IEC 60255-27:2013

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

IEC 60255-27:2013 describes the product safety requirements for measuring relays and protection equipment having a rated a.c. voltage up to 1 000 V with a rated frequency up to 65 Hz, or a rated d.c. 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 standard details essential safety requirements to minimize the risk of fire and hazards caused by electric shock or injury to the user. This new edition includes the following significant technical changes with respect to the previous edition:
- the removal of tables and diagrams which are from other standards and referring instead directly to the source standard;
- all aspects of IEC 60255-5 have been covered and this standard will be withdrawn.

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

La CEI 60255-27:2013 détaille les exigences de sécurité produit pour les relais de mesure et dispositifs de protection dont la tension assignée n'excède pas 1 000 V en courant alternatif avec une fréquence assignée jusqu'à 65 Hz ou 1 500 V en courant continu. Au-delà de ces limites, la CEI 60664-1 est applicable pour déterminer les distances dans l'air, les lignes de fuites et la valeur des tensions de tenue d'essai. Cette norme détaille les exigences fondamentales de sécurité pour minimiser le risque d'incendie et les dangers de choc électrique ou de blessure pour l'utilisateur. Cette nouvelle édition inclut les modifications techniques majeures suivantes par rapport à l'édition précédente:
- le retrait des tableaux et diagrammes qui proviennent d'autres normes et référant plutôt directement à ces standards sources;
- tous les aspects de la CEI 60255-5 ont été couvert et cette norme sera retirée.

General Information

• Status: Published
• Publication Date: 14-Oct-2013
• ICS: 29.120.70 - Relays
• Technical Committee: TC 95 - Measuring relays and protection equipment
• Drafting Committee: MT 3 - TC 95/MT 3
• Current Stage: DELPUB - Deleted Publication
• Start Date: 07-Feb-2023
• Completion Date: 29-Jun-2018

Relations

• Replaces: IEC 60255-5:2000 - Electrical Relays - Part 5: Insulation coordination for measuring relays and protection equipment - Requirements and tests
• Effective Date: 05-Sep-2023
• Revises: IEC 60255-27:2005 - Measuring relays and protection equipment - Part 27: Product safety requirements
• Effective Date: 05-Sep-2023
• Revises: IEC 60255-27:2005/COR1:2007 - Corrigendum 1 - Measuring relays and protection equipment - Part 27: Product safety requirements
• Effective Date: 05-Sep-2023
• Revised: IEC 60255-27:2023 - Measuring relays and protection equipment - Part 27: Product safety requirements
• Effective Date: 05-Sep-2023

Overview

IEC 60255-27:2013 is an international standard issued by the International Electrotechnical Commission (IEC) focusing on product safety requirements for measuring relays and protection equipment. This standard applies specifically to devices with rated alternating current (AC) voltages up to 1,000 V and frequencies up to 65 Hz, or direct current (DC) voltages up to 1,500 V. It is essential for ensuring that these devices minimize risks associated with electrical hazards such as electric shock, fire, and user injury.

Notably, IEC 60255-27:2013 supersedes the older IEC 60255-5 standard by incorporating all its safety provisions and eliminating redundant tables and diagrams through direct reference to source standards. This updated edition is critical for manufacturers, testers, and users aiming to ensure compliance with stringent product safety regulations in electrical protection systems.

Key Topics

• Scope and Voltage Limits
  Details safety requirements for measuring relays and protection equipment within specified AC and DC voltage and frequency ranges. Standards like IEC 60664-1 apply for equipment exceeding these parameters.

• Protection Against Electric Shock
  Covers design considerations such as insulation, earthing, protective impedances, and discharge of capacitors to prevent user exposure to hazardous live parts.

• Mechanical Safety
  Addresses protection against mechanical hazards including stability, secure terminations, and the safe construction of moving parts and edges.

• Fire and Flammability Risks
  Includes rigorous requirements for minimizing fire hazards through selection of flame-retardant materials, fire enclosures, temperature limits, and circuitry to prevent ignition sources.

• Design and Testing Procedures
  Defines general design requirements for components, electrical connections, and climatic conditions. Provides guidance on type tests and routine testing ensuring compliance with safety criteria.

• Marking and Documentation
  Specifies essential marking practices for equipment identification, warnings, voltage ratings, and documentation requirements covering installation, commissioning, maintenance, and operation.

