EN 61241-18:2004

SLOVENSKI SIST EN 61241-18:2005

STANDARD
april 2005
Električne naprave za uporabo v prisotnosti gorljivega prahu - 18. del: Zaščita
zalivanje z zalivno maso "mD" (IEC 61241-18:2004)
Electrical apparatus for use in the presence of combustible dust – Part 18:
Protection by encapsulation "mD"
ICS 29.260.20 Referenčna številka
©  Standard je založil in izdal Slovenski inštitut za standardizacijo. Razmnoževanje ali kopiranje celote ali delov tega dokumenta ni dovoljeno

EUROPEAN STANDARD EN 61241-18
NORME EUROPÉENNE
EUROPÄISCHE NORM November 2004

ICS 29.260.20
English version
Electrical apparatus for use in the presence of combustible dust
Part 18: Protection by encapsulation "mD"
(IEC 61241-18:2004)
Matériels électriques pour utilisation en Elektrische Betriebsmittel zur Verwendung
présence de poussières combustibles in Bereichen mit brennbarem Staub
Partie 18: Protection par encapsulage Teil 1: Schutz durch Vergusskapselung
"mD" "mD"
(CEI 61241-18:2004) (IEC 61241-18:2004)

This European Standard was approved by CENELEC on 2004-10-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and
notified to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden,
Switzerland and United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2004 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.

Ref. No. EN 61241-18:2004 E
Foreword
The text of document 31H/176/FDIS, future edition 1 of IEC 61241-18, prepared by SC 31H,
Apparatus for use in the presence of combustible dust, of IEC TC 31, Electrical apparatus for
explosive atmospheres, was submitted to the IEC-CENELEC parallel vote and was approved by
CENELEC as EN 61241-18 on 2004-10-01.
*)
This standard is to be used in conjunction with EN 61241-0 .
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2005-07-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2007-10-01
This European Standard has been prepared under a mandate given to CENELEC by the European
Commission and the European Free Trade Association and covers essential requirements of
EC Directive(s). See Annex ZZ.
Annexes ZA and ZZ have been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61241-18:2004 was approved by CENELEC as a European
Standard without any modification.
__________
*)
In preparation.
- 3 - EN 61241-18:2004
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE Where an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
IEC 60079-7 2001 Electrical apparatus for explosive gas EN 60079-7 2003
atmospheres
Part 7: Increased safety "e"
IEC 60079-11 1999 Part 11: Intrinsic safety "i" - -

1) 2)
IEC 60086-1 - Primary batteries EN 60086-1 2001
Part 1: General
IEC 60127 Series Miniature fuses EN 60127 Series

1) 2)
IEC 60243-1 - Electrical strength of insulating materials - EN 60243-1 1998
Test methods
Part 1: Tests at power frequencies

3)
IEC 60285 1993 Alkaline secondary cells and batteries - EN 60285 1994
Sealed nickel-cadmium cylindrical
rechargeable single cells
1) 2)
IEC 60622 - Secondary cells and batteries containing EN 60622 2003
alkaline or other non-acid electrolytes -
Sealed nickel-cadmium prismatic
rechargeable single cells
IEC 60664-1 1992 Insulation coordination for equipment
within low-voltage systems
Part 1: Principles, requirements and tests
+ A1 2000
+ A2 2002 EN 60664-1 2003
1) 2)
IEC 60691 - Thermal-links - Requirements and EN 60691 2003
application guide
1) 2)
IEC 61150 - Alkaline secondary cells and batteries - EN 61150 1993
Sealed nickel-cadmium rechargeable
monobloc batteries in button cell design

1)
Undated reference.
2)
Valid edition at date of issue.
3)
EN 60285 is superseded by EN 61951-1:2003 which is based on IEC 61951-1:2003.

Publication Year Title EN/HD Year
1)
IEC 61241-0 - Electrical apparatus for use in the - -
presence of combustible dust
Part 0: General requirements
1) 2)
IEC 61241-1 - Electrical apparatus for use in the EN 61241-1 2004
presence of combustible dust
Part 1: Protection by enclosures "tD"

4)
IEC 61241-11 - Part 11: Intrinsically safe apparatus 'iD' - -

5)
IEC 61436 1998 Secondary cells and batteries containing EN 61436 1998
alkaline or other non-acid electrolytes -
Sealed nickel-metal hydride rechargeable
single cells
1) 2)
IEC 61558-2-6 - Safety of power transformers, power EN 61558-2-6 1997
supply units and similar
Part 2-6: Particular requirements for
safety isolating transformers for general
use
1) 2)
IEC 61960-1 - Secondary lithium cells and batteries for EN 61960-1 2001
portable applications
Part 1: Secondary lithium cells

