IEC 60079-10-2 Explained: Classification of Areas – Combustible Dust Atmospheres
Hazardous area classification for combustible dust requires an assessment of dust release, accumulation, and operating conditions.
Combustible dust can create explosion hazards when fine particles become suspended in air under conditions that allow combustion to propagate. These hazards occur in industries such as grain handling, food processing, sugar production, woodworking, pharmaceutical manufacturing, coal handling, and metal powder processing.
IEC 60079-10-2 – Explosive Atmospheres – Part 10-2: Classification of Areas – Combustible Dust Atmospheres provides the methodology for classifying areas where combustible dust atmospheres may occur. The classification determines whether an area is designated Zone 20, Zone 21, or Zone 22.
This classification is an important input to hazardous-area drawings, equipment selection, and subsequent installation engineering. It must be based on the actual process and facility conditions rather than generic assumptions.
1. What Does IEC 60079-10-2 Cover?
IEC 60079-10-2 addresses the classification of areas where combustible dust atmospheres may be present. The assessment considers potential dust release sources, the likelihood and duration of explosive dust clouds, dust accumulation, and the extent of the hazardous area.
The standard establishes the classification. It does not, by itself, specify all construction and installation requirements for the equipment selected for that area.
The engineering sequence is generally:
Process Assessment → Dust Release Evaluation → Area Classification → Ex Equipment Selection → Installation
This work is relevant during process design, FEED, detailed engineering, and reviews of changes to existing facilities.
2. Understanding Combustible Dust Hazards
Materials handled as bulk solids may present a different hazard when finely divided and dispersed in air. Examples of potentially combustible dusts include:
- Grain and flour dust
- Sugar dust
- Wood dust
- Coal dust
- Aluminium dust
- Plastic powders
- Pharmaceutical powders
Not every dust has the same ignition or explosion characteristics. The assessment should use appropriate information about the actual material, including its relevant combustible properties, rather than relying only on its general name or appearance.
3. Dust Release Sources and Accumulation
A dust release source is a point where combustible material may escape from process equipment and become airborne. Typical examples include:
- Conveyor transfer points
- Bagging and filling stations
- Dust collectors and filter systems
- Bucket elevators
- Screw conveyors
- Mixers, blenders, and milling equipment
- Hoppers and silos
Each source must be evaluated according to its operating conditions and release characteristics. A single generic classification should not automatically be applied to an entire building or process area.
Dust clouds versus dust layers
A suspended dust cloud can form an explosive atmosphere when the material and surrounding conditions permit an explosion. Deposited dust layers are also important because they may be disturbed and become airborne, contribute to smouldering or ignition, or provide fuel for a secondary explosion.
Dust may accumulate on floors, cable trays, structural steel, equipment surfaces, lighting fixtures, and ventilation systems. The assessment should therefore consider the facility as operated, including credible dust accumulation and cleaning conditions.
Housekeeping and dust-control measures can influence the overall risk, but the classification should not assume ideal cleaning conditions unless those conditions are supported by the facility's actual operating arrangements.
4. Zone 20, Zone 21, and Zone 22
IEC 60079-10-2 uses three zones to describe the likelihood and duration of an explosive combustible dust atmosphere.
| Zone | General description |
|---|---|
| Zone 20 | An area where an explosive dust atmosphere, in the form of a cloud of combustible dust in air, is present continuously, frequently, or for long periods. |
| Zone 21 | An area where an explosive dust atmosphere is likely to occur occasionally during normal operation. |
| Zone 22 | An area where an explosive dust atmosphere is not likely to occur during normal operation and, if it does occur, persists only for a short period. |
These descriptions are the starting point for classification, not a substitute for an engineering assessment. The assigned zone depends on the release conditions, dust characteristics, process operation, and surrounding environment.
