ATEX zones, equipment groups and categories
ATEX equipment is classified into equipment-groups. Equipment groups are divided into categories. 
ATEX equipment group I:
Equipment in this category is only intended for use in the underground areas of mines, as well as those areas of surface installations of such mines endangered by firedamp and/or combustible dusts.
ATEX equipment group II:
Equipment in this category is intended for use in areas in which explosive atmospheres caused by gases, vapours, mists or air/dust mixtures are likely to occur occasionally. Think of production facilities in the food industry, chemical plants, oil & gas, and the pharmaceutical industry – to mention a few.
Categories within equipment group II:
Equipment category 1
Equipment in this category is intended for use in Zone 0 areas (gases) or Zone 20 (dusts), and must ensure a very high level of protection (i.e. the inside of tank truck or the interior of a dust filter). The design of the equipment must ensure protection, even in the event of rare incidents relating to the equipment. And safety must be guaranteed in the case of two faults occurring independently of each other.
Equipment category 2
This equipment is intended for use in Zone 1 (gases) or Zone 21 (dusts). A high level of protection is ensured.  You will find these zones at the manifold of a tank truck or the interior of a silo. Safety is ensured in the event of frequently occurring disturbances or equipment faults which normally have to be taken into account.
Equipment category 3
Is intended for us in Zone 2 (gases) or Zone 22 (dusts). Explosion protection must be ensured during normal operation. You will find this equipment in paint factories, a warehouse or around a bag dump station. Ways to ensure this level of safety are non-arcing design and limitation of the temperature of the equipment. Reproduced from Ex Machinery ATEX Blogs Overview of ATEX Zones and Equipment
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