1) 2)
IEC 62326-4-1 - Printed boards EN 62326-4-1 1997
Part 4: Rigid multilayer printed boards
with interlayer connections - Sectional
specification -- Section 1: Capability
Detail Specification - Performance levels
A, B and C
ISO 62 1999 Plastics - Determination of water EN ISO 62 1999
absorption
ANSI/UL 248-1 1995 Standard for low-voltage fuses - -
Part 1: General requirements
ANSI/UL 746B 2000 Polymeric Materials - Long-Term Property - -
Evaluations
4)
To be published.
5)
EN 61436 is superseded to EN 61951-2:2000, which is based on IEC 61951-2:2003.

- 5 - EN 61241-18:2004
Annex ZZ
(informative)
Coverage of Essential Requirements of EC Directives
This European Standard has been prepared under a mandate given to CENELEC by the European
Commission and the European Free Trade Association and within its scope the standard covers all
relevant essential requirements as given in Annex II of the EC Directive 94/9/EC.
Compliance with this standard provides one means of conformity with the specified essential
requirements of the Directive[s] concerned.
WARNING: Other requirements and other EC Directives may be applicable to the products falling
within the scope of this standard.

NORME CEI
INTERNATIONALE IEC
61241-18
INTERNATIONAL
Première édition
STANDARD
First edition
2004-08
Matériels électriques pour utilisation
en présence de poussières combustibles –
Partie 18:
Protection par encapsulage «mD»
Electrical apparatus for use in the
presence of combustible dust –
Part 18:
Protection by encapsulation "mD"
Numéro de référence
Reference number
CEI/IEC 61241-18:2004
61241-18” IEC:2004 – 3 –
CONTENTS
FOREWORD .7
INTRODUCTION .13
1 Scope .17
2 Normative references .17
3 Terms and definitions .21
4 General.23
4.1 Temperature classification.23
4.2 Level of protection.23
4.3 Level of protection "maD" .23
4.4 Level of protection "mbD" .23
4.5 Supply specifications.25
5 Requirements for compounds .25
5.1 General .25
5.2 Specification .25
6 Temperatures.25
6.1 General .25
6.2 Temperature limitation.27
6.3 Determination of the limiting temperature values.27
7 Constructional requirements .27
7.1 General .27
7.2 Determination of possible faults.29
7.3 Switching contacts.41
7.4 External connections .41
7.5 Protection of bare live parts .41
7.6 Cells and batteries .41
7.7 Protective devices .47
8 Type tests .49
8.1 Tests on the compound – water absorption test .49
8.2 Tests on the apparatus .49
9 Routine verifications and tests.57
9.1 Visual inspections .57
9.2 Dielectric strength test.57
10 Marking.59
Annex A (informative) Basic requirements for compounds for encapsulation “mD”
apparatus .61
Annex B (normative) Allocation of test samples.63
Annex C (normative) Test procedure during thermal cycling test.65
Figure 1 – Distances between free surface of compound and components or
conductors .33
Figure 2 – Distances between the wall or the free surface of the compound and the
components or conductors.35

61241-18” IEC:2004 – 5 –
Figure 3 – Distances between the wall or the free surface of the compound and the
components or conductors.37
Figure 4 – Minimum distances for multi-layer printed wiring boards .41
Figure A.1  Basic requirements for compounds for encapsulation “mD” apparatus .61
Figure C.1 – Test procedure during thermal cycling test .65
Table 1 – Distances through the compound.31
Table 2  Thickness of compound between the free surface of the compound and
components or conductors.33
Table 3 – Thickness of the compound between the wall or the free surface of the
compound and the components or conductors.35
Table 4 – Thickness of the compound between the wall or the free surface of the
compound and the components or conductors.37
Table 5 – Minimum distances for multi-layer printed wiring boards.39
Table 6 – Permissible primary cells.43
Table 7 – Permissible secondary cells .43
Table 8 – Test pressure.57
Table B.1  Allocation of test samples.63

61241-18” IEC:2004 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTRICAL APPARATUS FOR USE IN THE
PRESENCE OF COMBUSTIBLE DUST –
Part 18: Protection by encapsulation “mD”
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 provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61241-18 has been prepared by subcommittee 31H: Apparatus for
use in the presence of combustible dust, of IEC technical committee 31: Electrical apparatus
for explosive atmospheres.
The text of this standard is based on the following documents:
FDIS Report on voting
31H/176/FDIS 31H/180/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.