5. Typical Locations and Classification Considerations
The following examples illustrate locations that may require particular attention during a dust classification study.
| Location | Classification consideration |
|---|---|
| Inside a dust collector | May require assessment for Zone 20 where explosive dust clouds are present continuously, frequently, or for long periods. |
| Inside a silo or hopper | May require assessment for Zone 20, depending on the material and operating conditions. |
| Near bag-filling equipment | Potential Zone 21 or Zone 22 conditions depend on the frequency and duration of dust releases. |
| Near bucket elevator discharge points | Classification depends on the release source, containment, and normal operating conditions. |
| Adjacent to enclosed conveyors | Potential external dust releases and accumulation should be evaluated. |
These are not automatic zone assignments. Actual classification and the extent of each zone must be determined from the specific installation. Enclosed equipment and the surrounding external area may also require separate assessments.
6. Determining the Extent of a Dust Hazardous Area
After identifying a hazardous location, the study must determine how far the classified area extends from the relevant source. The result should reflect the actual process and physical layout.
Factors that may affect the assessment include:
- Quantity and frequency of dust released
- Particle size and relevant dust properties
- Containment and process equipment arrangement
- Air movement and ventilation
- Building geometry and surrounding structures
- Dust deposition and cleaning practices
- Normal operating conditions and foreseeable process changes
Hazardous-area drawings should be developed from facility-specific engineering information. Copying zone boundaries from another installation without verifying the underlying conditions can produce an unsuitable classification.
7. Indoor and Outdoor Dust Hazards
Indoor and outdoor installations can both present combustible dust hazards. Indoor areas may allow dust to accumulate on equipment and structures, while building geometry and limited air movement may influence dust dispersion.
Outdoor locations may benefit from natural air movement, but this does not automatically eliminate the hazard. Local containment, wind conditions, equipment arrangement, and dust deposition can still result in classified areas.
The classification should be based on the relevant conditions at the installation rather than on a simple indoor-versus-outdoor distinction.
8. Practical Workflow for a Dust Classification Study
A project team can organize the assessment around the following activities:
- Identify the materials. Establish which materials may form combustible dust atmospheres and obtain relevant material data.
- Review the process. Understand the equipment, operating modes, material transfer, and containment arrangements.
- Identify release sources. Locate openings, transfer points, filling stations, vents, and other potential release locations.
- Assess dust clouds and deposits. Consider airborne dust formation and locations where layers may accumulate.
- Evaluate the operating environment. Review ventilation, building geometry, cleaning practices, and credible operating conditions.
- Determine zone and extent. Establish the appropriate zone and its boundaries based on the assessment.
- Document the result. Record the classification and its supporting assumptions on the relevant engineering documents and drawings.
- Apply the classification. Use the result as an input to equipment selection and installation design.
Where key material or process information is unavailable, the assumptions and information gaps should be identified. They should not be silently replaced with generic zone assignments.
9. IEC 60079-10-2 and Ex Equipment Selection
Area classification establishes the type and extent of the hazardous location. Equipment selection must then consider the applicable equipment standards, the zone, equipment protection level (EPL), dust group, maximum surface temperature, protection concept, and installation requirements.
For combustible dust applications, IEC 60079-31 addresses equipment dust ignition protection by enclosure "t". It is an equipment protection standard, not the area-classification standard.
Selection should be checked against the equipment marking, certificate and its applicable conditions, manufacturer's instructions, and project requirements. A dust zone alone is not enough to establish that a particular product is suitable.
For related background, see Dust Groups IIIA vs IIIB vs IIIC Explained and EPL Da vs Db vs Dc Explained.
10. Procurement and RFQ Review
For EPC and procurement teams, the classification should be traceable to the project's hazardous-area drawings and supporting engineering documents before a quotation is technically evaluated.
A practical RFQ review should confirm:
- What dust zone applies at the equipment location?
- What material and dust characteristics are relevant?
- Which approved project drawing or document defines the classification?
- What dust group, EPL, and maximum surface temperature are required?
- Which protection concept and equipment marking are acceptable?
- Are environmental conditions, dust deposits, or cleaning practices relevant to selection?
- Do the certificate and manufacturer's instructions cover the proposed installation?
An RFQ that only states "Ex equipment for dust area" may not provide enough information for a complete technical evaluation. Missing classification or equipment requirements should be clarified with the responsible engineering team.
