SIST EN IEC 60255-27:2025

Measuring relays and protection equipment - Part 27: Product safety requirements

IEC 60255-27:2023 specifies the product safety requirements for measuring relays and protection equipment having a rated AC voltage up to 1 000 V, or a rated DC voltage up to 1 500 V. This document specifies essential safety requirements to minimize the risk of fire and hazards caused by electric shock or injury to the user and property. This document specifies only product safety requirements; functional performance of the equipment is not covered. This document covers all the ways in which the equipment can be mounted and used in cabinets, racks and panels. This document also applies to auxiliary devices such as shunts, series resistors, transformers, auxiliary control panels, display devices, etc., that are used in conjunction with measuring relays and protection equipment and are tested together.

Messrelais und Schutzeinrichtungen - Teil 27: Anforderungen an die Produktsicherheit

Relais de mesure et dispositifs de protection - Partie 27: Exigences de sécurité

L'IEC 60255-27:2023 spécifie les exigences de sécurité des produits pour les relais de mesure et les dispositifs de protection avec une tension assignée en courant alternatif inférieure ou égale à 1 000 V ou avec une tension assignée en courant continu inférieure ou égale à 1 500 V. Le présent document spécifie les exigences de sécurité fondamentales pour réduire le plus possible les risques d'incendie et de dangers causés par un choc électrique ou les risques de blessure de l'utilisateur et de dégradation matérielle. Le présent document spécifie uniquement les exigences de sécurité des produits; les performances fonctionnelles du matériel ne sont pas couvertes. En revanche, le présent document couvre tous les modes qui peuvent être utilisés pour le montage et l'utilisation du dispositif dans des baies, des racks ou des panneaux. Le présent document s'applique aussi aux dispositifs auxiliaires comme les shunts, résistances série, transformateurs, panneaux de commande auxiliaires, dispositifs d'affichage, etc., qui sont utilisés conjointement avec les relais de mesure et les dispositifs de protection et sont soumis à l'essai ensemble.

Merilni releji in zaščitna oprema - 27. del: Zahteve za varnost izdelka

Ta del standarda IEC 60255 določa zahteve za varnost izdelka za merilne releje in zaščitno opremo z nazivno izmenično napetostjo do 1000 V ali nazivno enosmerno napetostjo do 1500 V. Nad temi omejitvami se standard IEC 60664-1 uporablja za ugotavljanje zevi, plazilne razdalje in napetostne trdnosti.
Ta dokument določa bistvene varnostne zahteve za zmanjšanje nevarnosti požara in nevarnosti, ki jih povzroči električni udar, oziroma poškodb uporabnika ali materialne škode. Ta dokument določa samo zahteve za varnost izdelka; funkcionalne lastnosti opreme niso zajete.
Ta dokument zajema vse načine pritrditve ter uporabe opreme v kabinah, stojalih in ploščah. Ta dokument se uporablja tudi za pomožne naprave, kot so prespojnice, serijski upori, transformatorji, pomožne nadzorne plošče, prikazovalne naprave itd., ki se uporabljajo skupaj z merilnimi releji in zaščitno opremo ter se preskušajo skupaj.
Pomožna oprema (npr. omrežna stikala), uporabljena skupaj z merilnimi releji in zaščitno opremo, bo morda morala izpolnjevati dodatne varnostne zahteve.
Ta dokument ne določa implementacije posamezne opreme, vezij in komponent.
Ta dokument se uporablja za opremo, ki je zasnovana kot varna pod najmanj naslednjimi okoljskimi pogoji:
– uporaba v prostoru; – višina do 2000 m, v skladu s standardom IEC 60255-1; – razpon nazivne temperature okolja, v skladu s standardom IEC 60255-1; – največja zunanja relativna vlažnost, v skladu s standardom IEC 60255-1; – območje delovanja pomožne energetske napetosti, v skladu s standardom IEC 60255-1; – ustrezna kategorija prenapetosti; – ustrezna stopnja onesnaženosti predvidenega okolja (stopnja onesnaženosti 2 v večini primerov).

