What a hazardous area zone is
A hazardous area is any location where a flammable or explosive atmosphere may be present in quantities sufficient to require special precautions. The ATEX Workplace Directive (1999/92/EC) and its UK equivalent (DSEAR) require employers to:
- Identify locations where explosive atmospheres may form
- Classify those locations into zones based on the likelihood and duration of the explosive atmosphere
- Ensure that only appropriately certified equipment is installed in each zone
- Maintain the classification in an Explosive Atmosphere Document (EAD)
Zone classification is based on probability and duration, not on severity. A small amount of flammable gas present continuously creates a Zone 0; a larger amount present only occasionally and briefly creates a Zone 2. The physical extent of the zone depends on the source, ventilation, and the vapour density of the substance relative to air.
Gas and vapour zones (0, 1, 2)
| Zone | Definition | Typical frequency | Examples |
|---|---|---|---|
| Zone 0 | Explosive gas atmosphere present continuously, or for long periods, or frequently | More than 1000 hours/year | Inside a fuel tank or vessel containing volatile liquid; inside gas pipework |
| Zone 1 | Explosive gas atmosphere likely to occur during normal operation | 10–1000 hours/year | Within 1–3 m of open process connections; pump gland areas; around vents and relief valves during normal operation |
| Zone 2 | Explosive gas atmosphere not likely during normal operation; only under abnormal conditions | Less than 10 hours/year | Beyond the Zone 1 boundary; around storage areas; general areas of a process plant |
Zone 0 is almost always found only inside vessels, pipes, and tanks, not in open air. Zone 1 surrounds open process connections, vents, and areas where leaks occur during normal operation. Zone 2 is the most extensive zone and covers the general area around a process plant where a leak would only occur under abnormal circumstances.
Dust zones (20, 21, 22)
Combustible dust creates explosive atmospheres when suspended as a cloud at the right concentration. The dust zone classification mirrors the gas zone system but uses different numbers to avoid confusion:
| Zone | Definition | Examples |
|---|---|---|
| Zone 20 | Explosive dust cloud present continuously, frequently, or for long periods | Inside silos, hoppers, grinding mills, dryers, conveyors, during operation |
| Zone 21 | Explosive dust cloud likely during normal operation | Near filling and emptying points; near agitators; pneumatic conveying connections |
| Zone 22 | Explosive dust cloud unlikely; may occur only under abnormal conditions | Around dust collectors; general areas of food/grain processing where dust can accumulate |
Dust ignition requires additional consideration beyond gas classification. The relevant parameters are the Minimum Ignition Energy (MIE) of the dust cloud, the Minimum Ignition Temperature (MIT) of both the cloud and the layer, and the Minimum Explosive Concentration (MEC). These vary significantly between dust types, grain dust, coal dust, sugar, and metal dusts each have different explosion characteristics.
Equipment for dust zones is classified using the Ex t protection concept (enclosure method) defined in IEC 60079-31. The enclosure IP rating must be IP6X for Zone 20 and 21 (dust-tight); IP5X for Zone 22 (dust-protected).
Equipment Protection Levels
Equipment Protection Levels (EPLs) were introduced in the revised IEC 60079 standards to give a consistent measure of protection level across gas, dust, and mine applications:
| EPL | Atmosphere | Zone | Ignition risk basis |
|---|---|---|---|
| Ga | Gas | 0, 1, 2 | Very high protection, very low probability of ignition even with two independent faults |
| Gb | Gas | 1, 2 | High protection, low probability of ignition under normal or single-fault conditions |
| Gc | Gas | 2 | Enhanced protection, does not become a source of ignition under normal conditions |
| Da | Dust | 20, 21, 22 | Very high protection |
| Db | Dust | 21, 22 | High protection |
| Dc | Dust | 22 | Enhanced protection |
Equipment categories vs zones
Equipment categories (from the ATEX product Directive 2014/34/EU) define the level of protection built into equipment at manufacture. Zones define the level of protection required at a location:
| ATEX Category | Required zone | Example marking element |
|---|---|---|
| Category 1G | Zone 0 (gas) | II 1 G |
| Category 2G | Zone 1 (gas) | II 2 G |
| Category 3G | Zone 2 (gas) | II 3 G |
| Category 1D | Zone 20 (dust) | II 1 D |
| Category 2D | Zone 21 (dust) | II 2 D |
| Category 3D | Zone 22 (dust) | II 3 D |
Equipment of a higher category may always be installed in a lower category zone (Category 1 may be used in Zone 1 or Zone 2), but not the reverse. Category 3 equipment is not permitted in Zone 1, regardless of the manufacturer's marketing claims.
The zoning process
Zone classification is a formal engineering exercise, typically carried out by a competent person (a process engineer or safety engineer with relevant training). The process follows EN 60079-10-1 (gas) or EN 60079-10-2 (dust) and involves:
- Identify sources of release, valves, flanges, pump seals, open vessels, vents, drains, and any other point where flammable substance may escape
- Grade each source, continuous (likely Zone 0), primary (likely Zone 1), or secondary (likely Zone 2) grade of release
- Assess ventilation, dilution ventilation reduces zone extent and severity; poor ventilation extends it
- Determine zone type and extent, apply the standard guidance or model the dispersion using validated tools
- Document in area classification drawings, plan and section views with zones drawn to scale
Area classification drawings
Area classification drawings are the primary deliverable of the zoning exercise. They must show:
- The zone type and extent around each source of release
- Equipment already installed in each zone (during ATEX assessments of existing plant)
- The gas group and temperature class applicable to each zone
- Any non-zoned safe areas within the installation boundary
These drawings form part of the Explosive Atmosphere Document (EAD) required by DSEAR/1999/92/EC. They must be reviewed and updated whenever the process, layout, or materials change. An out-of-date area classification drawing is a significant compliance failure in a regulatory inspection.
Reducing zone extent
Zone classification is not fixed. The extent and severity of zones can be reduced by engineering controls:
- Improved sealing, replacing open vents with pressure relief valves, upgrading pump seals to double mechanical seals
- Ventilation, natural or forced ventilation at a source of release dilutes vapour below LEL more quickly, reducing zone extent
- Closed systems, fully enclosed processing with no normal release points eliminates Zone 0 and Zone 1
- Gas detection, in some applications, gas detectors may be used to classify a Zone 1 area as Zone 2 by automatically de-energising equipment on gas detection (subject to competent person sign-off)
Reducing zones through engineering controls reduces the cost and complexity of electrical equipment specification. It also reduces the risk of ignition, since the probability of a flammable atmosphere encountering an ignition source is reduced.