EN 50281-3:2002

SLOVENSKI SIST EN 50281-3:2003

STANDARD
marec 2003
Električne naprave za uporabo ob prisotnosti gorljivega prahu - 3. del:
Razdelitev območij, v katerih je ali je lahko prisoten gorljiv prah
Equipment for use in the presence of combustible dust -- Part 3: Classification of
areas where combustible dusts are or may be present
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 50281-3
NORME EUROPÉENNE
EUROPÄISCHE NORM September 2002
ICS 29.100.01
English version
Equipment for use in the presence of combustible dust
Part 3: Classification of areas where combustible dusts
are or may be present
Appareils pour utilisation en présence Betriebsmittel zur Verwendung in
de poussières combustibles Bereichen mit brennbarem Staub
Partie 3: Classement des emplacements Teil 3: Einteilung von
où des poussières combustibles sont staubexplosionsgefährdeten Bereichen
ou peuvent être présentes
This European Standard was approved by CENELEC on 2002-03-05. 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, Czech Republic,
Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta,
Netherlands, Norway, Portugal, Slovakia, 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
© 2002 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 50281-3:2002 E
Foreword
This European Standard was prepared by a joint Working Group (JWG 17) from CENELEC TC 31,
Electrical apparatus for explosive atmospheres - General requirements, and CEN TC 305.
The text of the draft was submitted to the Unique Acceptance Procedure and was approved by
CENELEC as EN 50281-3 on 2002-03-05.
This European Standard was prepared under a mandate given to CENELEC by the European
Commission and the European Free Trade Association and supports the essential safety
requirements of the EC Directive 94/9/EC.
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) 2003-03-01
- latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2005-03-01
Annexes designated "informative" are given for information only.
In this standard, annexes A to C are informative.
__________
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Contents
Introduction.4
1 Scope.5
2 Normative references.5
3 Definitions.6
4 Area classification for combustible dusts.7
5 Sources of release for explosive dust atmospheres.9
6 Zones for explosive dust atmospheres.10
7 Dust layer hazard .13
8 Documentation.13
Annex A (informative) Examples of area classification .16
Annex B (informative) Risk of fire from hot surface ignition of dust layer .20
Annex C (informative) Housekeeping.23
Figure 1 – Identification of zones on drawings .15
Figure A.1 – Area classification for a bag emptying station within a building and without
exhaust ventilation .16
Figure A.2 – Area classification for a bag emptying station within a building and with exhaust
ventilation.17
Figure A.3 – Area classification for cyclone and filter with clean outlet outside building .18
Figure A.4 – Area classification for a drum tipper within a building without exhaust
ventilation.19
Figure B.1 – Reduction in the maximum permissible surface temperature of
apparatus for increasing depth of dust layers.21
Table 1 - Designation of zones depending on presence of combustible dust.10

Introduction
Combustible dusts are hazardous because when they are dispersed in air by any means they form
potentially explosive atmospheres. Further, layers of combustible dust may ignite and act as ignition
sources for an explosive atmosphere.
Therefore, equipment placed in an environment where dust clouds can form should be dust ignition
protected and have a surface temperature limitation below the temperature at which a dust cloud or
layer will ignite.
This standard gives guidance on the identification of areas where hazards from combustible dust can
arise. The purpose is to permit selection of appropriate equipment for use in such areas. General and
special criteria are given, with examples, for the procedure used to identify areas.
By exercising ingenuity in the layout of equipment, it is frequently possible to locate much of the
equipment in less hazardous or in non-hazardous locations and thus, to reduce the amount of special
equipment required.
This standard contains an informative annex giving practical examples for classifying areas.

