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Chapter 3 ELECTRICAL RESISTIVITY A. Back~ro-und The panel reviewed two reports describing equipment used to determine the electrical resistivity of combustible dusts: I. Dr. S.H. Chiang, "Measurement of Electrical Conduc- tivity/Resistivity of Dust Samples," paper written in Septem- ber 1976 and given tot the Committee on Evaluation of Indus- trial Hazards on April 2S, 1977; 2. Instrument Society of America (ISA), Area Classifica- tion in Hazardous Dust Locations, ISA-S12.10-1973. The method developed was essentially the same as the ISA method except that a direct-current power supply was used, in lieu of the alternating-current power supply, to reduce the Jikelihood--of instrument error and reactance effects. B. Recommended Method of Measurement of Dust Resistivity 1. Scope . . The objective of this test method is to provide a con- venient, albeit not precise, determination of the resistivity of dust layers for the purpose of classifying dusts with re- gard to fire and explosion hazards. The result ts wit ~ be used to determine whether dusts have resistivities similar to met- allic dusts or to agricultural dusts; therefore minor differ- ences in measurement technique and experimental conditions are not likely to lead to differences in classification. 2. Sample Chamber The sample chamber shall be of open construction with rectangular electrodes similar to the chamber described in 19

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20 ISA-SI2.10-1973. This chamber uses stainl ess steel bars approximately 14 x ~ 4 nun and 100 Irma long, pi aced approxi- mately 12.5 mm apart, as illustrated in Figure 5. The exact dimensions of the cell are not crucial. Smaller cells are not recommended because they are difficult to fill and clean. A rectangular cell is recommended because it is simpler to construct than a cell made from concentric tubes and because calculations of resistivity can be made more easily. The open, rectangular cell construction also is filled and cleaned with less difficulty. H = Figure 5. Sample chamber The electrodes shall be mounted on a material of high surface resistivity. Any metallic mounting~screws shall be recessed or countersunk to avoid shunting effects of table tops, etc., during measurement. The dimensions of the cell are not standardized; rather, they are left to the discretion of the laboratory making the resistivity determination. Direct current measurement is re- commended to avoid possible complications of capacitance ef- fects. The measurement of resistance can be carried out with any convenient instrumentation suitable for resistance mea- surement in the range of 10 4 to loons, depending upon spe- cific cell dimensions. The method of resistance determina- tion may be based either on voltage and current measurement or use of a dc ohmmeter. If voltage and current are measured, the internal resistance of the voltmeter or ohmmeter must be considered in calculating the cell resistance. 3. Pre~arat~ on of Dust samn1 e The sample shall be dry sieved through a 200-mesh sieve :to~;exclude large partial es and shall be representative of duet that may deposit on the surface of electrical appara- tu~. No drying or other pretreatment of the sample is : ~

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21 required; however, if drying is necessary to facilitate siev- ing, the sample should be allowed to equilibrate at ambient humidity for at least 24 hours. For.this purpose, the sam- ple should be exposed in a thin (less than 6 mm) layer. 4. Procedure .. Rs. a. Measure the resistance, R0, of the empty sample chamber. b. Pour the dust into the sample chamber. Tap the sam- ple chamber several times to settle the dust. Remove excess dust by running a straightedge al ong the top of the sample chamber. I'f the dust does not remain in place at the ends of tile sample cell, tape may be used to keep the dust in place. If tape is used, the measurement of Ro shall be made with tape in place. c. Measure the resistance of the filled sample chamber, d. If Ro is greater than 10 Rs, compute dust resistiv- ity from the equation: . HE P = Rs L. where H. W. and L are cell dimensions in cm (see Figure 5), Rs is measured re.sis.tance-of sample-filled cell in ohms, and p is resistivity of dust sample in ohm-cm. e. If -Ro is less than 10 Rs, compute dust resistivity from the,equation: , R's Ro ~` P = R -R ' C. Validation of Test Method . . . . .. Since there is no data base for the recommended test .. . . method for determining electrical 'resistivity of-dust, two panel members constructed three different test ce'l~Is and tested four dusts (corn starch,' powdered sugar, activated' charc'oal, and powdered graphite)'under various' conditions in order to validate the method. Except for some-'differ- ences in treatment before the test, the dust samples were the same. The results obtained are given in Table 2.

