ATEX Zone Classifications Explained: Zones 0, 1, 2, 20, 21, 22 & Equipment Categories
Learn how zone classification determines equipment requirements and what category equipment you need.
Zone classification is the foundation of explosion protection. It determines exactly what equipment you can legally and safely use in different areas of your workplace. Get it wrong, and you're either wasting money on over-specified equipment or—far worse—creating the conditions for a catastrophic explosion.
Under UK law (DSEAR), employers must classify any area where an explosive atmosphere could form. This classification directly determines the ATEX equipment category required. There's no flexibility here: using equipment that isn't rated for the zone is both illegal and extremely dangerous. [LINK: DSEAR Compliance Guide]
This guide explains each zone classification, how to determine which zones apply to your workplace, and what equipment categories are required for each.
Understanding the Zone System
The ATEX zone system classifies areas based on how likely an explosive atmosphere is to occur and for how long. There are two parallel classification systems: one for gases, vapours, and mists (Zones 0, 1, 2), and one for combustible dusts (Zones 20, 21, 22). [LINK: What is ATEX?]
The numbering reflects the hazard level. Lower numbers mean higher risk—Zone 0 and Zone 20 are the most hazardous, where explosive atmospheres are present almost continuously. Higher numbers (Zone 2 and Zone 22) indicate lower risk areas where explosive atmospheres occur only rarely and briefly.
Understanding this system is essential because it drives every equipment decision. An industrial vacuum rated for Zone 22 cannot be used in Zone 21. A Zone 1 gas detector is inadequate for Zone 0. The zone classification is absolute.
Gas, Vapour, and Mist Zones (0, 1, 2)
These zones apply to areas where flammable gases, vapours from flammable liquids, or flammable mists could create an explosive atmosphere.
Zone 0: Continuous or Long-Period Hazard
Zone 0 is classified where an explosive gas atmosphere is present continuously, for long periods, or frequently. This is the highest hazard classification for gases.
Typical Zone 0 locations:
Inside tanks or vessels containing flammable liquids
Inside process equipment where flammable vapours are always present
Areas immediately above open flammable liquid surfaces
Zone 0 areas are relatively rare in most facilities. They're typically found inside enclosed process equipment rather than in general work areas. Equipment for Zone 0 must be Category 1G—the highest protection level for gas atmospheres.
Zone 1: Occasional Hazard in Normal Operation
Zone 1 applies where an explosive gas atmosphere is likely to occur occasionally during normal operation. 'Occasionally' typically means more than 10 hours per year but less than 1,000 hours per year.
Typical Zone 1 locations:
Areas immediately surrounding Zone 0 locations
Near filling or draining points for flammable liquids
Around pump seals and valve glands that may leak
Near sample points or drainage systems
Zone 1 requires Category 1G or 2G equipment. Category 2G equipment is designed to remain safe even with one fault condition.
Zone 2: Abnormal Conditions Only
Zone 2 is where an explosive gas atmosphere is not likely during normal operation, but if it does occur, will persist for only a short period. This typically means less than 10 hours per year.
Typical Zone 2 locations:
Areas surrounding Zone 1 locations
Near flanged connections that rarely leak
Storage areas for sealed containers of flammable materials
Areas where adequate ventilation normally prevents accumulation
Zone 2 permits Category 1G, 2G, or 3G equipment. Category 3G is the minimum requirement and provides protection during normal operation.
Combustible Dust Zones (20, 21, 22)
These zones apply to areas where combustible dusts could form an explosive cloud or where dust layers could ignite. Many common industrial materials create combustible dusts, including flour, sugar, wood, metals, plastics, and pharmaceuticals. [LINK: Complete Guide to Combustible Dusts]
Zone 20: Continuous or Long-Period Hazard
Zone 20 is where an explosive dust cloud is present continuously, for long periods, or frequently. This is the highest hazard classification for dusts.
Typical Zone 20 locations:
Inside dust collection equipment and hoppers
Inside silos and storage bins during filling
Inside cyclone separators and bag filters
Inside mixers and blenders processing powders
Like Zone 0, Zone 20 is typically found inside enclosed equipment rather than in general work areas. Equipment must be Category 1D—the highest protection level for dust atmospheres.
