Gas Groups and Dust Groups

Not all flammable gases and dusts behave the same way. The gas group and dust group classification systems categorise substances by their explosion characteristics, ensuring that equipment is designed and tested for the specific hazards it will encounter.

Gas Groups Explained

Gases and vapours are classified into groups based on two key properties: the Maximum Experimental Safe Gap (MESG) and the Minimum Igniting Current (MIC) ratio. These determine how easily an explosion can propagate through a narrow gap (relevant for flameproof enclosures) and how easily an electrical circuit can cause ignition (relevant for intrinsic safety).

For surface industries (Group II), gases are subdivided:

• Group IIA: Gases with MESG ≥ 0.9 mm. Examples: propane, butane, petrol vapour, ammonia, methane (surface use), acetone. These are the least demanding—equipment designed for IIA may not be suitable for more hazardous gases.
• Group IIB: Gases with MESG > 0.5 mm but < 0.9 mm. Examples: ethylene, ethyl ether, hydrogen sulphide, town gas. Requires tighter flamepath gaps and lower igniting currents than IIA.
• Group IIC: Gases with MESG ≤ 0.5 mm. Examples: hydrogen, acetylene, carbon disulphide. The most hazardous group—narrowest gaps, lowest igniting currents. Equipment certified for IIC is suitable for all Group II gases.

Group I for Mining

Group I applies to equipment for underground coal mines and their surface installations where methane (firedamp) and/or coal dust may be present. Methane in mining contexts is treated separately because of the specific conditions (confined spaces, limited escape routes) rather than its fundamental properties.

Equipment Marking for Gas Groups

The ATEX marking shows which gas group equipment is certified for: IIA, IIB, or IIC. A marking showing "IIC" means the equipment is suitable for all Group II gases. A marking showing "IIA" means it's only suitable for Group IIA gases—using it in a IIB or IIC atmosphere would be dangerous.

Some equipment is certified for specific gases rather than groups—typically where the substance has unusual properties. This appears in the marking (e.g., "IIB + H2" for equipment suitable for IIB gases plus hydrogen).

Dust Groups

Dusts are classified differently, primarily by whether they're conductive or non-conductive:

• Group IIIA: Combustible flyings (fibres and lint). Relatively low hazard but can form explosive atmospheres in textile mills, cotton processing, etc.
• Group IIIB: Non-conductive dust. Most organic dusts fall here: flour, grain, wood dust, pharmaceutical powders. The resistivity is >10³ Ω·m.
• Group IIIC: Conductive dust. Metal dusts (aluminium, magnesium, titanium), coal dust, carbon. Resistivity ≤10³ Ω·m. The most hazardous category because conductive dust can cause electrical faults and has specific ignition mechanisms.

Temperature Considerations for Dust

Unlike gases which use T-classes (T1-T6), dust equipment is often marked with a maximum surface temperature value directly (e.g., T125°C or T200°C). This is because the relevant temperature depends on the specific dust present—both the dust cloud ignition temperature and the dust layer ignition temperature must be considered. Dust layers can ignite at lower temperatures than dust clouds because smouldering can occur over extended periods.

Practical Selection

When selecting equipment, identify the most hazardous substance present and select equipment rated for that group. If multiple substances are present, choose equipment rated for the most demanding. The risk assessment should document which substances are present, their group classification, and the equipment suitability verification.

Reference Data

EN/IEC 60079-20-1 provides material characteristics for gas and vapour classification, including test methods and data tables. For many common substances, standard reference data is available. For unusual substances or mixtures, laboratory testing may be required to determine the appropriate group classification.