IEC 60079-18:2014

IEC 60079-18 ®
Edition 4.0 2014-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 18: Equipment protection by encapsulation “m”

Atmosphères explosives –
Partie 18: Protection du matériel par encapsulage "m"

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IEC 60079-18 ®
Edition 4.0 2014-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Explosive atmospheres –
Part 18: Equipment protection by encapsulation “m”

Atmosphères explosives –
Partie 18: Protection du matériel par encapsulage "m"

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX W
ICS 29.260.20 ISBN 978-2-8322-1994-2

– 2 – IEC 60079-18:2014  IEC 2014
CONTENTS
FOREWORD . 5
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 General . 10
4.1 Level of protection (equipment protection level (EPL)) . 10
4.2 Additional requirements for levels of protection “ma” and “mb” . 10
4.3 Additional requirements for level of protection “ma” . 10
4.4 Rated voltage and prospective short circuit current . 11
5 Requirements for compounds . 11
5.1 General . 11
5.2 Specification . 11
5.3 Properties of the compound . 11
5.3.1 Water absorption . 11
5.3.2 Dielectric strength . 11
6 Temperatures . 12
6.1 General . 12
6.2 Determination of the limiting temperatures . 12
6.2.1 Maximum surface temperature . 12
6.2.2 Temperature of the compound . 12
6.3 Temperature limitation . 12
7 Constructional requirements . 12
7.1 General . 12
7.2 Determination of faults . 13
7.2.1 Fault examination . 13
7.2.2 Components considered as not subject to fail . 13
7.2.3 Isolating components . 14
7.2.4 Infallible separation distances . 14
7.3 Free space in the encapsulation . 15
7.3.1 Group III “m” equipment . 15
7.3.2 Group I and Group II “m” equipment . 16
7.4 Thickness of the compound. 17
7.4.1 “m” equipment . 17
7.4.2 Windings for electrical machines . 19
7.4.3 Rigid, multi-layer printed wiring boards with through connections . 19
7.5 Switching contacts . 20
7.5.1 General . 20
7.5.2 Level of protection “ma” . 21
7.5.3 Level of protection “mb” . 21
7.5.4 Level of protection “mc” . 21
7.6 External connections . 21
7.6.1 General . 21
7.6.2 Additional requirements for “ma” equipment . 21
7.7 Protection of bare live parts . 21
7.8 Cells and batteries . 21
7.8.1 General . 21

7.8.2 Prevention of gassing . 22
7.8.3 Protection against inadmissible temperatures and damage to the cells
or batteries . 22
7.8.4 Reverse current . 22
7.8.5 Current limitation . 23
7.8.6 Protection against the polarity inversion and deep discharge of the cells . 23
7.8.7 Charging of cells or batteries . 23
7.8.8 Requirements for control safety devices for cells or batteries. 24
7.9 Protective devices . 24
7.9.1 General . 24
7.9.2 Electrical protective devices . 25
7.9.3 Thermal protective devices . 25
7.9.4 Built-in protective devices . 26
8 Type tests . 26
8.1 Tests on the compound . 26
8.1.1 Water absorption test . 26
8.1.2 Dielectric strength test . 26
8.2 Tests on the apparatus . 26
8.2.1 Test sequence . 26
8.2.2 Maximum temperature . 27
8.2.3 Thermal endurance test . 27
8.2.4 Dielectric strength test . 28
8.2.5 Cable pull test . 28
8.2.6 Pressure test for Group I and Group II electrical equipment . 29
8.2.7 Test for resettable thermal protective device. 30
8.2.8 Sealing test for built-in protective devices . 30
9 Routine verifications and tests . 30
9.1 Visual inspections . 30
9.2 Dielectric strength test . 30
10 Marking . 31
Annex A (informative) Basic requirements for compounds for “m” equipment . 32
Annex B (informative) Allocation of test samples . 33
Bibliography . 34

Figure 1 – Dimensional key for thickness through the compound . 18
Figure 2 – Minimum distances for multi-layer printed wiring boards . 20
Figure 3 – Fitting of blocking diodes . 23
Figure A.1 − Basic requirements for compounds for “m” equipment . 32

Table 1 – Distances through the compound . 15
Table 2 – Minimum thickness of compound adjacent to free space for Group III “m”
equipment . 16
Table 3 – Minimum tickness of compound adjacent to free space for Group I and Group
II “m” equipment . 17
Table 4 – Thickness of the compound . 19

– 4 – IEC 60079-18:2014  IEC 2014
Table 5 – Minimum distances for multi-layer printed wiring boards . 20
Table 6 – Test pressure . 29
Table B.1 − Allocation of test samples . 33

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 18: Equipment protection by encapsulation “m”

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.
Standard IEC 60079-18 has been prepared by IEC technical committee 31: Equipment for
explosive atmospheres.
This fourth edition cancels and replaces the third edition of IEC 60079-18 (2009), and
constitutes a technical revision.
This International Standard is to be used in conjunction with IEC 60079-0, Explosive
atmospheres – Part 0: Equipment-General requirements.

