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Electrical Apparatus for Use in the Presence of Combustible Dust consists of the following parts:
— Part 1: General Requirements;
— Part 2: Selection and Installation;
— Part 3: Classification of Areas Where Combustible Dusts are or May be Present;
— Part 4: Protection by Intrinsic Safety “iD”;
— Part 5: Protection by Enclosures “tD”;
— Part 6: Protection by Encapsulation “mD”;
— Part 7: Type of Protection “pD”;
— Part 8: Test Methods — Methods for Determining the Minimum Ignition Temperatures of Dust;
— Part 9: Test Methods — Method for Determining the Electrical Resistivity of Dust in Layers;
— Part 10: Test Methods — Method for Determining Minimum Ignition Energy of Dust/Air Mixtures;
This part is part 3 of Electrical Apparatus for Use in the Presence of Combustible Dust.
This part is drafted in accordance with the rules given in GB/T 1.1-2009.
This part replaces GB/T 12476.3-2007 Electrical Apparatus for Use in the Presence of Combustible Dust — Part 3: Classification of Areas Where Combustible Dusts Are or May Be Present in whole.
The following technical deviations have been made with respect to the GB GB/T 12476.3-2007:
— the hazards presented by dusts are listed (see 4.1);
— the dust classification is added (see 3.7, 3.8 and 3.9);
— the concept of “equipment protection levels” (EPLs) is added (see Annex D);
— the area of Zone 22 extending beyond Zone 21 is expanded from 1 m to 3 m (see 6.3.4; 6.4.3 of Edition 2007).
This part has been redrafted and modified adoption of International Standard IEC 60079-10-2:2009 Explosive Atmospheres — Part 10-2: Classification of Areas — Combustible Dust Atmospheres.
The technical deviations between this part and the International Standard IEC 60079-10-2:2009, together with their justifications, is given below:
— Adjustment has been made to "Normative References" of this part due to technical deviations so as to meet the technical specification in China; the adjustment situations are embodied in a concentrated way in Clause 2 "Normative References" and the specific adjustments are as follows:
● normative references GB 12476.1-2013 (see Clause 3) and ISO/IEC 80079-20-2 (see 4.2) are added;
● ISO 60079-0 is replaced by GB 3836.1-2010, which is redrafted and modified adoption of the international standard.
For the purposes of this part, the following editorial changes have also been made:
— the standard name is changed;
— Note 4 in 4.1 is modified as: Additional information on EPLs is given in Annex D. The equipment for use in Zone 20 is equivalent to Grade “Da” equipment, the equipment for use in Zone 21 is equivalent to Grade “Db” equipment, and the equipment for use in Zone 22 is equivalent to Grade “Dc” equipment.
This part was proposed by the China Electrical Equipment Industrial Association.
This part is under the jurisdiction of SAC/TC 9 National Technical Committee on Explosion Protected Electrical Apparatus of Standardization Administration of China.
The previous editions of this part are as follows:
— GB 12476.3-2007.
Introduction
Dusts, as defined in this part, are hazardous because when they are dispersed in air by any means, they form potentially explosive atmospheres. Furthermore, layers of dust may ignite and act as ignition sources for an explosive atmosphere.
This part gives guidance on the identification and classification of areas where such hazards from dust can arise. It sets out the essential criteria against which the ignition hazards can be assessed and gives guidance on the design and control parameters which can be used in order to reduce such a hazard. General and special criteria are given, with examples, for the procedure used to identify and classify areas.
This part contains an informative Annex A giving practical examples for classifying areas.
Electrical Apparatus for Use in the Presence of Combustible Dust — Part 3: Classification of Areas Where Combustible Dusts Are or May Be Present
1 Scope
This part specifies the identification and classification of areas where explosive dust atmospheres and combustible dust layers are present, in order to permit the proper assessment of ignition sources in such areas.
In this part, 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 examples in this part are based on a system of effective housekeeping being implemented in the plant to prevent dust layers from accumulating. Where effective housekeeping is not present, the area classification includes the possible formation of explosive dust clouds from dust layers.