Applications

IEC 60255-27:2013 is vital for industries and professionals involved in the design, manufacture, installation, and maintenance of protective relay equipment:

• Power Generation and Distribution
  Ensures safety and reliable operation of relays that protect transformers, switchgear, and transmission lines.

• Industrial Automation
  Applies to protection equipment safeguarding industrial control systems operating within defined voltage and frequency limits.

• Electrical Equipment Manufacturing
  Guides manufacturers in meeting international safety standards for measuring and protection relays, facilitating market approval and regulatory compliance.

• Testing and Certification Labs
  Provides test protocols and criteria essential for validating the safety features of protection devices during product certification.

• Utilities and Infrastructure
  Supports safe deployment of relays preventing electrical faults and hazards, thus enhancing system reliability and user safety.

Related Standards

• IEC 60664-1: Specifies requirements for clearance, creepage distances, and withstand voltages for electrical equipment exceeding rated voltage values covered in IEC 60255-27.

• IEC 60255-5 (withdrawn): Previous standard on relay safety now fully incorporated into IEC 60255-27.

• IEC 60255 Series: Covers various aspects of measuring relays and protection equipment testing, performance, and application.

• IEC 61010: General safety requirements for electrical equipment for measurement, control, and laboratory use.

Conclusion

IEC 60255-27:2013 is a cornerstone standard ensuring the safe design, construction, and operation of measuring relays and protection equipment. Its comprehensive coverage on electric shock prevention, fire safety, mechanical hazards, and marking requirements provides a robust framework to protect users and equipment. Compliance with this standard is essential for electrical industry stakeholders committed to safety, reliability, and international conformity in relay protection technology.

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Keywords: IEC 60255-27, measuring relays safety, product safety requirements, protection equipment, electric shock prevention, fire hazard reduction, electrical standards, relay testing, IEC standards, electrical protection devices.

Standards Content (Sample)

IEC 60255-27 ®
Edition 2.0 2013-10
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

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IEC 60255-27 ®
Edition 2.0 2013-10
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

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XD
ICS 29.120.70 ISBN 978-2-8322-1161-8

– 2 – 60255-27 © IEC:2013
CONTENTS
FOREWORD . 6
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 10
3 Terms and definitions . 11
4 General safety requirements . 19
4.1 General . 19
4.2 Earthing requirements . 20
5 Protection against electric shock . 20
5.1 General . 20
5.1.1 Introductory remark . 20
5.1.2 Protection from contact with hazardous live parts . 20
5.1.3 Discharge of capacitors . 21
5.1.4 Protective impedance . 22
5.1.5 Accessible parts . 22
5.1.6 Bonding to the protective conductor . 25
5.1.7 Protective conductor connection . 26
5.1.8 High leakage current . 26
5.1.9 Solid insulation . 26
5.1.10 Clearances and creepage distances . 27
5.1.11 Functional earthing . 29
5.2 Single-fault conditions . 29
5.2.1 Testing in single-fault condition . 29
5.2.2 Application of single-fault condition . 30
5.2.3 Duration of tests . 31
5.2.4 Compliance . 32
6 Mechanical aspects . 33
6.1 Protection against mechanical hazards . 33
6.1.1 Stability . 33
6.1.2 Moving parts . 33
6.1.3 Edges and corners . 33
6.2 Mechanical requirements . 33
6.3 Mechanical security of terminations . 33
7 Flammability and resistance to fire . 33
7.1 General . 33
7.2 Rationale . 34
7.3 General hazards from overheating and fire . 36
7.3.1 Equipment temperature limits . 36
7.3.2 Hazardous gases and chemicals. 36
7.4 Minimization of fire risk . 37
7.4.1 General . 37
7.4.2 Eliminating or reducing the sources of ignition within the equipment . 37
7.5 Cabling and fusing. 37
7.6 Flammability of materials and components . 38
7.6.1 General . 38
7.6.2 Materials for components and other parts inside fire enclosures . 38

