IEC 60079-31:2008

Explosive atmospheres - Part 31: Equipment dust ignition protection by enclosure "t"

IEC 60079-31:2008 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. This standard supplements and modifies the general requirements of IEC 60079-0. Where a requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of this standard shall take precedence. This first edition of IEC 60079-31 has been developed from the first edition of IEC 61241-1 (2004) which it now cancels and supersedes. The significant changes with respect to the previous edition are:
- Title changed to Equipment dust ignition protection by enclosure "t"
- Combination and rationalisation of practice A and B into a single practice, and some constructional requirements that may have applied to only one practice now apply to all enclosures
- Introduction of three levels of protection, "ta", "tb" and "tc"
- Defined test voltage ranges and overload conditions for thermal tests
- Introduction of a pressure test prior to the IP test
- Restrictions on power and voltage levels for level of protection "ta"
- Introduction of a variant of the IP6X test for level of protection "ta"
- Compulsory dust layer thermal test for protection level "ta" by surrounding the enclosure with dust to a depth of at least 500 mm on all available surfaces. The contents of the corrigendum of March 2009 and Interpretation Sheet 1 of February 2022 have been included in this copy.

Atmosphères explosives - Partie 31: Protection du matériel contre l'inflammation des poussières par enveloppe "t"

La CEI 60079-31:2008 est applicable au matériel électrique 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 le matériel électrique. La présente norme complète et modifie les exigences générales de la CEI 60079-0. Lorsqu'une exigence de la présente norme est en conflit avec une exigence de la CEI 60079-0, l'exigence de la présente norme prévaut. Cette première édition de la CEI 60079-31 est issue de la première édition de la CEI 61241-1 (2004) qu'elle annule et remplace. Les modifications importantes par rapport à l'édition précédente sont:
- Titre modifié pour Protection du matériel contre l'inflammation de poussières par enveloppe "t"
- Regroupement et rationalisation des méthodes A et B en une seule méthode, et de certaines exigences de constructions qui n'étaient appliquées qu'à une seule méthode et qui maintenant sont applicables à toutes les enveloppes
- Introduction des trois niveaux de protection, "ta", "tb" et "tc"
- Les gammes de tensions d'essai et les conditions de surcharge pour les essais thermiques ont été définies
- Introduction d'un essai de pression avant l'essai IP
- Réduction des niveaux de puissance et de tension pour le niveau de protection "ta"
- Introduction d'une variante de l'essai IP6X pour le niveau de protection "ta"
- Essai thermique de couche de poussière obligatoire pour le niveau de protection "ta" par enfouissement de l'enveloppe dans de la poussière dont la couche a une épaisseur d'au moins 500 mm sur toutes les surfaces accessibles. Le contenu du corrigendum de mars 2009 a été pris en considération dans cet exemplaire.

General Information

• Status: Published
• Publication Date: 12-Nov-2008
• ICS: 29.260.20 - Electrical apparatus for explosive atmospheres
• Technical Committee: TC 31 - Equipment for explosive atmospheres
• Drafting Committee: MT 60079-31 - TC 31/MT 60079-31
• Current Stage: DELPUB - Deleted Publication
• Completion Date: 27-Nov-2013

Relations

• Revised: IEC 60079-31:2013 - Explosive atmospheres - Part 31: Equipment dust ignition protection by enclosure "t"
• Effective Date: 05-Sep-2023
• Revised: IEC 60079-31:2008/ISH1:2022 - Interpretation Sheet 1 - Explosive atmospheres - Part 31: Equipment dust ignition protection by enclosure "t"
• Effective Date: 05-Sep-2023

Standards Content (Sample)

IEC 60079-31
Edition 1.0 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE


Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure “t”

Atmosphères explosives –
Partie 31: Protection du matériel contre l’inflammation des poussières par
enveloppe «t»

IEC 60079-31:2008

---------------------- Page: 1 ----------------------
THIS PUBLICATION IS COPYRIGHT PROTECTED
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---------------------- Page: 2 ----------------------
IEC 60079-31
Edition 1.0 2008-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE


