Gas Groups and Dust Groups

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 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 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)

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.

Grunnleggende — 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