IEC 60079-18:2009

IEC 60079-18 ®
Edition 3.0 2009-05
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 3.0 2009-05
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
V
CODE PRIX
ICS 29.260.20 ISBN 978-2-88910-046-0
– 2 – 60079-18 © IEC:2009
CONTENTS
FOREWORD.4

1 Scope.6

2 Normative references .6

3 Terms and definitions .7

4 General .9

4.1 Level of protection (Equipment protection level (EPL)) .9

4.2 Additional requirements for level of protection “ma”.9

4.3 Rated voltage and prospective short circuit current .9
5 Requirements for compounds .9
5.1 General .9
5.2 Specification .9
5.3 Properties of the compound.10
5.3.1 Water absorption .10
5.3.2 Dielectric strength .10
6 Temperatures .10
6.1 General .10
6.2 Determination of the limiting temperature .10
6.2.1 Maximum surface temperature.10
6.2.2 Temperature of the compound .10
6.3 Temperature limitation.10
7 Constructional requirements .11
7.1 General .11
7.2 Determination of faults .11
7.2.1 Fault examination .11
7.2.2 Components considered as not subject to fail .12
7.2.3 Isolating components.12
7.2.4 Infallible separation distances.12
7.3 Free space in the encapsulation .13
7.3.1 Group III “m” equipment.13
7.3.2 Group I and Group II “m” equipment .14
7.4 Thickness of the compound .15
7.4.1 “m” equipment .15

7.4.2 Windings for electrical machines .17
7.4.3 Rigid, multi-layer printed wiring boards with through connections .17
7.5 Switching contacts .18
7.5.1 Level of protection “ma”.18
7.5.2 Level of protection “mb”.18
7.5.3 Level of protection “mc” .18
7.6 External connections .19
7.6.1 General .19
7.6.2 Additional requirements for “ma” equipment .19
7.7 Protection of bare live parts.19
7.8 Cells and batteries .19
7.8.1 General .19
7.8.2 Prevention of gassing .19
7.8.3 Protection against inadmissible temperatures and damage to the cells .20

60079-18 © IEC:2009 – 3 –
7.8.4 Reverse current.20

7.8.5 Current limitation .20

7.8.6 Protection against the polarity inversion and deep discharge of the

cells .20

7.8.7 Charging of cells or batteries.21

7.8.8 Requirements for control safety devices for cells or batteries .21

7.9 Protective devices .21

7.9.1 General .21

7.9.2 Electrical protective devices .22

7.9.3 Thermal protective devices .23

7.9.4 Built-in protective devices.23
8 Type tests .23
8.1 Tests on the compound .23
8.1.1 Water absorption test .23
8.1.2 Dielectric strength test.24
8.2 Tests on the apparatus.24
8.2.1 Test sequence.24
8.2.2 Maximum temperature .24
8.2.3 Thermal endurance test.24
8.2.4 Dielectric strength test.25
8.2.5 Cable pull test .25
8.2.6 Pressure test for Group I and Group II electrical equipment.26
8.2.7 Test for resettable thermal protective device .26
8.2.8 Sealing test for build-in protective devices.27
9 Routine verifications and tests.27
9.1 Visual inspections .27
9.2 Dielectric strength test.27
10 Marking .27
Annex A (informative) Basic requirements for compounds for “m” equipment .29
Annex B (normative) Allocation of test samples .30
Bibliography.31

Figure 1 – Dimensional key for thickness through the compound .16
Figure 2 – Minimum distances for multi-layer printed wiring boards.18

Figure 3 – Fitting of blocking diodes .20
Figure A.1 − Basic requirements for compounds for “m” equipment.29

Table 1 – Distances through the compound .13
Table 2 – Minimum thickness of compound adjacent to free space for Group III “m”
equipment.13
Table 3 – Minimum thickness of compound adjacent to free space for Group I and
Group II “m” equipment.14
Table 4 – Thickness of the compound.16
Table 5 – Minimum distances for multi-layer printed wiring boards.17
Table 6 – Test pressure .26
Table B.1 − Allocation of test samples .30

– 4 – 60079-18 © IEC:2009
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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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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 60079-18 has been prepared by IEC technical committee 31:
Equipment for explosive atmospheres.
This third edition cancels and replaces the second edition of IEC 60079-18 (2004) and
IEC 61241-18 (2004), and constitutes a technical revision.
The significant technical changes with respect to the previous edition are as follows:
– Incorporation of level of protection “mc”
– Equipment protection levels (EPL Ma, Ga, Da, Mb, Gb, Db, Gc, Dc)
– Incorporation of the dust requirements
– Incorporation of switching contacts for level of protection “ma”

60079-18 © IEC:2009 – 5 –
The text of this standard is based on the following documents:

FDIS Report on voting
31/784/FDIS 31/801/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 standard is to be read in conjunction with IEC 60079-0:2007, Explosive atmospheres –

Part 0: Equipment – General requirements.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The list of all parts of 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 June 2009 have been included in this copy.