General Information

• Status: Published
• Public Enquiry End Date: 23-Feb-2022
• Publication Date: 15-May-2025
• ICS: 29.120.70 - Relays
• Technical Committee: ISS EIT.ERE - All-or-nothing electrical relays
• Current Stage: 6060 - National Implementation/Publication (Adopted Project)
• Start Date: 25-Apr-2025
• Due Date: 30-Jun-2025
• Completion Date: 16-May-2025
• Directive: 2014/35/EU - Directive 2014/35/EU of the European Parliament and of the Council of 26 February 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of electrical equipment designed for use within certain voltage limits
• Mandate: M/511 - Mandate to CEN, CENELEC and ETSI relating to harmonised standards in the field of the Low Voltage Directive
• Ref Project: EN IEC 60255-27:2025 - Measuring relays and protection equipment - Part 27: Product safety requirements

Overview

EN IEC 60255-27:2025 (CLC) specifies the product safety requirements for measuring relays and protection equipment (identical to IEC 60255-27:2023). It applies to devices with rated AC voltages up to 1 000 V and rated DC voltages up to 1 500 V and addresses essential safety measures to minimize fire risk, electric shock and injury. The standard covers equipment mounted in cabinets, racks and panels and also applies to auxiliary devices (shunts, series resistors, transformers, auxiliary control panels, displays) when tested together. It is a product-safety standard only - functional performance is explicitly out of scope.

Key topics and technical requirements

The standard organizes requirements across safety domains; major technical topics include:

• Protection against electric shock
  • Insulation, barriers, protective impedance and limits for accessible parts
  • Earthing/protective bonding, functional earthed circuits and protective conductor connections
• Clearances and creepage distances
  • Requirements to maintain safe dielectric separation under expected conditions
• Single-fault conditions
  • Design and testing guidance for safety when a single fault occurs
• Flammability and fire resistance
  • Fire enclosures, flame barriers, material selection, prevention of ignition sources and assessment of fire risk
• Mechanical safety
  • Stability, moving parts, edges, and secure terminations
• General design for safety
  • Component selection (including high-integrity parts), cabling and fusing, high leakage current and solid insulation
• Marking, documentation and packaging
  • Identification, auxiliary supply labeling, safety instructions and packaging to support safe installation and use

Practical applications

EN IEC 60255-27:2025 is used to:

• Guide design decisions to meet product safety expectations for measuring relays and protection devices
• Define test and evaluation criteria for safety conformity and certification
• Inform panel and switchgear manufacturers on safe mounting and integration practices
• Support risk assessments for fire and electric shock when relays/protection equipment are installed in cabinets, racks and panels
• Ensure auxiliary components tested with relays meet combined product safety criteria

Who should use this standard

• Relay and protection equipment manufacturers and designers
• Electrical safety engineers and product compliance teams
• Test laboratories and certification bodies (CE/CENELEC compliance)
• Utility protection engineers, panel builders and system integrators
• Regulatory bodies and procurement teams specifying safety requirements

Related standards

EN IEC 60255-27 references and aligns with several norms including IEC 60255-1, IEC 60255-26 (EMC), IEC 60664-1 (insulation coordination), IEC 61010-1, IEC 60529 (IP code) and relevant fire- and material-related IEC standards used for product safety conformity.

Standards Content (Sample)

SLOVENSKI STANDARD
01-junij-2025
Merilni releji in zaščitna oprema - 27. del: Zahteve za varnost izdelka
Measuring relays and protection equipment - Part 27: Product safety requirements
Messrelais und Schutzeinrichtungen - Teil 27: Anforderungen an die Produktsicherheit
Relais de mesure et dispositifs de protection - Partie 27: Exigences de sécurité
Ta slovenski standard je istoveten z: EN IEC 60255-27:2025
ICS:
29.120.70 Releji Relays
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60255-27

NORME EUROPÉENNE
EUROPÄISCHE NORM April 2025
ICS 29.120.70 Supersedes EN 60255-27:2014
English Version
Measuring relays and protection equipment - Part 27: Product
safety requirements
(IEC 60255-27:2023)
Relais de mesure et dispositifs de protection - Partie 27: Messrelais und Schutzeinrichtungen - Teil 27:
Exigences de sécurité des produits Anforderungen an die Produktsicherheit
(IEC 60255-27:2023) (IEC 60255-27:2023)
This European Standard was approved by CENELEC on 2024-10-16. 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
© 2025 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60255-27:2025 E

