ATEX Zone Classification Explained: A Practical Guide for Engineers and Facility Managers

Why zone classification is the starting point for everything

Every ATEX compliance obligation — equipment selection, inspection frequency, training requirements, the Explosion Protection Document — flows from the zone classification. If your classification is wrong, incomplete, or out of date, every downstream decision built on it is compromised. Yet zone drawings are among the most commonly neglected documents in industrial facilities. We regularly encounter sites where drawings are 10 or 15 years old and have not been updated to reflect process modifications, new equipment, or changes in materials handled.

This guide explains the zone types, how classification is performed under IEC 60079-10, and the most common mistakes to avoid.

The six ATEX zones: gas, vapour, mist and dust

ATEX distinguishes between two types of explosive atmosphere: those formed by flammable gases, vapours, or mists (Group II), and those formed by combustible dusts (Group III). Each type has three zone levels reflecting the frequency of occurrence:

The word "occasionally" in Zone 1 and Zone 21 definitions is intentionally non-prescriptive — IEC 60079-10-1 and -2 provide guidance but ultimately leave the judgement to the competent person performing the classification. This is where engineering experience matters most.

The classification process under IEC 60079-10

1. Identify release sources

The starting point is identifying every potential source of flammable material release. A release source is any point from which gas, vapour, mist, or dust can escape into the atmosphere. This includes: process flanges and valve glands in normal operation (continuous or primary grade releases); safety relief valves and rupture discs (secondary grade); and catastrophic failure points (which do not contribute to zone classification but inform risk assessment). All release sources must be documented with their release grade (continuous, primary, secondary) and release rate.

2. Determine zone extent

Zone extent — how far the classified zone extends from the release source — depends on: the release rate of flammable material; the lower explosive limit (LEL) of the substance; ventilation conditions (natural or forced, adequate or dilution ventilation); and the density of the vapour or gas relative to air (heavy vapours accumulate in low-lying areas; light gases like hydrogen rise and accumulate at ceiling level).

IEC 60079-10-1 provides worked examples and guidance on zone extent estimation. For complex geometries or non-standard ventilation scenarios, CFD gas dispersion modelling is increasingly used to determine zone extents with greater accuracy than the simplified analytical methods in the standard — this is an area where CHS Intl's combined ATEX and CFD capability adds particular value.

3. Produce zone drawings

Zone drawings show the classified zones in plan and elevation views, clearly delineating zone boundaries. They must be to a recognised drawing standard, uniquely identified, revision-controlled, and stored as part of the Explosion Protection Document (EPD). Zone drawings are living documents — they must be reviewed and updated whenever the process, materials, equipment, or building layout changes.

Gas vs dust: the key differences

While the zone numbering logic is parallel, gas and dust classifications have important practical differences:

• Dust layer thickness matters: Dust deposits on surfaces can be disturbed and form a cloud. IEC 60079-10-2 requires consideration of both the dust cloud zone and the additional hazard from accumulated dust layers — a 5 mm layer of combustible dust on a hot surface is itself an ignition risk even without a suspended cloud.
• Minimum Ignition Energy (MIE) varies widely: Fine metal dusts (aluminium, magnesium, titanium) can have MIE values in the millijoule range — meaning even electrostatic discharge from a person is sufficient to cause ignition. Grain and wood dusts are less sensitive but still require serious treatment.
• Cleaning and housekeeping become part of the safety case: A Zone 22 area can be maintained if cleaning regimes prevent dust accumulation above safe thresholds. This makes Zone 22 classification inherently dependent on documented and auditable cleaning procedures.

The five most common classification mistakes

1. Not updating drawings after process changes

A process modification that changes release rate, substance, ventilation, or geometry can change both zone type and extent. Many sites have a change management process that does not include a trigger for zone drawing review. This is the most common gap we find on site surveys.

2. Classifying without considering ventilation properly

Ventilation is the single biggest variable in zone classification. An outdoor release source with good natural ventilation may justify a Zone 2 (or even unclassified) designation. The same release source inside a poorly ventilated building with limited air changes per hour may be Zone 1 or even Zone 0. Zone classifications produced without a documented ventilation assessment are frequently wrong.

3. Using Zone 1 equipment in Zone 2 as a conservative default

Using higher-category equipment than required is not inherently wrong — Zone 1 (Category 2) equipment is permitted in Zone 2. However, routinely over-specifying equipment adds significant cost and, more importantly, masks the underlying classification uncertainty. If you do not know whether an area is Zone 1 or Zone 2, the answer is to do the classification properly — not to default to Zone 1 equipment everywhere.

4. Treating the LEL as the only relevant flammable property

The Lower Explosive Limit determines the concentration at which a mixture becomes flammable. But the vapour pressure and boiling point of the substance also matter — a high-boiling-point liquid with low vapour pressure at ambient temperature presents a very different release hazard to a low-boiling-point solvent. Flash point, autoignition temperature, and vapour density all need to be considered in a complete classification.

5. Classifying once and never reviewing

Zone classification is a living assessment, not a one-time exercise. IEC 60079-10-1 explicitly requires that classification is reviewed whenever changes occur that could affect the hazardous area. In practice, this means a formal review should be triggered by: any change to the process chemistry or operating conditions; any modification to containment equipment or pipework; changes to the building structure or ventilation system; and on a periodic basis (typically every 3–5 years) even without specific changes.