– 6 – IEC 60079-18:2014  IEC 2014
This edition includes the following significant technical changes with respect to the previous
edition:
Type
Explanation of the significance of the changes Clause Minor and Extension Major
editorial technical
changes changes
Definitions deleted and moved to IEC 60079-0 3 X
Heading modified /added to clarify which requirements are 4 X
additional requirements for “ma” level of protection only
Thermal conductivity added 5.2 X
Note added that it is not a requirement of this standard that 5.3.2 X
conformity to the manufacturer`s specification of the
compound needs to be verified
Clarification added 6.2.2 X
Clarification added 7.1 X
For the determination of faults options added and 7.2 X
clarification given
Additional information included in Figure 1 7.4.1 X
“Varnish and similar coatings are not considered to be solid 7.4.2 X
insulation.” was added in this section and deleted in the
definition on 3.8
For rigid, multi-layer printed wiring boards with through 7.4.3.1 X
connections additional standards added
Protection against inadmissible temperatures and damage 7.8.3  C1
to the cells
Electrical protective devices clarified and additional 7.9.2 X
possibilities added
Thermal protective devices clarified and additional 7.9.3 X
possibilities added
2/3 voltage limitation deleted 7.9.3 X
Determination of the maximum temperature for “Da” fixed 8.2.2  C2
Stabilization of the temperature 8.2.2  C3
Thermal endurance to heat 8.2.3.1 X
Temperature fixed as reference service temperatures and 8.2.3.1.1 X
tests given as alternatives
For the dielectric strength test procedure alternative 8.2.4.1 X
possibilities added
Alternative test methods for the required pressure test for 8.2.6 X
Group I and Group II electrical equipment added
Sealing test for build-in protective devices 8.2.8 X
For the dielectric strength test procedure alternative 9.2 X
possibilities added
Marking 10 X X
Explanation of the Types of Significant Changes:
A) Definitions
1. 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.
2. 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.
3. 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 item 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 Clause 7.8.3 modified and additional requirements added for cells or batteries
C2 The flexibility given in IEC 60079-0 is replaced by a min. requirement. For level of protection “ma”
equipment, designed for EPL “Da” 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 of at least 200 mm
C3 The increase of the temperature during the test can be a very slow process. The final temperature shall be
considered to have been reached when the rate of rise of temperature does not exceed 1 K/24 h

The text of this standard is based on the following documents:
FDIS Report on voting
31/1152/FDIS 31/1168/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 the parts in the IEC 60079 series, published 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 stability date indicated on the IEC website 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 July 2018 have been included in this copy.

– 8 – IEC 60079-18:2014  IEC 2014
EXPLOSIVE ATMOSPHERES –
Part 18: Equipment protection by encapsulation “m”

1 Scope
This part of IEC 60079 gives the specific requirements for the construction, testing and
marking of electrical equipment, parts of electrical equipment and Ex components with the
type of protection encapsulation “m” intended for use in explosive gas atmospheres or
explosive dust atmospheres.
This part applies only for encapsulated electrical equipment, encapsulated parts of electrical
equipment and encapsulated Ex components (hereinafter always referred to as “m”
equipment) where the rated voltage does not exceed 11 kV.
The application of electrical equipment in atmospheres, which may contain explosive gas as
well as combustible dust simultaneously, may require additional protective measures.
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 take account of any risk due to an emission of flammable or toxic gas
from the dust.
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 takes precedence.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60079-7, Explosive atmospheres – Part 7: Equipment protection by increased safety “e”
IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety “i”
IEC 60079-15, Explosive atmospheres – Part 15: Equipment protection by type of protection
"n"
IEC 60079-26, Explosive atmospheres – Part 26: Equipment with equipment protection level
(EPL) Ga
IEC 60079-31, Explosive atmospheres – Part 31: Equipment dust ignition protection by
enclosure “t”
IEC 60127 (all parts), Miniature fuses

IEC 60243-1, Electrical strength of insulating materials – Test methods – Part 1: Tests at
power frequencies
IEC 60691, Thermal-links – Requirements and application guide
IEC 60730-2-9, Automatic electrical controls for household and similar use – Part 2-9:
Particular requirements for temperature sensing controls
IEC 60738-1, Thermistors – Directly heated positive temperature coefficient – Part 1: Generic
specification
IEC 61140, Protection against electric shock – Common aspects for installation and
equipment
IEC 61558-1, Safety of power transformers, power supplies, reactors and similar products –
Part 1: General requirements and tests
IEC 61558-2-6, Safety of transformers, reactors, power supply units and similar products for
supply voltages up to 1 100 V − Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety isolating transformers
IEC 62326-4-1, Printed boards – Part 4: Rigid multilayer printed boards with interlayer
connections – Sectional specification – Section 1: Capability detail specification – Perform-
ance levels A, B and C
ANSI/UL 248 (all parts), Standard for low-voltage fuses
ANSI/UL 746B, Standard for polymeric materials – Long term property evaluations
ANSI/UL 796, Printed-Wiring Boards
IPC-A-600, Acceptability of Printed Boards
IPC-6012, Qualification and Performance Specification for Rigid Printed Boards
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0 and the
following definitions apply.
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426.
3.1
encapsulation “m”
type of protection whereby parts that are capable of igniting an explosive atmosphere by
either sparking or heating are fully enclosed in a compound or other non-metallic enclosure
with adhesion in such a way as to avoid ignition of a dust layer or explosive atmosphere under
operating or installation conditions
3.2
temperature range of the compound
range of temperatures within which the properties of the compound, in either operation or
storage, permit compliance with the requirements of IEC 60079-18