European foreword
The text of document 95/516/FDIS, future edition 3 of IEC 60255-27, prepared by TC 95 "Measuring
relays and protection equipment" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 60255-27:2025.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2026-04-30
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2028-04-30
document have to be withdrawn
This document supersedes EN 60255-27:2014 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 60255-27:2023 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 60332 series NOTE Approved as EN IEC 60332 series
IEC 60384-14:2013/A1:2016 NOTE Approved as EN 60384-14:2013/A1:2016 (not modified)
IEC 60695-2-12 NOTE Approved as EN IEC 60695-2-12
IEC 60695-11-5 NOTE Approved as EN 60695-11-5
IEC 61508 series NOTE Approved as EN 61508 series
IEC 61558 series NOTE Approved as EN IEC 61558 series
IEC 61810-1 NOTE Approved as EN 61810-1
IEC 61869-1 NOTE Approved as EN IEC 61869-1
IEC 62368-1 NOTE Approved as EN IEC 62368-1
IEC 62477-1:2022 NOTE Approved as EN IEC 62477-1:2023 (not modified)
IEC 62479 NOTE Approved as EN 62479
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 60085 - Electrical insulation - Thermal evaluation EN 60085 -
and designation
IEC 60127-1 - Miniature fuses - Part 1: Definitions for EN IEC 60127-1 -
miniature fuses and general requirements
for miniature fuse-links
IEC 60255-1 - Measuring relays and protection equipment EN IEC 60255-1 -
- Part 1: Common requirements
IEC 60255-26 - Measuring relays and protection equipment EN IEC 60255-26 -
- Part 26: Electromagnetic compatibility
requirements
IEC 60352-1 - Solderless connections - Part 1: Wrapped EN 60352-1 -
connections - General requirements, test
methods and practical guidance
IEC 60352-2 - Solderless connections - Part 2: Crimped EN IEC 60352-2 -
connections - General requirements, test
methods and practical guidance
IEC 60417 - Graphical symbols for use on equipment. - -
Index, survey and compilation of the single
sheets.
IEC 60529 1989 Degrees of protection provided by EN 60529 1991
enclosures (IP Code)
- - + AC 1993
+ A1 1999 + A1 2000
+ A2 2013 + A2 2013
- - + AC 2016
IEC 60664-1 2020 Insulation coordination for equipment EN IEC 60664-1 2020
within low-voltage supply systems – Part 1:
Principles, requirements and tests
- - + AC 2020
Publication Year Title EN/HD Year
IEC 60664-3 - Insulation coordination for equipment EN 60664-3 -
within low-voltage systems - Part 3: Use of
coating, potting or moulding for protection
against pollution
IEC/TS 60695-2-20 - Fire hazard testing -- Part 2-20: - -
Glowing/hot wire based test methods - Hot-
wire coil ignitability - Apparatus, test
method and guidance
IEC 60695-11-10 - Fire hazard testing - Part 11-10: Test EN 60695-11-10 -
flames - 50 W horizontal and vertical flame
test methods
IEC 60825-1 - Safety of laser products - Part 1: EN 60825-1 -
Equipment classification and requirements
IEC 60990 2016 Methods of measurement of touch current EN 60990 2016
and protective conductor current
IEC 61010-1 2010 Safety requirements for electrical EN 61010-1 2010
equipment for measurement, control, and
laboratory use - Part 1: General
requirements
+ A1 (mod) 2016 + A1 2019
- - + AC 2019
IEC 61032 - Protection of persons and equipment by EN 61032 -
enclosures - Probes for verification
IEC 61051-2 2021 Varistors for use in electronic equipment - EN IEC 61051-2 2021
Part 2: Sectional specification for surge
suppression varistors
IEC 61180 - High-voltage test techniques for low- EN 61180 -
voltage equipment - Definitions, test and
procedure requirements, test equipment
IEC 61869-6 - Instrument transformers - Part 6: Additional EN 61869-6 -
general requirements for low-power
instrument transformers
IEC 61869-10 - Instrument transformers - Part 10: EN IEC 61869-10 -
Additional requirements for low-power
passive current transformers
IEC 61869-11 - Instrument transformers - Part 11: EN IEC 61869-11 -
Additional requirements for low-power
passive voltage transformers
IEC 62151 - Safety of equipment electrically connected - -
to a telecommunication network

