IEC 60079-31:2022
(Main)Explosive atmospheres - Part 31: Equipment dust ignition protection by enclosure "t"
Explosive atmospheres - Part 31: Equipment dust ignition protection by enclosure "t"
IEC 60079-31:2022 is applicable to equipment protected by enclosure and surface temperature limitation for use in explosive dust atmospheres. It specifies requirements for design, construction and testing of Ex Equipment and Ex Components. This document supplements and modifies the general requirements of IEC 60079-0. Where a requirement of this document conflicts with a requirement of IEC 60079-0, the requirement of this document takes precedence. This document does not apply to dusts of explosives, which do not require atmospheric oxygen for combustion, or to pyrophoric substances. This document does not apply to Ex Equipment or Ex Components intended for use in underground parts of mines as well as those parts of surface installations of such mines endangered by firedamp and/or combustible dust. This document does not take account of any risk due to an emission of flammable or toxic gas from the dust. This third edition cancels and replaces the second edition published in 2013. This edition constitutes a technical revision. Main significant changes from the previous edition are:
1) Document has been restructured from edition 2
2) Fault current rating of interrupting contacts in clauses 4.3.1 and 4.4.1, major change type C1
3) Cells and batteries in clauses 4.3.5.1 and 4.4.5.1, major change type C2
4) Overload or malfunction condition for the determination of temperature class for "tb" converter fed rotating electric machines, major change type C3
5) Additional requirements for entry devices with dust ignition protection by enclosure "t", major change type C4
Information about the background of ‘Major Technical Changes’
C1 – Ex Equipment having Level of Protection "ta" shall be rated for connection to a circuit with a prospective short circuit current of not greater than 1.5 kA. For Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains connection and intended to interrupt fault current above 10kA, the equipment shall have a rated maximum short circuit withstand current, be tested according to 6.1.1.1, and be marked according to Clause 7.
C2 – For Ex Equipment having Level of Protection "ta" which contains a cell or battery, only a sealed cell or battery shall be used. For Ex Equipment having Level of Protection "tb" and "tc" where there are sparking contacts or hot surfaces, and which contains a cell or battery, only a sealed cell or battery shall be used.
C3 – Table 2 now includes malfunction conditions for temperature class determination of Level of Protection "tb" converter-fed electric machines.
C4 – Annex A added for entry devices with Type of Protection "t" including cable transit devices.
The contents of the corrigendum of October 2023 have been included in this copy.
Atmosphères explosives - Partie 31: Protection contre l’inflammation de poussières par enveloppe "t" relative à l’appareil
L'IEC 60079-31:2022 est applicable à l’appareil protégé par enveloppe et limitation de la température de surface, pour une utilisation en atmosphère de poussière explosive. Elle spécifie les exigences de conception, de construction et d'essai pour l’Appareil Ex et les Composants Ex. Le présent document complète et modifie les exigences générales de l’IEC 60079-0. Lorsqu'une exigence du présent document est en contradiction avec une exigence de l’IEC 60079-0, l’exigence du présent document prévaut. Le présent document ne s’applique pas aux poussières d’explosifs dont la combustion n’exige pas l’oxygène de l’air, ni aux substances pyrotechniques. Le présent document ne s’applique ni à l’Appareil Ex ni aux Composants Ex destinés à une utilisation dans les parties souterraines des mines, ni aux parties des installations de surface des mines où il existe des risques de grisou et/ou de poussières combustibles. Le présent document ne prend en compte aucun risque résultant d’une émission de gaz inflammable ou toxique provenant de la poussière.
Le contenu du corrigendum d'octobre 2023 a été pris en considération dans cet exemplaire.
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IEC 60079-31 ®
Edition 3.0 2022-01
REDLINE VERSION
INTERNATIONAL
STANDARD
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inside
Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure "t"
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IEC 60079-31 ®
Edition 3.0 2022-01
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure "t"
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.260.20 ISBN 978-2-8322-4590-3
– 2 – IEC 60079-31:2022 RLV © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 8
1 Scope . 9
2 Normative references . 9
3 Terms and definitions . 10
4 General . 10
4.1 Levels of protection. 10
4.2 Equipment groups and ingress protection . 11
4.3 Requirements for electrical equipment Ex Equipment with Level
of Protection "ta" . 11
4.3.1 Fault current . 11
4.3.2 Maximum surface temperature . 11
4.3.3 Overpressure .
4.3.3 Dust exclusion . 12
4.3.4 Protective Devices . 12
4.3.5 Supplementary internal enclosure . 13
4.3.6 Protection for arcing and sparking parts.
4.3.6 Cells and batteries . 13
4.4 Requirements for electrical equipment Ex Equipment with Level of Protection
"tb" and "tc" . 14
4.4.1 Fault current . 14
4.4.2 Maximum surface temperature . 14
4.4.3 Dust exclusion . 14
4.4.4 Thermal protection . 14
4.4.5 Cells and batteries . 15
4.4.6 External plug and socket connections for field wiring connection . 15
5 Construction . 15
5.1 Joints . 15
5.1.1 General . 15
5.1.2 Threaded joints . 15
5.1.3 Gaskets and seals . 16
5.1.4 Cemented joints. 16
5.1.5 Operating rods, spindles and shafts . 16
5.1.6 Windows . 17
5.2 Cable glands, cable transit devices and conduit sealing devices . 17
5.3 Entries . 17
5.3.1 Plain entries . 17
5.3.2 Threaded entries . 17
6 Verification and tests . 18
6.1 Type tests . 18
6.1.1 Type tests for dust exclusion by enclosures . 18
6.1.2 Thermal tests.
6.1.2 Tests to determine maximum surface temperature . 19
6.2 Routine tests . 20
7 Marking . 20
Annex A (normative) Supplementary requirements for entry devices . 21
A.1 General . 21
A.2 Construction requirements . 21
A.2.1 Cable glands, cable transit devices and conduit sealing devices . 21
A.2.2 Blanking elements and thread adapters . 21
A.3 Type tests . 21
A.3.1 Cable glands, cable transit devices and conduit sealing devices . 21
A.3.2 Blanking elements and thread adapters . 21
A.4 Marking . 21
Bibliography . 22
Table 1 – Level of Protection, equipment group and ingress protection (IP) relationship . 11
Table 2 – Overload or malfunction conditions for Level of Protection "tb" . 20
– 4 – IEC 60079-31:2022 RLV © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 31: Equipment dust ignition protection by enclosure "t"
FOREWORD
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rights. IEC shall not be held responsible for identifying any or all such patent rights.
This redline version of the official IEC Standard allows the user to identify the changes made to
the previous edition IEC 60603-7:2013. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.
International Standard IEC 60079-31 has been prepared by IEC technical committee 31:
Equipment for explosive atmospheres.
This third edition cancels and replaces the second edition published in 2013. This edition
constitutes a technical revision.