– 10 – IEC 60079-18:2014  IEC 2014
3.3
free surface
compound surface exposed to the explosive atmospheres and/or dust layers
3.4
switching contact
mechanical contact, which makes and breaks an electrical circuit
3.5
adhesion
moisture, gas and dust tight permanent bonding of a compound to a surface
3.6
countable fault
fault which occurs in parts of electrical equipment conforming to the constructional
requirements
3.7
infallible separation
separation between electrically conductive parts that is considered as not subject to short
circuits
3.8
solid insulation
insulation material which is extruded or moulded, but not poured
Note 1 to entry: Insulators fabricated from two or more pieces of electrical insulating material, which are solidly
bonded together may be considered as solid.
4 General
4.1 Level of protection (equipment protection level (EPL))
Electrical equipment with encapsulation “m” shall be either:
a) level of protection “ma” (EPL “Ma, Ga, Da”),
b) level of protection “mb” (EPL “Mb, Gb, Db”), or
c) level of protection “mc” (EPL “Gc, Dc”).
The requirements of this standard apply to all levels of protection for encapsulation “m” unless
otherwise stated.
4.2 Additional requirements for levels of protection “ma” and “mb”
Components without additional protection shall be used only if they cannot damage the
encapsulation mechanically or thermally in the case of any fault conditions specified in this
standard.
Alternatively, where a fault of an internal component may lead to failure of encapsulation “m”
due to increasing temperature, the requirements of 7.9 shall apply.
4.3 Additional requirements for level of protection “ma”
The working voltage at any point in the circuit shall not exceed 1 kV.

4.4 Rated voltage and prospective short circuit current
The rated voltage and the prospective short circuit current shall be specified such that the
limiting temperature is not exceeded for the relevant level of protection “ma”, “mb” or “mc”.
5 Requirements for compounds
5.1 General
The documentation shall specify the compound(s) used and the processing method(s),
including measures to prevent the formation of voids.
As a minimum, those properties of the compound(s) on which encapsulation “m” depends
shall be provided.
NOTE Proper selection of the compound allows for the expansion of components during operation and in the
event of allowable faults.
5.2 Specification
The specification for the compound shall include the following:
a) the name and address of the manufacturer of the compound,
b) the exact and complete reference of the compound and if relevant, percentage of fillers
and any other additives, the mixture ratios and the type designation,
c) if applicable, any treatment of the surface of the compound(s), for example varnishing,
d) if applicable, to obtain correct adhesion of the compound to a component, any requirement
for pre-treating of the component for example cleaning, etching,
e) the dielectric strength in accordance with IEC 60243-1 at the maximum service
temperature of the compound determined according to 8.2.2 a) if available; if not
available, the requirements of 5.3.2 shall be applied,
f) temperature range of the compound(s) (including maximum continuous operating
temperature (COT) and minimum continuous operating temperature (COT)),
g) in the case of “m” equipment where the compound is part of the external enclosure, the
temperature index TI value as defined by IEC 60079-0. As an alternative to the TI, the
relative thermal index (RTI-mechanical ) may be determined in accordance with ANSI/UL
746B,
h) the colour of the compound used for the test samples, where the compound specification
will be influenced by changing the colour,
i) Thermal conductivity if utilizing the alternative test method in 6.2.2.
NOTE It is not a requirement of this standard that conformity to the manufacturer’s specification of the compound
needs to be verified.
5.3 Properties of the compound
5.3.1 Water absorption
Either the compound shall be tested in accordance with 8.1.1 or, if this test is not performed,
the certificate number for the equipment 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 precautions necessary.
5.3.2 Dielectric strength
Where the dielectric strength according to IEC 60243-1 at the maximum service temperature
according to 8.2.2 a), of the compound is not available from the material manufacturer, a test
shall be performed in accordance with 8.1.2.

– 12 – IEC 60079-18:2014  IEC 2014
NOTE It is not a requirement of this standard that conformity to the manufacturer`s specification of the compound
needs to be verified.
6 Temperatures
6.1 General
The service temperature of the compound, determined in accordance with IEC 60079-0, shall
not exceed the maximum value of the COT of the compound. The maximum surface
temperature shall be determined in accordance with IEC 60079-0 under normal operation and
under fault conditions as defined in 7.2.1. The “m” equipment shall be protected in such a way
that encapsulation “m” is not adversely affected under these fault conditions.
6.2 Determination of the limiting temperatures
6.2.1 Maximum surface temperature
The maximum surface temperature shall be determined using the test method given in 8.2.2 in
accordance with the supply conditions specified in 4.4.
NOTE This temperature is used to determine either the temperature class for explosive gas atmospheres or the
maximum surface temperature in degree Celsius for explosive dust atmospheres of the equipment, or both.
6.2.2 Temperature of the compound
The hottest component shall be determined. The maximum temperature in the compound,
adjacent to the hottest component, shall be determined using the test method given in 8.2.2
for normal operation.
As an alternative the determination of the temperature of the hottest component in normal
operation may be done by calculation, manufacturer’s specification or by testing the
component under intended application conditions prior to encapsulating the component if the
thermal conductivity of the compound is greater than that of air.
NOTE Thermal conductivity of air is most often defined as 0.25 W/m*K (standard conditions).
6.3 Temperature limitation
Where the equipment may be subject to fault in accordance with 7.2.1, or where there is the
possibility of an increased temperature, for example by an unfavourable input voltage in
accordance with 7.2.1 or an unfavourable load, this shall be taken into account in determining
the limiting temperatures.
When a protective device is required to limit temperatures for safety reasons, it shall be an
electrical or thermal device external to the equipment or directly integrated into the
equipment, as defined in 7.9.
7 Constructional requirements
7.1 General
Where the compound forms part of the external enclosure it shall comply with the
requirements of IEC 60079-0 for non metallic enclosures and non metallic parts of enclosures.
If the surface of the compound is totally or partly surrounded by an enclosure and the
enclosure is part of the protection, the enclosure or parts of the enclosure shall comply with
the enclosure requirements of IEC 60079-0.
Additional protective measures may be required to be provided by the user in the installation
in order to comply with the requirements of this standard. For example, additional mechanical