61241-18” IEC:2004 – 9 –
This standard is to be read in conjunction with IEC 61241-0.
IEC 61241 consists of the following parts under the general title: Electrical apparatus for use
in the presence of combustible dust:
Part 0: General requirements
Part 1: Protection by enclosures ‘tD’
Part 2: Type of protection ‘pD’
Part 10: Classification of areas where combustible dusts are or may be present
Part 11: Protection by intrinsic safety ‘iD’
Part 14: Selection and installation
Part 17: Inspection and maintenance of electrical installations in hazardous areas (other
than mines)
Part 18: Protection by encapsulation ‘mD’
Part 20 : Test methods
Part 20-1: Methods for determining the minimum ignition temperatures of dust
Part 20-2: Method for determining the electrical resistivity of dust in layers
Part 20-3: Method for determining minimum ignition energy of dust/air mixtures
NOTE All references in this standard to the IEC 61241 series follows the proposed re-numbering of the dust
standards agreed by SC31H and TC31. It may be necessary to alter these numbers if the relevant standards are
not yet published.
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.
___________
To be published.
61241-18” IEC:2004 – 11 –
REFERENCE TABLE
Anticipated
New number
Standard Subject date of
assigned
change
IEC 61241-0 General requirements 2004
IEC 61241-1-1
IEC 61241-1 Protection by enclosure 2004
IEC 61241-1-2 IEC 61241-14 Selection and installation 2004
IEC 61241-2-1 IEC 61241-20-1 Test methods 2005
IEC 61241-2-2 IEC 61241-20-2 Test methods 2005
IEC 61241-2-3 IEC 61241-20-3 Test methods 2005
IEC 61241-3 IEC 61241-10 Classification 2004
IEC 61241-4 IEC 61241-2 Protection by pressurization 2005
IEC 61241-11 Protection by intrinsic safety 2005
IEC 61241-17 Inspection and maintenance 2004
IEC 61241-18 Protection by encapsulation 2004
IEC 61241-19 Repair and overhaul 2006

61241-18” IEC:2004 – 13 –
INTRODUCTION
Many dusts which are generated, processed, handled and stored, are combustible. When
ignited they can burn rapidly and with considerable explosive force if mixed with air in the
appropriate proportions. It is often necessary to use electrical apparatus in locations where
such combustible materials are present, and suitable precautions must therefore be taken to
ensure that all such apparatus is adequately protected so as to reduce the likelihood of
ignition of the external explosive atmosphere. In electrical apparatus, potential ignition
sources include electrical arcs and sparks, hot surfaces, and frictional sparks.
Areas where dusts, flyings and fibres in air occur in dangerous quantities are classified as
hazardous and are divided into 3 zones according to the level of risk.
Generally, electrical safety is ensured by the implementation of one of two considerations,
i.e. that electrical apparatus be located where reasonably practicable outside hazardous
areas and that electrical apparatus be designed, installed and maintained in accordance with
measures recommended for the area in which the apparatus is located.
Combustible dust can be ignited by electrical apparatus in several ways:
– by surfaces of the apparatus that are above the minimum ignition temperature of the dust
concerned. The temperature at which a type of dust ignites is a function of the properties
of the dust, whether the dust is in a cloud or layer, the thickness of the layer and the
geometry of the heat source;
– by arcing or sparking of electrical parts such as switches, contacts, commutators,
brushes, or the like;
– by discharge of an accumulated electrostatic charge;
– by radiated energy (for example, electromagnetic radiation);
– by mechanical sparking or frictional sparking or heating associated with the apparatus.
In order to avoid ignition hazards it is necessary that
– the temperature of surfaces, on which dust can be deposited, or which would be in
contact with a dust cloud, is kept below the temperature limitation specified in this
standard;
– any electrical sparking parts, or parts having a temperature above the temperature limit
specified in IEC 61241-14
- are contained in an enclosure which adequately prevents the ingress of dust, or
- the energy of electrical circuits is limited so as to avoid arcs, sparks or temperatures
capable to ignite combustible dust;
– any other ignition sources are avoided.
Compliance with this standard will only provide the required level of safety if the electrical
apparatus is operated within its rating and is installed and maintained according to the
relevant codes of practice or requirements, for example in respect of protection against over-
currents, internal short-circuits, and other electrical faults. In particular, it is essential that the
severity and duration of an internal or external fault be limited to values that can be sustained
by the electrical apparatus without damage.

