The design of electrical equipment and instrumentation is based on the nature of the flammable hazard in a process area. The hazardous area classification method used in the USA is defined in the National Electrical Code; it is a function of the nature and degree of the flammable hazards within a particular area. The rating method includes Classes I, II, and III, Groups A-G, and Divisions 1 or 2.
The classes are related to the nature of the flammable material:
· Class I: Locations where flammable gases or vapors are present.
· Class II: The same but for combustible dusts.
· Class III: Hazard locations where combustible fibers or dusts are present but not likely to be in suspension.
The groups designate the presence of specific flammable chemical types. Chemicals that are grouped have equivalent hazards:
· Group A: acetylene
· Group B: hydrogen, ethylene
· Group C: carbon monoxide, hydrogen sulfide
· Group D: butane, ethane, ethyl alcohol
· Group E: aluminum dust
· Group F: carbon black
· Group G: flour
Division designations are categorized in relation to the probability of the material being within its flammable or explosive regions:
· Division 1: Probability of ignition is high; that is, flammable concentrations are normally present.
· Division 2: Hazardous only under abnormal conditions. Flammable materials are normally contained in closed containers or systems.
Reference: adapted from Crowl, D. A., & Louvar, J. F. (2011). Chemical process safety: fundamentals with applications. Upper Saddle River, NJ: Prentice Hall, .
Note that the hazardous area classification method used in Europe is similar in intent but different in detail. The method is defined in the ATEX 95 and 137 Directives. It applies not just to electrical equipment and instrumentation but to any equipment that could act as an ignition source, e.g. a gearbox or a conveyor belt system.