Zone 21: Occasional Hazard in Normal Operation
Zone 21 applies where an explosive dust cloud is likely to occur occasionally during normal operation.
Typical Zone 21 locations:
Areas immediately around powder filling and emptying points
Near bag dump stations and manual powder handling
Around equipment that may release dust during operation
Where dust accumulation is disturbed regularly
Zone 21 is common in manufacturing environments where powders are handled. Equipment must be Category 1D or 2D. Many ATEX industrial vacuums are rated for Zone 21. [LINK: ATEX Industrial Vacuums Hub]
Zone 22: Abnormal Conditions Only
Zone 22 is where an explosive dust cloud is not likely during normal operation, but if it does occur, will persist for only a short period. This also includes areas where dust layers may accumulate and could form an explosive cloud if disturbed.
Typical Zone 22 locations:
General areas surrounding Zone 21 locations
Areas where dust layers accumulate on surfaces
•Near bagged product storage
Workshop areas in facilities handling dusty materials
Zone 22 is the most common dust zone classification and is frequently found throughout facilities that handle powders or generate dusts. Equipment can be Category 1D, 2D, or 3D.
Equipment Categories and Zone Compatibility
ATEX equipment is classified into categories that correspond to the zones where it can be used. The category indicates the level of protection built into the equipment. [LINK: Decoding ATEX Markings]
For gas atmospheres (Group II G):
Category 1G: Suitable for Zones 0, 1, and 2 — Very high level of protection
Category 2G: Suitable for Zones 1 and 2 — High level of protection
Category 3G: Suitable for Zone 2 only — Normal level of protection
For dust atmospheres (Group II D):
Category 1D: Suitable for Zones 20, 21, and 22 — Very high level of protection
Category 2D: Suitable for Zones 21 and 22 — High level of protection
Category 3D: Suitable for Zone 22 only — Normal level of protection
You can always use higher-category equipment in lower-hazard zones (e.g., Category 1D equipment in Zone 22), but never the reverse. Using Category 3D equipment in Zone 21 is illegal and dangerous.
Determining Zone Classifications for Your Workplace
Zone classification must be carried out as part of your DSEAR risk assessment. This isn't optional—it's a legal requirement. The assessment should be conducted by someone competent to evaluate the hazards and determine appropriate classifications.
Key factors in zone classification include:
Release characteristics
How often does a release occur? What quantity is released? What is the release velocity and direction?
Ventilation
Is the area enclosed or open? What is the air change rate? Is ventilation natural or mechanical, and is it reliable?
Substance properties
What is the flash point, lower explosive limit, vapour density, or dust particle size? How readily does the substance disperse?
Housekeeping
For dusts, how frequently do layers accumulate? How effectively are they removed? What thickness of accumulation is typical?
The HSE provides guidance on zone classification, and industry-specific standards may also apply. For complex situations, specialist consultants can provide expert assessment.
Common Zone Classification Mistakes
Forgetting dust layers
Zone 22 can apply even where dust clouds are rare if dust layers accumulate and could be disturbed. A 1mm layer of combustible dust on a hot surface can ignite.
Underestimating zone extent
Zones don't stop at arbitrary boundaries. They extend until dilution reduces concentrations below the lower explosive limit. Insufficient ventilation can extend zones significantly.
Ignoring maintenance activities
A location might be non-hazardous during normal operation but become Zone 1 or Zone 21 during maintenance when equipment is opened or cleaned.
Assuming standard cleaning equipment is acceptable
Using a standard industrial vacuum in a Zone 22 area is a DSEAR breach. The vacuum itself becomes an ignition source. [LINK: ATEX vs Standard Industrial Vacuums]
Selecting Equipment for Your Zone Classifications
Once you've determined your zone classifications, selecting the right ATEX-rated equipment is essential. CFM North East supplies industrial vacuums rated for Zone 1/21 and Zone 2/22 applications, helping you meet your DSEAR obligations with correctly specified equipment. [LINK: ATEX Vacuum Buying Guide]
Our team can help you match equipment specifications to your zone requirements, ensuring compliance and safety.