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1 Scope
This standard is concerned with the classification of areas where explosive dust/air mixtures and
combustible dust layers are present, in order to permit the proper selection of equipment for use in
such areas.
In this standard, explosive dust atmospheres and combustible dust layers are treated separately. In
Clause 4 area classification for explosive dusts clouds is described, with dust layers acting as one of
the possible sources of release. In Clause 7 the hazard of dust layer ignition is described.
The standard assumes effective housekeeping based on a system of cleaning for the plant.
The principles of the standard can also be followed when combustible fibres or flyings may cause a
hazard.
This standard is intended to be applied where there can be a risk due to the presence of explosive
dust/air mixtures or combustible dust layers under normal atmospheric conditions. It does not apply to
� underground mining areas,
� areas where a risk can arise due to the presence of hybrid mixtures,
� dusts of explosives which do not require atmospheric oxygen for combustion, or to pyrophoric
substances,
� catastrophic failures, which are beyond the concept of abnormality dealt with in this standard (see
NOTE 1),
� any risk arising from an emission of flammable or toxic gas from the dust.
This standard does not take into account the effects of consequential damage following a fire or an
explosion.
NOTE 1  Catastrophic failure in this context is applied, for example, to the rupture of a storage silo or a pneumatic conveyor.
NOTE 2  In any process plant, irrespective of size, there can be numerous sources of ignition apart from those associated with
equipment. Appropriate precautions will be necessary to ensure safety in this context, but these are outside the scope of this
standard.
2 Normative references
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
EN 1127-1 Explosive atmospheres - Explosion prevention and protection
Part 1: Basic concepts and methodology
1)
EN 13237-1 Potentially explosive atmosphere – Terms and definitions for equipment and protective
systems intended for use in potentially explosive atmospheres
EN 50281-1-1 Electrical apparatus for use in the presence of combustible dust
Part 1-1: Electrical apparatus protected by enclosures - Construction and testing
———————
1)
At draft stage.
EN 50281-1-2 Electrical apparatus for use in the presence of combustible dust
Part 1-2: Electrical apparatus protected by enclosures - Selection, installation and
maintenance
EN 50281-2-1 Electrical apparatus for use in the presence of combustible dust
Part 2-1: Test methods - Methods for determining the minimum ignition temperatures
of dust
IEC 61241-3 Electrical apparatus for use in the presence of combustible dust
Part 3: Classification of areas where combustible dusts are or may be present.
ISO 4225 Air quality - General aspects – Vocabulary
3 Definitions
For the purpose of this standard the following definitions apply.
3.1
area
a three-dimensional region or space
3.2
atmospheric conditions (surrounding conditions)
conditions that include variations in pressure and temperature above and below reference levels of
101,3 kPa (1 013 mbar) and 20 °C (293 K), provided that the variations have a negligible effect on the
explosive properties of the combustible dust (IEC 61241-3)
3.3
hybrid mixture
mixture of flammable substances in different physical states, with air (EN 1127-1)
NOTE  An example of a hybrid mixture is a mixture of methane, coal dust and air.
3.4
dust
small solid particles including fibres and flyings in the atmosphere which settle out under their own
weight, but which may remain suspended in air for some time (includes dust and grit as defined in
ISO 4225)
3.5
explosive dust atmosphere
a mixture with air, under atmospheric conditions, of flammable substances in the form of dust or fibres
in which, after ignition, combustion spreads throughout the unconsumed mixture (IEV 426-02-04)
3.6
combustible dust
dust that can burn or glow in air and could form explosive mixtures with air at atmospheric pressure
and normal temperatures
3.7
hazardous area (dust)
area in which combustible dust in cloud form is, or can be expected to be, present in quantities such
as to require special precautions for the construction, installation and use of equipment in order to
prevent ignition of an explosive dust/air mixture. Hazardous areas are divided into zones based upon
the frequency and duration of the occurrence of explosive dust/air mixture

