17 Classification criteria are the MaximumExperimental Safe Gap (MESG) and the “Minimum Ignition Current (MIC)”.The MESG and MIC are determined for the various gases and vapour according to a stipulated testing arrangement. The maximum experimental safe gap is the gap of the test apparatus with a width of flameproof joint of 25mm at which an internal ignition of an explosive mixture is not propagated to the exterior (IEC 60079-1-1).The minimum ignition current relates to the minimum ignition current for labora- tory methane.An overview of the maximum experimental safegaps and minimum ignition currents for the various explosion groups is shown in table 9.The dangerousness of the gases increases fromexplosion group IIA to IIC. The requirements for the electrical apparatus increase accordingly to these explosion groups. For this reason, the mark- ing of the electrical apparatus must show to which explosion group it belongs. Electrical apparatus approved for IIC may also be used for all other explosion groups.