– 6 – 60079-18 © IEC:2009
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 shall take precedence.
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 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
enclosures “t”
IEC 60127 (all parts), Miniature fuses

60079-18 © IEC:2009 – 7 –
IEC 60243-1, Electrical strength of insulating material – 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 61241-11, Electrical apparatus for use in the presence of combustible dust – Part 11:
Protection by intrinsic safety ‘iD’
IEC 61558-2-6, Safety of power transformers, power supply units and similar – Part 2:
Particular requirements for safety isolating transformers for general use
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
ISO 62, Plastics – Determination of water absorption
ANSI/UL 248-1, Standard for low-voltage fuses – Part 1: General requirements
ANSI/UL 746B, Standard for polymeric materials – Long term property evaluations
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-0 and the
following definitions specific to encapsulation “m” 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 enclosed in a compound in such a way as to avoid ignition of a
dust layer or explosive atmosphere under operating or installation conditions

3.2
compounds
any thermosetting, thermoplastic, epoxy resin or elastomeric materials with or without fillers
and/or additives, in their solid state
3.3
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 this standard
3.4
continuous operating temperature (COT) of the compound
temperature range within which, according to the details given by the manufacturer, the
properties of the compound, during operation, satisfy the requirements of this standard on a
permanent basis during the foreseen lifetime of the equipment

– 8 – 60079-18 © IEC:2009
3.5
encapsulation
process of applying the compound to enclose any electrical device(s) by suitable means

3.6
free surface
compound surface exposed to the explosive atmospheres and/or dust layers

3.7
normal operation
operation of equipment conforming electrically and mechanically with its design specification

and used within the limits specified by the manufacturer
NOTE 1 The limits specified by the manufacturer may include persistent operational conditions, for example
operation of a motor on a duty cycle.
NOTE 2 Variation of the supply specifications within stated limits and any other operational tolerance is part of
normal operation.
3.8
void
unintentional space created as a consequence of the encapsulation process
3.9
free space
intentionally created space surrounding components or space inside components
3.10
switching contact
mechanical contact, which makes and breaks an electrical circuit
3.11
adhesion
moisture, gas and dust tight permanent bonding of a compound to a surface
3.12
countable fault
fault, which occurs in parts of electrical equipment conforming to the constructional require-
ments
3.13
infallible separation or insulation
separation or insulation between electrically conductive parts that is considered as not subject

to short circuits as specified in IEC 60079-18. The probability of such fault modes occurring in
service or storage is considered to be so low that they are not to be taken into account
3.14
non-countable fault
fault, which occurs in parts of electrical equipment not conforming to the constructional
requirements of IEC 60079-18
3.15
solid insulation
insulation material, which is extruded or moulded, but not poured
NOTE Insulators fabricated from two or more pieces of electrical insulating material, which are solidly bonded
together may be considered as solid. Varnish and similar coatings are not considered to be solid insulation.

60079-18 © IEC:2009 – 9 –
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 in

c) level of protection “mc” (EPL “Gc, Dc”).

The requirements of this standard shall apply to all levels of protection “m” (EPL`s) unless

otherwise stated.
4.2 Additional requirements for level of protection “ma”
The working voltage at any point in the circuit shall not exceed 1 kV.
Components without additional protection shall be used only if they cannot damage the
encapsulation mechanically or thermally in the case of any specified fault.
Alternatively, where a fault of an internal component may lead to failure of the encapsulation
“m” due to increasing temperature, the requirements of 7.9 shall apply.
4.3 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).
As a minimum, those properties of the compound(s) on which the encapsulation “m” depends
shall be provided.
NOTE Due consideration should be given in the selection of compounds to allow 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 temperature of the
equipment determined according to 8.2.2 if available. If not available, the requirements of
5.3.2 shall be applied,
f) temperature range of the compound(s) (continuous operating temperature),
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

– 10 – 60079-18 © IEC:2009
relative thermal index (RTI-mechanical impact) 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.

