What Are ATEX Zones?

ATEX zones classify a hazardous area by how often, and for how long, an explosive atmosphere is likely to be present. The definitions come from the ATEX workplace directive 1999/92/EC, and the method for drawing them is set out in IEC 60079-10-1 for gas and IEC 60079-10-2 for dust.

There are six zones in two sets. Zones 0, 1 and 2 cover flammable gases, vapours and mists. Zones 20, 21 and 22 cover combustible dust. In both sets, the lower the number, the more often the hazard is present and the stricter the equipment rules.

The zone decides what equipment is allowed. Most computers and HMIs in hazardous areas are installed in Zone 2, near process equipment where a leak is possible but not expected. Zone 1 and Zone 0 call for more protective designs, such as intrinsically safe or flameproof equipment.

The table sets out each zone with a common rule of thumb for how long the atmosphere is present each year. The hours are industry guidance, not part of the directive, but they help when comparing areas.

Equipment rated for a stricter zone can always be used in a less strict one, which is why a category 1 instrument is acceptable in Zone 2. The reverse is never allowed.

Gas zones are drawn around each source of release, such as a vent, seal or sample point. Their size depends on how much can escape, how fast, how heavy the gas is and how well the area is ventilated.

Dust zones work in the same way but add one hazard gas does not have: layers. Dust that settles on a hot surface can smoulder and ignite, and a layer disturbed by a knock can form a cloud. For that reason, dust-rated equipment states a maximum surface temperature with dust layers in mind, and good housekeeping can shrink or remove a Zone 22.

Dust zones are common in food, grain, wood, pharmaceutical and plastics processing. The equipment is marked D rather than G, and dust groups IIIA, IIIB and IIIC separate combustible fibers, non-conductive dust and conductive dust.

Hazardous area classification is the assessment that turns a site’s process information into zones. It works source by source: for each place where a flammable gas, vapor or dust could escape, the assessor decides how often it will be released, how far the cloud can travel before it is diluted below its lower explosive limit, and how well the area is ventilated. In Great Britain the work sits inside the DSEAR risk assessment. The method comes from IEC 60079-10-1 for gas and IEC 60079-10-2 for dust, and many USA refineries, terminals and chemical plants also use the Energy Institute’s Model Code of Safe Practice Part 15, known as EI 15.

The result is a set of drawings showing the type and extent of every zone, usually annotated with the gas group and temperature class of the substances involved. Those three facts are what an equipment supplier needs. The main inputs are:

The zone says how often a hazard is present. The gas group and temperature class say how easily it ignites, and the equipment marking has to cover both. Equipment rated for a more demanding group can be used for a less demanding one, so IIC equipment also suits IIB and IIA.

Zoning is the employer’s job. Under 1999/92/EC, and under the DSEAR regulations in Great Britain, the site must carry out a hazardous area classification, record it on drawings and in an explosion protection document, and review it when the process changes. Specialist consultants often do the assessment, but the duty stays with the site.

The result is a zone drawing that the equipment supplier works from. When asking for a computer or display, give the zone, the gas or dust group and the temperature class from that drawing. Our ATEX explainer shows how those requirements appear in an equipment marking.

North America mainly uses classes and divisions under the US National Electrical Code and the Canadian Electrical Code. Division 1 covers what the zone system splits into Zone 0 and Zone 1, and Division 2 corresponds broadly to Zone 2. The two systems are certified separately, so a Zone 2 product is not automatically approved for Division 2. The US also permits a zone system of its own, in NEC Articles 505 for gas and 506 for dust, which uses Class I Zones 0, 1 and 2 and lines up closely with the IEC approach. Our Class 1 Division 2 explainer compares them in more detail.

Our hazardous area panel PCs cover Zone 2 and Class I Division 2 sites. We integrate and validate each system as standard, keeping to the certified configuration, so tell us the zone, gas group and temperature class and we will match the equipment.