Chapter 7
Gas groups and temperature classes
Published
- Gas groups IIA, IIB and IIC explained with typical gases
Gas groups IIA, IIB and IIC rank flammable gases by how easily an explosion passes a narrow gap and how little current ignites them. Propane is IIA, ethylene IIB, hydrogen and acetylene IIC. IIC equipment covers all three groups.
- Temperature classes T1 to T6: matching surface temperature to autoignition temperature
Temperature classes T1 to T6 cap the hottest surface of Ex equipment at 450, 300, 200, 135, 100 or 85 °C. Choose a class whose limit is below the autoignition temperature of every gas present. Table, rule, examples and margin practice.
- IIB+H2 explained: when is it enough for hydrogen?
IIB+H2 marks Ex equipment, usually flameproof, that meets gas group IIB and has also been proven for hydrogen. It is enough wherever hydrogen is the only IIC gas, but never for acetylene or carbon disulphide. Meaning, uses, limits and how it compares with IIC.
- Acetylene, carbon disulphide and hydrogen: the special cases of gas group IIC
Hydrogen, acetylene and carbon disulphide are all gas group IIC, but each adds its own problem: acetylene decomposes and attacks copper, carbon disulphide ignites at 90 °C and needs T6, hydrogen allows IIB+H2. What this means for Ex equipment.
- How to find the gas group and temperature class of a substance
Find a substance's gas group (from MESG or MIC ratio) and temperature class (from its autoignition temperature) in ISO/IEC 80079-20-1, NFPA 497, safety data sheets or the excodex gas library. Step-by-step lookup, worked example, and what to do when data are missing.
- Mixtures of flammable gases and vapours: choosing gas group and temperature class
For a gas or vapour mixture, take the most severe gas group and the lowest autoignition temperature of its components, or estimate the MESG and LEL with Le Chatelier-type formulas. Rules, exclusions, hydrogen blends, worked examples and when to test.
- Hot surfaces in hazardous areas: luminaires, motors, heat tracing and process heat
A temperature class is only valid under the conditions it was certified for. How heat tracing gets its T-class (stabilised or controlled design), why motors on variable speed drives need converter-rated certificates, safe lamp changes, and hot process surfaces.