If you are sourcing fixed gas detection equipment for a European plant, sooner or later a datasheet, a tender document or a safety consultant will refer to ATEX zone classification. Understanding what Zone 0, Zone 1 and Zone 2 actually mean is not just a compliance formality: it determines which equipment may legally be installed in each part of your site, how procurement specifications should be written, and which questions to ask a supplier before placing an order. This guide covers the zone definitions, the link to equipment categories, and the practical checks buyers can apply during selection.
Where the ATEX Zones Come From
ATEX is shorthand for the French term “atmospheres explosibles”. In the European Union, two directives share the name. Directive 2014/34/EU addresses the equipment side: it defines the requirements that apply to products intended for use in potentially explosive atmospheres. Directive 1999/92/EC addresses the workplace side: it requires employers to assess where explosive atmospheres may occur, classify those areas into zones, and select equipment appropriate to each zone.
The zoning itself is therefore the plant operator’s responsibility, usually documented in a hazardous area classification drawing prepared during the safety study. Equipment vendors do not classify your site; they supply products whose protection level can be matched to the zones you have already defined. No detector, however specified, can substitute for a proper area classification study.
Zone 0, Zone 1 and Zone 2: Definitions and Typical Examples
For flammable gases and vapours, the classification uses three zones based on how likely an explosive atmosphere is to be present and for how long:
- Zone 0: an explosive gas atmosphere is present continuously, for long periods, or frequently. Typical examples include the vapour space inside a solvent storage tank or the interior of process vessels handling flammable liquids.
- Zone 1: an explosive gas atmosphere is likely to occur occasionally in normal operation. Examples often include the immediate surroundings of pump seals, sampling points, vents and filling connections.
- Zone 2: an explosive gas atmosphere is not likely to occur in normal operation, and if it does occur it will persist only for a short period. Areas around flanged joints in well-maintained piping, or the wider surroundings of Zone 1 areas, are commonly classified as Zone 2.
A parallel scheme exists for combustible dusts, using Zone 20, Zone 21 and Zone 22 with analogous definitions; facilities handling both gases and dusts may carry both types of zoning on the same classification drawing.
Two practical points follow from these definitions. First, zones are three-dimensional volumes with defined extents, not vague labels for whole buildings; a single pump skid can have a small Zone 1 envelope surrounded by a larger Zone 2 envelope. Second, the zone reflects probability of an explosive atmosphere, not toxicity; a space can be dangerous from oxygen deficiency or toxic gas without being a classified explosion hazard at all.
How Zones Map to Equipment Categories and Ex Markings
Under the equipment directive, products are divided into categories that correspond to the zones in which they may be used. For gas atmospheres, Category 1G equipment is designed for Zone 0, Category 2G for Zone 1, and Category 3G for Zone 2. Equipment of a higher category may generally be used in a lower-risk zone, but never the other way around.
International practice, harmonised through the IEC 60079 series, expresses the same idea through Equipment Protection Levels: Ga aligns with Zone 0 suitability, Gb with Zone 1, and Gc with Zone 2. A marking such as Ex db IIC T6 Gb tells the reader four things: the protection concept (db, a flameproof enclosure that contains an internal ignition), the gas group (IIC, covering the most easily ignited gases such as hydrogen and acetylene), the temperature class (T6, meaning a maximum surface temperature of 85 degrees C), and the protection level (Gb, associated with Zone 1 use).
When comparing detectors, buyers should read the full marking rather than the headline. A device rated for gas group IIA cannot be assumed suitable where hydrogen is present, and a T3 temperature class may be unacceptable for low auto-ignition temperature vapours. The certificate behind the marking also matters: certification schemes are national or regional, and a certificate issued under one scheme does not automatically satisfy the requirements of another jurisdiction. For reference, ASA’s AN-O2-D, AN-CO2-D, AN-NH3-D, AN-CO-D, AN-VOC-D and AN-LEL-D fixed detectors carry China Ex Certificate EETI24.0463X (Ex db IIC T6 Gb, IP65), valid to Jan 2030; buyers in Europe or the Middle East should confirm with our team, and with their local authority, which certification basis applies to their specific project before specifying equipment for classified areas.
What Detector Buyers Should Check Before Ordering
A structured checklist helps turn zone theory into a defensible purchase decision:
- Obtain the area classification drawing first. Selection starts from the zone, gas group and temperature class defined for each installation point, not from the detector catalogue.
- Match the marking to the location. Confirm protection concept, gas group, temperature class and protection level against the zone requirements, and ask the supplier for the actual certificate documents rather than relying on datasheet summaries.
- Separate detection duty from mounting location. A detector monitoring a classified area is sometimes installed at its boundary or in adjacent unclassified space; controllers and alarm panels are normally placed in non-hazardous (safe) areas such as control rooms. Fixed combustible gas detectors for LEL monitoring are commonly used at the points where leaks are most likely, with signals routed back to a controller in the safe area.
- Consider the whole measurement task. Explosion risk is only one part of gas safety. Many sites pair LEL monitoring with oxygen and toxic gas measurement, and the full range of fixed gas alarm detectors can be evaluated against the site’s combined risk picture.
- Plan for documentation and maintenance. Classified-area installations typically require traceable certificates, correct cable glanding and periodic inspection. Clarify calibration intervals and spare parts availability at the procurement stage, not after commissioning.
Following this sequence helps avoid the two most common mistakes: over-specifying every point to the most demanding zone, which inflates cost, and under-specifying on price alone, which can leave equipment that is not permitted in its installed location.
Why ASA
ASA supplies fixed gas detectors, dust alarm detectors and gas alarm controllers to industrial buyers across Europe and the Middle East. Our product line covers oxygen, combustible, toxic and VOC measurement from a single source, which simplifies specification when a project spans multiple gas risks. Our engineering team can review your area classification drawings, advise on detector placement and controller architecture, and provide the certificate documentation your project file requires, with responsive pre-sales support and short quotation lead times. To discuss detector selection for your classified areas, contact the ASA team for a consultation.
Disclaimer: Product availability, specifications, measuring ranges, alarm configurations and certification coverage vary by model and region. The information in this article is provided for general reference only and does not constitute a performance guarantee, safety assurance or compliance statement. Always refer to the official product documentation and confirm suitability, certification status and local regulatory requirements with our team before selection, installation or use.