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
If the equipment is to be exposed to dampness, the compound shall be tested in accordance
with 8.1.1. If this test is not performed, the equipment shall be marked “X” in accordance with
the marking requirements of IEC 60079-0 and the restriction of use to dry environments
clarified in the instructions.
5.3.2 Dielectric strength
Where the dielectric strength according to IEC 60243-1 is not available at the maximum
temperature of the equipment as defined according to 8. 2. 2, s ee 5.2 e), a test shall be
performed in accordance with 8. 1. 2.
6 Temperatures
6.1 General
The maximum value of the continuous operating temperature of the compound shall not be
exceeded under normal operation. The maximum surface temperature, determined in
accordance with IEC 60079-0 shall not be exceeded under normal operation and under fault
conditions as defined in 7.2.1. The “m” equipment shall be protected in such a way that the
encapsulation “m” is not adversely affected under these fault conditions.
NOTE Normal operation includes operation at the extremes of voltage tolerances of the supply normally 90 % to
110 % if not otherwise specified.
6.2 Determination of the limiting temperature
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 4.3. This temperature shall be used to
determine the temperature class for explosive gas atmosphere or the maximum surface
temperature in degrees Celsius for explosive dust atmosphere of the equipment.

6.2.2 Temperature of the compound
The hottest component(s) shall be determined. The maximum temperature in the compound,
adjacent to the hottest component(s), 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 may be
done by calculation, manufacturer’s specification or by a practical test prior to encapsulating
the components.
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.
60079-18 © IEC:2009 – 11 –
When a protective device is required to limit temperatures for safety reasons, it shall be an

internal or external, electrical or thermal device, 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 parts of non metallic 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
If additional protective measures are required to be provided by the user in order to satisfy the
requirements of this standard, for example, additional mechanical protection, to indicate this
specific condition of use the equipment shall be in accordance with the “specific conditions of
use” marking requirements of IEC 60079-0.
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 t o 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.
NOTE The choice of the compound(s) to be used for a specific application is dependent on the task each
compound has to perform. In general, testing a compound once is not sufficient for universal use for encapsulation
“m”.
7.2 Determination of faults
7.2.1 Fault examination
The encapsulation “m” shall not be invalidated even under the most adverse input rating, but
between 90 % and 110 % of the rating and most adverse output load and up to two internal
countable faults for level of protection “ma”, and up to one internal countable fault for level of
protection “mb.
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 in the
printed wiring board.
Components meeting the requirements of 7.2.2 are not considered to fail and infallible
separation distances shall only be considered to fail in accordance with 7. 2. 4.
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.

– 12 – 60079-18 © IEC:2009
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 or rated power specified by the manufacturer of the respective
component:
– resistors, if they comply with the current limiting resistors of IEC 60079-11,

– single-layer, spirally wound coils,

– plastic foil capacitors,
– paper capacitors,
– ceramic capacitors,
– shunt semiconductors, if they are used in accordance with the shunt safety assemblies of
IEC 60079-11,
– series semiconductor devices used to limit current:
– a single device is adequate for level of protection “mb”
– two devices shall be used for level of protection “ma”.
For levels of protection “ma” and “mb” coils, motor windings and transformers 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:
• optocouplers and relays, if the rated insulation voltage conforms to 2U + 1 000 V r.m.s.
+5
% or 1 500 V r.m.s. whichever is greater (U is the sum of the rated r.m.s. voltages of
both circuits);
• transformers, complying with IEC 61558-2-6 or IEC 60079-11.
7.2.4 Infallible separation distances
It is not necessary to consider the possibility of a fault occurring as described in 7. 2 . 1 i n
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 7.2.4.1 and if applicable 7. 2. 4.2 .
7.2.4.1 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 the values in Table 1,
provided that the distances in the compound are fixed or secured mechanically before
encapsulation.
Distances between the minimum distances given for level of protection “mc” and the infallible
distances given for level of protection “ma” and “mb” are not considered infallible and shall be

60079-18 © IEC:2009 – 13 –
assessed as a “countable fault”. Distances less than those given for level of protection “mc”

are considered as short-circuits if this impairs the type of protection “m”.

For level of protection “mc” the values of Table 1 are the constructional requirements and may

be achieved by mechanically fixing before encapsulation.

Table 1 – Distances through the compound

Voltage U r.m.s. or d.c. (see note) Minimum distance

V mm
“ma” “mb ” “mc ”
0,5 0,5 0,2
≤32
≤63 0,5 0,5 0,3
1 1 0,6
≤400
≤500 1,5 1,5 0,8
2 2 0,9
≤630
≤1 000 2,5 2,5 1,7
- 4 4
≤1 600
- 7 7
≤3 200
≤6 300 - 12 12
- 20 20
≤10 000
NOTE Voltages shown are derived from IEC 60664-1. For all voltages, the actual
voltage may exceed the value given in the table by up to 10  %. This is based on the
rationalisation of supply voltages given in Table F.3b of IEC 60664-1.

