can be different, for example, for resistors installed
on the low voltage side of a step-down transformer or for resistors installed in series or parallel
to a neutral earthing reactor.
In the case of combined equipment, for example neutral grounding resistors and zig zag
transformers, IEC 62271-1 applies.

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NOTE Standard values of system voltages (U) are given in IEC 60071-1 for system voltages above 1 000 V and in
IEC 60664-1 for system voltages below 1 000 V.
5.6 Rated insulation level
The rated voltage U as defined in 5.5 shall be the basis to determine the rated insulation level
r
and clearance distances in air in Table 1 chosen as the next standard value of U equal to or
m
higher than U .
r
Table 1 – Rated insulation levels
Highest voltage for Rated short-duration Rated lightning impulse Minimum air clearance
power-frequency
equipment withstand voltage
withstand voltage
U U U mm
m d p
kV (RMS value) kV (RMS value) kV (peak value) (IEC 60076-3:2013,
Table 4), except for
U = 1kV
m
1,0 2,2 12 14
3,6 10 40 60
7,2 20 60 90
12 28 75 120
17,5 38 95 160
24 50 125 220
36 70 170 320
52 95 250 480
72,5 140 325 630
100 185 450 900
123 230 550 1 100
145 275 650 1 300
170 325 750 1 500
245 460 1 050 2 100

Other rated in
...