Explosive atmospheres –
Part 31: Equipment dust ignition protection by enclosure “t”

Atmosphères explosives –
Partie 31: Protection du matériel contre l’inflammation des poussières par
enveloppe «t»

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
L
CODE PRIX
ICS 29.260.20 ISBN 978-2-88910-050-7
® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale

---------------------- Page: 3 ----------------------
IEC 60079-31:2008/ISH1:2022 – 1 –

© IEC 2022


INTERNATIONAL ELECTROTECHNICAL COMMISSION


____________



IEC 60079-31
Edition 1.0  2008-11





EXPLOSIVE ATMOSPHERES –


Part 31: Equipment dust ignition protection by enclosure "t"


INTERPRETATION SHEET 1



This interpretation sheet has been prepared by IEC technical committee 31: Equipment for
explosive atmospheres.
The text of this interpretation sheet is based on the following documents:
DISH Report on voting
31/1605/DISH 31/1616/RVDISH

Full information on the voting for the approval of this interpretation sheet can be found in the
report on voting indicated in the above table.

___________

IEC 60079-31:2008 Edition 1.0, Explosive atmospheres – Part 31: Equipment dust ignition
protection by enclosure "t"

Question 1:
The requirements given in 5.2.2 of IEC 60079-31:2008 (Edition 1) permit threaded entries for
dust protected enclosures to be tapered threads or parallel threads with not less than five
threads, with a minimum tolerance of medium or fine according to ISO 956-1 and ISO 965-3.
Does the description of threads include other thread forms such as ISO 228-1 threads (these
threads are commonly referred to as BSPP or Type G threads and BSPT or Type R threads)?
Answer 1:
No, only metric and NPT threaded entries are permitted for enclosures having equipment dust
ignition protection by enclosure.
NOTE It has been determined that some sizes of Type G and NPT threads will engage but will result in a mismatched
thread engagement. The intention of the restriction to only metric and NPT regarding threaded entries for dust ignition
protection by enclosure is to minimize the risk of mismatch of thread forms in enclosure entries.
ICS 29.260.20
IEC 60079-31:2008-11/ISH1:2022-02(en)

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– 2 – IEC 60079-31:2008/ISH1:2022

© IEC 2022

Question 2:


Where a thread adapter is fitted and assessed as a factory-assembled part of the dust protected

enclosure, what are the permitted thread forms?


Answer 2:


A thread adapter may use thread forms other than metric and NPT, whether as an Ex Equipment

thread adapter or as a thread adapter fitted and assessed as a factory-assembled part of the

dust protected enclosure.


Question 3:
Can a blanking element be installed in a thread adapter which is fitted and assessed as a
factory-assembled part of the dust protected enclosure?
Answer 3:
Yes, a blanking element can be installed in a thread adapter which is fitted and assessed as a
factory-assembled part of the dust protected enclosure.
Question 4:
What is the required marking when a thread adapter is fitted and assessed as a factory
assembled part of the dust protected enclosure?
Answer 4:
Clause 16.2 of IEC 60079-0 (Editions 6 and 7) requires identification of the specific thread type
and size of threaded entries. In the case where the thread adapter is fitted and assessed as a
factory-assembled part of the dust protected enclosure, the requirement for identification
applies to the thread form of the adapter for the field wiring connection.

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– 2 – 60079-31 © IEC:2008
CONTENTS
FOREWORD.3
1 Scope.5
2 Normative references .5
3 Terms and definitions .6
4 Level of protection.6
4.1 General .6
4.2 Additional requirements for level of protection “ta”.6
4.2.1 Thermal protection .6
5 Construction .7
5.1 Joints .7
5.1.1 General .7
5.1.2 Gaskets and seals .7
5.1.3 Cemented joints .8
5.1.4 Operating rods, spindles and shafts.8
5.1.5 Windows.8
5.2 Cable glands and conduit entries.8
5.2.1 Cable glands .8
5.2.2 Conduit entries .8
6 Verification and tests .9
6.1 Type tests .9
6.1.1 Type tests for dust exclusion by enclosures.9
6.1.2 Thermal tests .9
6.1.3 Pressure test .10
6.2 Routine tests .10
7 Marking .10
Bibliography.11

