Document QX5D76b004ZvJNwNrxX8zV3rE

41--Electrical Hazards 500-2. Special Precaution. The intent of Ai tides 500 through 503 is to require a form of constiuction of equipment, and of installation that will insure safe peiformance undei conditions of pioper use and maintenance It, therefoie, is assumed that inspec tion authouties and users will exercise more than ordinary care with regard to instal lation and maintenance The explosion charactei istics of air mixtures of hazaidous gases, vapors, or dusts vary with the specific material involved Classification of a hazaidous mixtuie into a Class I hazaidous location, GOup A, B, C oi D, involves dete.minations of maximum explos.on pressure, maximum safe deaiance between parts of a clamped joint in an enclosuie, and the minimum ignition tempeintuie of the atmospheric mixture For Class II location, Cioups E, F, and G, the classification involves the tightness of the joints of assembly and shaft openings to pievent enhance of dust m the dust-igmhon-pioof enclosuie, the blanketing effect of laveis of dust on the equipment that mav cause ovei heating, elcctncal conductivity of the dust, and the ignition temperature of the dust It is necessaiy, theietoie, that equipment be appioved not only foi the class of location but also foi the specific gioup of the gas, vapoi or dust that will be piesent For purposes of testing and approval, various air mixtures (not oxygen enriched) have been grouped on the basis of their hazardous characteristics, and facilities have been made available for testing and approval of equipment for use in the following atmospheric groups For Groups A, B, C and D, see Table 500-2(c) Group E, Atmospheres containing metal dust, including aluminum, magnesium, and their commercial alloys, and other metals of similarly hazardous characteristics Group F, Atmospheres containing carbon black, coal or coke dust Group G, Atmospheres containing flour, starch, or grain dust Certain chemical atmospheres may have characteristics which would require safeguards beyond those required for any of the above groups Carbon disulfide is one of these chemicals because of its low ignition tempeiatnre (100 C) and the small joint clearance required to arrest its flame For a complete list noting propeities of flammable liquids, gases and solids refer to NFPA No 325M-1969 (a) Approval for Class and Properties Equipment shall be appioved not onlv for the class of location but also foi the explosion piopeities of the specific gas, vapoi, oi dust that will be piesent In addition, equipment shall not have exposed anv suiface that operates at a temperatuie m excess of the ignition tempeiatuie of the specific gas vapor or dust The characteristics of various atmospheuc mixtures of hazardous gases, vapors, and dusts depend on the specific hazardous material involved (b) Marking Appioved equipment shall be maiked to show the Class, Gioup and operating tempeiature, or temperatuie lange, based on operation in a 40 C ambient for which it is approved The tempeiature lange, if provided, shall be indicated m identification numbers, as shown in Table 500-2 (b) Identification numbers marked on equipment nameplates shall be in accordance with Table 500-2(b) Exception Equipment of the nonheat-producing type, such as junction boxes, con duit and fittings, are not required to have a marked operating temperature For purposes of testing and approval, various atmospheric mixtures (not oxygen enriched) have been grouped on the basis of their hazardous characteristics, and facilities have been made available for testing and approval of equipment for use in the atmospheric groups listed m Table 500-2(c) Since there is no consistent relationship between explosion properties and ignition temperature, the two must be regarded as independent requirements 1270