Closed storage drums containing flammable liquids in an inside storage room would not normally allow the hazardous vapors to escape into the atmosphere. Now let's move on to a discussion of the nature of hazardous substances. The only substance in Group A is acetylene. Acetylene makes up only a very small percentage of hazardous locations. Consequently, little equipment is available for this type of location.
Acetylene is a gas with extremely high explosion pressures.
Group B is another relatively small segment of classified areas. This group includes hydrogen and other materials with similar characteristics. If you follow certain specific restrictions in the Code, some of these Group B locations, other than hydrogen, can actually be satisfied with Group C and Group D equipment. Group C and Group D are by far the most usual Class I groups. They comprise the greatest percentage of all Class I hazardous locations.