The significance of changes between IEC Standard, IEC 60079-31, Edition 3.0 and
IEC 60079-31, Edition 2.0, are as listed below:
Type
Minor and Major technical
Changes Clause Extension
editorial changes
changes
Document has been restructured from edition 2 Numerous X
Withstand of prospective short-circuit current 4.3.1 X
Fault current rating of interrupting contacts rated 4.4.1 and C1
greater than 10 kA for mains connected circuits 6.1.1.1
Thermal protective device can include a thermal 4.4.4.1 X
protective circuit with an appropriate sensor.
Cells and batteries 4.3.6 and C2
4.4.5
Joints employing parallel threads with an additional 5.1.2 X
seal or gasket are permitted to have less than five
threads.
Gasket joints that interlock (not a butt joint) and are
5.1.3 X
designed such that under the intended compression
no gap between the pieces exist so that an
uninterrupted periphery is formed, these joints do not
need to be permanently joined.
Overload or malfunction condition for the Table 2 C3
determination of temperature class for "tb" converter
fed rotating electric machines
Additional requirements for entry devices with dust Annex A C4
ignition protection by enclosure "t"
Thermal tests are relocated to IEC 60079-0. Formerly A1
6.1.2
NOTE The technical changes referred to include the significance of technical changes in the revised IEC Standard,
but they do not form an exhaustive list of all modifications from the previous version. More guidance may be found
by referring to the Redline Version of the standard.
Explanations:
A) Definitions
Minor and editorial changes
clarification
decrease of technical requirements
minor technical change
editorial corrections
These are changes which modify requirements in an editorial or a minor technical way. They
include changes of the wording to clarify technical requirements without any technical change,
or a reduction in level of existing requirement.
Extension addition of technical options
– 6 – IEC 60079-31:2022 RLV © IEC 2022
These are changes which add new or modify existing technical requirements, in a way that new
options are given, but without increasing requirements for equipment that was fully compliant
with the previous standard. Therefore, these will not have to be considered for products in
conformity with the preceding edition.
Major technical changes
addition of technical requirements
increase of technical requirements
These are changes to technical requirements (addition, increase of the level or removal) made
in a way that a product in conformity with the preceding edition will not always be able to fulfil
the requirements given in the later edition. These changes have to be considered for products
in conformity with the preceding edition. For these changes additional information is provided
in clause B) below.
NOTE These changes represent current technological knowledge. However, these changes should not normally
have an influence on equipment already placed on the market.
B) Information about the background of ‘Major Technical Changes’
C1 – For Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains
connection and intended to interrupt fault current above 10kA is tested according to 6.1.1.1,
and is marked according to Clause 7.
C2 – For Ex Equipment having Level of Protection "ta" only sealed primary cells or batteries
shall be used. A control device shall be provided to prevent overheating of the cell or battery
during normal operation, expected malfunctions, or rare malfunctions. The control device may
also be considered as a thermal protective device or overcurrent protective device. For Ex
Equipment having Level of Protection "tb" and "tc" only sealed cells or batteries shall be used.
A control device shall be provided to prevent overheating of the cell or battery during normal
operation or expected malfunctions (“tb”) or during normal operation (“tc”). The control device
may also be considered as a thermal protective device or overcurrent protective device.
C3 – Table 2 now includes malfunction conditions for temperature class determination of Level
of Protection "tb" converter-fed electric machines.
C4 – Annex A added for entry devices with Type of Protection "t" including cable transit devices.
A1 – Thermal tests formerly located in 6.1.2 are relocated to IEC 60079-0 for the 2017 and later
editions.
The text of this International Standard is based on the following documents:
Draft Report on voting
31/1595/FDIS 31/1606/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
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• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it
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contents. Users should therefore print this document using a colour printer.
The contents of the corrigendum 1 (2023-10) have been included in this copy.
– 8 – IEC 60079-31:2022 RLV © IEC 2022
INTRODUCTION
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that compliance with this document may involve the use of a patent. IEC takes no position
concerning the evidence, validity, and scope of this patent right.
The holder of this patent right has assured IEC that s/he is willing to negotiate licences under
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subject of patent rights other than those in the patent database. IEC shall not be held
responsible for identifying any or all such patent rights.
EXPLOSIVE ATMOSPHERES –
Part 31: Equipment dust ignition protection by enclosure "t"
1 Scope
This part of IEC 60079 is applicable to electrical equipment protected by enclosure and surface
temperature limitation for use in explosive dust atmospheres. It specifies requirements for
design, construction and testing of electrical equipment Ex Equipment and Ex Components.
This document supplements and modifies the general requirements of IEC 60079-0. Where a
requirement of this document conflicts with a requirement of IEC 60079-0, the requirement of
this document takes precedence.
This document does not apply to dusts of explosives, which do not require atmospheric oxygen
for combustion, or to pyrophoric substances.
This document does not apply to electrical equipment Ex Equipment or Ex Components
intended for use in underground parts of mines as well as those parts of surface installations
of such mines endangered by firedamp and/or combustible dust.
This document does not take account of any risk hazard due to an emission of flammable or
toxic gas from the dust.
Consideration of additional protective measures is required where the application of electrical
equipment is in atmospheres, which can contain combustible dust and explosive gas, whether
simultaneously or separately.
This document does not contain requirements for Ex Equipment used in areas where both
combustible dust and explosive gas atmospheres can occur, whether simultaneously or
separately. Requirements for explosive gas atmospheres can be found in other parts of the
IEC 60079 series. Guidance on Ex Equipment to be used where combustible dust and explosive
gas atmospheres occur simultaneously ("hybrid mixtures") can be found in IEC 60079-14.
Where the electrical equipment Ex Equipment has to meet other environmental conditions, for
example, protection against ingress of water and resistance to corrosion, additional measures
which do not adversely affect the integrity of the enclosure can be necessary. The measures
used should not adversely affect the integrity of the enclosure.
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 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60127 (all parts), Miniature fuses
IEC 60269 (all parts), Low-voltage fuses
IEC 60691, Thermal-links – Requirements and application guide
– 10 – IEC 60079-31:2022 RLV © IEC 2022
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60034-5, Rotating electrical machines – Part 5: Degrees of protection provided by the
integral design of rotating electrical machines (IP code) – Classification
ISO 965-1, ISO general-purpose metric screw threads – Tolerances – Part 1: Principles and
basic data
ANSI/ASME B1.20.1, Pipe threads, general purpose (inch)
ANSI/UL 248 (all parts), Standard for Low-Voltage Fuses
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0, as well as
the following terms and definitions apply.
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426.