60255-27 © IEC:2013 – 3 –
7.6.3 Materials for fire enclosures . 39
7.6.4 Materials for components and other parts outside fire enclosures . 39
7.7 Fire ignition sources . 40
7.8 Conditions for a fire enclosure . 40
7.8.1 General . 40
7.8.2 Parts requiring a fire enclosure . 40
7.8.3 Parts not requiring a fire enclosure . 40
7.9 Requirements for primary circuits and circuits exceeding ELV limits . 41
7.10 Fire enclosures and flame barriers . 41
7.11 Assessment of the fire risk due to a single-fault condition . 43
7.11.1 Guidelines for maximum acceptable temperatures when subjecting a
circuit or component to a single-fault condition . 43
7.11.2 Temperature of windings under a normal condition or a single-fault
condition . 43
7.11.3 Compliance of equipment with requirements for protection against
the spread of fire . 43
7.12 Limited-energy circuit . 44
8 General and fundamental design requirements for safety . 45
8.1 Climatic conditions for safety . 45
8.2 Electrical connections . 45
8.3 Components . 45
8.3.1 General . 45
8.3.2 High-integrity part or component . 45
8.4 Connection to telecommunication networks . 46
8.5 Connection to other equipment . 46
8.6 Laser sources . 46
8.7 Explosion . 46
8.7.1 General . 46
8.7.2 Components at risk of explosion . 46
9 Marking, documentation and packaging . 47
9.1 Marking . 47
9.1.1 General . 47
9.1.2 Identification . 48
9.1.3 Auxiliary supplies, VT, CT, I/O (Input/Output) . 48
9.1.4 Fuses . 49
9.1.5 Measuring circuit terminals . 50
9.1.6 Terminals and operating devices . 50
9.1.7 Equipment protected by double or reinforced insulation . 51
9.1.8 Batteries . 51
9.1.9 Test voltage marking . 53
9.1.10 Warning markings . 53
9.1.11 Marking durability . 54
9.2 Documentation . 54
9.2.1 General . 54
9.2.2 Equipment ratings . 54
9.2.3 Equipment installation . 55
9.2.4 Equipment commissioning and maintenance . 55
9.2.5 Equipment operation . 56
9.3 Packaging . 56

– 4 – 60255-27 © IEC:2013
10 Type tests and routine tests. 56
10.1 General . 56
10.2 Safety type tests . 58
10.3 Routine testing or sample testing . 58
10.4 Conditions for testing . 58
10.5 Verification procedure . 58
10.6 Tests . 59
10.6.1 Climatic environmental tests . 59
10.6.2 Mechanical tests . 59
10.6.3 Clearances and creepage distances . 60
10.6.4 Safety-related electrical tests . 60
10.6.5 Electrical environment and flammability . 66
10.6.6 Reverse polarity and slow ramp test . 67
Annex A (normative) Isolation class requirements and example diagrams . 69
Annex B (normative) Rated impulse voltages . 77
Annex C (normative) Guidance for the determination of clearance, creepage distance
and withstand voltages . 78
Annex D (informative) Components . 88
Annex E (normative) External wiring terminations . 92
Annex F (informative) Examples of battery protection . 94
Bibliography . 95

Figure 1 – Flow chart showing requirements for protection against the spread of fire . 35
Figure 2 – Baffle . 42
Figure 3 – Location and extent of a flame barrier . 42
Figure 4 – Voltage ramp test . 68
Figure A.1 – Equipment with SELV input/output (I/O) . 73
Figure A.2 – Equipment with PELV input/output (I/O) . 74
Figure A.3 – Equipment with PEB input/output (I/O) . 75
Figure A.4 – Equipment with ELV input/output (I/O) . 76
Figure C.1 – Guidance for determination of clearances, creepage distances and
withstand voltages . 81
Figure F.1 – Non-rechargeable battery protection . 94
Figure F.2 – Rechargeable battery protection . 94

Table 1 – Current levels under normal conditions . 24
Table 2 – Charge or energy of capacitance levels under normal conditions . 24
Table 3 – Altitude multiplication factor . 28
Table 4 – Current levels in single-fault condition . 32
Table 5 – Maximum temperature under normal conditions and at an ambient
temperature of 40 °C . 36
Table 6 – Acceptable perforation in the bottom of an equipment case . 42
Table 7 – Insulation material of windings . 43
Table 8 – Limits of maximum available current . 44
Table 9 – Overcurrent protective device . 44

