Coffee Break Training: Inspection Techniques: Electrical 'Classified Locations': Combustible Dusts and Fibers Divisions (Part 4) [open pdf - 356KB]
"Last week's Coffee Break Training explained the National Fire Protection Association (NFPA) 70®, 'National Electrical Code'® hazard divisions for electrical wiring and equipment where flammable vapors and gases exist. This Coffee Break Training will explain the dusty environments that may need classification. Combustible dust is any finely divided solid material that is 420 microns (0.017 in) or smaller in diameter (material passing a U.S. No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed and ignited in air."
Coffee Break Training - Fire Protection Series No. FP-2011-22
United States Fire Administration: http://www.usfa.fema.gov/