Gas Groups (IIA / IIB / IIC)

Gas groups rank explosive gases by how easily equipment can ignite them and how violently they explode, setting construction requirements for Ex equipment. Group IIA (reference gas propane) covers many fuels and solvents; IIB (ethylene) covers more energetic chemistry; IIC (hydrogen and acetylene) is the most severe, demanding the tightest flameproof gaps and lowest ignition energies. Equipment certified IIC may be used in IIA and IIB areas, but not the reverse — and because IIC construction costs more, matching the group to the site’s actual gas study rather than defaulting to “IIC everywhere” is sound engineering economics. The group appears in the Ex marking, e.g. Ex db IIC T4 Gb.