Hazardous Areas Classification

A hazardous area is where an explosive atmosphere is present or may be expected to be present. Flammable liquids, gases, vapours, or combustible dust may cause an explosive atmosphere. 

Hazardous areas classification involves analysing and categorising areas based on the level of risk where an explosive atmosphere may exist. Hazardous areas classification must be carried out before any apparatus is installed. Once the area is classified, appropriate explosion protection techniques can be selected.

Hazardous areas can be classified into gas regions, dust regions, and sometimes combinations.

Type of Hazardous Area	ANZEx and IECEx	Definitions
Continuous Hazard	Zone0 (Gas) Zone20 (Dust)	An explosive atmosphere is present for a more significant period and has a high density.
Intermittent Hazard	Zone1 (Gas) Zone21 (Dust)	In normal operation, the Explosive atmosphere is present for a shorter period and at a lesser density compared to zone 1.
Hazard under abnormal conditions	Zone2 (Gas) Zone22 (Dust)	An Explosive atmosphere generally does not occur in normal operation, but if it does, it will exist for short periods.

Explosive gas atmosphere hazardous areas classification procedure –

• To classify gaseous hazardous areas, the AS/NZS 60079.10.1 standard is followed.
• The area classification must be performed by individuals who grasp the importance and relevance of flammable material properties. The personnel should be familiar with the process and equipment used and their safety, as well as electrical, mechanical, and other qualified engineering properties.
• The area classification should be done when the initial process, instrumentation line diagrams and layout plans are confirmed.
• Identification of source of release – Generally flammable gases and vapours are contained within the process equipment, which may or may not be totally enclosed. A process contains different types of equipment, such as tanks, pumps, pipelines, etc. Each item should be considered a potential source of flammable material while defining zones. These releases are classified as ‘continuous’, ‘primary’, or ‘secondary’.
• Once sources of release are established, the release rate is determined. Release rates influence the type and extent of the zone. A higher release rate will make the zone larger.
• The relative density of the gas or vapour also plays an important part in determining zones. If gas is lighter(relative density below 0.8) than air, it will be moved upwards. It will accumulate at ground level if it is heavier(relative density more than 1.2). The extent of the zone will increase at ground level.
• The lower Explosive(Flammable) Limit (LEL/LFL) and Upper Explosive(Flammable) Limit (UEL/UFL) of hazardous gas play important roles in deciding the extent of the zone. For a given release volume, the lower the LEL, the larger the zone area.
• The possibility of an explosive gas atmosphere depends on the grade of release and the ventilation. A continuous grade release typically corresponds to zone 0, a primary grade to zone 1, and a secondary grade to zone 2.
• By improving ventilation, the zone number can be decreased.

Zone 0 – Explosive atmosphere is present for a more significant period and has a high density.

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Zone 1 – Explosive atmosphere is present for a shorter period and has a lesser density than zone 1 in regular operation.

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Zone 2 – An explosive atmosphere generally does not occur in regular operation, but if it does, it will exist for short periods.

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For example, the image below shows a practical example of the classification of gas areas. It is an image of an adequately ventilated above-ground fuel storage tank.

Zone 0 – Inside tank.

Zone 1 – Outside the tank within a space from ground to 3m vertically above the tank shell.

Zone 2 – The outside extent of zone 1. Zone 2 extent depends upon the capacity of the tank.

Explosive dust atmosphere hazardous areas classification-

Dust forms an explosive atmosphere only at concentrations within its explosive range. Particles that are not removed by mechanical extraction or ventilation settle at a rate that depends on their properties, such as particle size, forming accumulations or layers. It is essential to consider that even a small and continuous release source can lead to dust buildup.

Classification procedure – 

• The AS/NZS 60079.10.2 standard is followed in Australia to classify hazardous areas.
• The area classification must be performed by individuals who grasp the importance and relevance of flammable material properties. The personnel should be familiar with the process and equipment used, as well as safety, electrical, mechanical, and other qualified engineering properties.
• Area classification depends upon different factors, such as whether dust is combustible or not. Laboratory tests can confirm this. Parameters such as particle size, moisture content, cloud and layer ignition temperature, and electrical resistivity are also considered.
• Operational and maintenance procedures for the plants must be studied. The housekeeping process is also included.
• Identification of the source of release. This is a point or location from which dust can be released or raised such that an explosive atmosphere could be formed. When dust containment is part of the process and dust is not vented into the outside atmosphere, such processes shall be studied for regular operation, abnormal operation and start-up and shutdown processes.
• Grading of release is identified. Grading of release is classified as continuous, primary, and secondary grade of release. The grade of each source of release will be determined.
• Conditions that may cause dust layers to be raised to form a dust cloud should be avoided by proper ventilation.

Zone 20 – The extent of zone 20 includes the interior of ducts and the production and handling equipment where explosive atmospheres are continuous, for long periods, or frequently present.

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Zone 21 – An explosive dust atmosphere is likely to occur periodically. It can be formed outside the equipment by a primary grade of release and depends on dust parameters such as dust amount, flow rate, particle size, and dust moisture content.

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Zone 22 – A secondary grade release can form zone 22.

Preferred symbol key –

The image below shows a practical example of the classification of dust areas. It depicts a bag emptying station outside the building without exhaust ventilation.

Zone 20 – Inside the hopper.

Zone 21 – Zone 21 is to be defined around the manhole, extending some distance from the edge of the manhole and down to the floor. An open manhole is a primary source of release. 

Zone 22 – Zone 22 could occur adjacent to Zone 21 due to dust accumulation as a layer or if the dust release is composed of very fine particles that could occasionally travel outside the normal Zone 21 boundary under abnormal operating conditions.

Last updated: 11/07/2025