ISO and IEC maintain terminological 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
dust ignition protection by enclosure "t"
Type of Protection for explosive dust atmospheres where electrical equipment is provided with
an enclosure providing dust ingress protection and a means to limit surface temperatures
3.2
joint
place where the corresponding surfaces of two parts of an enclosure, or the conjunction of
enclosures, come together
3.3
gasket
compressible element provided in a joint to provide a degree of protection against the ingress
of solid foreign objects and /or against ingress water/dust
4 General
4.1 Levels of protection
Type of Protection "t" is divided into three Levels of Protection based on the risk of the electrical
equipment Ex Equipment and Ex Components becoming an ignition source in an explosive dust
atmosphere. Electrical equipment Ex Equipment and Ex Components with dust ignition
protection by enclosure Type of Protection "t" shall be either one of the following:
• Level of Protection "ta" ( for EPL "Da"), or
• Level of Protection "tb" ( for EPL "Db"), or
• Level of Protection "tc" ( for EPL "Dc").
The construction and marking requirements apply to all electrical equipment, and in addition,
the requirements for "ta" as given in 4.3 and the requirements for "tb" and "tc" as given in 4.4.
The general requirements of Clause 4, the construction requirements of Clause 5, and the
marking requirements of Clause 7, apply to all Ex "t" Equipment and Ex "t" Components.
Failure modes as defined in the industrial standard for particular internal components affecting
the temperatures of the equipment shall be taken into account when considering applicable
fault conditions malfunctions.
When a resistor is used for current limiting, it shall not be considered to fail as a short circuit if
it is of metal film or of wire wound construction. The resistor shall be rated for the maximum
rated voltage of the equipment.
4.2 Equipment groups and ingress protection
The relationship between the Level of Protection, the group, and ingress protection required is
shown in Table 1.
Table 1 – Level of Protection, equipment group and
ingress protection (IP) relationship
Level of Protection Group IIIC Group IIIB Group IIIA
"ta" IP6X IP6X IP6X
"tb" IP6X IP6X IP5X
"tc" IP6X IP5X IP5X
Ingress protection shall be determined in accordance with
degree of protection (IP) of enclosures as specified in
IEC 60079-0 for level of protection “tb” and “tc”. For Level
of Protection “ta” the level of depression shall be
increased to at least 4 KPa for a period of least 8 h. Any
grease in the joints shall be removed before the IP test is
performed.
When IP5X is required, all enclosures including rotating
machines, shall satisfy the test and acceptance
requirements of IP5X, as specified in IEC 60529.
4.3 Requirements for electrical equipment Ex Equipment with Level of Protection "ta"
4.3.1 Fault current
For Level of Protection “ta”, the electrical equipment shall be rated for connection to a circuit
having a prospective short circuit current of not greater than 10 kA. Where the prospective short
circuit current withstand is less than 10 kA, it shall be marked according to Clause 7.
Ex Equipment shall be rated for connection to a circuit with a prospective short circuit current
of not greater than 1,5 kA and marked according to Clause 7.
4.3.2 Maximum surface temperature
These requirements for maximum surface temperature for “ta” electrical equipment modify and
supplement the requirements of IEC 60079-0.
The marked maximum surface temperature shall be measured on the external surfaces of the
enclosure and the surfaces of the internal components for electrical equipment with types of
protection “ta” in accordance with 6.1.2. The highest of the measured temperatures shall be the
basis for the maximum surface temperature marking.
The marked maximum surface temperature, as determined according to 6.1.2, shall be the
higher of the measured temperatures a) or b):
– 12 – IEC 60079-31:2022 RLV © IEC 2022
a) the surfaces of the internal components where there is no supplementary enclosure, or,
b) the temperature on the external surfaces of the supplementary enclosure.
NOTE 1 Partial rupture of the external enclosure is considered to be a potential rare malfunction and is considered
in the temperature determination for "ta" equipment.
NOTE 2 Level of Protection "ta" is typically applied for instrumentation where the maximum available short circuit
current is in the mA range. Due to the limited thermal dissipation available to Level of Protection "ta" equipment, the
maximum normal power dissipation is generally limited to a few watts.
4.3.3 Overpressure
A positive internal pressure of 4 kPa shall be applied to the enclosure in accordance with 6.1.1.3
prior to the dust exclusion test.
4.3.3 Dust exclusion
Dust exclusion by enclosure shall be carried out in accordance with 6.1.1.
4.3.4 Protective Devices
4.3.4.1 General
If the electrical equipment Ex Equipment is capable of exceeding the marked maximum surface
temperature as a result of the temperature test of 6.1.2 under expected malfunction or for rare
malfunction conditions, a protective device is required shall be used. The protective device may
be a thermal protective device according to 4.3.4.2 or an overcurrent protective device
according to 4.3.4.3 which may be directly integrated into the electrical equipment Ex
Equipment or be external to the electrical equipment Ex Equipment.
Where the external protective device is not provided by the manufacturer as part of the electrical
equipment Ex Equipment, the marking certificate number shall include the symbol "X" suffix in
accordance with the marking requirements of IEC 60079-0 and the Specific Conditions of Use
listed on the certificate shall detail the required ratings and performance characteristics of the
protective device. The protective device shall be capable of interrupting the maximum current
of the circuit in which it is installed. If the electrical equipment contains a cell or battery and a
control device is provided to prevent overheating of the cell or battery, the control device can
also be considered as a protective device, provided it also protects the complete electrical
equipment from exceeding the maximum surface temperature.
The response time of the thermal protective device or overcurrent protective device shall be
taken into account and be adequate for the necessary over temperature protection.
4.3.4.2 Thermal protective devices
When required by 4.3.4.1, the electrical equipment Ex Equipment shall be protected by one or
more integral thermal protective devices. Thermal protective devices shall not be of a self-
resettable type and shall be duplicated unless conforming to IEC 60127 series or IEC 60691,
in which case only one device is necessary.
Alternatively, if it can be demonstrated that an overcurrent protective device can be used to
provide thermal protection, such a device may be used. The overcurrent protective device used
in this way shall conform to IEC 60127 series and shall be rated at not more than 170 % of the
maximum rated current of the electrical equipment. When an overcurrent protective device is
not also used as a thermal protective device, it is permissible for the overcurrent protective
device to be located outside the enclosure of the electrical equipment. In this case, the marking
shall include the symbol "X" in accordance with IEC 60079-0 and the Specific Conditions of Use
shall detail the required overcurrent protective device.
The response time of the thermal protective devices should be taken into account and be
adequate for the necessary overtemperature protection.
NOTE When no such information exists, a common practice is to use 1.7 × rated current
4.3.4.3 Overcurrent protective devices
As an alternative to 4.3.4.2, if it is demonstrated that an overcurrent protective device or devices
provide thermal protection, such a device, or devices may be used. The overcurrent protective
device may be located outside the enclosure of the Ex Equipment. In this case, the certificate
number shall include the "X" suffix in accordance with the marking requirements of IEC 60079-0
and the Specific Conditions of Use listed on the certificate shall detail the performance
characteristics required by the overcurrent protective devices.
