Apparatus Groups
Classification of electrical equipment related to the explosive atmosphere for which it is to be used.
Due to the nature of the environment Mining and Surface industries are treated differently, within the Atex Coding, equipment is classified in groups relating to the explosive atmosphere for which it is to be used; Group I for mining and Group II for surface use with an additional Group III for surface Dust.

Group 1 electrical equipment for mines susceptible to firedamp1
Specifically for underground mines rather than surface mining predominantly for coal mines and has additional requirements over group 2 as coal dust generally has to be considered.

Group II is for Surface industries and excluded mines susceptible to firedamp

Gas

Split into 3 sub groups IIA, IIB, IIC based on the amount of energy required to ignite the gas (see Spark Energy)
Group IIA requires the highest energy to ignite with a typical gas being Methane, the main constituency of Firedamp which is a Group I gas
Group IIB the majority of gasses.
Group IIC includes only a small number of (the most easily ignited) gases2

Most petrochemical applications are designated Group IIB or occasionally IIB + Hydrogen. Certification for IIB + Hydrogen can often be easier to achieve than a full IIC particularly for Ex d flammable protected equipment.

Dust

Group III is for (Surface) Dust. It helps to think of it as Group II + I to tie up fuel groups with Mining or Surface Industries. Dusts are split into Groups based on dust types rather than Ignition energy as generally dust has approximately 1000 times the minimum ignition energy than gases do. Therefore ignition energy becomes largely irrelevant in terms of Atex certification as most protection techniques were devised around the lower ignition levels for gases and dust type becomes the important factor in the risk assessment.
The term 'Gas groups' is often (erroneously) used to include dusts.

IEC & CENELEC
Groups
Type Examples
IIIA Combustible Flying rayon, cotton, sisal, jute, hemp, cocoa fibre, oakum, and baled waste kapok.
IIIB Non-Conductive Dusts Foodstuffs (e.g. sugar, flour, grain & additives), paper and wood
IIIC Conductive Dusts Aluminium, Bronze, Zinc, Magnesium

Combustible Dust::- finely divided solid particles, 500 μm or less in nominal size, which may be suspended in air, may settle out of the atmosphere under their own weight, may burn or glow in air, and may form explosive mixtures with air at atmospheric pressure and normal temperatures.

Conductive Dust::- combustible dust with electrical resistivity equal to or less than 103 Ω · m

Combustible Flyings::- solid particles, including fibres, greater than 500 μm in nominal size which may be suspended in air and could settle out of the atmosphere under their own weight

Group IIIA dusts are considered to be lower risk and Group IIIB is by far the largest group, it contains almost all carbon based dusts e.g. foodstuffs which are explosive.
Group IIIC is considered high risk as static can be concentrated within the dust cloud and shorting of exposed electrical connections is a real hazard.
As with Gas groups an increasing level of protection is required going from group A to B to C e.g. Group IIIC equipment can be used in a IIB environment but not vice versa.


Notes


  1. Firedamp consists mainly of methane and traces of carbon dioxide, and hydrogen, and sometimes ethane and carbon monoxide. In mining practice firedamp is used as synonyms for Methane. 

  2. Group IIC contains 3 gases Hydrogen, Acetylene and Carbon Disulphide 

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