Zone 20 vs Zone 21 vs Zone 22 Explained
Under IEC 60079-10-2, hazardous dust locations are classified as Zone 20, Zone 21, or Zone 22 based on the frequency and duration of the presence of combustible dust clouds. These classifications help engineers select appropriate explosion-protected equipment and determine suitable Equipment Protection Levels (EPL).
Understanding the differences between Zone 20, Zone 21, and Zone 22 is essential for proper equipment selection, compliance, and risk reduction in combustible dust environments.
Quick Answer
Quick Answer
Zone 20 is an area where combustible dust is present continuously or for long periods, Zone 21 is an area where combustible dust may occur occasionally during normal operation, and Zone 22 is an area where combustible dust is not normally present but may occur infrequently and for short durations. These classifications are defined in IEC 60079-10-2 for hazardous dust atmospheres.
What Are Zone 20, Zone 21, and Zone 22?
Zone 20, Zone 21, and Zone 22 are hazardous area classifications used for combustible dust atmospheres. Similar to gas hazardous area classifications, the zone number reflects the likelihood and duration of an explosive atmosphere being present.
As the zone number increases, the probability of a hazardous dust atmosphere decreases. This classification directly influences equipment selection and the required Equipment Protection Level (EPL).
| Zone | Dust Presence | Required EPL |
|---|---|---|
| Zone 20 | Continuous or Long Duration | Da |
| Zone 21 | Occasional During Normal Operation | Db |
| Zone 22 | Abnormal Conditions Only | Dc |
What Is Zone 20?
Zone 20 is the area with the highest probability of combustible dust presence under IEC 60079-10-2. under IEC 60079-10-2. It refers to an area where a combustible dust cloud is present continuously, for long periods, or frequently enough that an explosive atmosphere is expected during normal operation.
Because of the high probability of ignition, equipment installed in Zone 20 must provide the highest level of dust protection and is typically certified with Equipment Protection Level (EPL) Da.
Zone 20 classifications are often found inside process equipment where combustible dust is routinely handled, transported, or stored.
Typical Zone 20 Locations
- Inside grain silos
- Inside flour milling equipment
- Inside dust collectors
- Inside pneumatic conveying systems
- Inside powder handling vessels
What Is Zone 21?
Zone 21 is an area where a combustible dust cloud may occur occasionally during normal operation. Unlike Zone 20, the presence of combustible dust is not continuous, but it is expected to occur periodically as part of normal process activities.
Equipment installed in Zone 21 generally requires Equipment Protection Level (EPL) Db and must be designed to prevent ignition during foreseeable operating conditions.
Many industrial facilities classify areas immediately surrounding dust-generating equipment as Zone 21.
Typical Zone 21 Locations
- Areas around bagging stations
- Areas near grain transfer points
- Dust handling equipment surroundings
- Areas near powder filling systems
- Locations adjacent to dust collectors
What Is Zone 22?
Zone 22 is the lowest-risk combustible dust classification. A combustible dust cloud is not expected during normal operation but may occur infrequently and only for short durations if abnormal conditions arise.
Equipment certified for Zone 22 applications generally requires Equipment Protection Level (EPL) Dc.
Although the probability of a hazardous dust atmosphere is lower, appropriate equipment selection and housekeeping practices remain important to prevent dust accumulation and ignition.
Typical Zone 22 Locations
- Areas surrounding Zone 21 locations
- Warehouses handling combustible powders
- General production areas with occasional dust release
- Packaging areas
- Storage facilities for combustible materials
Zone 20 vs Zone 21 vs Zone 22 Comparison
The table below summarizes the key differences between the three combustible dust hazardous area classifications.
| Feature | Zone 20 | Zone 21 | Zone 22 |
|---|---|---|---|
| Dust Presence | Continuous or Frequent | Occasional | Rare |
| Risk Level | Highest | Medium | Lowest |
| Required EPL | Da | Db | Dc |
| Typical Protection | Ex ta | Ex tb | Ex tc |
How Zone 20, Zone 21, and Zone 22 Relate to Equipment Protection Levels (EPL)
Equipment Protection Levels (EPL) provide an additional layer of equipment selection guidance beyond hazardous area classification. Each dust zone is associated with a minimum EPL requirement to ensure adequate protection against ignition risks.
| Dust Zone | Minimum EPL | Typical Marking |
|---|---|---|
| Zone 20 | Da | Ex ta IIIC T85°C Da |
| Zone 21 | Db | Ex tb IIIC T85°C Db |
| Zone 22 | Dc | Ex tc IIIB T120°C Dc |
In general, equipment with a higher EPL may be installed in lower-risk zones. For example, equipment certified as EPL Da can typically be used in Zone 21 and Zone 22 applications, subject to project requirements and local regulations.
Common Mistakes When Classifying Dust Hazardous Areas
Assuming Dust Hazards Are Less Dangerous Than Gas Hazards
Combustible dust explosions can be just as destructive as gas explosions. Dust clouds suspended in air can ignite rapidly and generate significant overpressure within enclosed facilities.
Ignoring Dust Accumulation
Even if airborne dust clouds are infrequent, accumulated dust layers can become hazardous when disturbed. Proper housekeeping is an important part of dust explosion prevention.
Confusing Dust Zones with Gas Zones
Zone 20, Zone 21, and Zone 22 apply only to combustible dust atmospheres. They are separate classifications from Zone 0, Zone 1, and Zone 2, which apply to flammable gas atmospheres.
Selecting Equipment Based Only on Zone Classification
Engineers must also consider dust groups, temperature limitations, Equipment Protection Levels (EPL), and equipment certification details when selecting hazardous area equipment.
Frequently Asked Questions
What Is Zone 20?
Zone 20 is a hazardous area where a combustible dust cloud is present continuously, frequently, or for long periods during normal operation.
What Is Zone 21?
Zone 21 is a hazardous area where a combustible dust cloud may occur occasionally during normal operation.
What Is Zone 22?
Zone 22 is a hazardous area where a combustible dust cloud is not normally present but may occur infrequently and for short durations.
Which EPL Is Required for Zone 20?
Zone 20 applications generally require equipment certified to Equipment Protection Level Da.
Can Zone 20 Equipment Be Used in Zone 21?
In most cases, yes. Equipment certified for the higher-risk Zone 20 environment may also be suitable for lower-risk dust zones, subject to project requirements.
What IEC Standard Defines Dust Zones?
Dust hazardous area classification is defined by IEC 60079-10-2.
Conclusion
Zone 20, Zone 21, and Zone 22 are hazardous area classifications used for combustible dust atmospheres under IEC 60079-10-2. Zone 20 represents the highest probability of combustible dust presence, Zone 21 covers occasional dust clouds during normal operation, and Zone 22 applies to areas where dust clouds occur only infrequently. Understanding these classifications helps engineers select compliant equipment and reduce ignition risks in combustible dust environments.
Technical Review
Technical Review
This article has been reviewed against IEC 60079-10-2 hazardous area classification requirements for combustible dust atmospheres and related IECEx certification practices. Interpretation aligns with current industry guidance regarding Zone 20, Zone 21, Zone 22 classifications and associated Equipment Protection Levels (EPL).
Final area classification and equipment selection must always consider dust characteristics, process conditions, ignition sources, dust accumulation, EPL requirements, certified documentation, and site-specific engineering studies.
References
- IEC 60079-10-2 – Explosive Atmospheres – Classification of Areas – Combustible Dust Atmospheres
- IEC 60079-0 – Explosive Atmospheres – General Requirements
- IECEx Certified Equipment Scheme Documentation
- IECEx Official Website (www.iecex.com)
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