At least one of the protective devices shall be capable of interrupting the maximum available
short circuit current of the circuit in which it is installed.
Overcurrent protective devices shall be duplicated unless conforming to the IEC 60127 series,
IEC 60269 series or ANSI/UL 248 series, in which case only one device is necessary.
4.3.5 Supplementary internal enclosure
Where normally arcing and sparking parts are incorporated, these parts shall have a
supplementary enclosure inside the main enclosure and the maximum surface temperature of
that enclosure is used to determine the marked surface temperature of the equipment in
accordance with 4.3.2.
When hot internal components are enclosed in a supplementary enclosure, the maximum
surface temperature of that enclosure shall be used to determine the marked surface
temperature of the equipment in accordance with 4.3.2.
The supplementary enclosure shall:
• have a continuous operating temperature at least equal to the lower specified ambient
temperature and at least 20 K greater than the maximum service temperature for non-
metallic materials,
• undergo the resistance to impact test in accordance with 6.1.1.2 with no hot and cold
impact testing required,
• be IP6X in accordance with IEC 60529 following the resistance to impact without thermal
endurance to heat and cold.
These requirements replace the requirements of IEC 60079-0 for such an enclosure.
4.3.6 Protection for arcing and sparking parts
Where normally arcing and sparking parts are incorporated, these parts shall have a
supplementary enclosure inside the main enclosure. This supplementary enclosure shall meet
the requirements for a “tc” enclosure with the following exceptions and modifications:
• The tests for thermal endurance to heat and cold and resistance to light, specified in 6.1.1.1
are not applicable,
• A COT of at least equal to the lower specified ambient temperature and at least 20 K greater
than the maximum service temperature applies for non-metallic materials,
• The internal enclosure is not considered to have external surfaces and the resistance to
ultraviolet light and electrostatic requirements are not applicable,
• The requirements for threaded entries, hinges, and requirements for threaded fasteners are
not applicable,
• Resistance to impact test is performed in accordance with 6.1.1.2 with no hot and cold
impact testing required,
• Pressure test is not applied,
– 14 – IEC 60079-31:2022 RLV © IEC 2022
• IP6X is required.
4.3.6 Cells and batteries
Only sealed primary cells or batteries shall be used. A control device shall be provided to
prevent overheating of the cell or battery. The control device may also be considered as a
thermal protective device or overcurrent protective device as given in 4.3.4, provided it also
protects the complete Ex Equipment from exceeding the maximum surface temperature.
4.4 Requirements for electrical equipment Ex Equipment with Level of Protection "tb"
and "tc"
4.4.1 Fault current
Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains connection to
a circuit with a maximum available short circuit current above 10 kA, shall be tested in
accordance with 6.1.1.1, and be marked according to Clause 7.
NOTE Limiting the maximum available short circuit current could be achieved by several means including the use
of current limiting fuses or current limiting circuit breakers.
4.4.2 Maximum surface temperature
The marked maximum surface temperature shall be measured on the external surfaces of the
enclosure for electrical equipment Ex Equipment with types Level of Protection "tb" or "tc" in
accordance with 6.1.2 with no dust layer on the external surfaces under normal operating
conditions.
4.4.3 Dust exclusion
Dust exclusion by enclosure shall be carried out in accordance with 6.1.1.
4.4.4 Thermal protection
4.4.4.1 General
For Level of Protection "tb", if the Ex Equipment is capable of exceeding the marked maximum
surface temperature as a result of the temperature test of 6.1.2, a protective device shall be
used. The protective device may be a thermal protective device according to 4.4.4.2 or an
overcurrent protective device according to 4.4.4.3 which may be directly integrated into the Ex
Equipment or be external to the Ex Equipment.
Where the external protective device is not provided by the manufacturer as part of the Ex
Equipment, the certificate number shall include the "X" suffix in accordance with the marking
requirements of IEC 60079-0 and the Specific Conditions of Use shall detail the required ratings
and characteristics of the protective device. The protective device shall be capable of
interrupting the maximum current of the circuit in which it is installed.
4.4.4.2 Thermal protective devices
When required by 4.4.4.1 the Ex Equipment shall be protected by one or more integral thermal
protective devices. Thermal protective devices shall not be of a self-resettable type and shall
be duplicated unless conforming to the IEC 60691 series, in which case only one device is
necessary.
The response time of the thermal protective devices shall be adequate for the necessary over
temperature protection.
4.4.4.3 Overcurrent protective devices
As an alternative to 4.4.4.2, if it is demonstrated that an overcurrent protective device can be
used to provide thermal protection, such a device may be used. It is permissible for the
overcurrent protective device to be located outside the enclosure of the Ex Equipment. In this
case, the certificate number shall include the "X" suffix in accordance with the marking
requirements of IEC 60079-0 and the Specific Conditions of Use shall detail the required
overcurrent protective device. The protective device shall be capable of interrupting the
maximum current of the circuit in which it is installed.
Overcurrent protective devices shall be duplicated unless conforming to the IEC 60127 series,
IEC 60269 series, or ANSI/UL 248 series, in which case only one device is necessary.
4.4.5 Cells and batteries
Only sealed cells or batteries shall be used. A control device shall be provided to prevent
overheating of the cell or battery. The control device may also be considered as a thermal
protective device or overcurrent protective device as given in 4.4.4.2 or 4.4.4.3, provided it also
protects the complete Ex Equipment from exceeding the maximum surface temperature.
4.4.6 External plug and socket connections for field wiring connection
External plug and socket connections for field wiring connection shall meet one of the following
requirements:
a) they shall be interlocked mechanically or electrically, or otherwise designed so that they
cannot be separated when the contacts are energized and the contacts cannot be energized
when plug and socket are separated; or
b) plug and socket connectors for Level of Protection "tc", shall be fixed together by a
mechanical means other than the friction between the plug and socket; and for Level of
Protection "tb", shall be fixed together by means of special fasteners.
5 Construction
5.1 Joints
5.1.1 General
All joints in the structure of the enclosure, whether permanently closed or designed to be
opened from time to time, shall fit closely together within the tolerances specified in the
documentation. They shall be effectively sealed against the ingress of dust and shall comply
with the following particular requirements in 5.1.2 to 5.1.6 and be subjected to the test of 6.1.1.
The use of grease alone to maintain the integrity of the seal is not considered to satisfy this
requirement.
5.1.2 Threaded joints
• The number of engaged threads for all threaded joints, employing parallel threads without
an additional seal or gasket shall be not less than five threads and with a tolerance quality
of medium or fine according to ISO 965-1. Tapered threaded joints without an additional
seal or gasket shall engage no less than 3½ threads.
• Hinges shall not be used as a means of maintaining a seal unless:
– correct compression of the gasket is achieved without causing undue movement, stress
or distortion to the gasket; and
– they are manufactured from materials that would not affect the correct function of the
sealing means.
