Comepex - Gas groups

Question 1

What are gas groups?

Answer 1

Gas groups classify flammable gases/vapors by how easily they can ignite and propagate a flame. More “severe” groups require more stringent equipment design.

Question 2

What properties are the gas groups based on?

Answer 2

• MESG (Maximum Experimental Safe Gap): how narrow a flameproof gap must be to stop flame transmission.
• MIC ratio (Minimum Ignition Current ratio): current needed to ignite the gas vs. a reference gas.
• Smaller MESG or lower MIC → more easily ignitable → higher subgroup (IIC).*

Question 3

What is group IIA?

Answer 3

Group IIA

Typical gases: propane, butane, methane/natural gas, gasoline vapors, acetone, ammonia

Criteria (indicative): MESG > 0.9 mm or MIC ratio > 0.8

Equipment can only be used in IIA

Question 4

What is group IIB?

Answer 4

Group IIB

Typical gases: ethylene, hydrogen sulfide (H₂S), ethyl ether

Criteria (indicative): 0.5 mm < MESG ≤ 0.9 mm or 0.45 < MIC ≤ 0.8

Equipment can be used in IIA and IIB

Question 5

What is group IIC?

Answer 5

Group IIC (most severe)

Typical gases: hydrogen, acetylene, carbon disulfide (CS₂)

Criteria (indicative): MESG ≤ 0.5 mm or MIC ≤ 0.45

Equipment can be used in IIA, IIB and IIC