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ification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (13) NFPA 820, Standard for Fire Protection in Wastewater Treatment and Collection Facilities (14) ANSI/API RP 500, Recommended Practice for Classification of Locations for E...

ification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas (13) NFPA 820, Standard for Fire Protection in Wastewater Treatment and Collection Facilities (14) ANSI/API RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2 (15) ISA-12.10, Area Classification in Hazardous (Classified) Dust Locations Informational Note No. 2: See NFPA 77, Recommended Practice on Static Electricity; NFPA 780, Standard for the Installation of Lightning Protection Systems; and API RP 2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, for information on protection against static electricity and lightning hazards in hazardous (classified) locations. Informational Note No. 3: See NFPA 30, Flammable and Combustible Liquids Code; and ANSI/API RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2, for information on ventilation. Informational Note No. 4: See ANSI/API RP 14F, Recommended Practice for Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class I, Division 1, and Division 2 Locations, for information on electrical systems for hazardous (classified) locations on offshore oil- and gas-producing platforms, drilling rigs, and workover rigs. Informational Note No. 5: See ANSI/UL 121203, Portable/Personal Electronic Products Suitable for Use in Class I, Division 2, Class I, Zone 2, Class II, Division 2, Class III, Division 1, Class III, Division 2, Zone 21 and Zone 22 Hazardous (Classified) Locations, for information on portable or transportable equipment having self-contained power supplies, such as battery-operated equipment, which could potentially become an ignition source in hazardous (classified) locations. Informational Note No. 6: See IEC/IEEE 60079-30-2, Explosive atmospheres — Part 30-2: Electrical resistance trace heating — Application guide for design, installation and maintenance, for information on electrical resistance trace heating for hazardous (classified) locations. Informational Note No. 7: See IEEE 844.2/CSA C293.2, IEEE/CSA Standard for Skin Effect Trace Heating of Pipelines, Vessels, Equipment, and Structures — Application Guide for Design, Installation, Testing, Commissioning, and Maintenance, for information on electric skin effect trace heating for hazardous (classified) locations. Informational Note No. 8: See IEEE 844.4/CSA C293.4, IEEE/CSA Standard for Impedance Heating of Pipelines and Equipment — Application Guide for Design, Installation, Testing, Commissioning, and Maintenance, for information on electric impedance heating for hazardous (classified) locations. ENHANCED CONTENT Collapse One type of documentation consists of area classification drawings. Once the hazardous area has been classified and the hazardous area documentation has been developed, the materials and installation methods of the NEC are used to design and construct the electrical system for use in the indicated classified area. This approach provides the necessary information for engineers, installers, service personnel, and AHJs to ensure that the electrical equipment installed in each classified area is of the proper type. An example of an area classification drawing is shown below. The standards referenced in Articles 500 through 517 are essential for proper application of the articles. In addition to those documents and others listed in each article’s Informational Notes, the following NFPA codes, standards, and recommended practices contain valuable information on hazardous locations in specific applications, occupancies, or industries: NFPA 30A, Code for Motor Fuel Dispensing Facilities and Repair Garages NFPA 51, Standard for the Design and Installation of Oxygen–Fuel Gas Systems for Welding, Cutting, and Allied Processes NFPA 52, Vehicular Natural Gas Fuel Systems Code NFPA 54, National Fuel Gas Code NFPA 59A, Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG) NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities NFPA 85, Boiler and Combustion Systems Hazards Code NFPA 99, Health Care Facilities Code NFPA 407, Standard for Aircraft Fuel Servicing NFPA 409, Standard on Aircraft Hangars NFPA 495, Explosive Materials Code NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids NFPA 655, Standard for Prevention of Sulfur Fires and Explosions 500.5 Classifications of Locations. General. Hazardous (Classified) Locations. Locations shall be classified depending on the properties of the flammable gas, flammable liquid–produced vapor, combustible liquid–produced vapors, combustible dusts, or fibers/flyings that could be present, and the likelihood that a flammable or combustible concentration or quantity is present. Each room, section, or area shall be considered individually in determining its classification. Informational Note: Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified) locations, it is frequently possible to locate much of the equipment in a reduced level of classification or in an unclassified location to reduce the amount of special equipment