IEC 60255-27 ®
Edition 3.0 2023-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Measuring relays and protection equipment –
Part 27: Product safety requirements
Relais de mesure et dispositifs de protection –
Partie 27: Exigences de sécurité des produits
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.120.70 ISBN 978-2-8322-6322-8
– 2 – IEC 60255-27:2023 © IEC 2023
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
2 Normative references . 10
3 Terms and definitions . 12
4 Protection against electric shock . 20
4.1 Protection from contact with hazardous-live-parts . 20
4.1.1 General . 20
4.1.2 Insulation . 21
4.1.3 Equipment enclosure and barriers . 21
4.1.4 Hazardous live terminations using stranded wire . 22
4.2 Protective impedance. 22
4.3 Accessible parts. 23
4.3.1 General . 23
4.3.2 Determination of accessible parts . 23
4.3.3 Permissible limits for accessible parts . 24
4.4 Earthing and protective bonding requirements . 25
4.4.1 General . 25
4.4.2 Insulation between live parts and accessible conductive parts . 25
4.4.3 Protective bonding . 26
4.5 Functional earthed circuits . 26
4.6 Protective conductor connection . 27
4.7 High leakage current . 27
4.8 Solid insulation . 27
4.8.1 General . 27
4.8.2 Requirements . 28
4.9 Clearances and creepage distances . 28
4.9.1 General . 28
4.9.2 Clearances . 28
4.9.3 Creepage distances . 29
4.10 Single fault conditions . 30
4.10.1 Testing in single fault condition . 30
4.10.2 Application of single fault condition . 30
4.10.3 Duration of tests . 32
4.10.4 Compliance . 32
5 Mechanical aspects . 33
5.1 Protection against mechanical hazards . 33
5.1.1 Stability . 33
5.1.2 Moving parts . 33
5.1.3 Edges and corners . 34
5.2 Mechanical requirements . 34
5.3 Mechanical security of terminations . 34
6 Flammability and resistance to fire . 34
6.1 General . 34
6.2 Requirements for protection against the spread of fire . 34
6.3 General hazards from overheating and fire . 36

IEC 60255-27:2023 © IEC 2023 – 3 –
6.3.1 Surface temperature limits for protection against burns . 36
6.3.2 Hazardous gases and chemicals . 36
6.4 Minimization of fire risk . 36
6.4.1 General . 36
6.4.2 Eliminating or reducing the sources of ignition within the equipment . 37
6.5 Cabling and fusing . 37
6.6 Flammability of materials and components . 37
6.6.1 General . 37
6.6.2 Materials for components and other parts inside fire enclosures . 38
6.6.3 Materials for fire enclosures . 38
6.6.4 Materials for components and other parts outside fire enclosures . 39
6.7 Fire ignition sources. 39
6.8 Conditions for a fire enclosure . 40
6.8.1 Parts requiring a fire enclosure . 40
6.8.2 Parts not requiring a fire enclosure . 40
6.9 Fire enclosures and flame barriers . 41
6.9.1 General . 41
6.9.2 Fire enclosures and flame barrier requirements . 41
6.10 Assessment of the fire risk due to a single fault condition . 43
6.10.1 Guidelines for maximum acceptable temperatures when subjecting a
circuit or component to a single fault condition . 43
6.10.2 Temperature of windings under normal use or a single fault condition . 43
6.10.3 Conformity of equipment with requirements for protection against the

spread of fire . 43
6.11 Limited-energy circuit . 44
7 General and fundamental design requirements for safety . 45
7.1 Climatic conditions for safety . 45
7.2 Electrical connections . 45
7.3 Components . 45
7.3.1 General . 45
7.3.2 High-integrity part or component . 45
7.4 Connection to other equipment . 46
7.5 High-intensity light sources . 46
7.6 Explosion . 46
7.6.1 General . 46
7.6.2 Components at risk of explosion . 46
8 Marking, documentation and packaging . 47
8.1 Marking . 47
8.1.1 General . 47
8.1.2 Identification . 47
8.1.3 Auxiliary supplies, measurands, inputs and outputs . 48
8.1.4 Fuses . 49
8.1.5 Measuring circuit terminals . 50
8.1.6 Terminals and operating devices . 50
8.1.7 Equipment protected by double or reinforced insulation . 51
8.1.8 Batteries . 51
8.1.9 Test voltage marking . 53
8.1.10 Warning markings . 53
8.1.11 Marking durability . 54

