Hazardous area classification in plain terms: what the zone on the drawing means for the equipment you buy

By Gido van Tienhoven, Ex-Machinery Explosion Proof Equipment B.V., Ridderkerk, the Netherlands

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Most process plants have a drawing that few people outside the safety department ever open: the hazardous area classification. It decides which equipment may be installed or carried into each part of the site, and it is the first thing a supplier should ask for. This article explains what the zones mean, how they translate into equipment categories and markings, and where projects typically go wrong.

Why the drawing exists

Wherever a flammable gas, vapour, mist or dust can mix with air in quantities that could ignite, European law requires the operator to assess the risk and to classify the affected areas into zones. In Great Britain this duty sits in the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR); in the rest of Europe it comes from Directive 1999/92/EC. The result is the classification drawing, usually accompanied by a short document that explains which substances, releases and ventilation assumptions it is based on. It is a statement about the plant, not about any product: a supplier cannot classify a site it does not operate.

What the zones mean

Zones describe how often an explosive atmosphere is expected. Zone 0 is an area where it is present continuously or for long periods; zone 1 where it is likely to occur occasionally in normal operation; zone 2 where it is not likely in normal operation and, if it does occur, exists only for a short time. For combustible dust the same logic applies with zones 20, 21 and 22. The extent of each zone depends on the release, the properties of the substance and the ventilation, which is why two plants handling the same product can have very different drawings.

Two details matter for equipment purchases. First, zones change when the plant changes: a new vent, a modified pump seal or a different cleaning procedure can move a boundary, and the drawing has to follow. Second, temporary zones exist during maintenance, when equipment is opened and the normal containment is gone. Those are the moments when portable and mobile equipment is most likely to be brought in.

From zone to category and marking

Equipment placed on the market for explosive atmospheres is certified under the ATEX equipment directive (2014/34/EU, retained in Great Britain as UKEX) and carries a marking that states what it is suitable for. The equipment category is the direct link to the zones: category 1 for zones 0 and 20, category 2 for zones 1 and 21, category 3 for zones 2 and 22. A category 3 device is properly certified, but only for zone 2 or 22; it does not become suitable for zone 1 because it looks robust.

The marking carries more than the category. The gas group (IIA, IIB or IIC) must cover the substances on site; hydrogen and acetylene require IIC, and a IIB device is not acceptable there. The temperature class, T1 to T6, must sit below the ignition temperature of the gases or dusts present. For dust, the maximum surface temperature is stated directly. Reading all of these against the drawing is the whole job of the person who approves a piece of equipment; the Ex symbol on its own says only that some certification exists.

The assembly, not only the parts

A certified motor, a certified sensor and a certified enclosure do not add up to a certified machine. Once components are combined, the assembly has to be assessed as a whole, including mechanical ignition sources such as hot bearings and friction, and the way it is installed. This is where converted machinery and packaged equipment such as air conditioning units differ from off-the-shelf components: the supplier assesses the complete unit, documents the protection concept and issues the certificate or declaration for the whole machine, together with a technical construction file.

For an operator, the practical consequence is to ask for the certificate of the complete assembly and to check that it names the same model and configuration that is being delivered. A certificate number ending in X indicates special conditions for safe use, which should be copied into the operating instructions.

Where projects go wrong

The specification says “ATEX” without naming the zone, so the supplier guesses, and the guess is usually zone 2 because it is cheaper.
The gas group is not checked. A device certified for IIB arrives at a hydrogen site.
The temperature class is ignored where substances with low ignition temperatures are handled.
Portable equipment is forgotten: phones, inspection cameras, lamps and the commissioning engineer’s laptop enter the zone without anyone looking at the drawing.
Equipment installed outside the zone is later surrounded by a new zone after a plant modification, and nobody re-checks it.

None of these is exotic. They are the ordinary consequence of treating the classification drawing as a document for the safety department instead of a purchasing document.

A short checklist for buyers

Get the classification drawing and the substance list before writing the specification, and state the zone, gas or dust group and temperature class in the enquiry.
Ask the supplier for the marking and the certificate of the complete equipment, not only of its components.
Check the category against the zone, the group against the substances and the temperature class against the ignition temperatures.
Agree who is responsible for the assembly assessment when equipment is combined or modified on site.
Repeat the check after any change to the plant or the process.

Explosion protection rarely fails because a certified product was faulty. It fails when the certified product and the place it was used do not match. Suppliers of explosion-protected equipment, such as Ex-Machinery in the Netherlands, which builds explosion-proof air conditioning and converts standard machinery for hazardous areas, start every enquiry with the same question for that reason: which zone, exactly, and what is in the air there?