The National Electrical Code, OSHA and NFPA have electrical wiring requirements for locations that may contain flammable vapors, liquids, and dusts. Depending on the material handled, the location is assigned one of three Classes.
A Class I locations contain flammable gases or vapors formed by flammable liquids. If a location instead has combustible dust present, it is a Class II location. Lastly, a location where combustible fibers or filings are easily ignited is a Class III location.
Each Class is further separated into Division I or II. In Division I locations, flammable vapors, dust, or fibers (depending on class) are present during normal operations. However, Division II locations only contain flammable vapor, dust, or fibers during abnormal situations.
– NFPA 70: National Electrical Code. 2014 Edition.
– 29 CFR 1926.407 Safety and Health Regulations for Construction, Electrical.