60255-27 © IEC:2013 – 5 –
Table 10 – Symbols . 52
Table 11 – Symbols for marking of test voltage(s) . 53
Table 12 – Overview of tests . 57
Table 13 – Guidance for routine and sample dielectric voltage testing for safety –
Informative . 63
Table 14 – AC test voltages . 64
Table A.1 – Circuit isolation class for product circuits/groups . 69
Table A.2 – Insulation requirement between any two circuits . 71
Table B.1 – Rated impulse voltages (waveform: 1,2/50 µs) . 77
Table C.1 – Functional insulation, pollution degree 1, overvoltage category I . 82
Table C.2 – Functional insulation, pollution degree 2, overvoltage category I . 83
Table C.3 – Functional, basic or supplementary insulation, pollution degree 1,
overvoltage category II . 83
Table C.4 – Functional, basic or supplementary insulation, pollution degree 2,
overvoltage category II . 84
Table C.5 – Functional, basic or supplementary insulation, pollution degree 1,
overvoltage category III . 84
Table C.6 – Functional, basic or supplementary insulation, pollution degree 2,
overvoltage category III . 85
Table C.7 – Double or reinforced insulation, pollution degree 1, overvoltage category
II . 85
Table C.8 – Double or reinforced insulation, pollution degree 2, overvoltage category
II . 86
Table C.9 – Double or reinforced insulation, pollution degree 1, overvoltage category
III . 86
Table C.10 – Double or reinforced insulation, pollution degree 2, overvoltage category
III . 87
Table C.11 – Test voltage multiplication factor for proving the clearance in air. 87
Table C.12 – 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 . 93
Table E.2 – Sizes of terminal studs or screws directly securing supply conductors . 93

– 6 – 60255-27 © IEC:2013
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
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60255-27 has been prepared by IEC technical committee 95:
Measuring relays and protection equipment.
This second edition cancels and replaces the first edition published in 2005. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition.
a) The removal of tables and diagrams which are from other standards and referring instead
directly to the source standard.
b) All aspects of IEC 60255-5 have been covered and this standard can be withdrawn.
c) Ambiguity within the standard has been removed.

60255-27 © IEC:2013 – 7 –
The text of this standard is based on the following documents:
FDIS Report on voting
95/316/FDIS 95/318/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 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 publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 8 – 60255-27 © IEC:2013
INTRODUCTION
In order to demonstrate that the equipment is safe, it was previously necessary to refer to
general safety standards such as IEC 61010-1 in addition to IEC 60664-1.
These general safety standards specify requirements for general product types or product
families in order to reduce the risk of fire, electric shock or injury to the user. The product
types do not include measuring relays and protection equipment. These standards also take
into account single-fault conditions.
Reference to all these various standards created confusion due to conflicting requirements,
for example, different clearances, creepage distances and test voltages etc., for the same
rated voltages.
The aim of this standard is:
• to remove confusion due to conflicting requirements between existing standards;
• to achieve a uniform approach throughout the international industry for measuring relays
and protection equipment.
This product safety standard for measuring relays and protection equipment takes the general
product safety standards and IEC 60664-1 as the base, defining those issues specific to
measuring relays and protection equipment.

60255-27 © IEC:2013 – 9 –
MEASURING RELAYS AND PROTECTION EQUIPMENT –

Part 27: Product safety requirements

1 Scope
This part of the IEC 60255 series describes the product safety requirements for measuring
relays and protection equipment having a rated a.c. voltage up to 1 000 V with a rated
frequency up to 65 Hz, or a rated d.c. 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 standard details essential safety requirements to minimize the risk of fire and hazards
caused by electric shock or injury to the user.
This standard does not cover the safety requirements of installations. It does cover all the
ways in which the equipment may be mounted and used in cubicles, racks and panels, and
also retesting. This standard also applies to auxiliary devices such as shunts, series resistors,
transformers, etc., that are used in conjunction with measuring relays and protection
equipment and are tested together.
Ancillary equipment used in conjunction with measuring relays and protection equipment may
need to comply with additional safety requirements.
This standard is intended to describe only product safety requirements; therefore, functional
performance of the equipment is not covered.
Functional safety requirements, including EMC functional safety, are not covered by this
standard. Functional safety risk analysis is not within the scope of this product safety
standard.
This standard does not specify the implementation of individual equipment, circuits and
components.
The object of this standard is to have a comprehensive standard that covers all aspects of
product safety and the related type and routine tests, for measuring relays and protection
equipment.
This standard 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;
– external operating temperature range, in accordance with IEC 60255-1;
– maximum external relative humidity 95 %, non-condensing, in accordance with
IEC 60255-1;
– supply fluctuations in accordance with IEC 60255-1;
– applicable supply overvoltage category;
– external pollution degree 1 and external pollution degree 2.
The equipment will normally be installed in a restricted access area within a power station,
substation or industrial/retail environment. The environmental conditions specified for the
equipment in IEC 60255-1 apply. This standard considers the normal environmental

