ATEX Zone 20, Zone 21, and Zone 22 classify the hazardous area risk from combustible dust clouds — separate from the gas and vapor zones (Zone 0/1/2). Zone 20 is continuous dust cloud presence (equivalent to NEC Class II Division 1 continuous exposure). Zone 21 is occasional dust cloud. Zone 22 is infrequent dust cloud. Equipment for dust zones must carry dust group (IIIA, IIIB, IIIC) and a maximum surface temperature that considers both cloud ignition and the more restrictive dust layer ignition temperature.
Combustible dust explosions are responsible for significant industrial accidents in grain handling, coal processing, sugar refining, flour milling, wood working, and metal powder manufacturing. The hazardous area classification for dust is often misunderstood — many facilities that have never classified their dust areas treat them as if they were unclassified. This guide covers the Zone 20/21/22 system, its NEC equivalents, and the specific requirements for surveillance cameras and electrical equipment in each zone.
Zone 20, 21, and 22 Defined
| IEC Zone | NEC Equivalent | Dust Cloud Presence | Typical Locations |
|---|---|---|---|
| Zone 20 | Class II Division 1 (continuous) | Continuously or frequently in normal operation | Inside grain bucket elevators, inside conveyors, filter interiors |
| Zone 21 | Class II Division 1 (intermittent) | Occasionally during normal operation | Around bucket elevator trunks, bagging areas, adjacent to conveyor transfers |
| Zone 22 | Class II Division 2 | Not likely; only in abnormal conditions | General interior of grain storage buildings, coal bunker rooms, areas outside Zone 21 |
Zone 20 is relatively rare in most facilities — it describes the interior of equipment where dust is continuously airborne (inside a bucket elevator, inside a mill, or inside a dust collector). Zone 21 is the most common classification for the immediate vicinity of dust-handling equipment. Zone 22 extends further from the dust source and covers areas where accumulated dust layers (rather than suspended dust clouds) are the primary hazard.
Dust Groups: IIIA, IIIB, and IIIC
IEC 60079 Group III covers combustible dusts, divided into three subgroups based on ignition characteristics:
Group IIIA — Combustible flyings and fibers: Textile lint, wood shavings, fibrous materials. Lower surface area per unit mass, somewhat easier to suppress. Group IIIB — Non-conductive combustible dust: Grain, flour, starch, coal dust, wood dust, sulfur dust. The most common industrial combustible dusts. Equipment rated for IIIB covers IIIA and IIIB dusts. Group IIIC — Conductive combustible dust / metal dust: Aluminum powder, magnesium dust, zinc dust, titanium dust. Metal dusts present additional hazard because they conduct electricity and can bridge clearances inside equipment. Group IIIC is the most demanding dust group. Equipment rated IIIC covers all three groups.
| IEC Group | NEC Equivalent | Examples | Covers Lower Groups? |
|---|---|---|---|
| IIIA | Class III (fibers) | Textile lint, wood shavings | IIIA only |
| IIIB | Class II Groups E, F, G | Grain, flour, coal, wood dust | IIIA and IIIB |
| IIIC | Class II Group E (metal dusts) | Aluminum, magnesium, zinc powder | All three groups (IIIA, IIIB, IIIC) |
Temperature Class for Dust: Cloud vs. Layer
This is the most critical — and most commonly misunderstood — aspect of dust zone equipment selection. Dust presents two distinct ignition temperatures: (1) the minimum ignition temperature of a dust cloud (MIT cloud), which is the temperature at which a suspended dust cloud ignites; and (2) the minimum ignition temperature of a dust layer (MIT layer), which is typically significantly lower than the cloud MIT. A 5mm layer of dust on a hot surface can ignite at a temperature 80–150°C lower than the cloud ignition temperature.
Camera housings and other electrical equipment in dust zones develop a surface temperature under normal operation and fault conditions. This surface temperature must be below both the cloud MIT and the layer MIT. ATEX-certified equipment for dust carries a maximum surface temperature specification (e.g., T135°C) rather than the T1-T6 class used for gas equipment. In practice, the layer MIT often governs equipment selection — a camera rated T135°C may be acceptable in a gas zone but unacceptable in a starch dust zone where the 5mm layer MIT is 120°C.
Always verify the MIT (both cloud and layer) for the specific dust present in the facility against the maximum surface temperature of the selected camera. The MIT data is available in NFPA 499 (Recommended Practice for the Classification of Combustible Dusts), IEC 61241-2-1, and the specific dust’s safety data sheet.
Frequently Asked Questions
What is the difference between Zone 21 and Zone 22?
Zone 21: dust cloud likely to occur occasionally during normal operation (around bucket elevator trunks, bagging areas). Zone 22: dust cloud not expected in normal operation but possible in abnormal conditions, or where dust layers accumulate. Zone 21 ≈ NEC Class II Division 1; Zone 22 ≈ Division 2.
What dust group covers grain and flour dust?
Group IIIB (non-conductive combustible dust). Equipment rated IIIB also covers IIIA (fibers). Metal dusts (aluminum, magnesium, zinc) require Group IIIC, which covers all dust groups.
Why is the dust layer temperature class more restrictive than cloud class?
A 5mm layer of dust on a hot surface ignites at 80–150°C lower than the same dust suspended in a cloud. Equipment surfaces accumulate dust layers during operation, so the maximum surface temperature must be below the layer MIT — often the governing constraint for equipment selection.
Can gas-zone ATEX cameras be used in dust zones?
No — not without a specific dust certification. Gas-zone equipment (marked II 2G) is not rated for dust zones. Look for the D marking (II 2D) or combined GD marking (II 2GD) for cameras approved in both gas and dust environments.
What does an ATEX dust zone camera marking look like?
Example: Ex II 2D Ex tb IIIC T135°C Db. Decoded: Group II surface industries; Category 2D (Zone 21); protection by enclosure (tb); Group IIIC (covers all dusts); max surface temperature 135°C; EPL Db (Zone 21).
Selecting surveillance cameras for combustible dust environments requires verification of both the dust group and the maximum surface temperature against the specific dust’s layer ignition temperature. Contact Veilux with your facility’s dust type and zone classification to confirm the appropriate camera specification for your Zone 20, 21, or 22 application.
Key Industry Standards and References
Dust zone classifications: IEC 60079-10-2 and ATEX Directive 2014/34/EU. NEC Class II equivalents: NFPA 70 Article 500. Combustible dust ignition temperatures: NFPA 499.
Related Resources
- Browse Explosion-Proof Cameras for Hazardous Locations
- ATEX Zone 0, Zone 1, Zone 2 Camera Selection Guide
- Class I vs Class II vs Class III Hazardous Locations Guide
- Explosion-Proof Cameras for Mining Operations
- IECEx vs ATEX Certification Differences
About the Author
Daniel Fernandez
Daniel Fernandez is a hazardous area security systems specialist with over a decade of experience specifying ATEX, IECEx, UL Class I Division 1, and cUL certified surveillance equipment for oil and gas, chemical, mining, pharmaceutical, and offshore environments. He holds expertise in NEC and IEC area classification standards and has consulted on explosion-proof camera system designs across North America, Europe, and the Middle East.