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Electrical equipment installation in atmosphere with flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers or flyings represents a risk for fire and explosion.<\/p>\n
Areas with possible fire or explosion risks due to explosive atmospheres and\/or mixtures – are called hazardous (or classified) locations or areas<\/strong>. These areas are in North America (United States and Canada) historically classified with the Class\/Division<\/strong> system. In Europe and the rest of the world<\/a> – but also more and more in North America – the Zone<\/strong> system is used.<\/p>\n The hazardous area classification system determines required protection techniques and methods for electrical installations in the location.<\/p>\n The Class\/Division\/Group system is based on Article 500 of the National Electrical Code (NEC) where<\/p>\n Class defines the general nature (or properties)<\/strong> of the hazardous material in the surrounding atmosphere.<\/p>\n Division defines the probability<\/strong> of the hazardous material being present in an ignitable concentration in the surrounding atmosphere.<\/p>\n Group defines the type<\/strong> of the hazardous material in the surrounding atmosphere.<\/p>\n 1)<\/sup> MESG (Maximum Experimental Safe Gap) – The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.<\/p>\n 2)<\/sup> MIC (Minimum Igniting Current) Ratio – The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.<\/p>\n Groups A, B, C, and D are for gases (Class I only). Groups E, F, and G are for dusts and flyings (Class II or III).<\/p>\n The specific hazardous materials within each group and their automatic ignition temperatures can be found in Article 500 of the National Electrical Code and in NFPA 497.<\/p>\nClass\/Division System<\/h3>\n
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Class<\/h4>\n
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\n \nClass<\/th>\n Nature of Hazardous Material<\/th>\n<\/tr>\n<\/thead>\n \n Class I<\/td>\n Hazardous because flammable gases or vapors<\/strong> are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.<\/td>\n<\/tr>\n \n Class II<\/td>\n Hazardous because combustible or conductive dusts<\/strong> are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.<\/td>\n<\/tr>\n \n Class III<\/td>\n Hazardous because ignitable fibers<\/strong> or flyings are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n Division<\/h4>\n
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\n \nDivision<\/th>\n Probability of Hazardous Material<\/th>\n<\/tr>\n<\/thead>\n \n Division 1<\/td>\n The substance referred to by class has a high probability<\/strong> of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from
\nthe equipment itself under normal operating conditions.<\/td>\n<\/tr>\n\n Division 2<\/td>\n The substance referred to by class has a low probability<\/strong> of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time – such as a container failure or system breakdown<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n Group<\/h4>\n
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\n \nGroup<\/th>\n Type of Hazardous Material<\/th>\n<\/tr>\n<\/thead>\n \n Group A<\/td>\n Atmosphere containing acetylene.<\/td>\n<\/tr>\n \n Group B<\/td>\n Atmosphere containing a flammable gas, a flammable liquid produced vapor, or a combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG (Maximum Experimental Safe Gap)1)<\/sup> value less than or equal to 0.45 mm or a MIC (Minimum Igniting Current)2)<\/sup> ratio less than or equal to 0.40 – such as hydrogen or fuel and combustible process gases containing more than 30% hydrogen by volume – or gases of equivalent hazard such as butadiene, ethylene, oxide, propylene oxide and acrolein.<\/td>\n<\/tr>\n \n Group C<\/td>\n Atmosphere containing a flammable gas, a flammable liquid produced vapor or a combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ratio is greater than 0.40 and less than 0.80 – such as carbon monoxide, ether, hydrogen sulfide, morphline, cyclopropane, ethyl and ethylene or gases of equivalent hazard.<\/td>\n<\/tr>\n \n Group D<\/td>\n Atmosphere containing flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG value greater than 0.75 mm or a MIC ratio greater than 0.80 – such as gasoline, acetone, ammonia, benzene, butane, cyclopropane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard.<\/td>\n<\/tr>\n \n Group E<\/td>\n Atmosphere containing combustible metal dusts, including aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment.<\/td>\n<\/tr>\n \n Group F<\/td>\n Atmosphere containing carbonaceous dusts, carbon black, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sesitized by other materials so they present an explosion hazard.<\/td>\n<\/tr>\n \n Group G<\/td>\n Atmosphere containing combustible dust not included in Group E & F – such as flour, grain, starch, sugar, wood, plastics and chemicals.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n Zone System<\/h3>\n