EN IEC 61643-361:2026

• Abstract:

  IEC 61643-361:2026 applies to surge isolation transformers (SITs) dedicated to surge mitigation and for connection to 50/60 Hz power circuits and equipment rated up to 1 000 V RMS. This document covers the surge and mitigation performance of SITs with an impulse withstand voltage performance of at least 30 kV, and provides standard methods for testing and rating. This document covers surge-related parameters but does not address typical transformer tests and parameters covered by the IEC 61558 series [13][1]. This document also does not cover SIT operation under differential mode lightning surge conditions. [1] Numbers in square brackets refer to the Bibliography.

• Status: Published
• Publication Date: 02-Jul-2026
• ICS: 29.180 - Transformers. Reactors
• Technical Committee: CLC/SR 37B - Specific components for surge arresters and surge protective devices
• Current Stage: 6060 - Document made available - Publishing
• Start Date: 03-Jul-2026
• Due Date: 31-Dec-2025
• Completion Date: 03-Jul-2026
• Ref Project: SIST EN IEC 61643-361:2026 - Low-voltage surge protective devices - Part 361: Surge isolation transformers (SITs) connected to low-voltage distribution system - Requirements and test methods (IEC 61643-361:2026)

• Refers: EN 60068-1:2014 - Environmental testing - Part 1: General and guidance
• Effective Date: 09-Jun-2026

Overview

EN IEC 61643-361:2026 specifies the requirements and test methods for surge isolation transformers (SITs) used in low-voltage distribution systems up to 1,000 V RMS at 50/60 Hz. Issued by CLC, this international standard focuses on the surge mitigation capabilities and performance parameters of SITs, including their impulse withstand voltage, which must be at least 30 kV. SITs are critical components in electrical infrastructure for protecting equipment and systems from harmful transient overvoltages due to lightning or switching surges.

This part of the IEC 61643 series outlines the essential properties unique to SITs-including enhanced insulation and shielding-to ensure robust protection in electrical installations. It does not cover general transformer tests or differential mode lightning surge conditions, which are addressed elsewhere.

Key Topics

• Definition and Purpose of SITs

  • SITs are specialized isolation transformers with enhanced insulation and electric screening between primary and secondary windings.
  • They are engineered to mitigate common-mode surges, greatly attenuating transient voltages from the primary to the secondary side.

• Service Conditions

  • Operating temperature: -25°C to +40°C (other ranges by agreement)
  • Relative humidity: Max 90%, with restrictions against condensation environments
  • Altitude: Up to 1,000 m (special considerations required above this)

• Electrical Requirements

  • Insulation Resistance: SITs must demonstrate a minimum of 100 MΩ at DC 500 V.
  • Power Frequency Withstand Voltage: Must achieve at least 10 kV for robust coordination with surge protective devices (SPDs).
  • Impulse Withstand Voltage: SITs must withstand surges of at least 30 kV, tested using the prescribed 1,2/50 μs waveform.

• Testing and Performance Verification

  • Specific procedures for type tests, including environmental and electrical test conditions, sample preparations, and assessment of pass/fail outcomes using voltage impulse generator templates.
  • Tests confirm that SITs provide the required insulation performance and surge attenuation.

Applications

The EN IEC 61643-361:2026 standard is particularly relevant in:

• Industrial and Commercial Power Distribution: Protects sensitive equipment and systems in manufacturing plants, data centers, and commercial facilities from voltage surges.
• Lightning Protection Schemes: SITs help establish lightning protection zones, forming part of a comprehensive strategy alongside SPDs and proper grounding.
• Critical Infrastructure: Ensures reliable operation of mission-critical utilities, telecommunications, and control systems by isolating and attenuating transient voltages.
• Electrical Panel and Cabinet Integration: Used by designers and panel builders to comply with surge protection regulations and improve overall safety and system resilience.

The practical value of SITs lies in reducing equipment downtime, minimizing maintenance needs, and extending the operational life of electrical assets by providing a high level of surge immunity.

Related Standards

Professionals applying this standard should also consider the following:

• IEC 61558 Series: Comprehensive requirements for the safety of general transformers, not specific to surge isolation transformers.
• IEC 61643-12: Guidelines for selection and application of SPDs in low-voltage power distribution systems.
• IEC 62305-4: Addresses protection of electrical and electronic systems within structures from lightning impacts.
• ITU-T K.95, K.126 & IEEE PC62.69: Standards for surge parameters and applications of isolating transformers in telecommunication and networking equipment.