– 16 – IEC 60079-31:2022 RLV © IEC 2022
The number of engaged threads for all threaded joints, employing parallel threads without an
additional seal or gasket shall be at least five threads and with a tolerance quality of medium
or fine according to ISO 965-1.
Joints employing parallel threads with an additional seal or gasket are permitted to have less
than five threads. In this case, a specific tolerance class is not required.
Tapered threaded joints without an additional seal or gasket shall engage no less than 3½
threads.
Where necessary, a means shall be provided to facilitate correct alignment of mating parts.
5.1.3 Gaskets and seals
Gaskets under compression in joints may be used to ensure the effectiveness of the enclosure
sealing.
All gaskets and seals shall be of one-piece continuous construction, that is with an uninterrupted
periphery.
One-piece construction also includes gaskets and seals that have been permanently joined to
form an uninterrupted periphery while maintaining the mechanical properties of the gasket or
seal material. Alternately, gasket joints that interlock (not a butt joint) and are designed such
that under the intended compression no gap between the pieces exist so that an uninterrupted
periphery is formed, these joints do not need to be permanently joined.
Unless all gaskets are secured to one face of the mating surface, either by adhesive or being
mechanically secured, the design of the enclosure shall be such that gaskets are correctly
positioned. Except for a slight amount of lubricant necessary for assembly or an adhesive
material on one side of the mating surfaces, joints using gaskets shall not be supplemented by
the application of a sealant material.
A flexible seal, for example a bellows, shall be such that it is not over-stressed at any point and
shall be protected from external mechanical damage and secured at each end by mechanical
means.
Hinges shall not be used as a means of maintaining a seal unless the correct compression of
the gasket is achieved without causing undue movement, stress or distortion to the gasket. The
hinges shall be manufactured from materials that would not affect the correct function of the
sealing means.
These requirements do not apply to internal seals of cable glands.
5.1.4 Cemented joints
Cemented joints shall not be used on mating parts which need to be removed to gain access to
field wiring connections or in-service adjusting facilities.
5.1.5 Operating rods, spindles and shafts
Openings in enclosures for rods, spindles or shafts shall have means to inhibit the ingress of
dust, other than only grease or compound, both when the spindles, rods or shafts are in motion
and when they are at rest.
IEC 600
...
IEC 60079-31 ®
Edition 3.0 2022-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure "t"
Atmosphères explosives –
Partie 31: Protection contre l’inflammation de poussières par enveloppe "t"
relative à l’appareil
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IEC 60079-31 ®
Edition 3.0 2022-01
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure "t"
Atmosphères explosives –
Partie 31: Protection contre l’inflammation de poussières par enveloppe "t"
relative à l’appareil
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.260.20 ISBN 978-2-8322-1062-5
– 2 – IEC 60079-31:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 General . 9
4.1 Levels of protection. 9
4.2 Equipment groups and ingress protection . 9
4.3 Requirements for Ex Equipment with Level of Protection "ta" . 10
4.3.1 Fault current . 10
4.3.2 Maximum surface temperature . 10
4.3.3 Dust exclusion . 10
4.3.4 Protective Devices . 10
4.3.5 Supplementary internal enclosure . 11
4.3.6 Cells and batteries . 11
4.4 Requirements for Ex Equipment with Level of Protection "tb" and "tc" . 11
4.4.1 Fault current . 11
4.4.2 Maximum surface temperature . 12
4.4.3 Dust exclusion . 12
4.4.4 Thermal protection . 12
4.4.5 Cells and batteries . 12
4.4.6 External plug and socket connections for field wiring connection . 13
5 Construction . 13
5.1 Joints . 13
5.1.1 General . 13
5.1.2 Threaded joints . 13
5.1.3 Gaskets and seals . 13
5.1.4 Cemented joints. 14
5.1.5 Operating rods, spindles and shafts . 14
5.1.6 Windows . 14
5.2 Cable glands, cable transit devices and conduit sealing devices . 14
5.3 Entries . 14
5.3.1 Plain entries . 14
5.3.2 Threaded entries . 14
6 Verification and tests . 15
6.1 Type tests . 15
6.1.1 Type tests for dust exclusion by enclosures . 15
6.1.2 Tests to determine maximum surface temperature . 16
6.2 Routine tests . 17
7 Marking . 17
Annex A (normative) Supplementary requirements for entry devices . 18
A.1 General . 18
A.2 Construction requirements . 18
A.2.1 Cable glands, cable transit devices and conduit sealing devices . 18
A.2.2 Blanking elements and thread adapters . 18
A.3 Type tests . 18
A.3.1 Cable glands, cable transit devices and conduit sealing devices . 18
A.3.2 Blanking elements and thread adapters . 18
A.4 Marking . 18
Bibliography . 19
Table 1 – Level of Protection, equipment group and ingress protection (IP) relationship . 10
Table 2 – Overload or malfunction conditions for Level of Protection "tb" . 16
– 4 – IEC 60079-31:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 31: Equipment dust ignition protection by enclosure "t"
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.
International Standard IEC 60079-31 has been prepared by IEC technical committee 31:
Equipment for explosive atmospheres.
This third edition cancels and replaces the second edition published in 2013. This edition
constitutes a technical revision.
The significance of changes between IEC Standard, IEC 60079-31, Edition 3.0 and
IEC 60079-31, Edition 2.0, are as listed below:
Type
Changes Clause Minor and Extension Major technical
editorial changes
changes
Document has been restructured from edition 2 Numerous X
Withstand of prospective short-circuit current 4.3.1 X
Fault current rating of interrupting contacts rated 4.4.1 and C1
greater than 10 kA for mains connected circuits 6.1.1.1
Thermal protective device can include a thermal 4.4.4.1 X
protective circuit with an appropriate sensor.
Cells and batteries 4.3.6 and C2
4.4.5
Joints employing parallel threads with an additional 5.1.2 X
seal or gasket are permitted to have less than five
threads.
Gasket joints that interlock (not a butt joint) and are
5.1.3 X
designed such that under the intended compression
no gap between the pieces exist so that an
uninterrupted periphery is formed, these joints do not
need to be permanently joined.
Overload or malfunction condition for the Table 2 C3
determination of temperature class for "tb" converter
fed rotating electric machines
Additional requirements for entry devices with dust Annex A C4
ignition protection by enclosure "t"
Thermal tests are relocated to IEC 60079-0. Formerly A1
6.1.2
NOTE The technical changes referred to include the significance of technical changes in the revised IEC Standard,
but they do not form an exhaustive list of all modifications from the previous version. More guidance may be found
by referring to the Redline Version of the standard.
Explanations:
A) Definitions
Minor and editorial changes
clarification
decrease of technical requirements
minor technical change
editorial corrections
These are changes which modify requirements in an editorial or a minor technical way. They
include changes of the wording to clarify technical requirements without any technical change,
or a reduction in level of existing requirement.