11. Common Classification Mistakes
Common issues that can undermine a dust hazardous-area study include:
- Assuming a material is non-combustible without adequate technical information.
- Ignoring dust layers and deposits outside the process equipment.
- Assuming housekeeping is always effective without reviewing actual practice.
- Assigning zones based only on equipment type or generic examples.
- Applying gas-area classification principles directly to dust hazards.
- Assuming outdoor locations are automatically non-hazardous.
- Using generic drawings without checking the actual process and facility.
- Failing to review the classification after material, process, or layout changes.
Classification documents should be reviewed when changes could affect dust release, accumulation, dispersion, or the boundaries of the hazardous area.
12. IEC 60079-10-2 vs IEC 60079-10-1
These standards classify different types of explosive atmospheres.
| Standard | Application | Zones |
|---|---|---|
| IEC 60079-10-1 | Explosive gas atmospheres | Zone 0, Zone 1, Zone 2 |
| IEC 60079-10-2 | Combustible dust atmospheres | Zone 20, Zone 21, Zone 22 |
Gas and dust hazards have different physical characteristics and require the applicable classification methodology. Where both hazards are present, each relevant hazard should be assessed using the appropriate standard.
13. Related IEC 60079 Standards
| Standard | Primary role |
|---|---|
| IEC 60079-10-1 | Classification of explosive gas atmospheres. |
| IEC 60079-10-2 | Classification of combustible dust atmospheres. |
| IEC 60079-31 | Equipment dust ignition protection by enclosure "t". |
| IEC 60079-14 | Design, selection, and installation of electrical installations. |
| IEC 60079-17 | Inspection and maintenance of electrical installations. |
Together, these standards support different stages of the hazardous-area lifecycle, from classification through equipment selection, installation, and subsequent inspection and maintenance.
Frequently Asked Questions
What is IEC 60079-10-2?
It provides the methodology for classifying areas where combustible dust atmospheres may occur and determining Zone 20, Zone 21, and Zone 22.
What is the difference between Zone 20, Zone 21, and Zone 22?
The zones distinguish the likelihood and duration of an explosive dust atmosphere. Zone 20 represents continuous, frequent, or long-duration presence; Zone 21 represents likely occasional presence during normal operation; and Zone 22 represents an atmosphere not likely during normal operation and present only briefly if it occurs.
Why are dust layers important?
Deposited dust can be disturbed and become airborne, contributing to an explosive dust cloud. Accumulations can also contribute to ignition, overheating, and secondary explosion hazards.
Does IEC 60079-10-2 cover gas atmospheres?
No. IEC 60079-10-2 addresses combustible dust atmospheres. IEC 60079-10-1 addresses explosive gas atmospheres.
Does a dust zone determine which equipment to buy?
The zone is an important selection input, but it is not sufficient on its own. Equipment suitability also depends on applicable marking, EPL, dust group, surface-temperature requirements, protection concept, certification, installation conditions, and project specifications.
Conclusion
IEC 60079-10-2 provides the framework for classifying areas where combustible dust atmospheres may occur. A sound assessment considers release sources, dust clouds, accumulation, operating conditions, and the extent of the hazardous area.
The resulting Zone 20, Zone 21, or Zone 22 classification supports downstream equipment selection and installation decisions. The classification must remain specific to the actual facility and should be reviewed when relevant process or operating conditions change.
References
- IEC 60079-10-2 – Explosive Atmospheres – Part 10-2: Classification of Areas – Combustible Dust Atmospheres.
- IEC 60079-10-1 – Explosive Atmospheres – Part 10-1: Classification of Areas – Explosive Gas Atmospheres.
- IEC 60079-31 – Explosive Atmospheres – Equipment Dust Ignition Protection by Enclosure "t".
- IEC 60079-14 – Explosive Atmospheres – Electrical Installations Design, Selection and Installation.
- IEC 60079-17 – Explosive Atmospheres – Electrical Installations Inspection and Maintenance.
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