– 10 – 60255-27 © IEC:2013
conditions of corrosion caused by humidity but does not cover corrosion by atmospheric
pollution.
It is assumed that access to the equipment during installation, maintenance, normal service
and decommissioning is restricted to users aware of working procedures necessary to ensure
safety.
This product safety standard takes precedence over general standards for matters of safety.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60050 (all parts), International Electrotechnical Vocabulary (available at

IEC 60085, Electrical insulation – Thermal evaluation and designation
IEC 60255-1, Measuring relays and protection equipment – Part 1: Common requirements
IEC 60255-21-1, Electrical relays – Part 21: Vibration, shock, bump and seismic tests on
measuring relays and protection equipment – Section One: Vibration tests (sinusoidal)
IEC 60255-21-2, Electrical relays – Part 21: Vibration, shock, bump and seismic tests on
measuring relays and protection equipment – Section Two: Shock and bump tests
IEC 60255-21-3, Electrical relays – Part 21: Vibration, shock, bump and seismic tests on
measuring relays and protection equipment – Section 3: Seismic tests
IEC 60255-26:2013, Measuring relays and protection equipment – Part 26: Electromagnetic
compatibility requirements
IEC 60352-1, Solderless connections – Part 1: Wrapped connections – General requirements,
test methods and practical guidance
IEC 60352-2, Solderless connections – Part 2: Crimped connections – General requirements,
test methods and practical guidance
IEC 60417, Graphical symbols for use on equipment. Available at: http://www.graphical-
symbols.info/equipment
IEC 60529:1989, Degrees of protection provided by enclosures (IP Code)
Amendment 1:1999
IEC 60664-1:2007, Insulation coordination for equipment within low-voltage systems – Part 1:
Principles, requirements and tests
IEC 60664-3:2003, Insulation coordination for equipment within low-voltage systems – Part 3:
Use of coating, potting or moulding for protection against pollution
Amendment 1:2010
_______________
1 There exists a consolidated Edition 2.1 (2001) that includes Edition 2.0 and its Amendment 1.

60255-27 © IEC:2013 – 11 –
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 flames – 50 W horizontal and vertical
flame test methods
IEC 60825-1, Safety of laser products – Part 1: Equipment classification and requirements
IEC 60990:1999, Methods of measurement of touch current and protective conductor current
IEC 61010-1:2010, Safety requirements for electrical equipment for measurement, control and
laboratory use – Part 1: General requirements
IEC 61032, Protection of persons and equipment by enclosures – Probes for verification
IEC 61140, Protection against electric shock – Common aspects for installation and
equipment
IEC 61180-1:1992, High-voltage test techniques for low-voltage equipment – Part 1:
Definitions, test and procedure requirements
IEC 61180-2, High-voltage test techniques for low-voltage equipment – Part 2: Test
equipment
IEC 62151, Safety of equipment electrically connected to a telecommunication network
ISO 7000, Graphical symbols for use on equipment – Index and synopsis. Available at:
http://www.graphical-symbols.info/equipment
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60664-1 and
IEC 60050-151-448 as well as the following apply.
3.1
accessible part
part which can be touched under normal conditions with a standard rigid or jointed test finger
as specified in 3.5.1 of IEC 61010-1:2010
Note 1 to entry: A communication circuit/network, which may be connected and taken outside the cubicle housing
the equipment, or on the front of the panel without the need to open a cover or flap to access it, should be
considered to be accessible, i.e. should it be PEB, PELV, SELV or equivalent.
[SOURCE: IEC 60050:1998, 442.01.15, modified – More details about the test finger and the
note to entry has been added.]
3.2
adjacent circuits
electric circuits which are separated from the considered circuit by the necessary basic or
double/reinforced insulation
Note 1 to entry: Circuits which are separated by more than double or reinforced insulation are not considered to
be adjacent.
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Edition 3.0 currently under preparation.

