Classification of Explosive Atmospheres

1. Atmosphere Categories

ATEX applies strict classification, grouping and temperature classes to explosive atmospheres and equipment, which form the basis for equipment selection and area management.

1) Category I: Explosive methane atmospheres in underground coal mines

2) Category II: Gas/vapour explosive atmospheres excluding coal mines (e.g. petroleum, chemical industry)

3) Category III: Dust explosive atmospheres (e.g. grain processing, plastics manufacturing)

2. Temperature Classes

If the surface temperature of equipment during operation exceeds the ignition temperature of flammable substances, an explosion may occur.

For this reason, ATEX defines the following 6 temperature classes (from T1 to T6: maximum surface temperature decreases, safety level increases).

1) T1 — Hydrogen (ignition temperature: 500 °C)

2) T2 — Ethylene (ignition temperature: 425 °C)

3) T3 — Gasoline (ignition temperature: 280 °C)

4) T4 — Acetaldehyde (ignition temperature: 140 °C)

5) T5 — Carbon disulfide (ignition temperature: 90 °C)

6) T6 — Ethyl nitrate (ignition temperature: 85 °C)

3. Gas Groups

For Category II gas atmospheres, common flammable gases are divided into groups according to their minimum ignition energy and ignition temperature:

1) Group IIA: Methane, propane, gasoline, etc. (lower hazard)

2) Group IIB: Ethylene, acrylonitrile, etc.

3) Group IIC: Hydrogen, acetylene, carbon disulfide, etc. (highest hazard)