– 4 – IEC 60255-27:2023 © IEC 2023
8.2 Documentation . 54
8.2.1 General . 54
8.2.2 Equipment ratings . 54
8.2.3 Equipment installation . 55
8.2.4 Equipment commissioning and maintenance . 55
8.2.5 Equipment operation . 56
8.3 Packaging . 56
9 Type tests and routine tests . 56
9.1 General . 56
9.2 Safety type tests . 58
9.3 Routine testing . 58
9.4 Conditions for testing . 58
9.5 Verification procedure . 58
9.6 Tests . 59
9.6.1 Climatic environmental tests . 59
9.6.2 Mechanical tests . 60
9.6.3 Clearances and creepage distances . 60
9.6.4 Safety-related electrical tests. 60
9.6.5 Electrical environment and flammability . 65
9.6.6 Reverse polarity and slow ramp test . 67
9.6.7 Resistance to mechanical stress . 68
Annex A (normative)  Isolation class requirements and example diagrams . 70
A.1 General . 70
A.2 Hazardous live voltage (HLV) . 70
A.3 Symbols . 71
A.4 Typical insulation examples complying with the requirements of Table A.2 . 73
Annex B (normative) Rated impulse voltages . 76
Annex C (normative) Guidance for the determination of clearance, creepage distance
and withstand voltages . 77
C.1 General . 77
C.1.1 Introductory remark . 77
C.1.2 Rated insulation voltages . 77
C.1.3 Determination of rated insulation voltage . 78
C.1.4 Determination of rated impulse voltage . 78
C.1.5 Switching overvoltages generated by equipment . 79
C.1.6 lnsulation material . 79
C.1.7 Overvoltage categories . 79
C.2 Determination of clearances, creepage distances and withstand voltages . 80
C.2.1 Guidance for determination of clearances, creepage distances and

withstand voltages . 80
C.2.2 Determination of clearances, creepage distances and withstand
voltages . 82
Annex D (informative) Components . 88
D.1 General . 88
D.2 Introductory remark . 88
D.3 Transformers . 88
D.4 Equipment primary circuit capacitors . 88

IEC 60255-27:2023 © IEC 2023 – 5 –
D.5 Coil devices – transformers, instrument transformers and transducers,
reactors, and operating coils of relays and contactors with multiple windings
and/or screen . 89
D.5.1 Coil windings . 89
D.5.2 Insulating foil . 90
D.5.3 Interposing protective screen . 90
D.5.4 Safety isolation transformers . 90
D.6 Electromechanical components . 90
D.7 Semiconductor components and semiconductor configurations . 90
D.8 Connectors and terminal blocks . 91
D.9 Voltage dependent resistors (VDRs) . 91
D.10 Intentional radio transmitters . 92
Annex E (normative) External wiring terminations . 93
E.1 General . 93
E.2 Permanently connected equipment . 93
E.3 Conductors . 93
E.4 Terminals . 94
Annex F (informative) Examples of battery protection . 95
Annex G (informative) Risk assessment . 96
G.1 General . 96
G.2 Risk assessment procedure . 96
G.3 Achieving tolerable risk . 97
G.4 An application of risk assessment procedures . 98
Bibliography . 99

Figure 1 – Flow chart showing requirements for protection against the spread of fire . 35
Figure 2 – Baffle . 41
Figure 3 – Location and extent of a flame barrier . 42
Figure 4 – Voltage ramp test . 67
Figure 5 – Impact test with sphere . 69
Figure A.1 – Equipment with SELV input and/or output (I/O) . 74
Figure A.2 – Equipment with PELV input and/or output (I/O) . 74
Figure A.3 – Equipment with PEB input and/or output (I/O) . 75
Figure A.4 – Equipment with ELV input and/or output (I/O) . 75
Figure C.1 – Guidance for determination of clearances, creepage distances and
withstand voltages . 81
Figure F.1 – Non-rechargeable battery protection . 95
Figure F.2 – Rechargeable battery protection . 95
Figure G.1 – Iterative process of risk assessment and risk reduction . 96
Figure G.2 – Risk reduction . 97

Table 1 – Equipment enclosure requirements for protection against electric shock . 21
Table 2 – Current levels under normal use . 25
Table 3 – Charge or energy of capacitance levels under normal use . 25
Table 4 – Altitude multiplication factor . 29
Table 5 – Current levels in single fault condition . 33

– 6 – IEC 60255-27:2023 © IEC 2023
Table 6 – Maximum temperature under normal use and at an ambient temperature of
40 °C . 36
Table 7 – Acceptable perforation in the bottom of an equipment case . 42
Table 8 – Insulation material of windings . 43
Table 9 – Limits of maximum available current . 44
Table 10 – Overcurrent protective device . 44
Table 11 – Fuse types . 49
Table 12 – Symbols . 52
Table 13 – Symbols for marking of test voltage(s) . 53
Table 14 – Overview of tests . 57
Table 15 – Impulse generator characteristics . 61
Table 16 – Guidance for routine dielectric voltage testing for safety . 64
Table 17 – Limiting dynamic values . 66
Table 18 – Impact energy levels, test height and corresponding IK codes . 69
Table A.1 – Circuit isolation class for product circuits and/or groups . 70
Table A.2 – Insulation requirement between any two circuits . 72
Table B.1 – Rated impulse voltages (waveform: 1,2/50 µs) . 76
Table C.1 – Comparative tracking index (CTI) . 79
Table C.2 – Functional insulation, pollution degree 1, overvoltage category I . 82
Table C.3 – Functional insulation, pollution degree 2, overvoltage category I . 83
Table C.4 – Functional, basic or supplementary insulation, pollution degree 1,

