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I NTRODUCT I ON HAZARDOUS LOCATION DATA

Hazardous Location – Area Classification for Explosive Dust Atmospheres

North America – General Information The NEC and CE Code contain two systems that deal with combustible or explosive dusts. Similar to the Rules for explosive gas atmospheres, the NEC allows the user to use either the Class/Division System or Zone System. The CE Code mandates use of the Zone System for all new construction, with facilities constructed before the introduction of the Zone System for dust atmospheres being permitted to continue using the Class/Division Rules. NEC Article 502 contains the Rules for areas classified using the Division System and Article 506 contains the Rules for the Zone System. The CE Code uses Section 18 for the Zone Rules and Annex J18 for the Division Rules. Similar to Class I locations, Class II locations are separated into Divisions or Zones based on the likelihood combustible dust being suspended in the atmosphere in sufficient quantities to produce ignitable mixtures or where a failure or abnormal operation of equipment might produce a hazardous concentration of dust. Unlike gases and vapors, particle size is a key factor in identifying Class II materials. The definition of a Class II dust is any finely divided solid material that is 420 microns or smaller in diameter (material passing through a No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed and ignited in air. An example of the importance of particle size is wood; a log could burn but not explode, and the size of the wood decreases so does its ability to burn. When the wood reaches the size of a fine sawdust is it easier for it to become suspended in air at which point it can become highly explosive.

Typical Class II Dust Location

Class II, Division 2 A Class ll, Division 2 location is one where combustible dust is not normally in the air in quantities sufficient to produce explosive or ignitable mixtures, and dust accumulations are not normally sufficient to interfere with the normal operation of electrical equipment, such as clogging ventilating openings or causing bearing failure. It includes locations where combustible dust may be in suspension in the air only as a result of infrequent malfunctioning of handling or processing equipment, and those locations where dust accumulation may be on or in the vicinity of the electrical equipment and may be sufficient to interfere with the safe dissipation of heat from the equipment, or may be ignitable by abnormal operation or failure of the electrical equipment. GUIDE TO CLASSIFICATION OF CLASS II LOCATIONS BY DIVISION* THICKNESS OF DUST LAYER ON EQUIPMENT** DUST GROUP DIVISION Greater than 1/8 in. E, F, G 1 1/8 in or less but surface color not discernible E 1 1/8 in or less but surface color not discernible F, G 2 1/8 in or less and surface color discernible under dust layer E, F, G non-classified * From NFPA 497B ** Based on build-up of dust level in a 24-hr period on the major portions of the horizontal surfaces.

Class II, Division System Area Classification Definitions

Class II, Division 1 A Class II, Division 1 location is one where combustible dust is normally in suspension in the air in sufficient quantities to produce ignitable mixtures, or where mechanical failure or abnormal operation of equipment or machinery might cause an explosive or ignitable dust-air mixture to be produced and might also provide a source of ignition through simultaneous failure of electrical equipment.

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