7.2.4.2 Distances through solid insulation
The distance through solid insulation, on which the type of protection “m” depends, shall be at
least 0,1 mm and shall comply with the dielectric strength test of 8.2.4.
7.3 Free space in the encapsulation
7.3.1 Group III “m” equipment
The compound shall be free of voids.
The sum of the free spaces is not limited, but the volume of each individual free space is

limited to 100 cm . The thickness of the compound surrounding such free spaces shall meet
the requirements of Table 2.
Table 2 – Minimum thickness of compound adjacent
to free space for Group III “m” equipment

Level of Minimum thickness of Free space Free space
3 3 3
protection compound adjacent to
≤ 1 cm > 1 cm ≤ 100 cm
free space to:
Free space or free surface 3 mm 3 mm
Non-metallic or metal 3 mm (enclosure + 3 mm (enclosure +
a a
enclosure with adhesion compound) compound)
“ma”
Non-metallic or metal 3 mm 3 mm
enclosure without
adhesion
– 14 – 60079-18 © IEC:2009
Level of Minimum thickness of Free space Free space

3 3 3
protection compound adjacent to
≤ 1 cm > 1 cm ≤ 100 cm
free space to:
Free space or free surface 1 mm 3 mm

Non-metallic or metal 1 mm (enclosure + 3 mm (enclosure +
a
enclosure with adhesion compound) compound)

“mb”
Non-metallic or metal 1 mm 3 mm

enclosure without
adhesion
Free space or free surface 1 mm 1 mm
“mc” Non-metallic or metal 1 mm (enclosure + 1 mm (enclosure +
enclosure with adhesion compound) compound)
Non-metallic or metal 1 mm 1 mm
enclosure without
adhesion
a
Wall thickness of the enclosure ≥ 1 mm.
NOTE The thickness of the materials quoted in this table does not imply compliance with
other mechanical tests required by IEC 60079-0.
7.3.2 Group I and Group II “m” equipment
The compound shall be free of voids.
The sum of the free spaces shall not exceed
• 100 cm for level of protection “mb” and “mc”;
• 10 cm for level of protection “ma”.
The minimum thickness of the compound surrounding such free spaces shall comply with
Table 3.
Table 3 – Minimum thickness of compound adjacent
to free space for Group I and Group II “m” equipment
Level of Minimum thickness of Free space Free space Free space
3 3 3 3 3
protection compound adjacent to
≤ 1 cm > 1 cm ≤ 10 cm > 10 cm ≤ 100 cm
free space to:
Free space or free surface 3 mm 3 mm Not permitted
(pressure test
in accordance with 8.2.6)
Non-metallic or metal 3 mm (enclosure + 3 mm (enclosure + Not permitted
a a
enclosure with adhesion compound) compound)
“ma”
(pressure test
in accordance with 8.2.6)
3 mm 3 mm Not permitted
Non-metallic or metal
enclosure without (pressure test
adhesion in accordance with 8.2.6)
Free space or free surface 1 mm 3 mm 3 mm
“mb”
(pressure test
in accordance with 8.2.6)
60079-18  IEC:2009 – 15 –
Level of Minimum thickness of Free space Free space Free space

3 3 3 3 3
protection compound adjacent to ≤ 1 cm > 1 cm ≤ 10 cm > 10 cm ≤ 100 cm

free space to:
Non-metallic or metal 1 mm (enclosure + 3 mm (enclosure + 3 mm (enclosure +

a a
enclosure with adhesion compound) compound) compound)

(pressure test
in accordance with 8.2.6)
Non-metallic or metal 1 mm 3 mm 3 mm

enclosure without (pressure test

adhesion in accordance with 8.2.6)
Free space or free surface 1 mm 1 mm 3 mm

“mc ” Non-metallic or metal 1 mm (enclosure + 1 mm (enclosure + 3 mm (enclosure +
enclosure with adhesion compound) compound) compound)
See note
Non-metallic or metal 1 mm 1 mm 3 mm
enclosure without
adhesion
a
Wall thickness of the enclosure ≥ 1 mm.
NOTE The thickness of the materials quoted in this table does not imply compliance with other mechanical tests
required by IEC 60079-0.
7.4 Thickness of the compound
7.4.1 “m” equipment
The minimum thickness of compound surrounding the electrical components and circuit shall
be in accordance with Table 4 and Figure 1.
If solid insulation according to 7.2.4.2 is used in an enclosure with metallic walls as shown in
Figure 1, the compound shall adhere to the wall.
NOTE Figure 1 does not necessarily represent a practical construction, but is intended to assist in understanding
Table 4 by showing an encapsulated circuit with all of: a free surface; a
...