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3.8
non-hazardous area (dust)
area in which combustible dust in cloud form is not expected to be present in quantities such as to
require special precautions for the construction, installation and use of apparatus
3.9
dust containment
those parts of the process equipment inside which materials are handled, processed, transported or
stored e.g. to prevent the release of dust to the surrounding atmosphere
3.10
source of dust release
a point or location from which combustible dust can be released to the atmosphere. This can be either
from a dust containment or a dust layer.
Sources of release will be divided into the following grades depending on the order of decreasing
severity:
� continuous formation of a dust cloud: Locations in which a dust cloud may exist continuously, or
may be expected to continue for long periods or for short periods which occur frequently;
� primary grade of release: A source can be expected to release combustible dust in normal
operation occasionally;
� secondary grade of release: A source which is not expected to release combustible dust during
normal operation but if it releases, is likely to do so only infrequently and for short periods only
3.11
extent of zone
distance in any direction from the edge of a source of release to the point where the hazard
associated with the release is considered to exist no longer
3.12
normal operation
the situation when the process equipment is operating within its design parameters. Minor releases of
dust which may form a cloud or layer (e.g. releases from filters) can be part of normal operation
3.13
abnormal operation
expected process linked malfunctions that can occur infrequently
3.14
equipment
machines, apparatus, fixed or mobile devices, control components and instrumentation thereof and
detection or prevention systems which, separately or jointly, are intended for the generation, transfer,
storage, measurement, control and conversion of energy or the processing of material and which are
capable of causing an explosion through their own potential sources of ignition
4 Area classification for combustible dusts
4.1 General
This standard adopts the concept, similar to that used for flammable gases and vapours, of using area
classification to give an assessment of the risk of fire and/or explosion from dust clouds.
Hazardous and non hazardous areas are defined in 3.7 and 3.8, respectively.

Combustible dusts form explosive atmospheres only at concentrations in the explosion range.
Although a cloud with a very high concentration may not be explosive, however the danger exists that
should the concentration fall, it may enter the explosion range. Depending on the circumstances, not
every source of release will necessarily produce an explosive dust/air mixture.
Dusts which are not removed by mechanical extraction ventilation settle out, at a rate depending on
among other things particle size, into layers or accumulations. It has to be taken into account that a
dilute or small continuous source of release in time is able to produce a potentially hazardous dust
layer.
The hazards presented by combustible dusts are
� the formation of a dust cloud from any source of release including a layer or accumulation to form
an explosive atmosphere (see clause 5),
� the formation of dust layers which are not likely to form a dust cloud but which may ignite due to
self heating or hot surfaces and cause a fire hazard or over heating of equipment. The ignited
layer may also act as an ignition source for an explosive atmosphere (see clause 7).
Explosive dust clouds and combustible dust layer may exist therefore sources of ignition should be
avoided.
If this cannot be done, then measures should be taken to reduce the likelihood of combustible dust
and/or ignition sources so that the likelihood of coincidence is so small as to be acceptable. In some
cases, it can be necessary to employ some form of explosion protection such as explosion venting or
explosion suppression.
In this standard, explosive dust atmospheres and ignitable dust layers are treated separately. In this
clause area classification for explosive dust clouds is described, with dust layers acting as one of the
possible sources of release. In clause 7 the hazard of dust layer ignition is described.
4.2 Area classification objectives for explosive dust atmospheres
In most practical situations where combustible dusts are present, it is difficult to ensure that an
explosive dust/air mixture will never occur. It can also be difficult to ensure that equipment will never
give rise to a source of ignition. Therefore, in situations where an explosive dust/air mixture has a high
likelihood of occurring, reliance is placed on using equipment which is designed to have an extremely
low likelihood of creating a source of ignition.
Conversely, where the likelihood of an explosive dust/air mixture occurring is lower, equipment
constructed to a less rigorous specification can be used.
4.3 Area classification procedure for explosive dust atmospheres
Area classification is based on an informed input from a number of sources. The decision to area
classify depends on whether the dust is combustible or not. Dust combustibility can be confirmed by
laboratory tests. An understanding of the material characteristics to be used in the process is required
and these should be obtained from a process specialist. Account has to be taken of the operating and
maintenance regime for the plant including the housekeeping. Specialist engineering knowledge may
also be necessary to provide information on the nature of releases from particular items of plant. Close
co-operation is necessary from specialists in safety and equipment. The definitions for zones of risk
deal only with the cloud risk.
a) The first step is to identify the material characteristics, e.g. particle size, moisture content, cloud
and layer minimum ignition temperature and the electrical resistivity.
b) The second step is to identify where dust containment or sources of dust release can be present,
as given in 5.2. It may be necessary to consult process line diagrams and drawings of plant layout.
This step should include the identification of the possibility of the formation of dust layers as given
in clause 7.
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c) The third step is to determine the likelihood that dust will be released from those sources and thus,
the likelihood of explosive dust/air mixtures in various parts of the installation as given in 5.2.2
It is only after these steps that the zones can be identified and their extents defined. The decisions on
the zone types and extents and the presence of dust layers has to be recorded on the area
classification drawing. (The drawing is to be used subsequently as the basis for the selection of
equipment).
The reasons for the decisions taken should be recorded in notes of the area classification study to
facilitate understanding at future area classification reviews. Reviews of the area classification shall
take place following changes to the process or changes to process materials or if dust escapes
become more common due to deterioration of the plant. It may be appropriate to hold reviews on a
periodic basis.
Because this standard is to cover a wide range of circumstances, no exact identification of necessary
measures can be given for each individual case. It is important, therefore, that the recommended
procedure should be carried out by personnel having knowledge of the principles of area classification,
the process material used, the plant involved and its functioning.
5 Sources of release for explosive dust atmospheres
5.1 General
Explosive dust atmospheres are formed from sources of dust release. Sources of dust release are a
point or location from which combustible dust can be released or raised, such that an explosive
dust/air atmosphere can be formed. This includes layers of combustible dust capable of being
dispersed to form a dust cloud. Depending on the circumstances, not every source of release will
necessarily produce an explosive dust/air mixture. On the other hand a dilute or small continuous
source of release in time can produce a potentially hazardous dust layer.
The types of sources of release are defined in 3.10.
5.2 Identification of sources of release
The conditions need to be identified in which process equipment, process steps or other actions that
can be expected in plants, can form explosive dust/air mixtures or create combustible dust layers. It is
necessary to consider separately the inside and outside of a dust containment.
5.2.1 Dust containment
Inside a dust containment dust is not released to the atmosphere but as part of the process
continuous dust clouds may be formed. These may exist continuously or may be expected to continue
for long periods or for short periods which occur frequently depending on the process cycle. The
equipment should be studied for normal operation, abnormal operation and in the startup and
shutdown condition so that the incidence of cloud and layer presence can be identified. Where thick
layers are formed these should be noted (see clause 7 for dust layers).
5.2.2 Sources of release
Outside the dust containment many factors can influence the area classification. Where higher than
atmospheric pressures are used within the dust containment (positive pressure pneumatic transfer)
dust can easily be blown out of leaking equipment. In the case of negative pressure within the dust
containment, the likelihood of formation of dusty areas outside the equipment is very low. Dust particle
size, moisture content and where applicable transport velocity, dust extraction rate and fall height can
influence release rate potential. Once the process potential for release is known, each source of
release must be identified and its grade of release determined.

