NFPA 496-1998 Standard for Purged and Pressurized Enclosures for Electrical Equipment.
Class 11, Division I. A kwation (1) in which combustible dust is in the air undei’ normal operating conditions in qtumti— tics stifliciriti to pr(MILi— explosive or ignitiblr mixtures; or
(2) where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitible mixturc—s 1(1 lw pro1ieesI awl niight also providr a source of ignition through simultaneous failure of electrical equipment. operation of protection devices, or from other causes; or (3) in which combustible lusis ofan eleciticalb conductive nature I tnav be preselit in l,aiaidou.s quanutws. 1Sn’ Serw?? 500-8(a) of M’J4 70, .\qiionaI Hnina! Co.’kj
Class II, Division 2. A locatioji where c(,nhlnLsul)le (ILLSI 1.5 in e lit ,iinallv iii the air in quantities suffidciit to produce explosive or ignitible mixtures, and dust accumulations are normalls insuflicient to interfere with the nol mal operation of electrical equipment or oilier apparatus. iltit combustible dust ma he iii suspension in the air as a result of infrequent malfunctioning of handling o processing equipment and wlierr combtistil,lr clint accuiiiiilatic,iis on. iii, or iii dir vicinity of the electrical equipment may be sttrncient to iiiterfrre with the safe dissipation o’ heat from electrical equipnlenl or may be ignitible b abnormal operation or lsilure of electrical I equipment. [tw Spriian 500-8(b) of NH4 70, Nathmal titrtriral Code’. I
Class I, Zone 0. A locaihnI (1) in which igmuitibir conceit— tnttuins of tLiiiini;thle gases orv;tporsarr present coil iiiimously; or (2) in which ignitible concentrations of flainmahlL- gases or vapors are present lot bug petio(k of time.
Class I, Zone 1. A location (I in which ignitible concentrations of Ilainmable gases or vapors are likely to exist untlei normal operatilig comulitmotis; cit (2) iii which igmuitible c.oticeii— tuitions of fl;umniahbe gases or Vapors ma exist flTqIiCIltlV hecatise of repair or maintenance operations or because of leakage: or (3) in which r(buipmc’Iit is operated or presses are carried on, of such a nature that equipment breakdown or lault operations could result in the release of ignitible concentrations of flammable gases or sapols and also cause siniiiltatieouis lilutrr of electrical rqlnnhieuiI ill a umale to cause the electrical equipment to become a source of ignition; 01. (-I) that is adjacent to a Class I. Zone (I location from which igititiblc— (-oiicentratiolts Of 5al)Ot% (0111(1 lW c4,uIimuLlnicatecl. unless communication is prevented by adequate positive—pressure vetitilation from a source of clean air and effective safeguards against ventilation failure are 1)lOvide(l.
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