Combustible Dust Glossary
Class II locations. Class II locations are those that are hazardous because of the presence of combustible dust. The following are Class II locations where the combustible dust atmospheres are present:
Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, and other
combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.
Group F. Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles (see ASTM D 3175,
Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, for coal and coke dusts) or that have been sensitized by other
materials so that they present an explosion hazard. Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts.
Group G. Atmospheres containing other combustible dusts, including flour, grain, wood flour, plastic and chemicals.
Combustible Dust. A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.
Combustible Particulate Solid. Any combustible solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition.
Hybrid Mixture. A mixture of a flammable gas with either a combustible dust or a combustible mist.
Deflagration. Propagation of a combustion zone at a speed that is less than the speed of sound in the unreacted medium.
Deflagration Isolation. A method employing equipment and procedures that interrupts the propagation of a deflagration of a flame front, past a predetermined point.
Deflagration Suppression. The technique of detecting and arresting combustion in a confined space while the combustion is still in its incipient stage, thus preventing the development of pressures that could result in an explosion.
Detonation. Propagation of a combustion zone at a velocity that is greater than the speed of sound in the unreacted medium.
Dust-ignitionproof. Equipment enclosed in a manner that excludes dusts and does not permit arcs, sparks, or heat otherwise generated or liberated inside of the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust on or in the vicinity of the enclosure.
Dusttight. Enclosures constructed so that dust will not enter under specified test conditions.
Explosion. The bursting or rupture of an enclosure or a container due to the development of internal pressure from deflagration.
Lower Flammable Limit (LFL). The lower flammable limit is the lowest concentration of a combustible substance in an oxidizing medium that will propagate a flame (vapors & gases).
Minimum Explosible Concentration (MEC). The minimum concentration of combustible dust suspended in air, measured in mass per unit volume that will support a deflagration.
Minimum Ignition Energy (MIE). The lowest electrostatic spark energy that is capable of igniting a dust cloud.
Minimum Ignition Temperature (MIT). The lowest temperature at which ignition occurs.
Upper Flammable Limit (UFL). The upper flammable limit is the highest concentration of a combustible substance in an oxidizing medium that will propagate a flame (vapors & gases).
Source: OSHA Combustible Dust National Emphasis Program (Reissued 3/11/08)