Gas Groups IIA, IIB, IIC & Dust Groups: Classification Explained
Last updated: March 2026 · Based on IEC 60079 (2020 edition) and ATEX 2014/34/EU
Flammable gases, vapours, and dusts are classified into groups based on their ignition properties. Equipment must be certified for the specific gas or dust group present (the group appears on the equipment marking) in the hazardous area.
Equipment Groups
Group I. Mining (Firedamp)
Equipment for underground mines and surface installations where firedamp (methane from coal seams) and/or combustible dust may be present.
Group II. Surface Industries (Gas/Vapour)
Equipment for all other locations with explosive gas/vapour atmospheres. Subdivided into IIA, IIB, and IIC.
Group III. Surface Industries (Dust)
Equipment for locations with explosive dust atmospheres. Subdivided into IIIA, IIIB, and IIIC.
Gas Groups (Group II)
Gases are classified based on two measurable properties:
- MESG (Maximum Experimental Safe Gap). the maximum width of a gap through which an internal explosion will not propagate (critical for flameproof enclosure design) to an external explosive atmosphere. Tested in a standardized 25ml spherical chamber with a 25mm long gap. Smaller MESG = more dangerous gas.
- MIC Ratio (Minimum Igniting Current Ratio). the ratio of the minimum igniting current of the gas to that of methane, measured in a standard intrinsic safety test apparatus. Smaller MIC ratio = more dangerous gas.
| Gas Group | MESG (mm) | MIC Ratio | Typical Gases | Hazard Level |
|---|---|---|---|---|
| IIA | >0.90 | >0.80 | Methane, propane, butane, acetone, ethanol, benzene, diesel, ammonia | Least hazardous |
| IIB | 0.50–0.90 | 0.45–0.80 | Ethylene, hydrogen sulphide, diethyl ether, ethylene oxide, town gas | Intermediate |
| IIC | <0.50 | <0.45 | Hydrogen, acetylene, carbon disulphide | Most hazardous |
Important Notes
- Equipment certified for IIC can be used in IIA and IIB atmospheres (IIC is the most stringent)
- Equipment certified for IIB can be used in IIA atmospheres
- Equipment certified for IIA can only be used in IIA atmospheres
- The hierarchy is: IIC > IIB > IIA (IIC covers all). See the equipment selection guide for practical advice
Special Case: IIB + H₂
Some equipment is certified for "IIB + H₂". this means it is tested for all IIB gases plus hydrogen specifically, but not for acetylene or carbon disulphide. This is a common and cost-effective approach when hydrogen is present but acetylene/CS₂ are not.
For Ex d (flameproof) equipment marked IIB + H₂, the installation clearance distances must follow IIC requirements (40mm), not IIB (30mm).
Representative Gases by Group
Group IIA:
- Methane (CH₄). natural gas, biogas
- Propane (C₃H₈). LPG
- Butane (C₄H₁₀). LPG
- Acetone. solvents
- Ethanol. pharmaceutical, food/beverage
- Benzene, toluene, xylene. petrochemical
- Ammonia (NH₃). refrigeration, agriculture
- n-Hexane. extraction processes
Group IIB:
- Ethylene (C₂H₄). petrochemical feedstock
- Hydrogen sulphide (H₂S). oil & gas, wastewater
- Diethyl ether. pharmaceutical
- Ethylene oxide. sterilization
- Town gas / manufactured gas
Group IIC:
- Hydrogen (H₂). refineries, electrolyzer plants, fuel cells
- Acetylene (C₂H₂). welding, chemical synthesis
- Carbon disulphide (CS₂). viscose industry
Hydrogen: A Special Challenge
Hydrogen is in the most hazardous gas group (IIC) due to its extremely small MESG (0.29mm) and very low minimum ignition energy (0.017 mJ. about 10× less than methane). However, its auto-ignition temperature is relatively high (560°C), placing it only in temperature class T1.
This means hydrogen requires the most stringent protection against sparks/arcs (IIC) but is relatively forgiving on surface temperature (T1). This is the opposite of some IIA gases.
Dust Groups (Group III)
| Dust Group | Type | Examples | Hazard Level |
|---|---|---|---|
| IIIA | Combustible flyings | Textile fibres, cotton lint, rayon, jute | Least hazardous |
| IIIB | Non-conductive dust | Grain flour, sugar, wood dust, coal, plastic powder, pharmaceutical powders | Intermediate |
| IIIC | Conductive dust | Aluminium powder, magnesium, titanium, iron, carbon black | Most hazardous |
Conductive vs Non-Conductive Dust
The distinction matters because conductive dust particles can bridge insulation gaps and create short circuits, providing an ignition source even through Ex e (increased safety) protection. Conductive dusts require more stringent protection.
A dust is considered conductive if its resistivity is ≤10³ Ω·m.
Dust Ignition Properties
Two distinct ignition mechanisms exist for dust:
- Dust cloud ignition. airborne dust within its explosive range
- Dust layer ignition. settled dust ignited by a hot surface at a much lower temperature than the cloud
The temperature class for dust equipment must consider both the dust cloud ignition temperature (T_CL) and the dust layer ignition temperature (T_5mm).
Equipment maximum surface temperature must not exceed:
- ⅔ × T_CL (dust cloud). measured in Kelvin
- T_5mm − 75°C (for 5mm layer thickness)
Whichever gives the lower value determines the maximum allowable surface temperature.
NEC/CEC Gas Groups (North America)
| NEC/CEC Group | Equivalent IEC Group | Typical Gases |
|---|---|---|
| Group A | IIC | Acetylene |
| Group B | IIC | Hydrogen, butadiene |
| Group C | IIB | Ethylene, hydrogen sulphide |
| Group D | IIA | Methane, propane, ethanol |
| Group E | IIIC | Metal dusts (aluminium, magnesium) |
| Group F | IIIB/IIIC | Carbon black, coal dust, coke dust |
| Group G | IIIB | Grain dust, flour, sugar, wood dust |
Note: The NEC/CEC system uses letters (A–G) rather than Roman numerals, and the order is roughly reversed (A = most dangerous gas).
Selecting Equipment by Gas/Dust Group
When selecting equipment:
- Identify all flammable substances that may be present in the area
- Determine the gas/dust group for each substance
- Select equipment certified for the most hazardous group present
- Verify the temperature class is also suitable
If multiple gases are present (e.g., methane IIA + hydrogen IIC), the equipment must be rated for IIC.
Deep Dive: Gas Group IIC
For an in-depth look at why IIC is the most demanding gas group. including hydrogen's invisible flame, acetylene's decomposition hazard, carbon disulfide's extreme sensitivity, and when the IIB+H₂ compromise is acceptable. see our dedicated article:
Why Gas Group IIC Demands Special Attention: Hydrogen, Acetylene, and Carbon Disulfide
Related Files
- Fundamentals. explosion triangle, LEL/UEL
- Temperature Classes. max surface temperatures per gas
- Zone Classification. where these gases might be present
- Protection Methods. how equipment handles different gas groups
- Ex Markings. how gas group appears on the equipment label
Compiled from IEC 60079 series, ATEX 2014/34/EU, and IECEx operational documents. This reference guide does not replace official standards or certified site assessments. Always consult the applicable standard edition and a qualified Ex engineer for your specific application.