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Chapter 1 SUMMARY OF RECOMMh~DAT IONS The pre sent method of grouping combustible dusts ~ i . e., the 1981 NEC 500 ~ is based solely on electrical resistivity. In addition, the present NED 1 imits the maximum surf ace temperature that should be obtained with the equipment used for a particular group ~ see Chapter 2 ~ . This maximum temperature is related to dust-layer ignition temperature s, values that are not well known generally . The ref ore, the committee, based on its study and the work of its panels, recommends that: 1. Dusts having an ignition sensitivity less than 0.2 (i.e., a low probability of ignition) and an explosion severity less than 0.5 (i.e., a low consequence ~ should be considered as constituting only a weak explosion hazard, and electrical equipment suitable for Class II locations should not be required for these dust atmo sphere s . 2 . Du sts with an ignition sensitivity equal to or greater than 0. 2 or an explosion severity equal to or greater than 0.5 should be classified solely on the basis of re sistivity into two arbitrary groups, E and G. 3. The specification of maximum permitted surface temperatures, such a s is pre sentry done in the National Electrical Code, should be continued. However, to provide for safe use of electrical apparatus in the presence of dusts with layer ignition temperatures lower than these maximum permitted temperatures, the maximum surface temperature of electrical equipment should be required to be lower than the layer ignition temperature by a fixed differential (e.g., 25°C) . 4. The test procedures for measuring resistivity and layer ignition temperature should be those de scribed in this document . 5. There is a need f or consistency in dealing with the hazard of hot surfaces, and those responsible for safety in locations made hazardous by the pre sence of combustible dusts should apply the same limitations on maximum surface temperature to all equipment, not only electrical equipment. 6. Analytical and experimental re search should be undertaken to develop and validate a predictive model that can be used as a basis for classifying the ignition and explosion hazards of a dust in the workup ace based on its properties and conditions. Such research also could lead to development of more adequate testing techniques and could suggest new methods for preventing and controlling dust explosions. (A potential approach is described in NMAB 353-6. ~ 1
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2 7. A laboratory should be established to evaluate the explosion hazard of dusts in the workplace.* This laboratory should be capable of testing and evaluating 150 to 200 samples each year using the current standard procedures. The laboratory also should perform comparative testing (i.e., use various experimental techniques) and should determine properties of dusts, as needed. Further, it should be active in some of the research activities recommended in item 6 above and should act as a clearinghouse for domestic and foreign testing and research results in support of affected industries and regulatory bodies. ) *The O.S. Bureau of Mines' Dust Explosions Research Laboratory closed operations in this field in the late 1960s.
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