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Chapter l SUMMARY AND RECOMMENDATIONS A. Background The Occupational Safety and Health Administration (OSHA) requested that the Committee on Evaluation of Industrial Hazards of the National Research Council's (NRC) National Materials Advisory Board (NMAB) classify a submitted list of several hundred gases, vapors, and dusts in accordance with the groups presented in Article 500 of the National Electrical Code (NEC). In work previously conducted for the U.S. Coast Guard by the NRC Committee on Hazardous Mate- rials, a method to classify flammable gases and vapors had been established and this method was used by the NMAB com- mittee to prepare a matrix of combustion-relevant properties and classifications of gases, vapors, and selected solids. It appeared that no system for classifying the identi- fied combustible dusts was available, but a review by the committee of the few dusts classified in Class II, Groups E, F, and G of NEC Article 500 indicated that the layer ignition temperature and the electrical conductivity of the dusts were the crucial factors. There were, however, no well-established test methods to determine either of these properties for dusts. Accordingly, the committee established its Panel on Dust Test Equipment to recommended the needed methods. Layer ignition temperature and electrical resistivity were deemed to be the crucial properties for the following reasons: l. It was assumed that the equipment used in the areas exposed to the dust would be dust-ignition-proof since NEC Section 502-l requires that the equipment be dust-tight. Accordingly, electrically energized parts would not be exposed to the combustible dust, and the minimum ignition energy of the dust, an important consideration for intrin- sically safe equipment and for protection against ignition
by discharge of static electricity, would not have to be considered in the classification scheme. 2. A review of published data3-8 indicated that, with some exceptions (approximately 50 out of l500) , the temper- ature of a heated surface required to cause ignition was always lower for a layer of dust on the surface than it was for a dust cloud in a heated chamber. In those cases in which the cloud ignition temperature was lower than the layer ignition temperature, it was apparent that either: 0 There was a change in state of the dust from a solid to a gas (a typical example would be asphalt), or 0 The material in the cloud ignition temperature test was somewhat different from that in the layer ignition temperature test because of oxi- dation of each particle (an example would be atomized aluminum), or 0 The difference in temperature was within the test tolerance and, as such, is probably not significant. The panel reasoned that dust cloud ignition temperatures need not be considered in the classification scheme because of the small number of dusts having cloud ignition tempera- tures that might be lower than layer ignition temperatures. See Appendix C for a list of dusts which have been tested and found, under test conditions, to have cloud ignition temperatures lower than the layer ignition temperatures). 3. Although particle size and shape are known to affect the critical properties of dust, the availability of a rela- tively simple and specific test method to determine these properties would permit the specific dust available in the area under consideration to be evaluated, should there be any question as to the group classification. 4. The thermal insulating properties of a dust will have some effect on the surface temperature of the electri- cal equipment on which it collects, particularly equipment that generates considerable heat such as lighting fixtures and motors. However, this effect was judged to be relative- ly minor for most combustible dusts.
5. The abrasiveness of the dust was not judged to be a problem if the dust is kept out of moving parts such as motor bearings. 6. The previous history of a dust, particularly metal dusts, was judged to be a critical factor with relation to electrical resistivity. The panel felt that for such highly conductive dusts, the electrical resistivity of the solid from which the dust is formed generally should be consid- ered the electrical resistivity of the dust itself.* 7. 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 when the dust is in deep layers. The effect of moisture content on the ignition properties was judged to be of only minor sig- nificance with respect to ignition by the heated surface of electrical equipment because the heat of the surface would drive off the moisture quickly.^ B. Objectives The panel set forth the following objectives: 1. Evaluate methods presently used to test for the layer ignition temperature and electrical resistivity of combustible dusts. 2. Suggest any modification of these methods needed to account for particle size and shape and the need to characterize and define dust. 3. Recommend the test methods to be used to classify the identified combustible dusts according to the groups given in NEC Article 500. 4. Describe testing needed to validate these recom- mended test methods. *In special situations, the method recommended in this re- port can be used if the samples tested are representative of those actually present in the workplace. That samples are representative is, of course, subject to agreement by the responsible code enforcing authority.
C. Recommendations The panel reviewed the methods presently used to deter- mine the layer ignition temperature and the electrical resistivity of dusts. The panel recommended: l. A modified version of the proposed International Electrotechnical Commission hot plate method to determine the layer ignition temperature, and 2. A modified version of the method presented in the Instrument Society of America's Area Classification in Hazardous Dust Locations, ISA-Sl2.l0-l973, to determine the electrical resistivity. The recommended methods are described in detail along with the panel's rationale in Chapters 2 and 3 of this report. The panel initially considered methods of testing for thermal/oxidative stability as well as layer ignition tem- perature and electrical resistivity. A review of the liter- ature disclosed several different techniques for the deter- mination of long-term stability, including thermogravi- metric analysis, an active oxygen method, a modified ASTM oxygen bomb method, differential scanning calorimetry, and pressure differential scanning calorimetry. It appeared, however, that no single standardized analysis technique was suitable for all types of materials and that no such tech- nique could be established. The panel, therefore, decided that the method recommended for determining the ignition temperature of a dust layer would provide sufficient infor- mation on short-term thermal/oxidative stability and that no technique should be recommended at this time for deter- mining long-term thermal/oxidative stability.