For any deployment involving surge protection in low-voltage distribution systems, following EN IEC 61643-361:2026 ensures alignment with international best practices, enhances system resilience, and supports safe, compliant installations. This document is indispensable for manufacturers, system integrators, consultants, and facility managers aiming to mitigate the risks of transient overvoltages.

SLOVENSKI STANDARD
01-september-2026
Nizkonapetostne naprave za zaščito pred prenapetostnimi udari - 361. del:
Prenapetostni izolacijski transformatorji (SIT) za nizkonapetostne napajalne
sisteme - Zahteve in preskusne metode (IEC 61643-361:2026)
Low-voltage surge protective devices - Part 361: Surge isolation transformers (SITs)
connected to low-voltage distribution system - Requirements and test methods (IEC
61643-361:2026)
Bauelemente für den Überspannungsschutz bei Niederspannungen - Teil 361:
Überspannungs-Trenntransformatoren (SIT) in Niederspannungsnetzen - Anforderungen
und Prüfverfahren (IEC 61643-361:2026)
Parafoudres basse tension - Partie 361: Transformateurs d’isolement contre les
surtensions (SIT) connectés aux réseaux de distribution basse tension - Exigences et
méthodes d’essai (IEC 61643-361:2026)
Ta slovenski standard je istoveten z: EN IEC 61643-361:2026
ICS:
29.120.50 Varovalke in druga Fuses and other overcurrent
nadtokovna zaščita protection devices
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 61643-361

NORME EUROPÉENNE
EUROPÄISCHE NORM July 2026
ICS 29.180
English Version
Low-voltage surge protective devices - Part 361: Surge isolation
transformers (SITs) connected to low-voltage distribution system
- Requirements and test methods
(IEC 61643-361:2026)
Parafoudres basse tension - Partie 361: Transformateurs Bauelemente für den Überspannungsschutz bei
d'isolement contre les surtensions (SIT) connectés aux Niederspannungen - Teil 361: Überspannungs-
réseaux de distribution basse tension - Exigences et Trenntransformatoren (SIT) in Niederspannungsnetzen -
méthodes d'essai Anforderungen und Prüfverfahren
(IEC 61643-361:2026) (IEC 61643-361:2026)
This European Standard was approved by CENELEC on 2026-06-02. 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 61643-361:2026 E

European foreword
The text of document 37B/267/FDIS, future edition 1 of IEC 61643-361, prepared by SC 37B
"Components for low-voltage surge protection" of IEC/TC 37 "Surge arresters" was submitted to the
IEC-CENELEC parallel vote and approved by CENELEC as EN IEC 61643-361:2026.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-07-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-07-31
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 61643-361:2026 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 60060-1:2025 NOTE Approved as EN IEC 60060-1:2025 (not modified)
IEC 61643-12 NOTE Approved as CLC/TS 61643-12
IEC 62305-4 NOTE Approved as EN IEC 62305-4
IEC 61643-351:2016 NOTE Approved as EN 61643-351:2017 (not modified)
IEC 60664-1:2020 NOTE Approved as EN IEC 60664-1:2020 (not modified)
IEC 60099-4:2014 NOTE Approved as EN 60099-4:2014 (not modified)
IEC 61558 (series) NOTE Approved as EN IEC 61558 (series)
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 60068-1 - Environmental testing - Part 1: General EN 60068-1 -
and guidance
IEC 61643-361 ®
Edition 1.0 2026-04
INTERNATIONAL
STANDARD
Low-voltage surge protective devices -
Part 361: Surge isolation transformers (SITs) connected to low-voltage
distribution system - Requirements and test methods
ICS 29.180  ISBN 978-2-8327-1224-5