overvoltage category II . 83
Table C.5 – Functional, basic or supplementary insulation, pollution degree 2,
overvoltage category II . 84
Table C.6 – Functional, basic or supplementary insulation, pollution degree 1,
overvoltage category III . 84
Table C.7 – Functional, basic or supplementary insulation, pollution degree 2,

overvoltage category III . 85
Table C.8 – Double or reinforced insulation, pollution degree 1,
overvoltage category II . 85
Table C.9 – Double or reinforced insulation, pollution degree 2,
overvoltage category II . 86
Table C.10 – Double or reinforced insulation, pollution degree 1,

overvoltage category III . 86
Table C.11 – Double or reinforced insulation, pollution degree 2,
overvoltage category III . 87
Table C.12 – Test site correction factor for proving the clearance in air . 87
Table C.13 – Reduction of the pollution degree of internal environment through the
use of additional protection within the equipment . 87
Table E.1 – Range of conductor sizes to be accepted by terminals . 94
Table E.2 – Sizes of terminal studs or screws directly securing supply conductors . 94
Table G.1 – Severity of harm . 98
Table G.2 – Probability of harm . 98
Table G.3 – Risk category . 98

IEC 60255-27:2023 © IEC 2023 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 27: Product safety requirements

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
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6) All users should ensure that they have the latest edition of this publication.
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IEC 60255-27 has been prepared by IEC technical committee 95: Measuring relays and
protection equipment. It is an International Standard.
This third edition cancels and replaces the second edition published in 2013. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) conflicting statements removed;
b) scope clarified and statement added that all clauses of the standard are required not just
type tests;
c) terminology, definitions and documentation requirements aligned with IEC 60255-1;
d) alignment with IEC 61010-1, e.g. HLV definitions;
e) ingress protection clarified;
f) dielectric and impulse tests added to mechanical and environmental test requirements;

– 8 – IEC 60255-27:2023 © IEC 2023
g) insulation resistance requirements updated for alignment with other product safety
standards;
h) sample testing removed;
i) short time limiting thermal overload added;
j) resistance to mechanical stress added;
k) low-power voltage and current transformer ports added;
l) Annex C tables updated to align with base standards;
m) Annex D voltage dependent resistors and radio transmitters added;
n) Annex G for risk assessment added.
The text of this International Standard is based on the following documents:
Draft Report on voting
95/516/FDIS 95/526/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 60255 series, published under the general title Measuring relays
and protection equipment, 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.
IEC 60255-27:2023 © IEC 2023 – 9 –
INTRODUCTION
This document specifies the safety requirements that are generally applicable to all equipment
within its scope. These requirements may be supplemented by general product safety standards
and IEC 60664-1.
– 10 – IEC 60255-27:2023 © IEC 2023
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 27: Product safety requirements

1 Scope
This part of IEC 60255 specifies the product safety requirements for measuring relays and
protection equipment having a rated AC voltage up to 1 000 V, or a rated DC voltage up to
1 500 V. Above these limits, IEC 60664-1 is applicable for the determination of clearance,
creepage distance and withstand test voltage.
This document specifies essential safety requirements to minimize the risk of fire and hazards
caused by electric shock or injury to the user and property. This document specifies only product
safety requirements; functional performance of the equipment is not covered.
This document covers all the ways in which the equipment can be mounted and used in
cabinets, racks and panels. This document also applies to auxiliary devices such as shunts,
series resistors, transformers, auxiliary control panels, display devices, etc., that are used in
conjunction with measuring relays and protection equipment and are tested together.
It is possible that ancillary equipment such as network switches used in conjunction with
measuring relays and protection equipment needs to comply with additional safety
requirements.
This document does not specify the implementation of individual equipment, circuits and
components.
This document applies to equipment designed to be safe at least under the following
environmental conditions:
– indoor use;
– altitude up to 2 000 m, in accordance with IEC 60255-1;
– rated ambient temperature range, in accordance with IEC 60255-1;
– maximum external relative humidity, in accordance with IEC 60255-1;
– operating range of auxiliary energizing voltage in accordance with IEC 60255-1;
– applicable overvoltage categ
...