required. Refrigerant Machinery Rooms Using Ammonia. Refrigerant machinery rooms that contain ammonia refrigeration systems and are equipped with adequate mechanical ventilation that operates continuously or is initiated by a detection system at a concentration not exceeding 150 ppm shall be permitted to be classified as “unclassified” locations. Informational Note: See ANSI/IIAR 2, Standard for Design of Safe Closed-Circuit Ammonia Refrigeration Systems, for information on classification and ventilation of areas involving closed-circuit ammonia refrigeration systems. ENHANCED CONTENT Collapse Determining the appropriate classification for an area requires an understanding of the following factors: (1) the type and amount of material present; (2) the potential for that presence under normal and abnormal conditions; and (3) the entire process that the material will or might undergo in given circumstances. The NEC does not classify specific Class I, II, and III locations. Other standards and recommended practices of NFPA technical committees and other organizations such as American Petroleum Institute (API) with expertise in working with flammable and combustible materials can be used to classify locations. Classification requires consideration of the specific equipment and process and depends on the materials and physical equipment and location (for example, possible leaks at flanges and machinery seals). Care must be taken when classifying an area that is likely to become hazardous and in determining those portions of the premises to be classified Division 1 or Division 2. If different types of material — such as a flammable gas and a combustible dust — simultaneously exist in a process or location, the area must be classified as both a Class I and a Class II location, and protection must be provided for both hazards. Equipment that is identified for use in a Class I location may or may not be suitable for use in a Class II location and vice versa. Below is an example of a coal-handling operation classified as both a Class I (methane) and Class II (coal dust) location. (Courtesy of Noel Williams) Class I Locations. Class I locations are those in which flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures. Class I locations shall include those specified in 500.5(B)(1) and (B)(2). ENHANCED CONTENT Collapse For a given vapor, the vapor-in-air ratio must be within the flammable limits to be deemed a hazard. Many chemicals reach the lower limit within a few percentage points of the vapor-to-air ratio, and some are within less than 1 percentage point. The flammable range may be very narrow or very wide (such as for acetylene, which has a flammable range of 2.5 percent to nearly 100 percent). A Class I location that dispenses compressed natural gas (CNG) is pictured below. Class I, Division 1. A Class I, Division 1 location is a location: In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors can exist under normal operating conditions, or In which ignitible concentrations of such flammable gases, flammable liquid–produced vapors, or combustible liquids above their flash points might exist frequently because of repair or maintenance operations or because of leakage, or In which breakdown or faulty operation of equipment or processes might release ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become a source of ignition Informational Note: This classification usually includes the following locations: (1) Where volatile flammable liquids or liquefied flammable gases are transferred from one container to another (2) Interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flammable solvents are used (3) Locations containing open tanks or vats of volatile flammable liquids (4) Drying rooms or compartments for the evaporation of flammable solvents (5) Locations containing fat- and oil-extraction equipment using volatile flammable solvents (6) Portions of cleaning and dyeing plants where flammable liquids are used (7) Gas generator rooms and other portions of gas manufacturing plants where flammable gas might escape (8) Inadequately ventilated pump rooms for flammable gas or for volatile flammable liquids (9) Interiors of refrigerators and freezers in which volatile flammable materials are stored in open, lightly stoppered, or easily ruptured containers (10) Inside of inadequately vented enclosures containing instruments normally venting flammable gases or vapors to the interior of the enclosure (11) Inside of vented tanks containing volatile flammable liquids (12) Area between inner and outer roof sections of floating roof tanks containing volatile flammable fluids (13) Inadequately ventilated areas within spraying or coating operations using volatile flammable fluids (14) Interior of exhaust ducts used to vent ignitible concentrations of gases or vapors (15) All other locations where ignitible concentrations of flammable vapors or gases are likely to occur during normal operations Experience has demonstrated the prudence of avoiding the installation of instrumentation or other electrical equipment in the areas covered in list items (11) through (15). Where it cannot be avoided because it is essential to the process and other locations are not