IEC 61643-361:2026-04(en)
IEC 61643-361:2026 © IEC 2026
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and symbols . 6
3.1 Terms and definitions . 6
3.2 Symbols . 8
3.3 Abbreviated terms and acronyms . 9
4 Service condition . 9
4.1 Temperature range . 9
4.2 Humidity range . 9
4.3 Altitude and air pressure . 9
5 SIT surge condition . 10
5.1 General . 10
5.2 SITs surge mitigation . 10
6 Electrical requirements . 11
6.1 General . 11
6.2 Insulation resistance . 11
6.3 Power frequency withstand voltage . 11
6.4 Impulse withstand voltage . 12
7 Type tests . 12
7.1 General . 12
7.2 Test condition . 12
7.3 Sample preparation. 12
7.4 Insulation resistance . 13
7.5 Power frequency withstand voltage . 13
7.6 Impulse withstand voltage . 14
Annex A (informative) 1,2/50 impulse . 17
Bibliography . 18

Figure 1 – Symbol of SIT for single phase low-voltage power system (single-line
diagram) . 8
Figure 2 – Symbol of SIT for single phase low-voltage power system (multi-line
diagram) . 8
Figure 3 – SITs under common mode surge condition . 11
Figure 4 – Sample preparation . 13
Figure 5 – Test circuit of insulation resistance . 13
Figure 6 – Power frequency withstand voltage test circuit . 14
Figure 7 – Construction of pass/fail template from the 1,2/50 open-circuit waveform . 15
Figure 8 – Pass/fail template and test waveforms . 15
Figure 9 – Impulse withstand voltage test circuit . 16
Figure A.1 – 1,2/50 time periods and voltage amplitudes . 17

IEC 61643-361:2026 © IEC 2026
Table 1 – List of abbreviations and acronyms used in this standard . 9
Table 2 – Conditions used for test . 12
Table A.1 – 1,2/50 voltage impulse generator parameters . 17

IEC 61643-361:2026 © IEC 2026
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Low-voltage surge protective devices -
Part 361: Surge isolation transformers (SITs) connected
to low-voltage distribution system - Requirements and test methods

FOREWORD
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC 61643-361 has been prepared by subcommittee 37B: Components for low-voltage surge
protection, of IEC technical committee 37: Surge arresters. It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
37B/267/FDIS 37B/270/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.
IEC 61643-361:2026 © IEC 2026
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 61643 series, published under the general title Low-voltage surge
protective devices, 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 61643-361:2026 © IEC 2026
INTRODUCTION
This part of IEC 61643 covers surge isolation transformers (SITs) whose rated impulse
withstands voltage coordinates with the expected surge environment of the installation location.
This type of surge protective component (SPC) isolates and attenuates transient voltage in
conjunction with current diverting components (or SPDs).
SITs are a product with specific enhanced insulation layers between the primary and secondary
windings and an electric screen (ES) inserted between primary and secondary windings. With
these ES and specific enhanced insulation layers, the surge that appears between the primary
winding and earth (common-mode) is greatly attenuated and appears between the secondary
winding and earth. SITs are products with a particularly improved impulse withstand voltage
compared to ordinary isolating transformers.
It is essential that SITs are protected by an appropriate SPD at their primary side, to avoid any
damage to the SIT itself. SITs are intended to be used in surge protection to establish lightning
protection zones.
IEC 61643-361:2026 © IEC 2026
1 Scope
This part of IEC 61643 applies to surge isolation transformers (SITs) dedicated to surge
mitigation and for connection to 50/60 Hz power circuits and equipment rated up to
1 000 V RMS. This document covers the surge and mitigation performance of SITs with an
impulse withstand voltage performance of at least 30 kV, and provides standard methods for
testing and rating.
This document covers surge-related parameters but does not address typical transformer tests
and parameters covered by the IEC 61558 series [13] . This document also does not cover SIT
operation under differential mode lightning surge conditions.
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 60068-1, Environmental testing - Part 1: General and guidance
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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
3.1.1
surge isolation transformer for power
SIT
isolation transformer which has high impulse withstand voltage with/without electric screen
between input and output windings
Note 1 to entry: SITs for power systems require electric screen.
[SOURCE: IEC 61643-351:2016 [8], 3.1.1, modified – the alternative without electric screen has
been removed.]
3.1.2
electric screen
electric shield (US)
screen of conductive material intended to reduce the penetration of an electric field into a given
region
[SOURCE: IEC 60050-151:2001 [9], 151-13-10]
___________
Numbers in square brackets refer to the Bibliography.
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