In de IEC / CENELEC marking of the protection method there is also an apparatus group (I for mines, II other). In the NEC 500 this is called the apparatus class (Class I for gasses, Class II for dust and Class III for fibers).
The apparatus group is supplemented by the IEC / CENELEC with the gas group (A, B or C) and by the NEC 500 met de gas group (A - D for gasses and E - F for dust).
The gas group related for Ex d to the Minimum Experimental Safe Gap (MESG) and for Ex i to the Minimum Ignition Current (MIC).
The temperature class is related to the Auto Ignition Temperature (AIT) of the gas.
* Not within scope of NEC. Under
jurisdiction of MSHA
The end-users area classification into Zones or Divisions determines the demands on the equipment that may be used in those Zones or Division. When the types of gasses that may occur vary (bas in chemical tankers) the end user will demand IIC T6 , but may also be satisfied with IIC T4. In the petrochemical and en bulk storage industry is IIB T4 often satisfactory.