Extension addition of technical options
These are changes which add new or modify existing technical requirements, in a way that new
options are given, but without increasing requirements for equipment that was fully compliant
with the previous standard. Therefore, these will not have to be considered for products in
conformity with the preceding edition.
– 6 – IEC 60079-31:2022 © IEC 2022
Major technical changes
addition of technical requirements
increase of technical requirements
These are changes to technical requirements (addition, increase of the level or removal) made
in a way that a product in conformity with the preceding edition will not always be able to fulfil
the requirements given in the later edition. These changes have to be considered for products
in conformity with the preceding edition. For these changes additional information is provided
in clause B) below.
NOTE These changes represent current technological knowledge. However, these changes should not normally
have an influence on equipment already placed on the market.
B) Information about the background of ‘Major Technical Changes’
C1 – For Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains
connection and intended to interrupt fault current above 10kA is tested according to 6.1.1.1,
and is marked according to Clause 7.
C2 – For Ex Equipment having Level of Protection "ta" which contains a cell or battery, only a
sealed cell or battery shall be used. For Ex Equipment having Level of Protection "tb" and "tc"
where there are sparking contacts or hot surfaces, and which contains a cell or battery, only a
sealed cell or battery shall be used.
C3 – Table 2 now includes malfunction conditions for temperature class determination of Level
of Protection "tb" converter-fed electric machines.
C4 – Annex A added for entry devices with Type of Protection "t" including cable transit devices.
A1 – Thermal tests formerly located in 6.1.2 are relocated to IEC 60079-0 for the 2017 and later
editions.
The text of this International Standard is based on the following documents:
Draft Report on voting
31/1595/FDIS 31/1606/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.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://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.
INTRODUCTION
The International Electrotechnical Commission (IEC) draws attention to the fact that it is claimed
that compliance with this document may involve the use of a patent. IEC takes no position
concerning the evidence, validity, and scope of this patent right.
The holder of this patent right has assured IEC that s/he is willing to negotiate licences under
reasonable and non-discriminatory terms and conditions with applicants throughout the world.
In this respect, the statement of the holder of this patent right is registered with IEC. Information
may be obtained from the patent database available at http://patents.iec.ch.
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those in the patent database. IEC shall not be held
responsible for identifying any or all such patent rights.
– 8 – IEC 60079-31:2022 © IEC 2022
EXPLOSIVE ATMOSPHERES –
Part 31: Equipment dust ignition protection by enclosure "t"
1 Scope
This part of IEC 60079 is applicable to equipment protected by enclosure and surface
temperature limitation for use in explosive dust atmospheres. It specifies requirements for
design, construction and testing of Ex Equipment and Ex Components.
This document supplements and modifies the general requirements of IEC 60079-0. Where a
requirement of this document conflicts with a requirement of IEC 60079-0, the requirement of
this document takes precedence.
This document does not apply to dusts of explosives, which do not require atmospheric oxygen
for combustion, or to pyrophoric substances.
This document does not apply to Ex Equipment or Ex Components intended for use in
underground parts of mines as well as those parts of surface installations of such mines
endangered by firedamp and/or combustible dust.
This document does not take account of any hazard due to an emission of flammable or toxic
gas from the dust.
This document does not contain requirements for Ex Equipment used in areas where both
combustible dust and explosive gas atmospheres can occur, whether simultaneously or
separately. Requirements for explosive gas atmospheres can be found in other parts of the
IEC 60079 series. Guidance on Ex Equipment to be used where combustible dust and explosive
gas atmospheres occur simultaneously ("hybrid mixtures") can be found in IEC 60079-14.
Where the Ex Equipment has to meet other environmental conditions, for example, protection
against ingress of water and resistance to corrosion, additional measures which do not
adversely affect the integrity of the enclosure can be necessary.
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 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60127 (all parts), Miniature fuses
IEC 60269 (all parts), Low-voltage fuses
IEC 60691, Thermal-links – Requirements and application guide
IEC 60529, Degrees of protection provided by enclosures (IP Code)
IEC 60034-5, Rotating electrical machines – Part 5: Degrees of protection provided by the
integral design of rotating electrical machines (IP code) – Classification
ISO 965-1, ISO general-purpose metric screw threads – Tolerances – Part 1: Principles and
basic data
ANSI/ASME B1.20.1, Pipe threads, general purpose (inch)
ANSI/UL 248 (all parts), Standard for Low-Voltage Fuses
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological 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
dust ignition protection by enclosure "t"
Type of Protection for explosive dust atmospheres where equipment is provided with an
enclosure providing dust ingress protection and a means to limit surface temperatures
4 General
4.1 Levels of protection
Type of Protection "t" is divided into three Levels of Protection based on the risk of the Ex
Equipment and Ex Components becoming an ignition source in an explosive dust atmosphere.
Ex Equipment and Ex Components with Type of Protection "t" shall be one of the following:
• Level of Protection "ta" for EPL "Da";
• Level of Protection "tb" for EPL "Db";
• Level of Protection "tc" for EPL "Dc".
The general requirements of Clause 4, the construction requirements of Clause 5, and the
marking requirements of Clause 7, apply to all Ex "t" Equipment and Ex "t" Components.
Failure modes as defined in the industrial standard for particular internal components affecting
the temperatures of the equipment shall be taken into account when considering applicable
malfunctions.
When a resistor is used for current limiting, it shall not be considered to fail as a short circuit if
it is of metal film or of wire wound construction. The resistor shall be rated for the maximum
rated voltage of the equipment.
4.2 Equipment groups and ingress protection
The relationship between the Level of Protection, the group, and ingress protection required is
shown in Table 1.
– 10 – IEC 60079-31:2022 © IEC 2022
Table 1 – Level of Protection, equipment group and
ingress protection (IP) relationship
Level of Protection Group IIIC Group IIIB Group IIIA
"ta" IP6X IP6X IP6X
"tb" IP6X IP6X IP5X
"tc" IP6X IP5X IP5X
4.3 Requirements for Ex Equipment with Level of Protection "ta"
4.3.1 Fault current
Ex Equipment shall be rated for connection to a circuit with a prospective short circuit current
of not greater than 1,5 kA and marked according to Clause 7.
4.3.2 Maximum surface temperature
These requirements modify and supplement the requirements of IEC 60079-0.
The marked maximum surface temperature, as determined according to 6.1.2, shall be the
higher of the measured temperatures a) or b):
a) the surfaces of the internal components where there is no supplementary enclosure, or,
b) the temperature on the external surfaces of the supplementary enclosure.
NOTE 1 Partial rupture of the external enclosure is considered to be a potential rare malfunction and is considered
in the temperature determination for "ta" equipment.