protection may be required to protect the equipment from a direct impact. In such cases, the
certificate number for the equipment shall include the “X” suffix in accordance with the
marking requirements of IEC 60079-0 and the specific conditions of use listed in the
certificate shall detail the precautions necessary.
Appropriate action shall be taken to accommodate the expansion of components during
normal operation and in the event of faults according to 7.2.
In 7.2 to 7.9 the requirements differ according to whether the compound adheres to the
enclosure. Where adhesion is specified, the aim is to prevent the ingress of explosive
atmospheres and moisture at the boundary surfaces (for example enclosure-compound,
compound-parts that are not completely embedded in the compound, such as printed wiring
boards, connection terminals, etc.). Where adhesion is required to maintain the type of
protection, it shall be maintained after completion of all the prescribed tests. The choice of the
compound(s) to be used for a specific application is dependent on the task each compound
has to perform. Testing a compound once for a given application will not qualify that
compound for all applications.
NOTE Tests for adhesion are under consideration.
7.2 Determination of faults
7.2.1 Fault examination
When tested in accordance with IEC 60079-0, encapsulation “m” shall be maintained in the
case of
a) the most unfavourable output load and
b) up to two internal countable faults for level of protection “ma”, and up to one internal
countable fault for level of protection “mb”, taking into account 7.2.2, 7.2.3 and 7.2.4.
No faults are taken into account for level of protection “mc”.
NOTE Examples of faults are: a short circuit in any component, the failure of any component and a fault between
tracks in the printed wiring board, but not the opening of a track.
The failure of some components may result in an unstable condition, for example, alternating
between high and low resistance. In those cases, the most onerous condition shall be
considered.
If a fault leads to one or more subsequent faults, for example, due to the overload of a
component, the primary and subsequent fault(s) shall be considered to be a single fault.
7.2.2 Components considered as not subject to fail
For levels of protection “ma” and “mb” the following components shall be considered as not to
fail if they are encapsulated according to the requirements of this standard, if they are
suitable for the service temperature and if they are not operated at more than 2/3 of their
rated voltage, rated current and rated power related to the rating of the device, the mounting
conditions and the temperature range specified:
– resistors,
– single-layer, spirally wound coils,
– plastic foil capacitors,
– paper capacitors,
– ceramic capacitors,
– semiconductors,
– semiconductor devices used as a protective device according to 7.9,

– 14 – IEC 60079-18:2014  IEC 2014
– resistors used as a protective device according to 7.9, if they comply with the current
limiting resistors of IEC 60079-11 for level of protection “ia” or “ib”.
For levels of protection “ma” and “mb” windings that comply with IEC 60079-7, including also
those that have wire diameters of less than 0,25 mm shall be considered as not subject to
failure if they are encapsulated according to the requirements of this standard.
7.2.3 Isolating components
The following components for the segregation of different circuits shall be considered to
provide isolation and are not considered to fail across the segregation:
• Galvanically separating components (e.g. optocouplers and relays),
+5
– if the rated insulation voltage conforms to 2U + 1 000 V r.m.s. % or 1 500 V r.m.s.
whichever is greater (U is the sum of the rated r.m.s. voltages of both circuits), or
– for a rated insulation voltage across the segregation of more than 60 V (sum of the
rated r.m.s. voltages of both circuits), optocouplers and relays providing a double or
reinforced insulation between the circuits per IEC 61140, or
– complying with IEC 60079-11 for level of protection “ia” or “ib”.
• Transformers,
– complying with IEC 61558-2-6, or
– providing a double or reinforced insulation between the circuit per IEC 61558-1, or
– complying with IEC 60079-11 for level of protection “ia” or “ib”.
NOTE 1 It is not a requirement of this standard that conformity to the above standards per the manufacturer´s
specification regarding segregation needs to be verified.
NOTE 2 Galvanically separating components providing double or reinforced insulation according to a product
standard are considered to meet the requirements of IEC 61140 e.g. IEC 60747-5-5 for optocouplers.
7.2.4 Infallible separation distances
7.2.4.1 General
It is not necessary to consider the possibility of a fault occurring as described in 7.2.1 in
respect of voltage breakdown, if the distances between bare current-carrying parts:
– of the same circuit, or
– of a circuit and earthed metal parts, or
– of two separate circuits (sum of the working voltages shall be taken as the voltage for
Table 1; where one of the working voltages is less than 20 % of the other, it shall be
ignored),
comply with the requirements of 7.2.4.2 and if applicable 7.2.4.3.
7.2.4.2 Distances through the compound
Distances through compound shall be considered to be infallible against short circuit for level
of protection “ma” and level of protection “mb” if they comply with t
...

