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Chapter 3 DUST CLAS SIFICATION METHODS C LAS SI EYING USING THE 1981 NEC RESISTIVITY GUIDELINES Dust is classified into Group E, F. or G according to the 1981 NEC resistivity guidelines by using the decision tree approach shown in Figure 3-1. It is first determined whether a specific material is an oxidizer, an explosive, or a pyrophoric material; a noncombustible material; or a combustible material. If the dust is an oxidizer, explosive, or pyrophoric material, it may require safeguards beyond those required for atmospheres in which equipment designed for use in Class II locations as defined in the NEC can be used and the electrical equipment should not be utilized without further testing or evaluation. If the dust is noncombustible, classification need not proceed further since electrical equipment suitable for Class II locations is not required. . . If the dust is combustible, the next step is to consider whether the data needed to determine ignition sensitivity and/or explosion severity (see Appendix A for definitions) are available. If ignition sensitivity data and/or explosion severity data are not available, the dust can be assigned a group classification based on resistivity data. The classification will err on the side of safety because the dust may not present an explosion hazard requiring Class II equipment. It is not always necessary to test a dust to determine resistivity in order to classify it. Resistivity can be estimated for purposes of classification except in doubtful cases. Metal dusts should be presumed be Group E unless tests have been conducted. Plastic and chemical dusts agricultural dusts can usually be classified as Group G because of their similarity to other dusts that have been classified. If it is desirable to assess the relative hazard of the dust, the data required to determine ignition sensitivity and explosion severity should be acquired. A dust that has an ignition sensitivity less than 0.2 and an explosion severity less than 0.5 poses a weak explosion hazard and does not require Class II electrical equipment. Any dust having an ignition sensitivity equal to or greater than 0.2 or an explosion severity equal to or greater than 0.5 requires electrical equipment suitable for Class II hazardous locations. 9 to and

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11 Test procedures for quantifying ignition sensitivity and explosion severity are described in Appendix A, and were used to derive the values presented in Table 4-1. It is recognized, however, that any change in test equipment may affect the numerical values for explosion severity or ignition sensitivity (e.g., in a chamber larger than that of the Hartmann apparatus, the values of the maximum pressure and the rate of pressure rise may be different). Ignition sensitivity and explosion severity are not fundamental combustion parameters of a combustible dust but are apparatus dependent values. If, in the future, standard test methods different from those discussed in Appendix A are adopted for use, the values of ignition sensitivity and explosion severity used in classifying dusts must be re-established. It should be emphasized that the values are not sacred. What is important is that dusts from such materials as chlorinated rubber, milled antimony, Pennsylvania anthracite, egg white, and cereal grass (indicated in Table 4-1 by d) are not explosion hazards. The criteria in Figure 3-1 and in Figure 3-2 have been established based on experience with such dusts, and if new test methods yield different values, the criteria must be changed accordingly. PRCE,OSED NEW CLASSIFICATION OF DUSTS The committee proposes that future classifications be made as outlined in Figure 3-2. This proposal separates dusts into two rather than three groups. Group F is considered unnecessary and undesirable in the proposed new classification scheme. It now includes dusts with electrical resistivities both greater and less than the dividing line between conductive and nonconductive combustible dusts, and electrical equipment selected and installed according to the 1981 NEC is essentially the same for atmospheres containing Group E and electrically conductive Group F dusts. Therefore, under the proposed scheme, Group F is eliminated and equipment selection is restricted to Group E and Group G dusts. The committee believes, however, that equipment operated at over 600 V may present a problem if electrically semiconductive dust is present on uninsulated live parts. It therefore recommends that if Group F is eliminated in some future edition of the NEC, consideration be given to . requiring: 1. That equipment containing uninsulated live metal parts operating at greater than 600 V comply with the requirements for equipment for use in Division 1 locations and 2. That plugs and receptacles connected to circuits rated over 600 V be considered unacceptable where conductive dusts are present.

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13 This recommendation is based on reports to the committee indicating that plugs and receptacles in mines have short-circuited when operating in excess of 600 Nt. Because dust settles and forms layers on operating equipment, the present NEC states that the maximum surface temperatures under actual operating conditions should not exceed those shown in Table 3-1. The committee proposes that a different approach be considered. TABLE 3-1 Maximum Surface Temperatures Equipment (e.g.' Motors or Power Transformers) Subject to Overloading Equipment Not Normal Abnormal Class II Subject to Overloading Operation Operation Group C OF C OF C OF E 200 392 200 392 200 392 F 200 392 150 302 200 392 G 165 329 120 248 165 329 The method employed to measure dust layer ignition temperature parallels what happens to dust that has settled on electrical equipment that generates heat. It is realized that surface temperature and ignition temperature depend on layer thickness and dust compactness; therefore, the committee recorurnends that the maximum surface temperature of the electrical equipment be lower than the specified dust layer ignition temperature by some differential value (e.g., 25C).* The committee believes that such a safety factor is appropriate and will give the user some flexibility in judgment. Furthermore, the committee recommends that the NEC continue to specify an independent upper limit, based on experience, for the surface temperature of the electrical equipment. *Field experience or other special conditions may dictate use of another differential value in some specific cases.

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14 TEST APPARATUS Based on the rationale given in Chapter 2 of this report, only data for layer ignition temperature and resistivity are essential for the classification of a combustible dust. The Panel on Dust Test Equipment reviewed the methods presently used to determine the layer ignition temperature and the electrical resistivity of dusts. Based on the panel's findings, the committee recommends use of: 1. A modified version of the proposed International Electrochemical Commission hot plate method to dete,-~ine 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-S12.10-1973, to determine electrical resistivity. These recommended methods and the panel's rationale are described in detail in Appendixes B and C. The panel initially considered methods of testing for thermal and oxidative stability as well as layer ignition temperature and electrical resistivity. A review of the literature disclosed several different techniques for the determination of long-term stability, including thermogravimetric 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 material and that no such technique could be established. The panel, therefore, decided that the method recommended for determining the ignition temperature of a dust layer would provide sufficient information on short-te,~u thermal and oxidative stability and that no technique should be recommended at this time for determining long term thermal and oxidative stability. TESTING AND RESEAP~H The U.S. Bureau of Mines' Dust Explosions Research Laboratory closed operations in the late 1960s and there now is no laboratory available to conduct routine determinations of the combustion parameters of dusts. Such a laboratory is needed to evaluate the explosion hazard of dusts found in the workplace using currently recognized testing methods. This laboratory also could carry out research needed to specify more adequate testing techniques and to develop methods of classifying dusts in accordance with the severity of explosion hazard based on the properties and physical condition of the dust. This laboratory also could act as a clearinghouse for other domestic and overseas research and testing results in support of affected industries and regulatory bodies.