What the group measures
The gas group carries two things at once: the minimum energy needed to ignite the mixture, and the flame's ability to pass through a narrow gap. For equipment using flameproof protection the second directly drives the design — the gap in the enclosure must be narrow enough to stop the flame escaping.
| Group | Demand | Representative gas |
|---|---|---|
| IIA | Least | Propane |
| IIB | Medium | Ethylene |
| IIC | Most demanding | Hydrogen, acetylene |
The coverage rule
Equipment certified for IIC may be used in IIB and IIA atmospheres; IIB equipment may be used in IIA. The reverse is never valid. That is why IIB equipment cannot be treated as sufficient in places where hydrogen is released, such as battery charging rooms.
Group I is separate
Group I is for firedamp atmospheres in mines and is not a lower rung of the surface classification (Group II); it is a separate family. Dust atmospheres are handled with their own group (Group III), where what decides is the dust itself, including whether it is conductive.
Mixed atmospheres
Where a volume holds more than one flammable substance, the equipment must satisfy the most demanding group and the lowest ignition temperature at the same time. The worst case of each criterion is taken separately; they are not averaged.