Hazardous Area References

CLASS I: Locations in which flammable gas/vapors may be present and could produce explosive or ignitable mixtures
CLASS II: Locations in which combustible dust may be present and could produce explosive or ignitable mixtures
CLASS III: Locations in which ignitable fibers may be present and could produce explosive or ignitable mixtures

DIV 1: High probability of hazardous material producing an explosive or ignitable mixture due to it being present during normal operating conditions
DIV 2: Low probability of hazardous material producing an explosive or ignitable mixture due to its presence only during abnormal operting conditions

GROUP A: Acetylene
GROUP B: Flammable gas or vapor whose MESG less than 0.45mm (i.e. butadiene, ethylene oxide, propylene oxide, and acrolein)
GROUP C: Flammable gas or vapor whose MESG greater than 0.45 mm but less than or equal to 0.75mm (i.e. ethyl ether, ethylene, acetaldehyde, and cyclopropane)
GROUP D: Flammable gas or vapor whose MESG greater than 0.75 mm (i.e. acetone, ammonia, benzene, butane, ethanol, gasoline, methane, natural gas, naphtha, and propane)
GROUP E: Combustible metal dusts such as aluminum, magnesium, and their commercial alloys
GROUP F: Combustible carbonaceous dusts with 8% or more trapped volatiles such as carbon black, coal, or coke dust
GROUP G: Combustible dusts not included in Group E or Group F. Typical dusts include flour, starch, grain, wood, plastic and chemicals

A mixture of hazardous gases and air may ignite in contact with a hot surface. The condition for ignition depends on several factors as surface area, temperature and concentration of gas.