Zone Classification: Zones 0, 1, 2, 20, 21, 22 for Hazardous Areas

Last updated: March 2026 · Based on IEC 60079 (2020 edition) and ATEX 2014/34/EU

Zone classification is the process of dividing a facility into areas based on the probability and duration of an explosive atmosphere being present. For a step-by-step walkthrough, see the hazardous area classification guide. It determines what type of Ex-protected equipment is required in each location.

Gas and Vapour Zones (IEC 60079-10-1)

Zone 0 Examples

• Inside storage tanks containing flammable liquids above their flash point
• Inside process vessels during normal production
• Inside pipelines carrying flammable gases

Zone 1 Examples

• Area surrounding Zone 0 (e.g., around tank vents)
• Areas near equipment that frequently releases flammable substances
• Pump and compressor seal areas
• Sample points and drainage systems
• Loading/unloading areas for flammable liquids

Zone 2 Examples

• Areas surrounding Zone 1
• Areas where release only occurs during maintenance, breakdown, or abnormal conditions
• Flange connections in good condition
• Valve stems under normal conditions
• Areas near Zone 1 with adequate ventilation

Dust Zones (IEC 60079-10-2)

Zone 20 Examples

• Inside silos, hoppers, and cyclone filters
• Inside pneumatic conveying lines
• Inside mixers and blenders processing combustible dusts

Zone 21 Examples

• Areas around dust filling/emptying points
• Near conveyor transfer points with frequent spillage
• Bag filter rooms during normal operation

Zone 22 Examples

• Areas surrounding Zone 21 with adequate housekeeping
• Near equipment that rarely releases dust
• Warehouse areas storing bags of combustible powder

Important: Dust layers on surfaces can form Zone 22 (or higher) even without airborne dust. A layer of 5mm of combustible dust, if disturbed, can create an explosive cloud.

North American Classification (NEC/CEC)

The US (NEC Article 500/505) and Canada (CEC Section 18) use a parallel system:

NEC Article 505 also recognizes the Zone system directly. The Division system is less granular. Division 1 covers both Zone 0 and Zone 1.

Class III covers fibers and flyings (textile mills, woodworking). no direct Zone equivalent.

For a detailed comparison of the NEC Class/Division system and the international Zone system, including conversion tables and dual-certification guidance for multinational facilities, see our NEC 500/505 vs ATEX/IECEx guide.

Zone Classification Methodology

Classification is typically performed by a team including:

• Process engineers (source and release characterization)
• Safety/HSE engineers
• Electrical engineers
• Operations personnel

Key Factors

1. Source of release. where flammable material can escape (flanges, valves, seals, vents)
2. Grade of release
3. Continuous → forms Zone 0/20
4. Primary (expected during normal operation) → forms Zone 1/21
5. Secondary (not expected during normal operation) → forms Zone 2/22
6. Ventilation. affects zone extent and possibly zone type
7. Good ventilation reduces zone size and may downgrade zone type
8. Poor ventilation extends zone size and may upgrade zone type
9. Negligible ventilation (enclosed spaces). zone extends to fill the entire space
10. Relative density of gas. lighter gases (hydrogen) rise; heavier gases (propane) pool at ground level

Zone Extent

The physical size of a zone depends on:

• Release rate and velocity
• Ventilation rate and effectiveness
• Relative density of the flammable substance
• Flash point and vapour pressure
• LEL of the substance

Standards provide calculation methods: IEC 60079-10-1 Annex B (gases), IEC 60079-10-2 Annex B (dusts).

Zone Documentation

The output of a zone classification study is:

1. Hazardous Area Classification drawings. plans and sections showing zone boundaries
2. Schedule of release sources. tabulated list of all potential sources
3. Data sheets. substance properties, release characteristics
4. Zone classification report. methodology, assumptions, results

These documents form the basis for selecting equipment with the correct EPL and protection method.

Common Pitfalls

• Forgetting that lighter-than-air gases (hydrogen, methane) accumulate at ceiling level
• Ignoring dust layers. a 5mm layer disturbed becomes a dust cloud
• Not reassessing when processes change
• Confusing "Zone 2" with "non-hazardous". it still requires Ex equipment
• Under-estimating ventilation failures (what happens when the fan stops?)

Related Files

• Fundamentals. explosion triangle, basic concepts
• Gas Groups. how different fuels are classified
• EPL. equipment protection levels required per zone
• Protection Methods. types of protection suitable per zone
• Standards. IEC 60079-10-1 and 60079-10-2
• NEC vs ATEX/IECEx. North American vs international classification systems
• DSEAR (UK). UK workplace regulations for explosive atmospheres