IEC 60079-18 ®
Edition 4.1 2017-08
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 18: Equipment protection by encapsulation “m”
Atmosphères explosives –
Partie 18: Protection du matériel par encapsulage "m"
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IEC 60079-18 ®
Edition 4.1 2017-08
CONSOLIDATED VERSION
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Explosive atmospheres –
Part 18: Equipment protection by encapsulation “m”

Atmosphères explosives –
Partie 18: Protection du matériel par encapsulage "m"

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.260.20 ISBN 978-2-8322-4770-9

IEC 60079-18 ®
Edition 4.1 2017-08
CONSOLIDATED VERSION
REDLINE VERSION
VERSION REDLINE
colour
inside
Explosive atmospheres –
Part 18: Equipment protection by encapsulation “m”

Atmosphères explosives –
Partie 18: Protection du matériel par encapsulage "m"

– 2 – IEC 60079-18:2014+AMD1:2017 CSV
 IEC 2017
CONTENTS
FOREWORD . 5
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 General . 10
4.1 Level of protection (equipment protection level (EPL)) . 10
4.2 Additional requirements for levels of protection “ma” and “mb” . 10
4.3 Additional requirements for level of protection “ma” . 10
4.4 Rated voltage and prospective short circuit current . 11
5 Requirements for compounds . 11
5.1 General . 11
5.2 Specification . 11
5.3 Properties of the compound . 11
5.3.1 Water absorption . 11
5.3.2 Dielectric strength . 11
6 Temperatures . 12
6.1 General . 12
6.2 Determination of the limiting temperatures . 12
6.2.1 Maximum surface temperature . 12
6.2.2 Temperature of the compound . 12
6.3 Temperature limitation . 12
7 Constructional requirements . 12
7.1 General . 12
7.2 Determination of faults . 13
7.2.1 Fault examination . 13
7.2.2 Components considered as not subject to fail . 13
7.2.3 Isolating components . 14
7.2.4 Infallible separation distances . 14
7.3 Free space in the encapsulation . 15
7.3.1 Group III “m” equipment . 15
7.3.2 Group I and Group II “m” equipment . 16
7.4 Thickness of the compound. 17
7.4.1 “m” equipment . 17
7.4.2 Windings for electrical machines . 19
7.4.3 Rigid, multi-layer printed wiring boards with through connections . 19
7.5 Switching contacts . 20
7.5.1 General . 20
7.5.2 Level of protection “ma” . 21
7.5.3 Level of protection “mb” . 21
7.5.4 Level of protection “mc” . 21
7.6 External connections . 21
7.6.1 General . 21
7.6.2 Additional requirements for “ma” equipment . 21
7.7 Protection of bare live parts . 21
7.8 Cells and batteries . 21
7.8.1 General . 21

 IEC 2017
7.8.2 Prevention of gassing . 22
7.8.3 Protection against inadmissible temperatures and damage to the cells
or batteries . 22
7.8.4 Reverse current . 22
7.8.5 Current limitation . 23
7.8.6 Protection against the polarity inversion and deep discharge of the cells . 23
7.8.7 Charging of cells or batteries . 23
7.8.8 Requirements for control safety devices for cells or batteries. 24
7.9 Protective devices . 24
7.9.1 General . 24
7.9.2 Electrical protective devices . 25
7.9.3 Thermal protective devices . 25
7.9.4 Built-in protective devices . 26
8 Type tests . 26
8.1 Tests on the compound . 26
8.1.1 Water absorption test . 26
8.1.2 Dielectric strength test . 26
8.2 Tests on the apparatus . 26
8.2.1 Test sequence . 26
8.2.2 Maximum temperature . 27
8.2.3 Thermal endurance test . 27
8.2.4 Dielectric strength test . 28
8.2.5 Cable pull test . 28
8.2.6 Pressure test for Group I and Group II electrical equipment . 29
8.2.7 Test for resettable thermal protective device. 30
8.2.8 Sealing test for built-in protective devices . 30
9 Routine verifications and tests . 30
9.1 Visual inspections . 30
9.2 Dielectric strength test . 30
10 Marking . 31
Annex A (informative) Basic requirements for compounds for “m” equipment . 32
Annex B (informative) Allocation of test samples . 33
Annex C (normative) Dielectric strength test between circuits and environment . 34
C.1 General . 34
C.2 Batch test procedure . 34
Bibliography . 35

Figure 1 – Dimensional key for thickness through the compound . 18
Figure 2 – Minimum distances for multi-layer printed wiring boards . 20
Figure 3 – Fitting of blocking diodes . 23
Figure A.1 − Basic requirements for compounds for “m” equipment . 32

Table 1 – Distances through the compound . 15
Table 2 – Minimum thickness of compound adjacent to free space for Group III “m”
equipment . 16
Table 3 – Minimum thickness of compound adjacent to free space for Group I and
Group II “m” equipment . 17

– 4 – IEC 60079-18:2014+AMD1:2017 CSV
 IEC 2017
Table 4 – Thickness of the compound . 19
Table 5 – Minimum distances for multi-layer printed wiring boards . 20
Table 6 – Test pressure . 29
Table B.1 − Allocation of test samples . 33