Grades of release are:
� primary grade of release: examples are the close vicinity around an open bag filling or emptying
point;
� secondary grade of release: an example is a dust handling plant where deposits of dust are
present outside.
The following items should not be regarded as sources of release during normal and abnormal
operation:
� pressure vessels, the main structure of the shell including its closed nozzles and manholes;
� pipes, ducting and trunking without joints;
� valve glands and flanged joints, provided, in the design and construction adequate consideration
has been given to the prevention of leakage of dust.
Based on the likelihood of the formation of potentially explosive dust/air mixtures the areas can be
designated according to Table 1.
Table 1 - Designation of zones depending on presence of combustible dust
Presence of combustible dust Resulting zone classification of area of dust clouds
Continuous presence of a dust cloud 20
Primary grade of release 21
Secondary grade of release 22
NOTE 1  Some silos may be filled or emptied only infrequently, and the inside may then be classified as zone 21. Equipment
inside the silo may be used only when the silo is being emptied or filled. Equipment selection should take account of the fact
that the dust cloud is likely to be present while the equipment is in operation.
NOTE 2  The rare event of a burst of a large container of dust may cause a deep layer to form. If any deep layer formed in this
way is removed quickly or the equipment isolated, it may not be necessary to classify the area as zone 22.
NOTE 3  Many products such as grain and sugar contain a small amount of dust mixed into a large amount of granular
material. Equipment selection should take account of the risk that the coarse material can be overheated and start to burn, even
if no dust explosion is possible at that location. Burning granular material may be transported on through a process, to create
the risk of an explosion elsewhere.
6 Zones for explosive dust atmospheres
6.1 General
Areas classified for explosive dust atmospheres are divided into zones; based upon the frequency and
duration of the occurrence of explosive dust/air atmospheres.
The classificati
...