Table 1 – Ingress protection .9
Table 2 – Conditions for the determination of maximum surface temperature.9

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60079-31 © IEC:2008 – 3 –
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
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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
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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 60079-31 has been prepared by IEC technical committee 31:
Equipment for explosive atmospheres.
This first edition of IEC 60079-31 has been developed from the first edition of IEC 61241-1
(2004) which it now cancels and supersedes.
The significant changes with respect to the previous edition are listed below:
– Title changed to Equipment dust ignition protection by enclosure "t"
– Combination and rationalisation of practice A and B into a single practice, and some
constructional requirements that may have applied to only one practice now apply to all
enclosures
– Introduction of three levels of protection, “ta”, “tb” and “tc”
– Defined test voltage ranges and overload conditions for thermal tests.
– Introduction of a pressure test prior to the IP test
– Restrictions on power and voltage levels for level of protection “ta”

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– 4 – 60079-31  IEC:2008


– Introduction of a variant of the IP6X test for level of protection “ta”

– Compulsory dust layer thermal test for protection level “ta” by surrounding the enclosure

with dust to a depth of at least 500 mm on all available surfaces


The text of this standard is based on the following documents:


FDIS Report on voting


31/765/FDIS 31/775/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 of the IEC 60079 series, under the general title Explosive atmospheres, can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
The contents of the corrigendum of March 2009 and Interpretation Sheet 1 of February 2022
have been included in this copy.

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60079-31 © IEC:2008 – 5 –
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.
This standard supplements and modifies the general requirements of IEC 60079-0. Where a
requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of
this standard shall take precedence.
This standard does not apply to dusts of explosives, which do not require atmospheric oxygen
for combustion, or to pyrophoric substances.
This standard does not apply to electrical equipment 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 standard does not take account of any risk due to an emission of flammable or toxic gas
from the dust.
NOTE 1 The application of electrical equipment in atmospheres, which may contain combustible dust as well as
explosive gas, whether simultaneously or separately, may require additional protective measures.
NOTE 2 Where the equipment has to meet other environmental conditions, for example, protection against
ingress of water and resistance to corrosion, additional methods of protection may be necessary. The method used
should not adversely affect the integrity of the enclosure.
2 Normative references
The following referenced documents are indispensable for the application 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 60034-1, Rotating electrical machines – Part 1: Rating and performance
IEC 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60079-7, Explosive atmospheres – Part 7: Equipment protection by increased safety “e”
IEC 60127 series, Miniature fuses
IEC 60691, Thermal-links – Requirements and application guide
ISO 965-1, ISO general-purpose metric screw threads – Tolerances – Part 1: Principles and
basic data
ISO 965-3, ISO general-purpose metric screw threads – Tolerances – Part 3: Deviation for
constructional screw threads

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– 6 – 60079-31 © IEC:2008
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0, as well as
the following definitions, apply.
1
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426[1] .
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
a compressible element provided in a joint to provide protection against the ingress of dust
4 Level of protection
Type of protection “t” is divided into three levels of protection, “ta”, “tb” and “tc”. For each
level of protection, an equipment protection level (EPL) has been assigned based on the risk
of the equipment becoming an ignition source in a hazardous atmosphere.
4.1 General
Equipment with dust ignition protection by enclosure “t” shall be either
• level of protection ta (EPL “Da”), or
• level of protection tb (EPL “Db”), or
• level of protection tc (EPL “Dc”).
The requirements of this standard shall apply to all levels of protection “t” unless otherwise
stated.
4.2 Additional requirements for level of protection “ta”
For level of protection “ta”, the requirements of IEC 60079-31 shall apply, in addition to the
following. The power supply to level of protection “ta” equipment shall be rated for a
prospective short circuit current of not more than 10 kA.
4.2.1 Thermal protection
4.2.1.1 General
Where the equipment is capable of exceeding the maximum surface temperature as a result
of an abnormal operating condition, a protective device shall be provided either external to
the equipment or directly integrated into the equipment. The protective device shall be
capable of interrupting the maximum current of the circuit in which it is installed. Where the
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, providing
it also protects the complete equipment from exceeding the maximum surface temperature.
—————————
1
 Figures in square brackets refer to the bibliography.