61241-18” IEC:2004 – 15 –
Several techniques are available for the explosion protection of electrical apparatus in
hazardous areas. This standard describes the safety features of these types of explosion-
protection techniques and specifies the installation procedures to be adopted. It is most
important that the correct selection and installation procedures be followed to ensure the safe
use of electrical apparatus in hazardous areas.

61241-18” IEC:2004 – 17 –
ELECTRICAL APPARATUS FOR USE IN THE
PRESENCE OF COMBUSTIBLE DUST –
Part 18: Protection by encapsulation “mD”
1 Scope
This standard is to be read in conjunction with IEC 61241-0, the requirements of which apply
to electrical apparatus protected by encapsulation and surface temperature limitation unless
specifically excluded.
This part of IEC 61241 is applicable to electrical apparatus protected by encapsulation type
of protection “mD" and surface temperature limitation for use in areas where combustible
dust may be present in quantities which could lead to a fire or explosion hazard. It specifies
requirements for design, construction and testing of electrical apparatus, parts of electrical
apparatus and Ex components where the rated voltage does not exceed 10 kV.
NOTE 1 The actual working voltage may exceed the value given above by up to 10 %.
NOTE 2 IEC 61241-14 (“Electrical apparatus for use in the presence of combustible dust – Part 14: Selection and
installation”) gives guidance on the selection and installation of the apparatus. Apparatus within the scope of this
standard may also be subjected to additional requirements in other standards – for example, IEC 60079-0
(“Electrical apparatus for explosive gas atmospheres - Part 0: General requirements”).
The application of electrical apparatus in atmospheres which may contain explosive gas as
well as combustible dust, whether simultaneously or separately, requires 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 is not applicable to electrical apparatus 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.
This standard does not include other types of protection and is only applicable to protection
by encapsulation.
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 60079-7:2001, Electrical apparatus for explosive gas atmospheres – Part 7: Increased
safety "e“
IEC 60079-11:1999, Electrical apparatus for explosive gas atmospheres – Part 11: Intrinsic
safety "i“
IEC 60086-1, Primary batteries – Part 1: General

61241-18” IEC:2004 – 19 –
IEC 60127 (all parts), Miniature fuses
IEC 60243–1, Electrical strength of insulating materials – Test methods – Part 1: Tests at
power frequencies
IEC 60285, Alkaline secondary cells and batteries – Sealed nickel-cadmium cylindrical
rechargeable single cells
IEC 60622, Secondary cells and batteries containing alkaline or other non-acid electrolytes –
Sealed nickel-cadmium prismatic rechargeable single cells
IEC 60664–1:1992, Insulation co-ordination for equipment within low-voltage systems – Part
1: Principles, requirements and tests
Amendment 1 (2000)
Amendment 2 (2002)
IEC 60691, Thermal links – Requirements and application guide
IEC 61150, Alkaline secondary cells and batteries – Sealed nickel cadmium rechargeable
monobloc batteries in button cell design
IEC 61241-0:, Electrical apparatus for use in the presence of combustible dust – Part 0:
General requirements
IEC 61241-1:, Electrical apparatus for use in the presence of combustible dust – Part 1:
Protection by enclosures “tD”
IEC 61241-11:, Electrical apparatus for use in the presence of combustible dust – Part 11:
Protection by intrinsic safety “iD”
IEC 61436, Secondary cells and batteries containing alkaline or other non-acid electrolytes –
Sealed nickel-metal hydride rechargeable single cells
IEC 61558-2-6, Safety of power transformers, power supply units and similar – Part 2:
Particular requirements for safety isolation transformers for general use
IEC 61960-1, Secondary lithium cells and batteries for portable applications – Part 1:
Secondary lithium cells
IEC 62326-4-1, Printed boards – Part 4: Rigid multilayer printed boards with interlayer
connections – Sectional specification – Section 1: Capability detail specification –
Performance 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, Polymeric Materials – Long-term Property Evaluations
___________
Cancelled and replaced by IEC 61951-1 (2003).
To be published.
Cancelled and replaced by by IEC 61951-2 (2003).

