Zone 20 vs Zone 21 vs Zone 22 Explained
Dust collectors, silos, bagging stations, and powder transfer systems can all create combustible dust hazards, but they do not necessarily require the same hazardous area classification. The classification depends on how often an explosive dust atmosphere can occur and how long it is expected to remain present.
IEC 60079-10-2 provides the framework for classifying areas where combustible dust atmospheres may occur. It distinguishes three zones: Zone 20, Zone 21, and Zone 22.
For equipment selection, the distinction matters because the zone classification helps establish the required Equipment Protection Level (EPL). The final decision must also account for the dust properties, process conditions, equipment certification, and installation requirements.
Zone 20, Zone 21, and Zone 22 at a Glance
| Classification | When an explosive dust atmosphere may occur | Associated EPL |
|---|---|---|
| Zone 20 | Continuously, for long periods, or frequently | Da |
| Zone 21 | Occasionally during normal operation | Db |
| Zone 22 | Not likely during normal operation; if it occurs, it is expected to be infrequent and short-lived | Dc |
These are the general relationships used when selecting equipment for dust zones. The classification is based on the likelihood and duration of an explosive dust atmosphere, not simply on whether dust can be seen in the area.
What Is Zone 20?
Zone 20 is an area where an explosive dust atmosphere, in the form of a cloud of combustible dust in air, is present continuously, for long periods, or frequently.
It is commonly considered during the assessment of process equipment in which dust is routinely contained, conveyed, separated, or handled. The exact extent of the classified area depends on the process and the results of the area classification study.
Examples that may require Zone 20 assessment
- Interiors of grain or flour silos.
- Dust collector interiors.
- Enclosed powder handling vessels.
- Parts of pneumatic conveying systems where explosive dust clouds are expected frequently.
- Enclosed milling or mixing equipment that regularly contains explosive dust clouds.
These are examples, not automatic zone assignments. The actual classification must be established from the operating conditions, dust release characteristics, and relevant engineering assessment.
Equipment selection for Zone 20
Zone 20 installations generally require equipment suitable for EPL Da. Equipment marking, dust group, maximum surface temperature, ambient conditions, and certificate scope must all be checked against the application.
What Is Zone 21?
Zone 21 is an area where an explosive dust atmosphere is likely to occur occasionally during normal operation.
This can happen around process points where dust is released as part of routine activities, such as transferring, filling, emptying, or handling powder. Whether a particular location qualifies as Zone 21 depends on the frequency and duration of the dust cloud and the effectiveness of containment or extraction.
Examples that may require Zone 21 assessment
- Areas around some bag filling or emptying operations.
- Dust release points at grain or powder transfer systems.
- Locations near equipment that releases dust periodically during normal operation.
- Areas around powder processing equipment where explosive dust clouds can occur occasionally.
Being close to a dust-generating machine does not automatically make an area Zone 21. The classification depends on the actual release scenario, ventilation, containment, and the expected presence of an explosive dust cloud.
Equipment selection for Zone 21
Zone 21 applications generally require equipment suitable for EPL Db or a higher protection level that is appropriate for the installation. The equipment must also be certified for the relevant dust group and surface temperature limits.
What Is Zone 22?
Zone 22 is an area where an explosive dust atmosphere is not likely to occur during normal operation. If a dust cloud does occur, it is expected to be infrequent and to persist only for a short period.
Zone 22 does not mean that dust is harmless or that the area requires no special precautions. Dust releases, equipment failures, cleaning activities, and accumulated dust may still create hazards that need to be assessed.
Examples that may require Zone 22 assessment
- Some areas around enclosed powder handling equipment where dust escape is infrequent.
- Locations near transfer points with effective containment and occasional abnormal releases.
- Parts of packaging or storage facilities where an explosive dust cloud is expected only infrequently.
- Areas where a dust release is possible following an abnormal event, subject to the classification study.
The boundaries of Zone 22 must be established from the release scenario and site conditions. A warehouse or packaging area should not be assigned Zone 22 solely because it handles combustible material.