 IEC 2017
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 18: Equipment protection by encapsulation “m”
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.
This consolidated version of the official IEC Standard and its amendment(s) has been
prepared for user convenience.
IEC 60079-18 edition 4.1 contains the fourth edition (2014-12) [documents 31/1152/FDIS
and 31/1168/RVD] and its amendment 1 (2017-08) [documents 31/1323/FDIS and
31/1336/RVD].
In this Redline version, a vertical line in the margin shows where the technical content
is modified by amendment 1. Additions are in green text, deletions are in strikethrough
red text. A separate Final version with all changes accepted is available in
this publication.
– 6 – IEC 60079-18:2014+AMD1:2017 CSV
 IEC 2017
Standard IEC 60079-18 has been prepared by IEC technical committee 31: Equipment for
explosive atmospheres.
This fourth edition constitutes a technical revision.
This International Standard is to be used in conjunction with IEC 60079-0, Explosive
atmospheres – Part 0: Equipment-General requirements.
This edition includes the following significant technical changes with respect to the previous
edition:
Type
Explanation of the significance of the changes Clause Minor and Extension Major
editorial technical
changes changes
Definitions deleted and moved to IEC 60079-0 3 X
Heading modified /added to clarify which requirements are 4 X
additional requirements for “ma” level of protection only
Thermal conductivity added 5.2 X
Note added that it is not a requirement of this standard that 5.3.2 X
conformity to the manufacturer`s specification of the
compound needs to be verified
Clarification added 6.2.2 X
Clarification added 7.1 X
For the determination of faults options added and 7.2 X
clarification given
Additional information included in Figure 1 7.4.1 X
“Varnish and similar coatings are not considered to be solid 7.4.2 X
insulation.” was added in this section and deleted in the
definition on 3.8
For rigid, multi-layer printed wiring boards with through 7.4.3.1 X
connections additional standards added
Protection against inadmissible temperatures and damage 7.8.3  C1
to the cells
Electrical protective devices clarified and additional 7.9.2 X
possibilities added
Thermal protective devices clarified and additional 7.9.3 X
possibilities added
2/3 voltage limitation deleted 7.9.3 X
Determination of the maximum temperature for “Da” fixed 8.2.2  C2
Stabilization of the temperature 8.2.2  C3
Thermal endurance to heat 8.2.3.1 X
Temperature fixed as reference service temperatures and 8.2.3.1.1 X
tests given as alternatives
For the dielectric strength test procedure alternative 8.2.4.1 X
possibilities added
Alternative test methods for the required pressure test for 8.2.6 X
Group I and Group II electrical equipment added
Sealing test for build-in protective devices 8.2.8 X
For the dielectric strength test procedure alternative 9.2 X
possibilities added
Marking 10 X X
 IEC 2017
Explanation of the Types of Significant Changes:
A) Definitions
1. 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.
2. 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.
3. 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 item 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 Clause 7.8.3 modified and additional requirements added for cells or batteries
C2 The flexibility given in IEC 60079-0 is replaced by a min. requirement. For level of protection “ma”
equipment, designed for EPL “Da” 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 of at least 200 mm
C3 The increase of the temperature during the test can be a very slow process. The final temperature shall be
considered to have been reached when the rate of rise of temperature does not exceed 1 K/24 h

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all the parts in the IEC 60079 series, published under the general title Explosive
atmospheres, can be found on the IEC website.
The committee has decided that the contents of the base publication and its amendment will
remain unchanged until the stability date indicated on the IEC web site under
"http://webstore.iec.ch" in the data related to the specific publication. At this date, the
publication will be
 reconfirmed,
 withdrawn,
 replaced by a revised edition, or
 amended.
The contents of the corrigendum of July 2018 have been included in this copy.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 8 – IEC 60079-18:2014+AMD1:2017 CSV
 IEC 2017
EXPLOSIVE ATMOSPHERES –
Part 18: Equipment protection by encapsulation “m”

1 Scope
This part of IEC 60079 gives the specific requirements for the construction, testing and
marking of electrical equipment, parts of electrical equipment and Ex components with the
type of protection encapsulation “m” intended for use in explosive gas atmospheres or
explosive dust atmospheres.
This part applies only for encapsulated electrical equipment, encapsulated parts of electrical
equipment and encapsulated Ex components (hereinafter always referred to as “m”
equipment) where the rated voltage does not exceed 11 kV.
The application of electrical equipment in atmospheres, which may contain explosive gas as
well as combustible dust simultaneously, may require additional protective measures.
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 take account of any risk due to an emission of flammable or toxic gas
from the dust.
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 takes precedence.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60079-0, Explosive atmospheres – Part 0: Equipment – General requirements
IEC 60079-7, Explosive atmospheres – Part 7: Equipment protection by increased safety “e”
IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety “i”
IEC 60079-15, Explosive atmospheres – Part 15: Equipment protection by type of protection
"n"
IEC 60079-26, Explosive atmospheres – Part 26: Equipment with equipment protection level
(EPL) Ga
IEC 60079-31, Explosive atmospheres – Part 31: Equipment dust ignition protection by
enclosure “t”
IEC 60127 (all parts), Miniature fuses