NOTE 2 Level of Protection "ta" is typically applied for instrumentation where the maximum available short circuit
current is in the mA range. Due to the limited thermal dissipation available to Level of Protection "ta" equipment, the
maximum normal power dissipation is generally limited to a few watts.
4.3.3 Dust exclusion
Dust exclusion by enclosure shall be carried out in accordance with 6.1.1.
4.3.4 Protective Devices
4.3.4.1 General
If the Ex Equipment is capable of exceeding the marked maximum surface temperature as a
result of the temperature test of 6.1.2 under expected malfunction or for rare malfunction
conditions, a protective device shall be used. The protective device may be a thermal protective
device according to 4.3.4.2 or an overcurrent protective device according to 4.3.4.3 which may
be directly integrated into the Ex Equipment or be external to the Ex Equipment.
Where the external protective device is not provided by the manufacturer as part of the Ex
Equipment, the certificate number shall include the "X" suffix in accordance with the marking
requirements of IEC 60079-0 and the Specific Conditions of Use listed on the certificate shall
detail the required ratings and performance characteristics of the protective device. The
protective device shall be capable of interrupting the maximum current of the circuit in which it
is installed.
The response time of the thermal protective device or overcurrent protective device shall be
taken into account and be adequate for the necessary over temperature protection.
4.3.4.2 Thermal protective devices
When required by 4.3.4.1, the Ex Equipment shall be protected by one or more integral thermal
protective devices. Thermal protective devices shall not be of a self-resettable type and shall
be duplicated unless conforming to IEC 60691, in which case only one device is necessary.
4.3.4.3 Overcurrent protective devices
As an alternative to 4.3.4.2, if it is demonstrated that an overcurrent protective device or devices
provide thermal protection, such a device, or devices may be used. The overcurrent protective
device may be located outside the enclosure of the Ex Equipment. In this case, the certificate
number shall include the "X" suffix in accordance with the marking requirements of IEC 60079-0
and the Specific Conditions of Use listed on the certificate shall detail the performance
characteristics required by the overcurrent protective devices.
At least one of the protective devices shall be capable of interrupting the maximum available
short circuit current of the circuit in which it is installed.
Overcurrent protective devices shall be duplicated unless conforming to the IEC 60127 series,
IEC 60269 series or ANSI/UL 248 series, in which case only one device is necessary.
4.3.5 Supplementary internal enclosure
Where normally arcing and sparking parts are incorporated, these parts shall have a
supplementary enclosure inside the main enclosure and the maximum surface temperature of
that enclosure is used to determine the marked surface temperature of the equipment in
accordance with 4.3.2.
When hot internal components are enclosed in a supplementary enclosure, the maximum
surface temperature of that enclosure shall be used to determine the marked surface
temperature of the equipment in accordance with 4.3.2.
The supplementary enclosure shall:
• have a continuous operating temperature at least equal to the lower specified ambient
temperature and at least 20 K greater than the maximum service temperature for non-
metallic materials,
• undergo the resistance to impact test in accordance with 6.1.1.2 with no hot and cold
impact testing required,
• be IP6X in accordance with IEC 60529 following the resistance to impact without thermal
endurance to heat and cold.
These requirements replace the requirements of IEC 60079-0 for such an enclosure.
4.3.6 Cells and batteries
Only sealed primary cells or batteries shall be used. A control device shall be provided to
prevent overheating of the cell or battery. The control device may also be considered as a
thermal protective device or overcurrent protective device as given in 4.3.4, provided it also
protects the complete Ex Equipment from exceeding the maximum surface temperature.
4.4 Requirements for Ex Equipment with Level of Protection "tb" and "tc"
4.4.1 Fault current
Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains connection to
a circuit with a maximum available short circuit current above 10 kA, shall be tested in
accordance with 6.1.1.1, and be marked according to Clause 7.
– 12 – IEC 60079-31:2022 © IEC 2022
NOTE Limiting the maximum available short circuit current could be achieved by several means including the use
of current limiting fuses or current limiting circuit breakers.
4.4.2 Maximum surface temperature
The marked maximum surface temperature shall be measured on the external surfaces of the
enclosure for Ex Equipment with Level of Protection "tb" or "tc" in accordance with 6.1.2.
4.4.3 Dust exclusion
Dust exclusion by enclosure shall be carried out in accordance with 6.1.1.
4.4.4 Thermal protection
4.4.4.1 General
For Level of Protection "tb", if the Ex Equipment is capable of exceeding the marked maximum
surface temperature as a result of the temperature test of 6.1.2, a protective device shall be
used. The protective device may be a thermal protective device according to 4.4.4.2 or an
overcurrent protective device according to 4.4.4.3 which may be directly integrated into the Ex
Equipment or be external to the Ex Equipment.
Where the external protective device is not provided by the manufacturer as part of the Ex
Equipment, the certificate number shall include the "X" suffix in accordance with the marking
requirements of IEC 60079-0 and the Specific Conditions of Use shall detail the required ratings
and characteristics of the protective device. The protective device shall be capable of
interrupting the maximum current of the circuit in which it is installed.
4.4.4.2 Thermal protective devices
When required by 4.4.4.1 the Ex Equipment shall be protected by one or more integral thermal
protective devices. Thermal protective devices shall not be of a self-resettable type and shall
be duplicated unless conforming to the IEC 60691 series, in which case only one device is
necessary.
The response time of the thermal protective devices shall be adequate for the necessary over
temperature protection.
4.4.4.3 Overcurrent protective devices
As an alternative to 4.4.4.2, if it is demonstrated that an overcurrent protective device can be
used to provide thermal protection, such a device may be used. It is permissible for the
overcurrent protective device to be located outside the enclosure of the Ex Equipment. In this
case, the certificate number shall include the "X" suffix in accordance with the marking
requirements of IEC 60079-0 and the Specific Conditions of Use shall detail the required
overcurrent protective device. The protective device shall be capable of interrupting the
maximum current of the circuit in which it is installed.
Overcurrent protective devices shall be duplicated unless conforming to the IEC 60127 series,
IEC 60269 series, or ANSI/UL 248 series, in which case only one device is necessary.
4.4.5 Cells and batteries
Only sealed cells or batteries shall be used. A control device shall be provided to prevent
overheating of the cell or battery. The control device may also be considered as a thermal
protective device or overcurrent protective device as given in 4.4.4.2 or 4.4.4.3, provided it also
protects the complete Ex Equipment from exceeding the maximum surface temperature.
4.4.6 External plug and socket connections for field wiring connection
External plug and socket connections for field wiring connection shall meet one of the following
requirements:
a) they shall be interlocked mechanically or electrically, or otherwise designed so that they
cannot be separated when the contacts are energized and the contacts cannot be energized
when plug and socket are separated; or
b) plug and socket connectors for Level of Protection "tc", shall be fixed together by a
mechanical means other than the friction between the plug and socket; and for Level of
Protection "tb", shall be fixed together by means of special fasteners.