Equipment selection for Zone 22
Zone 22 applications generally require equipment suitable for EPL Dc or a higher protection level appropriate to the installation. Dust ingress protection, surface temperature, certification, and environmental conditions remain important selection criteria.
How Zone Classification Relates to EPL
Equipment Protection Level identifies the level of protection provided by equipment in relation to ignition risk. For combustible dust atmospheres, the commonly associated zone and EPL relationships are:
| Dust zone | Associated EPL | General selection principle |
|---|---|---|
| Zone 20 | Da | Equipment suitable for the highest protection level in dust applications. |
| Zone 21 | Db | Equipment suitable for the required protection level in Zone 21. |
| Zone 22 | Dc | Equipment suitable for the required protection level in Zone 22. |
Equipment with a higher EPL may be suitable for a zone requiring a lower EPL, provided that its certificate, complete marking, and conditions of use permit the application. Do not assume suitability based on EPL alone.
For more detail, see EPL Da vs Db vs Dc Explained and Equipment Protection Level (EPL) Explained.
Does Zone Classification Determine the Ex t Marking?
No. The zone classification and the equipment protection marking are related, but they are not the same thing.
For dust protection, IEC 60079-31 addresses equipment protection by enclosure “t”. Depending on the equipment design and certification, markings may include Ex ta, Ex tb, or Ex tc. These designations must be read together with the complete marking and certificate.
| Marking example | What it indicates | What still needs checking |
|---|---|---|
Ex ta ... Da |
A dust protection concept using enclosure “t” with EPL Da. | Dust group, surface temperature, certificate, and installation conditions. |
Ex tb ... Db |
A dust protection concept using enclosure “t” with EPL Db. | Permitted zone, dust group, surface temperature, and certificate scope. |
Ex tc ... Dc |
A dust protection concept using enclosure “t” with EPL Dc. | Permitted zone, dust group, surface temperature, and certificate scope. |
The ellipses in these examples represent other marking elements that have not been specified. They are illustrative only, not complete equipment markings or a substitute for certificate review.
Do not select equipment solely by matching the zone number to an Ex t designation. Verify the actual certification and all application parameters.
Related guide: What Is Ex t Dust Protection?
Zone Classification Is Not the Same as Dust Group
Zone classification describes how frequently and for how long an explosive dust atmosphere may be present. Dust groups describe characteristics relevant to equipment selection.
Under the IEC framework, dust groups include:
- IIIA: combustible flyings.
- IIIB: non-conductive dust.
- IIIC: conductive dust.
A Zone 21 location can involve different dust groups depending on the material being handled. Knowing the zone alone does not establish whether a product is suitable for the dust involved.
See Dust Groups IIIA vs IIIB vs IIIC Explained for more information.
Dust Layers Must Also Be Considered
Area classification focuses on the likelihood of an explosive dust atmosphere, but settled dust is also relevant to ignition risk. A dust layer may be disturbed and become airborne, while accumulated dust can affect heat dissipation and contribute to elevated surface temperatures.
For this reason, equipment selection and installation should consider both airborne dust and the potential for dust accumulation. The required surface temperature limit depends on the characteristics of the dust and the applicable assessment, not on a universal temperature value for each zone.
Housekeeping, enclosure condition, maintenance, and control of dust releases are part of the broader risk management approach. They do not replace correct area classification or certified equipment selection.
How Engineers Should Determine the Zone
Zone assignment should follow the process conditions rather than a generic list of equipment locations. A practical engineering review should address the following questions:
- Identify the combustible dust. Confirm the material and obtain relevant information about its properties.
- Identify dust release sources. Review filling, emptying, conveying, milling, mixing, collection, and potential leakage points.
- Understand normal operation. Establish how frequently dust clouds may occur and how long they are likely to persist.
- Review abnormal conditions. Consider foreseeable equipment failures, loss of containment, and maintenance activities as appropriate.
- Assess containment and ventilation. Determine how these affect the likelihood and extent of an explosive dust atmosphere.
- Document the classification. Establish the zone boundaries and the technical basis for the decision.
- Use the classification for equipment selection. Confirm EPL
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