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60079-31 © IEC:2008 – 7 –
NOTE The response time of the thermal protective devices should be taken into account and demonstrated by the
manufacturer that they are adequate for the necessary overtemperature protection.
4.2.1.2 Protective devices
The equipment shall be protected by an integral thermal protective device. This thermal
protective device shall not be a self-resettable type and must be duplicated unless it complies
with IEC 60127 or IEC 60691, then 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 to combine temperature protection and overcurrent protection.
The overcurrent protective device shall comply with IEC 60127 and shall be rated at not more
than 170 % of the maximum rated current. 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 of 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.
4.2.1.3 Temperature limitation
The temperature of internal devices and components shall not exceed the temperature rating
of the equipment under the test conditions of the thermal test in 6.1.2.
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 and be subjected to the test of 6.1.1:
NOTE The use of grease alone to maintain the integrity of the seal is not considered to satisfy this requirement.
• 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, with a minimum tolerance
of medium or fine according to ISO 965-1 and ISO 965-3.
• 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 give rise to sufficient wear that
may affect the correct function of the sealing means .
Where necessary, a means shall be provided for mating parts to facilitate correct alignment.
5.1.2 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, i.e. with an uninterrupted
periphery.
NOTE 1 One-piece construction would also include gaskets and seals that have been permanently joined to form
an uninterrupted periphery while maintaining the mechanical properties of the gasket or seal material.
NOTE 2 Unless all gaskets are secured to one face of the mating surface, either by adhesive or mechanically
secured, the design of the enclosure should be such that determination of the correct location of the gaskets can
be made.

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– 8 – 60079-31 © IEC:2008
Except for a slight amount of lubricant necessary for assembly, joints using gaskets shall not
be supplemented by the application of a sealant material except for an adhesive material on
one side of the mating surfaces.
A flexible seal, e.g. 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.
5.1.3 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; and
• have a width less than 3 mm.
5.1.4 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.
5.1.5 Windows
5.1.5.1 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.
5.1.5.2 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. No separate detachable frame is required.
5.2 Cable glands and threaded entries
5.2.1 Cable glands
Cable glands, whether integral or separate, shall meet the requirements of IEC 60079-0, and
the joint requirements of 5.1. In addition to these requirements, cable glands for “ta” shall be
subjected to the tests of 6.1.1.
Where cable glands are integral with the enclosure or specific to the enclosure they shall be
tested as part of the enclosure.
Where cable glands are separate:
• threaded Ex cable glands can be evaluated as equipment
• other cable glands can be evaluated only as an Ex component.
5.2.2 Threaded entries
Threaded entries with tapered threads are considered to meet the requirements for “ta”, “tb”
and “tc” equipment. Threaded entries with parallel threads shall be provided with not less than
five threads, with a minimum tolerance of medium or fine according to ISO 965-1 and ISO
965-3.

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60079-31 © IEC:2008 – 9 –
6 Verification and tests
6.1 Type tests
6.1.1 Type tests for dust exclusion by enclosures
Samples of the equipment shall be subjected to the thermal endurance to heat, thermal
endurance to cold and mechanical tests of IEC 60079-0.
The samples shall then each be subjected to the pressure test of 6.1.3, Finally, the samples
shall each satisfy the requirements shown in Table 1.
a
Table 1 – Ingress protection
Level of protection IIIC IIIB IIIA
“ta” IP6X IP6X IP6X
“tb” IP6X IP6X IP5X
“tc” IP6X IP5X IP5X
a
Ingress protection is to 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 should be
removed before the IP test is performed.

6.1.2 Thermal tests
This test shall be carried out as described in IEC 60079-0 with the test voltages and overload
or fault conditions applied as specified in Table 2. For level or protection “ta” equipment, the
requirements of 4.2 apply, and the maximum surface temperature shall be determined with
the equipment mounted in accordance with the manufacturer’s instructions, and surrounded
on all available surfaces by dust with a layer thickness o
...