feasible,electrical equipment or instrumentation approved for the specific application or consisting of intrinsically safe systems might be considered. Class I, Division 2. A Class I, Division 2 location is a location: In which volatile flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems or in case of abnormal operation of equipment, or In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors are normally prevented by positive mechanical ventilation and which might become hazardous through failure or abnormal operation of the ventilating equipment, or That is adjacent to a Class I, Division 1 location, and to which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors above their flash points might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided. Informational Note No. 1: This classification usually includes locations where volatile flammable liquids or flammable gases or vapors are used but that, in the judgment of the authority having jurisdiction, would become hazardous only in case of an accident or of some unusual operating condition. The quantity of flammable material that might escape in case of accident, the adequacy of ventilating equipment, the total area involved, and the record of the industry or business with respect to explosions or fires are all factors that merit consideration in determining the classification and extent of each location. Informational Note No. 2: See NFPA 30, Flammable and Combustible Liquids Code, and NFPA 58, Liquefied Petroleum Gas Code. Piping without valves, checks, meters, and similar devices would not ordinarily introduce a hazardous condition even if used for flammable liquids or gases. Depending on factors such as the quantity and size of the containers and ventilation, locations used for the storage of flammable liquids or liquefied or compressed gases in sealed containers might be considered either hazardous (classified) or unclassified locations. (C) Class II Locations. Class II locations are those that are hazardous because of the presence of combustible dust. Class II locations shall include those specified in 500.5(C)(1) and (C)⁠(2). ENHANCED CONTENT Collapse Housekeeping, settlement rates, and air velocity are all factors in determining the need for, or the extent of, a Class II classified location. A settled layer of dust could ignite at a temperature different from that of the same dust dispersed into the air as a cloud. Classification of dust layers is based on the thickness of the dust or on the amount of dust expected to settle out, usually over a set period of time. Class II, Division 1. A Class II, Division 1 location is a location: In which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures, or Where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitible mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes, or In which Group E combustible dusts may be present in quantities sufficient to be hazardous in normal or abnormal operating conditions. Informational Note: Dusts containing magnesium or aluminum are particularly hazardous, and the use of extreme precaution is necessary to avoid ignition and explosion. ENHANCED CONTENT Collapse Group E dusts (metal dusts, e.g., aluminum or magnesium) are particularly hazardous. For example, current through the dust can cause a sufficient temperature rise or electrical arc to trigger ignition. The classification of an area where a Group E dust is or may be present will be Division 1. Class II, Division 2. A Class II, Division 2 location is a location: In which combustible dust due to abnormal operations may be present in the air in quantities sufficient to produce explosive or ignitible mixtures; or Where combustible dust accumulations are present but are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but could as a result of infrequent malfunctioning of handling or processing equipment become suspended in the air; or In which combustible dust accumulations on, in, or in the vicinity of the electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitible by abnormal operation or failure of electrical equipment. Informational Note No. 1: The quantity of combustible dust that may be present and the adequacy of dust removal systems are factors that merit consideration in determining the classification and may result in an unclassified area. Informational Note No. 2: Where products such as seed are handled in a manner that produces low quantities of dust, the amount of dust deposited may not warrant classification. (D) Class III Locations. Class III locations shall be locations meeting the requirements of 500.5(D)(1) and (D)(2). Class III, Division 1. Class III, Division 1 locations shall include those locations specified in 500.5(D)(1)(a) and (D)(1)(b). Combustible Fibers/Flyings. Locations where nonmetal combustible fibers/flyings are in the air under normal operating conditions in quantities sufficient to produce explosible mixtures or where mechanical failure or abnormal operation of machinery or equipment might cause combustible fibers/flyings to be produced and might also provide a source of ignition through simultaneous failure of electrical equipment, through operation of protection devices, or from other causes shall