Explosive atmosphere zones 0, 1, 2 and 20, 21, 22 explained

Updated: 2026-08-13 · ISKRIT

A zone number answers one question only: how often, and for how long, an explosive atmosphere is present in that place. It says nothing about how large the zone is, and confusing the two is the most expensive mistake in area classification.

The three gas zones

Zone 0 is a place where an explosive gas or vapour atmosphere is present continuously or for long periods, conventionally more than 1000 hours a year. Zone 1 is a place where it is likely to occur occasionally in normal operation, between 10 and 1000 hours. Zone 2 is a place where it is not expected in normal operation and, if it occurs, persists only briefly, under 10 hours a year.

The hour figures are a convention rather than a measurement. In practice the zone follows from the grade of release: a continuous grade of release gives zone 0, a primary grade gives zone 1, a secondary grade gives zone 2, and the classification argument is about which grade a given source has.

The dust series

Dust gets a parallel series with the same logic: zone 20 for a cloud present continuously, zone 21 for one arising in normal operation, zone 22 for one arising only on a fault. Dust differs from gas in two ways that matter. A layer that is not airborne today can be raised into a cloud tomorrow, so housekeeping is part of the classification. And a dust only forms an explosive cloud at all if its lower explosive limit is low enough, in the range of tens of grams per cubic metre.

Fire-hazardous areas are a separate question

Not every place with combustibles has an explosive atmosphere. Combustible liquids with a high flash point, solid combustibles and fibres that never go airborne create a fire risk without an explosion risk, and Russian practice classifies them in their own series, P-I to P-III under federal law 123-FZ article 18, rather than forcing them into the explosive zones. The distinction is worth keeping in any jurisdiction: equipment selected for a dust cloud is not what a stack of pallets requires.

Ventilation moves the number

The same release in the same room classifies differently depending on how reliably the air moves. Good dilution with reliable ventilation can hold a secondary release at zone 2 or even leave the volume unclassified; poor dilution promotes it. This is why ventilation availability, graded as good, fair or poor, is an input to the classification and not an afterthought, and why an interlock that stops the process when a fan fails is a classification measure as much as a safety one.

Number against extent

The zone number and the zone extent are two separate results. The number comes from frequency and duration. The extent, the distance in metres to which the zone reaches, comes from a quantitative calculation of the release rate and the hypothetical volume of the flammable cloud under IEC 60079-10-1. A small release into a well ventilated space can be declared of negligible extent and the place treated as unclassified, while the same release in a still corner reaches several metres. Equipment cost follows the extent, which is why that calculation is the part the market charges for.

Questions

Does the zone number set the equipment category? Yes: zone 0 and 20 need category 1 equipment, zone 1 and 21 category 2, zone 2 and 22 category 3.

Is the inside of a tank a zone? Usually zone 0 for a flammable liquid, since the vapour space is above the explosive limit or within it for long periods.

Determine the zone class from the medium and the frequency of the mixture, alongside the room category and the printable signs, in the fire load density calculator.