Zone classification

The purpose of zone classification is to categorise, by the probability of the hazard, every space where an explosive atmosphere may form. There is a separate system for gases and vapours and another for dusts. The classification is not an end in itself: it determines which equipment category and Ex protection type is permitted in a given place, and it goes into the explosion protection document as well.

Three zones apply to an explosive atmosphere formed by gas, vapour or mist:

Zone 0

A space where an explosive atmosphere is present continuously, for long periods or frequently. Typically the inside of closed process spaces, e.g. the vapour space of tanks and vessels.

Zone 1

A space where an explosive atmosphere may occasionally form during normal operation. For example, the immediate surroundings of filling and decanting points.

Zone 2

A space where an explosive atmosphere is not likely in normal operation and, if it does form, persists only for a short time. Typically the wider surroundings around Zone 1.

A system analogous to the gas zones, but independent of them, applies to an explosive atmosphere formed by flammable dust:

Zone 20

A dust atmosphere is present continuously, for long periods or frequently — e.g. inside silos, filters, closed conveyors.

Zone 21

A dust atmosphere may occasionally form in normal operation — e.g. filling points, the surroundings of frequently opened equipment.

Zone 22

A dust atmosphere is not likely in normal operation and, if it forms, persists only for a short time. Often results from the disturbance of settled dust.

With dust zones, settled dust deserves special attention: dust accumulated on surfaces can form an explosive cloud on its own when stirred up, so the cleaning regime is part of the protection.

The classification follows the relevant standards: MSZ EN 60079-10-1 gives the methodology for gas spaces and MSZ EN 60079-10-2 for dusty spaces. The essence of the process:

1. Identifying the flammable substances and their properties.
2. Locating the sources of release (where and how often a substance may escape).
3. Evaluating the grade of release (continuous, primary, secondary).
4. Taking the effect of ventilation into account.
5. Determining the type and extent of the zones.

This requires expert judgement — too broad a classification means unnecessary cost, too narrow a one means risk.

The result of the classification must be recorded on a zone classification drawing: shown on a plan and, where needed, a section, where each zone begins and how far it extends. This drawing makes the classification usable in practice — equipment is selected and maintenance is planned according to it.

Zone classification quick-guide (PDF) — a one-page summary of gas and dust zones with the equipment categories assigned to them. Download →