Apparatus are classified into groups and categories:

Group of apparatus I – applies to apparatus for mining operations above ground and underground that may be endangered by methane gas and/or inflammable dusts. This group is subdivided into the categories M1and M2

Group of apparatus II – applies to apparatus for use in all other areas which may be subject to the hazard of an explosive atmosphere. This group is subdivided into the categories 1, 2 and 3.

Graphic examples are compliments of the The Wolf Safety Lamp Company Ltd., Sheffield, England