"...the identification of dusts that are a fire hazard but not a serious explosion hazard. Such dusts need not be classified according to NEC guidelines." page 4 NMAB 353-4. In contrast, an excerpt from the CSB Hoeganaes Case Study, " Although values indicate that the dust produces a weak explosion relative to other dusts, the dust is considered combustible by the OSHA definition and can result in a flash fire capable of causing injuries and fatalities" http://www.csb.gov/assets/document/CSB_Case_Study_Hoeganaes_Feb3_300-1.pdf
Classification of Dusts Relative to Electrical Equipment in Class II
National Research Council. "Chapter 2: Rationale for Classification of Combustible Dusts." Classification of Dusts Relative to Electrical Equipment in Class II Hazardous Locations. Washington, DC: The National Academies Press,...http://www.nap.edu/openbook.php?record_id=10952&page=4&goback=%2Egde_1184577_member_102193046