61241-18” IEC:2004 – 21 –
3 Terms and definitions
For the purposes of this document, the following terms and definitions, specific to protection
by encapsulation "mD“, apply. They supplement the definitions given in IEC 61241-0.
3.1
encapsulation "mD"
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 cloud 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
the 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
the temperature 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 apparatus
3.5
encapsulation
the 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
3.7
normal operation
the operation of apparatus 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

61241-18” IEC:2004 – 23 –
3.10
switching contact
mechanical contact which is intended for making and breaking of an electrical circuit
4 General
4.1 Temperature classification
Encapsulation “mD” apparatus shall be classified in accordance with Clause 5 of IEC 61241-
0.
4.2 Level of protection
Electrical apparatus with encapsulation "mD“ shall be either level of protection "maD“ or level
of protection "mbD“.
The requirements of this standard shall apply to both levels of protection unless otherwise
stated.
If a non-resetting protective device, according to IEC 60127 or IEC 60691, is used, then only
one device is necessary for both levels of protection.
4.3 Level of protection "maD"
Level of protection “maD” shall not be capable of causing ignition in each of the following
circumstances:
a) in normal operation and installation conditions;
b) any specified abnormal conditions;
c) in defined failure conditions.
For level of protection “maD” the working voltage at any point in the encapsulation circuit
shall not exceed 1 kV.
For level of protection "maD", components without additional protection shall only be used if
they cannot damage the encapsulation mechanically or thermally in the case of any fault.
Alternatively, where a fault of an internal component may lead to failure of the encapsulation
system due to increasing temperature, the requirements of 6.2 shall apply.
NOTE Certain components whose application is allowed according to this standard for level of protection “mbD”
can possibly make the protective measure ‘encapsulation’ ineffective by mechanical or thermal damage as a
consequence of internal reactions. This risk should be excluded for level of protection “maD” apparatus.
4.4 Level of protection "mbD"
Level of protection “mbD” shall not be capable of causing ignition in each of the following
circumstances:
a) in normal operation and installation conditions;
b) in defined failure conditions.

61241-18” IEC:2004 – 25 –
4.5 Supply specifications
The limiting values of the supply (rated voltage and the prospective short circuit current) shall
be stated to ensure that under the relevant level of protection, “maD” or “mbD”, the limiting
temperature is not exceeded. Any protective device used shall comply with the requirements
of 7.7.
5 Requirements for compounds
5.1 General
The documentation in accordance with 23.2 of IEC 61241-0 shall specify the compound(s)
used and the processing method(s).
As a minimum, those properties of the compound(s) on which the encapsulation "mD“
depends shall be provided.
Due consideration shall be taken in the selection of encapsulating materials to allow for the
expansion of components during operation and in the event of allowable faults.
5.2 Specification
The manufacturer shall attest on his own responsibility that the material complies with the
compound specification.
The specification shall include:
a) the name and address of the manufacturer of the compound;
b) the exact and complete reference of the material, 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) if appropriate, the result of the water absorption test in accordance with 8.1. Where this
test has not been performed, the apparatus shall be marked "X“ in accordance with item l)
of 29.2 of IEC 61241-0;
f) the dielectric strength in accordance with IEC 60243-1 at the maximum temperature of
the apparatus determined according to 8.2.2;
g) temperature range of the compound(s) (upper and lower continuous operating
temperature);
h) in the case of encapsulation "mD“ apparatus where the compound is part of the outside
enclosure, the temperature index TI value as defined in 6.1.4 of IEC 61241-0. As an
alternative to the TI, the Relative Thermal Index (RTI-mechanical impact) may be
determined in accordance with ANSI/UL 746B;
i) the colour of the compound used for the test samples where the compound specification
will be influenced by changing the colour.
6 Temperatures
6.1 General
The maximum surface temperature and the maximum value of the continuous operating
temperature of the compound shall not be exceeded during normal operation. The
encapsulation "mD“ apparatus shall be protected in such a way that the encapsulation "mD“
is not affected under specified fault conditions.

