The Zones

Hazardous area classification divides a facility into zones according to how likely a flammable atmosphere is to be present, so that ignition source control can be applied proportionately.

Gases, vapours and mists

Zone	Definition
Zone 0	Explosive atmosphere present continuously, for long periods or frequently
Zone 1	Likely to occur in normal operation occasionally
Zone 2	Not likely in normal operation, and if it does occur will persist only for a short period

Dusts

The equivalent dust zones are 20, 21 and 22, with the same meanings applied to clouds of combustible dust. Dust classification must also consider layers, because a settled layer can be raised into a cloud by disturbance and can ignite on a hot surface.

What drives the classification

• Source of release — continuous, primary (expected in normal operation) or secondary (not expected in normal operation).
• Release rate, driven by pressure, hole size, fluid properties and volatility.
• Ventilation — degree and availability. Good ventilation reduces zone extent and can reduce zone type; loss of ventilation can upgrade an area.
• Fluid properties — flash point, vapour density, lower explosive limit.

Note that a flammable liquid handled well below its flash point may not produce a hazardous area at all — but a mist release of the same liquid can be flammable regardless of flash point.