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3.3
ambient temperature
ambient air temperature
temperature, determined under prescribed conditions, of the air surrounding the complete
equipment
Note 1 to entry: For equipment installed inside an enclosure, it is the temperature of the air outside the enclosure.
Note 2 to entry: The ambient temperature is measured at half the distance from any neighbouring equipment, but
not more than 300 mm distance from the equipment case, at middle height of the equipment, protected from direct
heat radiation from the equipment.
[SOURCE: IEC 60050:2000, 441.11.13, modified – "switching device or fuse" has been
replaced by "equipment" and a second note to entry has been added.]
3.4
barrier
electrically protective barrier
part providing protection against direct contact from any usual direction of access
Note 1 to entry: Barriers may provide protection against the spread of fire (see Clause 7).
[SOURCE: IEC 60050:2004, 826.12.23]
3.5
basic insulation
insulation of hazardous live parts to provide basic protection
Note 1 to entry: This concept does not apply to insulation used exclusively for functional purposes.
[SOURCE: IEC 60050:2004, 826.12.14, modified – the word "which" is replaced by "to".]
3.6
bounding surface
outer surface of the equipment case, considered as though metal foil were pressed into
contact with accessible surfaces of insulating material
3.7
class I equipment
equipment with basic insulation as provision for basic protection against electric shock and
protective bonding as provision for fault protection, such that conductive parts on the outside
of the equipment case, cannot become live in the event of a failure of the basic insulation
3.8
class II equipment
equipment with
• basic insulation as provision for basic protection against electric shock, and
• supplementary insulation as provision for fault protection; or
• in which basic protection and fault protection are provided by reinforced insulation
Note 1 to entry: There should be no provision for a protective conductor or reliance upon installation conditions
for safety purposes. It is, however, possible to connect an earth conductor to Class II equipment for functional (for
example, EMC) purposes.
[SOURCE: IEC 60050:2008, 851.15.11, modified – The phrase "against electrical shock" and
a note to entry have been added while the reference to IEC 61140:2001, 7.3 has been
omitted.]
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3.9
class III equipment
equipment, or parts of equipment, in which protection against electric shock relies upon
supply from SELV or PELV circuits and in which hazardous voltages are not generated
3.10
clearance
shortest distance, measured in air, between two conductive parts, or between a conductive
part and the outer bounding surface of the equipment, whether conductive or not
3.11
CTI
comparative tracking index
numerical value of the maximum voltage in volts which a material can withstand without
tracking and without a persistent flame occurring under specified test conditions
[SOURCE: IEC 60050:2010, 212.11.59]
3.12
communication circuit/network
circuit/network for receiving and/or transmitting, digital or analogue signals
Note 1 to entry: It may communicate with other circuits via optical, magnetic or electromagnetic radiation means,
or metallic connections.
3.13
creepage distance
shortest distance along the surface of a solid insulating material between two conductive
parts, or between a conductive part and the bounding surface (accessible part) of the
equipment, measured along the surface of insulation
[SOURCE: IEC 60050:2001, 151.15.50, modified – The phrase "or between a conductive part
and the bounding surface (accessible part) of the equipment, measured along the surface of
insulation" has been added.]
3.14
direct contact
electrical contact of persons with live parts
[SOURCE: IEC 60050:2004, 826.03.05, modified – The words "or animals" have been
omitted.]
3.15
double insulation
insulation comprising both basic insulation and supplementary insulation
Note 1 to entry: Basic and supplementary insulation are separate, each designed for basic protection against
electric shock.
[SOURCE: IEC 60050:1998, 195.06.08, modified – The note to entry has been added.]
3.16
ELV
extra low voltage
SEE: Table A.1
3.17
enclosure
housing affording the type and degree of protection suitable for the intended application

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Note 1 to entry: Enclosures may provide protection against the spread of fire (see Clause 7).
[SOURCE: IEC 60050:1998, 195.02.35, modified – The note to entry has been added.]
3.18
accessible conductive part
exposed conductive part
conductive part of electrical equipment, which can be touched and which is not normally live,
but which can become live when basic insulation fails
Note 1 to entry: For equipment which is not enclosed, the frame, the fixing devices, etc., may form the accessible
conductive parts.
Note 2 to entry: For equipment which is enclosed, the conductive parts which are accessible when the equipment
is mounted in its normal position of use, including those of its fixing surface, form the accessible conductive parts.
[SOURCE: IEC 60050:1998, 422.01.21, modified – The notes to entry have been added.]
3.19
fire enclosure
part of the equipment intended to minimize the spread of fire or flames
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