be classified as Class III, Division 1. Locations where metal combustible fibers/flyings are present shall be classified as Class II, Division 1, Group E. Informational Note No. 1: Such locations usually include some parts of rayon, cotton, and other textile mills; associated manufacturing and processing plants; cotton gins and cotton-seed mills; flax-processing plants; clothing manufacturing plants; woodworking plants; and establishments and industries involving similar hazardous processes or conditions. Informational Note No. 2: Combustible fibers/flyings include flat platelet-shaped particulates, such as metal flakes, and fibrous board, such as particle board. (b) Ignitible Fibers/Flyings. Locations where ignitible fibers/flyings are handled, manufactured, or used shall be classified as Class III, Division 1. Informational Note No. 1: Such locations usually include some parts of rayon, cotton, and other textile mills; associated manufacturing and processing plants; cotton gins and cotton-seed mills; flax-processing plants; clothing manufacturing plants; woodworking plants; and establishments and industries involving similar hazardous processes or conditions. Informational Note No. 2: Ignitible fibers/flyings can include rayon, cotton (including cotton linters and cotton waste), sisal or henequen, istle, jute, hemp, tow, cocoa fiber, oakum, baled waste kapok, Spanish moss, excelsior, and other materials of similar nature. Class III, Division 2. Class III, Division 2 locations shall include those locations specified in 500.5(D)(2)(a) and (D)(2)(b). Combustible Fibers/Flyings. Locations where nonmetal combustible fibers/flyings might be present in the air in quantities sufficient to produce explosible mixtures due to abnormal operations or where accumulations of nonmetal combustible fibers/flyings accumulations are present but are insufficient to interfere with the normal operation of electrical equipment or other apparatus but could, as a result of infrequent malfunctioning of handling or processing equipment, become suspended in the air shall be classified as Class III, Division 2. Ignitible Fibers/Flyings. Locations where ignitible fibers/flyings are stored or handled, other than in the process of manufacture, shall be classified as Class III, Division 2. 500.6 Materials. ENHANCED CONTENT Collapse In a hazardous location, oxygen enrichment can drastically change the explosion characteristics of materials. It lowers minimum ignition energies, increases explosion pressures, and can reduce the maximum experimental safe gap. This can render both intrinsically safe and explosionproof equipment unsafe unless the equipment has been tested for the specific conditions involved. An oxygen-rich environment is not, in itself, a hazardous location according to Article 500; however, such an environment does pose specific design and installation challenges. Equipment found to be safe in ordinary atmospheric conditions is not necessarily safe in oxygen concentrations or pressures higher than those of the standard atmosphere. NFPA 53, Recommended Practice on Materials, Equipment, and Systems Used in Oxygen- Enriched Atmospheres, recommends that no electrical equipment be used in such atmospheres unless approved for use in the specific hazardous atmospheres at the maximum proposed pressure and oxygen concentration. Class I Group Classifications. Class I groups shall be in accordance with 500.6(A)(1) through (A)(4). Informational Note No. 1: The explosion characteristics of air mixtures of gases or vapors vary with the specific material involved. For Class I locations, Groups A, B, C, and D, the classification involves determinations of maximum explosion pressure and maximum safe clearance between parts of a clamped joint in an enclosure. It is necessary, therefore, that equipment be identified not only for class but also for the specific group of the gas or vapor that will be present. Informational Note No. 2: Certain chemical atmospheres may have characteristics that require safeguards beyond those required for any of the Class I groups. Carbon disulfide is one of these chemicals because of its low autoignition temperature (90°C) and the small joint clearance permitted to arrest its flame. ENHANCED CONTENT Collapse NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, contains a list of selected chemicals categorized by group. For the complete table, refer to NFPA 497, Table 4.4.2. For dust materials, NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, contains a list of dust materials categorized by group. For the complete table, refer to NFPA 499, Table 5.2.3. Group A. Acetylene. [497:3.3.5.1.1] Group B. Flammable gas, flammable liquid–produced vapor, or combustible liquid–produced vapor mixed with air that may burn or explode, having either a maximum experimental safe gap (MESG) value less than or equal to 0.45 mm or a minimum igniting current ratio (MIC ratio) less than or equal to 0.40. [497:3.3.5.1.2] Informational Note: A typical Class I, Group B material is hydrogen. (3) Group C. Flammable gas, flammable liquid–produced vapor, or combustible liquid–produced vapor mixed with air that may burn or explode, having either a maximum experimental safe gap (ME