|
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. Areas with possible fire or explosion risks due to explosive atmospheres and/or mixtures - are called hazardous (or classified) locations or areas. These areas are in North America (United States and Canada) historically classified with the Class/Division system. In Europe and the rest of the world - but also more and more in North America - the Zone system is used. The hazardous area classification system determines required protection techniques and methods for electrical installations in the location. Class/Division SystemThe Class/Division/Group system is based on Article 500 of the National Electrical Code (NEC) where Classes - defines the general nature of the hazardous material in the surrounding atmosphere Divisions - defines the probability of hazardous material being present the surrounding atmosphere Groups - defines the type of the hazardous material in the surrounding atmosphere ClassClass defines the general nature (or properties) of the hazardous material in the surrounding atmosphere.
Hazardous Areas - North America Classification - Classes
ClassNature of Hazardous Material
Division Division defines the probability of the hazardous material being present in an ignitable concentration in the surrounding atmosphere.
Hazardous Areas - North America Classification - Divisions
DivisionProbability of Hazardous Material The substance referred to by class has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from The substance referred to by class has a low probability 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 Group defines the type of the hazardous material in the surrounding atmosphere.
Hazardous Areas - North America Classification - Groups
GroupType of Hazardous Material 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) value less than or equal to 0.45 mm or a MIC (Minimum Igniting Current)2) 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, ethyleneoxide, propylene oxide and acrolein. 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, isoprene, acetaldhyde and ethylene or gases of equivalent hazard. 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, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard. 1) 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. 2) 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. 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). 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.
Zone System The Zone system is based on Article 505/506 of the National Electrical Code (NEC) and follows the international method of area classification as developed by the International Electrotechnical Commission (IEC). Zones - defines the general nature (or properties) of the hazardous material - if its gas or dust, and the probability of the hazardous material in the surrounding atmosphere Groups - defines the type of the hazardous material and (partly) the location of the surrounding atmosphere ZoneZone defines the general nature - if it is a gas or dust - and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. The Zone system has three levels of hazard for gas or dust where the Division system has two. Gases, Vapors and MistsArticle 505 National Electrical Code (NEC)
Hazardous Areas - North America Classification - Gases, Vapors and Mists
ZoneNature and Probability of Hazard Material Article 506 National Electrical Code (NEC) Hazardous Areas - North America Classification - Dusts ZoneNature and Probability of Hazard MaterialZone 20 An area where combustible dusts or ignitable fibers and flyings are present continuously or for long periods of time. Zone 21 An area where combustible dusts or ignitable fibers and flyings are likely to occur under normal operating conditions. Zone 22 An area where combustible dusts or ignitable fibers and flyings are not likely to occur under normal operating conditions and do so only for a short period of time. Zones compares to classes and divisions in the Class/Division system. GroupGroup define the type of hazardous material and (partly) the location of the surrounding atmosphere. Group is divided in three groups where Group I is reserved for mining locations. Group II is for explosive gases (Zone 0, 1 and 2) and Group III is for explosive dusts (Zone 20, 21 and 22). Hazardous Areas - North America Classification - Groups GroupType of Hazardous Material and Location of AtmosphereGroup I Mines susceptible to firedamp (flammable mixture of gases naturally occurring in a mine). Group II Explosive gas atmosphere other than mines susceptible to firedamp. Group II equipment is subdivided into three subgroups. A Atmospheres containing propane, acetone, benzene, butane, methane, petrol, hexane, paint solvents or gases and vapors of equivalent hazard. B Atmospheres containing ethylene, propylene oxide, ethylene oxide, butadiene, cyclopropane, ethyl ether, or gases and vapors of equivalent hazard. C Atmospheres containing acetylene, hydrogen, carbon disulphide or gases and vapors of equivalent hazard. Group III Explosive dust atmosphere. Group III equipment is subdivided into three subgroups. A Atmospheres containing combustible flyings. B Atmospheres containing non-conductive dust. C Atmospheres containing conductive dust. Example - Hazardous Area Classification A room with a propane gas installation will typically be classified with the Class/Division system as: Class I, Division 2, Group D Zone system as: Zone 2, Group IIA Recommended reading for this topic: National Electrical Code, NFPA 70, Chapter 5, Article 500 29 CFR 1910 Subpart S, Electrical 1910.307 NFPA 497, "Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous Classified Locations" NFPA Handbook, "Electrical Installations in Hazardous Locations, " by P. J. Schram and M. W. Earley NFPA 70E, Chapter 5, "Hazardous (Classified) Locations" NFPA (Fire) HAZ-10, "Fire Protection Guide to Hazardous Materials" ANSI/UL 913, "Intrinsically Safe Apparatus" NFPA 496, "Purged and Pressurized Enclosure for Electrical Equipment in Hazardous Locations." (责任编辑：) |