Gases are grouped into Mining and other (surface) industries and then by the energy required to cause ignition.
| IEC & CENELEC Gas Groups |
Representative Gas |
Ignition Energy |
|
|---|---|---|---|
| I | Methane | 180µJ | Mining |
| IIA | Propane | 180µJ | |
| IIB | Ethylene | 60µJ | |
| IIC | Hydrogen | 20µJ |
The above is the traditional method of explaining spark ignition parameters and is easily understandable in terms of electrical parameters. Electrical equipment is then certified for the relevant group.
Due to the nature of the environment Mining and Surface industries are treated differently, within the Atex and UKEX 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 must be considered.
Group II is for Surface industries and excluded mines susceptible to firedamp
Group II is split into 3 sub groups IIA, IIB, IIC based on the amount of energy required to ignite the gas which is evaluated by the MESG & MIC tests.
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 gases.
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.
*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 NOT 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 certification as most protection techniques were devised around the lower ignition levels for gases with the type of dust rather than the ignition energy being the important factor in the risk assessment.
The primary form of protection for dust is Ex t, Protection by enclosure where in most cases ignition is not an issue.
The term 'Gas groups' is often (erroneously) used to include dust groups.
| 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.
The way that the group members are decided, like most aspects of hazardous area assessement, is by testing. (see EN 60079-20-1) Gases and vapours are classified according to their Maximum Experimental Safe Gaps (MESG).
and/or their Minimum Ignition Current (MIC)
| Group | MESG | MIC Ration | One Determination | Both MESG and MIC |
|---|---|---|---|---|
| IIA | ≥ 0.9mm | > 0.8 | MESG ≥ 0.9 mm or MIC > 0.9 |
0.8 ≤ MIC ≤ 0.9 need to confirm by MESG |
| IIB | > 0.5 and < 0.9 mm |
≥ 0,45 and ≤ 0.8 |
0.55 mm ≤ MESG < 0,9 mm or 0.5 ≤ MIC ≤ 0.8 |
0.45 ≤ MIC ≤ 0.5 need to confirm by MESG |
| IIC | ≤ 0.5 | < 0.45 | MESG < 0.5 mm or MIC < 0.45 |
0.5 ≤ MESG < 0.55 need to confirm by MIC |
For a mix of gases there are a number of ways of determining characteristics based on known gases, refere to EN 60079-20-1 clauses 4.5 and 4.6
Notes
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. ↩
Group IIC contains 4 gases Hydrogen, Acetylene, Ethyl Nitrate and Carbon Disulphide ↩
Detailed in EN 60079-20-1 Explosive atmospheres. Material characteristics for gas and vapour classification. Test methods and data ↩