It is not possible to choose the right equipment without knowing how often and for how long an explosive gas atmosphere can occur in a facility. For this reason, ATEX legislation defines a three-level zoning system for explosive atmospheres caused by gas, vapor and mist: Zone 0, Zone 1 and Zone 2. In this article we explain what each zone means, how it is determined, and which equipment category it is matched with.
The Purpose of Zone Classification
Zone classification is a facility analysis carried out under the responsibility of the employer, within the scope of Workplace Directive 1999/92/EC (ATEX 137). Its purpose is to systematically assess the probability and duration of an explosive atmosphere occurring, and to determine the protection level required for equipment operating in that area. This analysis is generally documented through an "explosion protection document" prepared in accordance with EN 60079-10-1.
Definitions of Zone 0, Zone 1 and Zone 2
| Zone | Definition | Required Equipment Category |
|---|---|---|
| Zone 0 | An area in which an explosive gas/vapor atmosphere is present continuously, for long periods or frequently (e.g. the interior volume of a tank) | Category 1G |
| Zone 1 | An area in which an explosive atmosphere is likely to occur occasionally in normal operation | Category 1G or 2G |
| Zone 2 | An area in which an explosive atmosphere is not expected to occur in normal operation, and if it does, only for a short period | Category 1G, 2G or 3G |
An important rule to note: equipment with a higher level of protection can always be used in a lower-risk zone (Category 1G equipment works fine in Zone 2), but the reverse is not possible; Category 3G equipment can never be used in Zone 0 or Zone 1.
Factors Considered in Determining a Zone
- Grade of release: Possible gas/vapor sources classified as continuous grade, primary grade or secondary grade.
- Ventilation: The degree and availability of natural or mechanical ventilation in the area; well-ventilated open areas are generally assessed at a lower risk level.
- Gas properties: Evaporation rate, density (heavier/lighter than air) and ignition limits.
- Facility layout: The location of equipment, pipelines, tanks and connection points, and possible leak scenarios.
Confusion With Dust Environments
The Zone 0/1/2 classification applies only to explosive atmospheres caused by gas, vapor and mist. A separate classification system (Zone 20/21/22) is used for environments created by combustible dusts such as flour, coal dust or metal dust. Not confusing the two systems is critical for correct equipment selection; for detailed information on dust environments, see our article What Are Zones 20, 21 and 22?.
The Next Step After a Zone Is Determined
Once zone classification is clarified, every piece of equipment to be installed or used in that area (including motors, lighting fixtures, sensors and cable connection elements) must hold Ex certification appropriate to the relevant category. We explain how to read the marking on the equipment in our article How Is the Ex Mark Read?.
Frequently Asked Questions
Who carries out zone classification?
Zone classification is the responsibility of the employer operating the facility and is generally documented through an explosive atmosphere report prepared by a specialized engineering team in accordance with EN 60079-10-1.
Is Zone 0 the most hazardous area?
Yes, for gas environments the highest risk level is Zone 0, because an explosive atmosphere is present there continuously or for long periods. For this reason, only Category 1G equipment, with the highest level of protection, can be used in Zone 0.
Does a facility have only a single zone?
No, more than one zone is generally present together in the same facility; for example, the interior volume of a tank may be classified as Zone 0, the immediate surroundings of the tank as Zone 1, and a more distant area as Zone 2.
What factors are taken into account when determining a zone?
The grade of the release source, the ventilation condition and degree of the environment, the density of the gas, and the facility's overall layout are the fundamental factors taken into account in determining a zone.
Is equipment selection more flexible for Zone 2?
Because Zone 2 is the lowest-risk gas environment, Category 3G equipment is sufficient; however, this equipment must still hold the relevant Ex certification, and ordinary industrial equipment cannot be used.
Conclusion
Zone 0/1/2 classification ensures the systematic grading of gas-based explosive atmosphere risk and the selection of equipment with the correct level of protection. An incorrect or incomplete zone determination can lead either to unnecessarily costly over-protection or, far more seriously, to insufficient protection and a real explosion risk.
For more information, see our ATEX consultancy service.
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