 IEC 2017
IEC 60243-1, Electrical strength of insulating materials – Test methods – Part 1: Tests at
power frequencies
IEC 60691, Thermal-links – Requirements and application guide
IEC 60730-2-9, Automatic electrical controls for household and similar use – Part 2-9:
Particular requirements for temperature sensing controls
IEC 60738-1, Thermistors – Directly heated positive temperature coefficient – Part 1: Generic
specification
IEC 61140, Protection against electric shock – Common aspects for installation and
equipment
IEC 61558-1, Safety of power transformers, power supplies, reactors and similar products –
Part 1: General requirements and tests
IEC 61558-2-6, Safety of transformers, reactors, power supply units and similar products for
supply voltages up to 1 100 V − Part 2-6: Particular requirements and tests for safety isolating
transformers and power supply units incorporating safety isolating transformers
IEC 62326-4-1, Printed boards – Part 4: Rigid multilayer printed boards with interlayer
connections – Sectional specification – Section 1: Capability detail specification – Perform-
ance levels A, B and C
ANSI/UL 248 (all parts), Standard for low-voltage fuses
ANSI/UL 746B, Standard for polymeric materials – Long term property evaluations
ANSI/UL 796, Printed-Wiring Boards
IPC-A-600, Acceptability of Printed Boards
IPC-6012, Qualification and Performance Specification for Rigid Printed Boards
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0 and the
following definitions apply.
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426.
3.1
encapsulation “m”
type of protection whereby parts that are capable of igniting an explosive atmosphere by
either sparking or heating are fully enclosed in a compound or other non-metallic enclosure
with adhesion in such a way as to avoid ignition of a dust layer or explosive atmosphere under
operating or installation conditions
3.2
temperature range of the compound
range of temperatures within which the properties of the compound, in either operation or
storage, permit compliance with the requirements of IEC 60079-18

– 10 – IEC 60079-18:2014+AMD1:2017 CSV
 IEC 2017
3.3
free surface
compound surface exposed to the explosive atmospheres and/or dust layers
3.4
switching contact
mechanical contact, which makes and breaks an electrical circuit
3.5
adhesion
moisture, gas and dust tight permanent bonding of a compound to a surface
3.6
countable fault
fault which occurs in parts of electrical equipment conforming to the constructional
requirements
3.7
infallible separation
separation between electrically conductive parts that is considered as not subject to short
circuits
3.8
solid insulation
insulation material which is extruded or moulded, but not poured
Note 1 to entry: Insulators fabricated from two or more pieces of electrical insulating material, which are solidly
bonded together may be considered as solid.
4 General
4.1 Level of protection (equipment protection level (EPL))
Electrical equipment with encapsulation “m” shall be either:
a) level of protection “ma” (EPL “Ma, Ga, Da”),
b) level of protection “mb” (EPL “Mb, Gb, Db”), or
c) level of protection “mc” (EPL “Gc, Dc”).
The requirements of this standard apply to all levels of protection for encapsulation “m” unless
otherwise stated.
4.2 Additional requirements for levels of protection “ma” and “mb”
Components without additional protection shall be used only if they cannot damage the
encapsulation mechanically or thermally in the case of any fault conditions specified in this
standard.
Alternatively, where a fault of an internal component may lead to failure of encapsulation “m”
due to increasing temperature, the requirements of 7.9 shall apply.
4.3 Additional requirements for level of protection “ma”
The working voltage at any point in the circuit shall not exceed 1 kV.

 IEC 2017
4.4 Rated voltage and prospective short circuit current
The rated voltage and the prospective short circuit current shall be specified such that the
limiting temperature is not exceeded for the relevant level of protection “ma”, “mb” or “mc”.
5 Requirements for compounds
5.1 General
The documentation shall specify the compound(s) used and the processing method(s),
including measures to prevent the formation of voids.
As a minimum, those properties of the compound(s) on which encapsulation “m” depends
shall be provided.
NOTE Proper selection of the compound allows for the expansion of components during operation and in the
event of allowable faults.
5.2 Specification
The specification for the compound shall include the following:
a) the name and address of the manufacturer of the compound,
b) the exact and complete reference of the compound and if relevant, percentage of fillers
and any other additives, the mixture ratios and the type designation,
c) if applicable, any treatment of the surface of the compound(s), for example varnishing,
d) if applicable, to obtain correct adhesion of the compound to a component, any requirement
for pre-treating of the component for example cleaning, etching,
e) the dielectric strength in accordance with IEC 60243-1 at the maximum service
temperature of the compound determined according to 8.2.2 a) if available; if not
available, the requirements of 5.3.2 shall be applied,
f) temperature range of the compound(s) (including maximum continuous operating
temperature (COT) and minimum continuous operating temperature (COT)),
g) in the case of “m” equipment where the compound is part of the external enclosure, the
temperature index TI value as defined by IEC 60079-0. As an alternative to the TI, the
relative thermal index (RTI-mechanical ) may be determined in accordance with ANSI/UL
746B,
h) the colour of the compound used for the test samples, where the compound specification
will be influenced by changing the colour,
i) Thermal conductivity if utilizing the alternative test method in 6.2.2.
NOTE It is not a requirement of this standard that conformity to the manufacturer’s specification of the compound
needs to be verified.
5.3 Properties of the compound
5.3.1 Water absorption
Either the compound shall be tested in accordance with 8.1.1 or, if this test is not performed,
the certificate number for the equipment 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 precautions necessary.
5.3.2 Dielectric strength
Where the dielectric strength according to IEC 60243-1 at the maximum service temperature
according to 8.2.2 a), of the compound is not available from the material manufacturer, a test
shall be performed in accordance with 8.1.2.

