Classification of Dusts Relative to Electrical Equipment in Class II Hazardous Locations (1982) / Chapter Skim
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Chapter 2: Rationale for Classification of Combustible Dusts
Chapter 2: Rationale for Classification of Combustible Dusts
Pages 3-8
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From page 3... ...
Selection and installation rules for electrical equipment recognized that the hazards posed by electrically conductive dusts and electrically nonconductive dusts were different, but it was not until the 1981 edition was published that the NEC quantified the boundary values of electrical resistivity for Groups E, F and G dusts and explicitly stated a value of resistivity below which a dust must be considered electrically conductive.
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From page 4... ...
CONSEQUENCES OF THESE PREMISES Premise 1 NEC selection and installation requirements for Class II locations specify use of dust-ignition-proof enclosures or dust-tight enclosures, depending on the type of equipment enclosed and the relative hazard of the location. In addition, the NEC specifies maximum surface temperatures for electrical apparatus.
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From page 5... ...
can be fire hazards but are only weak explosion hazards because they are relatively difficult to ignite and, if ignited, produce relatively weak explosions. The classification of these dusts based on resistivity alone would impose economic penalties on the user of electrical apparatus (i.e., the user would have to select and install special electrical apparatus despite the fact that, in practice, the hazard level is so low that general-purpose apparatus could be used safely)
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From page 6... ...
, and moisture content may affect explosion parameters significantly in laboratory tests, these effects are not judged to be significant enough to change the classification of a dust. Particle size and shape are known to affect the critical properties of dust, but the availability of a relatively simple and specific test method to determine resistivity and layer ignition
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From page 7... ...
The committee felt that for such highly conductive dusts the electrical resistivity of the solid from which the dust is formed generally should be considered to be the electrical resistivity of the dust itself.* Although moisture content is known to have an effect on the ignition properties of dust, the major effect appears to be to accelerate spontaneous ignition of non-metallic dusts when the dust is in deep layers.
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From page 8... ...
Committee on Hazardous Materials, Fire Hazard Classif ication of Chemical Vapors Re lative to Explosion Proof Equipment vol . 1, National Academy of Sciences, Washington, D.C., Reports I (February 1970 )
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