The principles of this part can also be followed when combustible fibres or flyings may cause a hazard.
This part is intended to be applied where there can be a risk due to the presence of explosive dust atmospheres or combustible dust layers under normal atmospheric conditions.
This part does not apply to:
— underground mining areas;
— areas where a risk can arise due to the presence of hybrid mixtures;
— dusts of explosives that do not require atmospheric oxygen for combustion, or to pyrophoric substances;
— catastrophic failures which are beyond the concept of abnormality dealt with in this part (see Note 1);
— any risk arising from an emission of flammable or toxic gas from the dust.
This part 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 part.
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.
GB 3836.1-2010 Explosive Atmospheres — Part 1: Equipment — General Requirements (IEC 60079-0: 2007, MOD)
GB 12476.1-2013 Electrical Apparatus for Use in the Presence of Combustible Dust — Part 1: General Requirements (IEC 61241-0: 2004, MOD)
ISO/IEC 80079-20-2 Explosive Atmospheres — Part 20-2: Material Characteristics — Combustible Dusts Test Methods
3 Terms and Definitions
For the purposes of this document, the terms and definitions given in GB 3836.1-2010 and GB 12476.1-2013 and the following apply.
Note: Additional definitions applicable to explosive atmospheres can be found in GB/T 2900.35-2008.
3.1
area
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
3.3
hybrid mixture
mixture of flammable substances in different physical states, with air
Note: An example of a hybrid mixture is a mixture of methane, coal dust and air.
3.4
dust
generic term including both combustible dust and combustible flyings
3.5
combustible dust
finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures
Note 1: This definition may also include dust and grit as defined in HJ 492.
Note 2: The term ‘solid particle’ is intended to address particles in the solid phase and not the gaseous or liquid phase, but does not preclude a hollow particle.
3.6
explosive dust atmosphere
mixture with air, under atmospheric conditions, of flammable substances in the form of dust, or flyings in which, after ignition, permits self-sustaining flame propagation
3.7
conductive dust
combustible dust with electrical resistivity equal to or less than 103 Ω·m
3.8
non-conductive dust
combustible dust with electrical resistivity greater than 103 Ω·m
3.9
combustible flyings
solid particles, including fibres, greater than 500 μm in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, can burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures
Note: Examples of fibres and flyings include rayon, cotton (including cotton linters and cotton waste), sisal, jute, hemp, cocoa fibre, oakum, and baled waste kapok.
3.10
hazardous area (dust)
area in which combustible dust, in the form of a cloud is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment
Note 1: Hazardous areas are divided into zones based upon the frequency and duration of the occurrence of explosive dust atmospheres (see 6.2 and 6.3).
Note 2: The potential of creating an explosive dust cloud from a dust layer also needs to be considered.
3.11
non-hazardous area (dust)
an area in which combustible dust in the form of a cloud is not expected to be present in quantities such as to require special precautions for the construction, installation and use of equipment
3.12
dust containment
process equipment housing which is intended to handle, process, transport or store materials inside of it, while preventing the release of combustible dust to the surrounding atmosphere
3.13
source of dust release
point or location from which combustible dust can be released into the atmosphere
Note: This can be from a dust containment or a dust layer.
3.14
continuous grade of release
release which is continuous or is expected to occur frequently or for long periods
3.15
primary grade of release
release which can be expected to occur periodically or occasionally during normal operation
3.16
secondary grade of release
release which is not expected to occur in normal operation and, if it does occur, is likely to do so only infrequently and for short periods
3.17
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
Foreword II
Introduction IV
1 Scope
2 Normative References
3 Terms and Definitions
4 Area Classification
5 Sources of Release
6 Zones
7 Dust Layer Hazard
8 Documentation
Annex A (Informative) Area Classification Application
Annex B (Informative) Risk of Fire from Hot Surface Ignition of Dust Layer
Annex C (Informative) Housekeeping
Annex D (Informative) Introduction of an Alternative Risk Assessment Method Encompassing ‘Equipment Protection Levels’ for Ex Equipment
Bibliography
