European standards or IEC
Group 1: For coal mining (methane) Group II: For gases and vapors encountered in the surface industries. Sub groups: II A (Propane) II B (Ethylene) II C (Hydrogen)
D (Propane), C (ethylene), B (Hydrogen), A (Acetylene)
The danger of gas increases from explosion group IIA to IIC according to IEC and from Group D to Group A to the NEC classifications. The requirements for electrical apparatus for these groups increase accordingly. Electrical apparatus certified for IIC for example is suitable for all other explosion group.