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NOTE It is not a requirement of this standard that conformity to the manufacturer`s specification of the compound
needs to be verified.
6 Temperatures
6.1 General
The service temperature of the compound, determined in accordance with IEC 60079-0, shall
not exceed the maximum value of the COT of the compound. The maximum surface
temperature shall be determined in accordance with IEC 60079-0 under normal operation and
under fault conditions as defined in 7.2.1. The “m” equipment shall be protected in such a way
that encapsulation “m” is not adversely affected under these fault conditions.
6.2 Determination of the limiting temperatures
6.2.1 Maximum surface temperature
The maximum surface temperature shall be determined using the test method given in 8.2.2 in
accordance with the supply conditions specified in 4.4.
NOTE This temperature is used to determine either the temperature class for explosive gas atmospheres or the
maximum surface temperature in degree Celsius for explosive dust atmospheres of the equipment, or both.
6.2.2 Temperature of the compound
The hottest component shall be determined. The maximum temperature in the compound,
adjacent to the hottest component, shall be determined using the test method given in 8.2.2
for normal operation.
As an alternative the determination of the temperature of the hottest component in normal
operation may be done by calculation, manufacturer’s specification or by testing the
component under intended application conditions prior to encapsulating the component if the
thermal conductivity of the compound is greater than that of air.
NOTE Thermal conductivity of air is most often defined as 0.25 W/m*K (standard conditions).
6.3 Temperature limitation
Where the equipment may be subject to fault in accordance with 7.2.1, or where there is the
possibility of an increased temperature, for example by an unfavourable input voltage in
accordance with 7.2.1 or an unfavourable load, this shall be taken into account in determining
the limiting temperatures.
When a protective device is required to limit temperatures for safety reasons, it shall be an
electrical or thermal device external to the equipment or directly integrated into the
equipment, as defined in 7.9.
7 Constructional requirements
7.1 General
Where the compound forms part of the external enclosure it shall comply with the
requirements of IEC 60079-0 for non metallic enclosures and non metallic parts of enclosures.
If the surface of the compound is totally or partly surrounded by an enclosure and the
enclosure is part of the protection, the enclosure or parts of the enclosure shall comply with
the enclosure requirements of IEC 60079-0.
Additional protective measures may be required to be provided by the user in the installation
in order to comply with the requirements of this standard. For example, additional mechanical

 IEC 2017
protection may be required to protect the equipment from a direct impact. In such cases, the
certificate number for the equipment shall include the “X” suffix in accordance with the
marking requirements of IEC 60079-0 and the specific conditions of use listed in the
certificate shall detail the precautions necessary.
Appropriate action shall be taken to accommodate the expansion of components during
normal operation and in the event of faults according to 7.2.
In 7.2 to 7.9 the requirements differ according to whether the compound adheres to the
enclosure. Where adhesion is specified, the aim is to prevent the ingress of explosive
atmospheres and moisture at the boundary surfaces (for example enclosure-compound,
compound-parts that are not completely embedded in the compound, such as printed wiring
boards, connection terminals, etc.). Where adhesion is required to maintain the type of
protection, it shall be maintained after completion of all the prescribed tests. The choice of the
compound(s) to be used for a specific application is dependent on the task each compound
has to perform. Testing a compound once for a given application will not qualify that
compound for all applications.
NOTE Tests for adhesion are under consideration.
7.2 Determination of faults
7.2.1 Fault examination
When tested in accordance with IEC 60079-0, encapsulation “m” shall be maintained in the
case of
a) the most unfavourable output load and
b) up to two internal countable faults for level of protection “ma”, and up to one internal
countable fault for level of protection “mb”, taking into account 7.2.2, 7.2.3 and 7.2.4.
No faults are taken into account for level of protection “mc”.
NOTE Examples of faults are: a short circuit in any component, the failure of any component and a fault between
tracks in the printed wiring board, but not the opening of a track.
The failure of some components may result in an unstable condition, for example, alternating
between high and low resistance. In those cases, the most onerous condition shall be
considered.
If a fault leads to one or more subsequent faults, for example, due to the overload of a
component, the primary and subsequent fault(s) shall be considered to be a single fault.
7.2.2 Components considered as not subject to fail
For levels of protection “ma” and “mb” the following components shall be considered as not to
fail if they are encapsulated according to the requirements of this standard, if they are
suitable for the service temperature and if they are not operated at more than 2/3 of their
rated voltage, rated current and rated power related to the rating of the device, the mounting
conditions and the temperature range specified:
– resistors,
– single-layer, spirally wound coils,
– plastic foil capacitors,
– paper capacitors,
– ceramic capacitors,
– semiconductors,
– semiconductor devices used as a protective device according to 7.9,

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– resistors used as a protective device according to 7.9, if they comply with the current
limiting resistors of IEC 60079-11 for level of protection “ia” or “ib”.
For levels of protection “ma” and “mb” windings that comply with IEC 60079-7, including also
those that have wire diameters of less than 0,25 mm shall be considered as not subject to
failure if they are encapsulated according to the requirements of this standard.
7.2.3 Isolating components
The following components for the segregation of different circuits shall be considered to
provide isolation and are not considered to fail across the segregation:
• Galvanically separating components (e.g. optocouplers and relays),
+5
– if the rated insulation voltage conforms to 2U + 1 000 V r.m.s. % or 1 500 V r.m.s.
whichever is greater (U is the sum of the rated r.m.s. voltages of both circuits), or
– for a rated insulation voltage across the segregation of more than 60 V (sum of the
rated r.m.s. voltages of both circuits), optocouplers and relays providing a double or
reinforced insulation between the circuits per IEC 61140, or
– complying with IEC 60079-11 for level of protection “ia” or “ib”.
• Transformers,
– complying with IEC 61558-2-6, or
– providing a double or reinforced insulation between the circuit per IEC 61558-1, or
– complying with IEC 60079-11 for level of protection “ia” or “ib”.
NOTE 1 It is not a requirement of this standard that conformity to the above standards per the manufacturer´s
specification regarding segregation needs to be verified.
NOTE 2 Galvanically separating components providing double or reinforced insulation according to
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