5 Construction
5.1 Joints
5.1.1 General
All joints in the structure of the enclosure, whether permanently closed or designed to be
opened from time to time, shall be effectively sealed against the ingress of dust and shall comply
with the particular requirements in 5.1.2 to 5.1.6 and be subjected to the test of 6.1.1.
The use of grease alone to maintain the integrity of the seal is not considered to satisfy this
requirement.
5.1.2 Threaded joints
The number of engaged threads for all threaded joints, employing parallel threads without an
additional seal or gasket shall be at least five threads and with a tolerance quality of medium
or fine according to ISO 965-1.
Joints employing parallel threads with an additional seal or gasket are permitted to have less
than five threads. In this case, a specific tolerance class is not required.
Tapered threaded joints without an additional seal or gasket shall engage no less than 3½
threads.
Where necessary, a means shall be provided to facilitate correct alignment of mating parts.
5.1.3 Gaskets and seals
Gaskets under compression in joints may be used to ensure the effectiveness of the enclosure
sealing.
All gaskets and seals shall be of one-piece continuous construction, that is with an uninterrupted
periphery.
One-piece construction also includes gaskets and seals that have been permanently joined to
form an uninterrupted periphery while maintaining the mechanical properties of the gasket or
seal material. Alternately, gasket joints that interlock (not a butt joint) and are designed such
that under the intended compression no gap between the pieces exist so that an uninterrupted
periphery is formed, these joints do not need to be permanently joined.
Unless all gaskets are secured to one face of the mating surface, either by adhesive or being
mechanically secured, the design of the enclosure shall be such that gaskets are correctly
positioned. Except for a slight amount of lubricant necessary for assembly or an adhesive
material on one side of the mating surfaces, joints using gaskets shall not be supplemented by
the application of a sealant material.
– 14 – IEC 60079-31:2022 © IEC 2022
A flexible seal, for example a bellows, shall be such that it is not over-stressed at any point and
shall be protected from external mechanical damage and secured at each end by mechanical
means.
Hinges shall not be used as a means of maintaining a seal unless the correct compression of
the gasket is achieved without causing undue movement, stress or distortion to the gasket. The
hinges shall be manufactured from materials that would not affect the correct function of the
sealing means.
These requirements do not apply to internal seals of cable glands.
5.1.4 Cemented joints
Cemented joints shall not be used on mating parts which need to be removed to gain access to
field wiring connections or in-service adjusting facilities.
5.1.5 Operating rods, spindles and shafts
Openings in enclosures for rods, spindles or shafts shall have means to inhibit the ingress of
dust, other than only grease or compound, both when the spindles, rods or shafts are in motion
and when they are at rest.
5.1.6 Windows
5.1.6.1 Windows employing a cemented joint
A window design employing a cemented joint shall be such that it is cemented either directly
into the wall of the enclosure so as to form with the latter an inseparable assembly, or into a
frame such that the assembly can be replaced as a unit.
5.1.6.2 Windows employing a gasket joint
A window design employing a gasket for dust exclusion shall be such that it is mounted directly
in the wall or cover of the enclosure. Alternatively, a window employing a gasket may be
mounted in a separate frame such that the assembly can be replaced as a unit.
5.2 Cable glands, cable transit devices and conduit sealing devices
Cable glands, cable transit devices, and conduit sealing devices, whether integral or separate,
shall meet the requirements of IEC 60079-0 and Annex A.
5.3 Entries
5.3.1 Plain entries
The clearance holes for plain entries shall be specified in the instructions to allow the proper
selection of a gland or fitting. The inside of the enclosure shall be provided with sufficient room
to attach a locknut to the gland or fitting.
5.3.2 Threaded entries
Threaded entries are considered to meet the requirements for Levels of Protection "ta", "tb" and
"tc" Ex Equipment if they are one of the following:
• tapered threads with no fewer than 3½ threads according to ANSI/ASME B1.20.1;
• parallel threads with no fewer than five threads, with a tolerance class of 6H or better
according to ISO 965-1;
• parallel threads with fewer than five threads with a tolerance class of 6H or better according
to ISO 965-1 and are provided with an additional seal or gasket. If the additional seal is not
an integral part of the Ex Equipment, the certificate number shall include the "X" suffix in
accordance with the marking requirements of IEC 60079-0 and the Specific Conditions of
Use shall detail the required use of a seal or gasket. An advisory marking of the requirement
for a seal or gasket may appear on the Ex Equipment as an alternative to the requirement
for the "X" marking.
6 Verification and tests
6.1 Type tests
6.1.1 Type tests for dust exclusion by enclosures
6.1.1.1 General
Samples of the Ex Equipment shall be subjected to the thermal endurance to heat, thermal
endurance to cold and impact tests specified in IEC 60079-0, and the drop test if applicable. If
there is a supplementary internal enclosure, there shall be no visible damage to the
supplementary internal enclosure caused by the impact testing of the equipment enclosure.
The supplementary internal enclosure may be removed during the thermal endurance to heat
and thermal endurance to cold tests.
After conducting the tests of enclosures in accordance with IEC 60079-0, all of the samples
shall then be subjected to the pressure test of 6.1.1.3 followed by the IP test of 6.1.1.4.
Ex Equipment having Level of Protection "tb" or "tc" which is intended for mains connection and
intended to interrupt fault current above 10 kA, shall be subjected to short circuit interrupting
tests in accordance with the relevant industrial standard after the pressure test of 6.1.1.3 and
prior to the IP tests of 6.1.1.4.
NOTE 1 Limiting the maximum available short circuit current could be achieved by several means including the use
of current limiting fuses or current limiting circuit breakers.
NOTE 2 When the volume of the switching device displaces a large portion of the free internal volume of the
enclosure, damage from a short circuit event can occur due to the venting of arc products from the switching device
into the enclosure volume. The increase in enclosure pressure resulting from this venting could impact the ability of
the enclosure seal to meet the IP requirement of this document.
6.1.1.2 Impact test on supplementary internal enclosures
An impact test on supplementary internal enclosures shall be performed in accordance with the
resistance to impact test of IEC 60079-0 using the nominal 1 kg mass dropped from a nominal
height of 0,2 m. There shall be no damage which invalidates the required ingress protection.
6.1.1.3 Pressure test
A positive internal pressure of at least:
• (4 ± 0,4) kPa for Level of Protection "ta", or
• (2 ± 0,2) kPa for Level of Protection "tb" and "tc",
shall be applied to the Ex Equipment for at least 60 seconds but not more than 70 seconds.
Any breathing or draining device may be sealed for this test if the pressure cannot be
maintained. Any seals of the breathing or draining device shall be removed and the sample
shall be subjected to the IP test in the condition it is in after the completion of this test. This
test is not required for cable glands evaluated as Ex Equipment cable gland.
If the design of the Ex Equipment is such that any gaske
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