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22 . Table 2.. Resistivity of Tes;t Dusts Under Various Conditions . ~ . . . - Resistivity, ohin-cin Test Cell ~ Test Cell 2 Test Cell 3 . Dust As Received . Corn Starch (a) 2 X 10 6 2 X 1011 101: - 10 12 Powdered Sugar (b) 6 X 1014 10 14 1012 _ 1013 Activated Charcoal (c) 1.2 X 10 4 (g) 2 X 10 3 Graphite (d) 10 3 (g) lO 4 Dry (e) . ~ . . . . . Corn Starch (a) 4 X 1012 5 X 10= (g) Powdered Sugar (b) 8 X 10 13 1o 15 (I) Activated Charcoal (c) 8 ~ 10 3 lo 5 (g) Graphite id) 2 X 10 3 4 X 10 3 fig) Moist (f) Corn Starch (a) 6 X 108 7 X i01 (g) Powdered Sugar (b) 7 X 10 7 6 X 10 6 (g) Activated Charcoal (c) 6 X 10 3 7 X 10 3 (g Graphite (d) 6 ~ 10: 8 X 10 2 ma) NOTE: On Test Cell 3 only, voltage varied 10 to lO00 volts do for~corn starch and powdered sugar. Test Cells 1 and 2, testing corn starch and powdered sugar, used General Radio No. 1644A Megohm Bridge at 500 volts dc; Test Cells 1 and 2, testing activated charcoal and graphite, used Fluke digital ohmmeter. Test Cell 3, testing charcoal and graphite, used Triplett multi- meter. (a) Argo "pure corn starch'." (b) Domino "confectioners 10-X powdered sugar.:' (c) Apache Chemicals No. 1599 carbon powder, activated decolonizing, Lot No. 10077. (~) Wickes Engineered Materials No. 205 ''lubricating graphite." (e) Dried in desiccator at least 24 hours. (f) Stored over water in closed container at room ambient at least 24 hours. (g) Not tested.

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23 The results of the recommended method would be consid- ered subject .to.excessive error when compared to American Society. for Testing and Materials methods and to well-con- trolled laboratory experiments. However, the intent of the testing is to be able to group dusts of similar resistivity. Data published in ISA-S12.10-1973 and the panel.tests do not indicate that the effect of moisture, aging, testing, test voltage, etc., are likely to be sufficiently great to cause a misclassification of dusts. D. groups: Recommended Resistivity Criteria for NEC Article 500 The 1978 National Electrical Code recognizes three dust Group E--metallic dusts, Group Fe-carbonaceous dusts and Group G--agricultural dusts. These groups.can be considered to be conducting, semi- conducting, and non-conducting, respectively. If these three groups are maintained, the panel recom- mends tilat they be defined in terms of resistivity as fol- lows: Group E--p < 102 ohm-cm, Group F--1.02 ohm-cm < p < 108 ol~m-cm, and Group G--p > 108 ohm-cm. . These values are based on data in ISA-S12.-10-1973. Since test data have indicated that the resistivity.of dust is dependent upon particle size and shape, bulk density, and past history (e.g., surface oxidation), it is to be expec- ted that varying results will be observed when testing dif- ferent samples of dust from the same material. The panel therefore recommends that a dust be considered to be electri- cally conductive or electrically semi-conductive (Groups E and F. respectively, in accordance with the 1978 National Electrical Code) if the resistivity of the solid maternal (if any) from which the dust is formed has a value falling within the ranges specified for Group E or Group F dusts. The preference of the panel, however, is to see the three groups replaced by two: Group E--p < 1Os ohm-cm, and Group G--p > 10~ ohm-cm. The basis for this preference is that the requirements given. ~ ~ ~~ :~: ~~ in the NEC recognize the greater hazard of conductive dust~.~;-~.~-.~:~;:

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24 as compared to non-conductive dusts, and NEC equipment selec- tion and installation rules are essentially the same for Group E and F dusts (even though some Group F dusts may not be sufficiently conductive to be considered of the same hazard level as Group E dusts). Thus, there seems to be no clear rationale for maintaining three groups of dusts. However, the panel also believes that equipment opera- ted at over 600 V may present a problem if electrically semi-conductive dust is present on uninsulated live parts. Therefore, the panel recommends that, if Group F is elimina- ted in some future edition of the NEC, consideration be given to: 1. Requiring that equipment containing uninsulated live metal parts operating at greater than 600 V be re- quired to comply with the requirements for equipment for use in Division 1 locations; and 2. Requiring that plugs and receptacles connected to circuits rated over 600 V be considered unacceptable where semi-conductive dusts are present. The above recommendation has been based on experience re- ported to the panel indicating that there has been short- circuiting of plugs and receptacles in mines where the plugs and receptacles are operating in excess of 600 V.