1. Basic Classification

• Class I: Flammable gases/vapors → Groups A, B, C, D.
• Class II: Combustible dusts → Groups E, F, G.
• Class III: Ignitable fibers/flyings (no subgroups).

Gas groups (A-D) and dust groups (E-G) are different classes and cannot be interchanged. Equipment must be clearly marked with the correct Class and Group.

2. Gas Groups (Class I, Groups A-D)

Classified by explosion pressure, MESG and MIC. Hazard decreases from A to D:

• Group A: Acetylene – Highest explosion pressure, most hazardous.
• Group B: Hydrogen, butadiene, ethylene oxide – MESG ≤0.45mm or MIC ≤0.40.
• Group C: Ethylene, diethyl ether, cyclopropane – MESG 0.45–0.75mm or MIC 0.40–0.80.
• Group D: Propane, gasoline, methane, acetone, natural gas – MESG >0.75mm or MIC >0.80 (most common).

3. Dust Groups (Class II, Groups E-G)

• Group E: Metal dusts (aluminum, magnesium) – Conductive, highly abrasive.
• Group F: Carbonaceous dusts (coal, coke) – High explosion risk.
• Group G: Other dusts (flour, grain, wood, plastics) – Non-conductive.

4. How to Select Equipment

Check the nameplate for Class I (A-D) or Class II (E-G). Gas-rated equipment cannot be used for dust and vice versa. Higher hazard groups (e.g. B) can often cover lower ones (C, D), but not reverse. T-Code must match the substance's auto-ignition temperature.

International Equivalents (IEC/ATEX): Gases: IIC (≈A+B), IIB (≈C), IIA (≈D). Dusts: IIIC (≈E), IIIB (≈F+G).

Practical Advice: Determine gas or dust hazard (Class I or II). Identify specific group using NFPA 497/499 or NEC tables. Equipment must meet Class + Group + Division/Zone + T-Code.