61241-18” IEC:2004 – 27 –
6.2 Temperature limitation
When a protective device is required to limit temperatures for safety reasons, it shall be a
non-resetting internal or external, electrical or thermal device. One device is required for
level of protection “mbD” and two devices for level of protection “maD”. The thermal coupling
of the device to the component being monitored shall be sufficient. For functional reasons the
“mD” apparatus may also contain an additional resettable protective device.
If the non-resetting protective device complies with IEC 60127 or IEC 60691, only one device
is necessary for both levels of protection.
Where the apparatus may be subject to fault, see 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, these shall be taken into account in determining the limiting
temperature.
6.3 Determination of the limiting temperature values
6.3.1 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 4.5.
6.3.2 Temperature of the components in 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.
NOTE The determination of the hottest component may be done by calculation or manufacturer's specification or
by a practical test prior to encapsulating the components.
7 Constructional requirements
7.1 General
Where the encapsulation forms part of the external enclosure it shall comply with the
requirements for non-metallic enclosures and parts of non-metallic enclosures.
If additional protective measures are required by the user in order to satisfy the requirements
of this standard, for example, additional mechanical protection, to indicate this special
condition of use, the apparatus shall be marked “X” in accordance with item l) of 29.2 of
IEC 61241-0.
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 to 7.7 the requirements differ according to whether the compound adheres to the
enclosure. Where adhesion is specified the aim is to prevent the ingress of combustible dust
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,
connections terminals, etc.). If additional treatment for the boundary surfaces is required in
order to ensure adhesion, this shall be included in the manufacturer's documentation.

61241-18” IEC:2004 – 29 –
Where adhesion is specified, 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
“mD”.
7.2 Determination of possible faults
7.2.1 Fault examination
The encapsulation protection shall be maintained even in the case of an unfavourable input
condition (but between 90 % and 110 % of the rated parameter) and unfavourable output load
and of any internal electrical fault (two faults for level of protection “maD” and one fault for
level of protection “mbD”), for example, in the event of
– a short circuit in any component;
– the failure of any component;
– a fault in the printed circuitry.
Infallible components and infallible separation distances shall not be subject to fault.
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) are considered to be a single fault.
7.2.2 Infallible components
The following components shall be considered to be infallible if they are encapsulated
according to the requirements of this standard, are suitable for the temperature range at the
site of installation and if they are not loaded with more than 2/3 of their rated voltage, rated
current and rated power specified by the manufacturer of the respective component:
x resistors, if they comply with 8.4 of IEC 60079-11,
x single-layer, spirally wound coils,
x plastic foil capacitors,
x paper capacitors,
x ceramic capacitors,
x semiconductors, if they are used in accordance with 8.6 of IEC 60079-11.
Where semiconductor devices are used to limit current, a single device is adequate for level
of protection "mbD“ but two devices shall be used for level of protection "maD“.
NOTE In contrast to the requirements for intrinsically safe apparatus with level of protection “iaD” according to
IEC 61241-11, it is not necessary to prohibit the use of active semiconductor regulating circuits, as the effect of a
short term transient disturbance is significantly less for encapsulated apparatus.
The following components for the segregation of different circuits shall be considered
infallible.
a) Optocouplers and relays, if the rated insulation voltage conforms to (2U + 1 000 V) or
1 500 V a.c. whichever is greater (U is the sum of the rated r.m.s. voltages of both
circuits).
b) Transformers, complying with IEC 61558-2-6 or IEC 61241-11.
c) Coils, transformers and motor windings that comply with IEC 60079-7, including those
that have wire diameters of less than 0,25 mm when they are also protected against
inadmissible internal temperatures, are considered infallible.

61241-18” IEC:2004 – 31 –
7.2.3 Infallible separation distances
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 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 7.2.3.1 and if applicable 7.2.3.2.
7.2.3.1 Distances through the compound
Distances through compound shall be considered to be infallible if they comply with the
values in Table 1, provided that they were fixed mechanically before encapsulation.
Table 1 – Distances through the compound
Voltage (see Note) Minimum distance
U r.m.s. or d.c., V mm
0,5
d63
d400
1,5
d500
d630 2
2,5
d 1 000
d 1 600 4
d 3 200
d 6 300
d 10 000 20
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 3b of IEC 60664-1.
7.2.3.2 Distances through solid insulation
Distances through solid insulation shall be considered to be infallible if the minimum
thickness of solid insulation is 0,1 mm and meets the dielectric strength test of 8.2.4.
7.2.4 Thickness of the compound
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 61241-1.
The minimum thickness of the compound with or without a surrounding enclosure shall
comply with the requirements of 7.2.5 to 7.2.7 as applicable.
In all cases the encapsulant is additionally subjected to the dielectric strength test of 8.2.4.

61241-18” IEC:2004 – 33 –
7.2.5 Encapsulation "mD" apparatus with free su
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