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SG) value greater than 0.45 mm and less than or equal to 0.75 mm, or a minimum igniting current (MIC) ratio greater than 0.40 and less than or equal to 0.80. [497:3.3.5.1.3] Informational Note: A typical Class I, Group C material is ethylene. Group D. Flammable gas, flammable liquid–produced vapo...

SG) value greater than 0.45 mm and less than or equal to 0.75 mm, or a minimum igniting current (MIC) ratio greater than 0.40 and less than or equal to 0.80. [497:3.3.5.1.3] Informational Note: A typical Class I, Group C material is ethylene. Group D. 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 greater than 0.75 mm or a minimum igniting current (MIC) ratio greater than 0.80. [497:3.3.5.1.4] Informational Note No. 1: A typical Class I, Group D material is propane. [497:3.3.5.1.4] Informational Note No. 2: See ANSI/ASHRAE 15, Safety Standard for Refrigeration Systems, for information on the classification of areas involving ammonia atmospheres. ENHANCED CONTENT Collapse Flammable gases, flammable liquid–produced vapors, and combustible liquid–produced vapors are separated into four Class I groups — A, B, C, and D (or three Class I Zone 0 or Zone 1 groups — IIC, IIB, and IIA; see 505.6), depending on their properties. By grouping explosive mixtures that have similar igniting current ratios and maximum safe clearances between parts of a joint in an enclosure, equipment can be designed for the entire group rather than an individual chemical. This method makes it easier to properly select equipment designed for use in the particular group involved. Selected combustible materials have been evaluated for the purpose of designating the appropriate gas group — A, B, C, or D (or IIC, IIB, or IIA) — and this information is used to properly select electrical equipment for use in Class I locations. These materials, with their group classification and relevant physical properties, are listed in 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. For the complete table, refer to NFPA 497, Table 4.4.2. Class II Combustible Dust Group Classifications. Combustible dust shall be grouped in accordance with 500.6(B)(1) through (B)(3). Group E. Atmospheres 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 the use of electrical equipment. [499:3.3.8.1.1] Informational Note: Certain metal dusts may have characteristics that require safeguards beyond those required for atmospheres containing the dusts of aluminum, magnesium, and their commercial alloys. For example, zirconium, thorium, and uranium dusts have extremely low ignition temperatures [as low as 20°C (68°F)] and minimum ignition energies lower than any material classified in any of the Class I or Class II groups. (2) Group F. Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles (see ASTM D3175-2017, Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. [499:3.3.8.1.2] Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts. [499:A.‍3.3.8.1.2] Informational Note: Testing of specific dust samples, following established ASTM testing procedures, is a method used to identify the combustibility of a specific dust and the need to classify those locations containing that material as Group F. Group G. Atmospheres containing combustible dusts not included in Group E or Group F, including flour, grain, wood, plastic, and chemicals. [499:3.3.8.1.3] Informational Note No. 1: See NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, for information on group classification of Class II materials. Informational Note No. 2: The explosion characteristics of air mixtures of dust vary with the materials involved. For Class II locations, Groups E, F, and G, the classification involves the tightness of the joints of assembly and shaft openings to prevent the entrance of dust in the dust-ignitionproof enclosure, the blanketing effect of layers of dust on the equipment that may cause overheating, and the ignition temperature of the dust. It is necessary, therefore, that equipment be identified not only for the class but also for the specific group of dust that will be present. Informational Note No. 3: See ANSI/IEEE C2, National Electrical Safety Code, Section 127A, Coal Handling Areas. Certain dusts might require additional precautions due to chemical phenomena that can result in the generation of ignitible gases. ENHANCED CONTENT Collapse NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities, establishes minimum requirements for industrial, commercial, or institutional facilities that process wood or that manufacture wood products, creating wood chips, particles, or dust. Section 500.6(B) separates combustible dusts into three Class II groups — E, F, and G — depending on their properties. Selected combustible dusts have been evaluated for the purpose of designating the appropriate dust group — E, F, or G — and this information is used to select electrical equipment for use in Class II locations. The selected materials, with their group classification and relevant physical properties, are listed in NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. For the complete table, refer to NFPA 499, Table 5.2.3. Class III Combustible Fibers/Flyings. Combustible fibers/flyings shall not be further grouped. Class III Ignitible Fibers/Flyings. Ignitible fibers/flyings shall not be further grouped. 500.7 Protection Techniques. Electrical and electronic equipment in hazardous (classified) locations shall be protected by one or more of the techniques in 500.7(A) through (P). Suitability of the protection techniques for specific hazardous locations is shown in Chapter 9, Table 13. Explosionproof Equipment. This protection technique shall be permitted for equipment in Class I, Division 1 or 2 locations. Dust Ignitionproof. This protection technique shall be permitted for equipment in Class II, Division 1 or 2 locations. Dusttight. This protection technique shall be permitted for equipment in Class II, Division 2 or Class III, Division 1 or 2 locations. Purged and Pressurized. This protection technique shall be permitted for equipment in any hazardous (classified) location for which it is identified. ENHANCED CONTENT Collapse Purging is the process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the initial concentration of any flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors to an acceptable level. Pressurization is the process of supplying an enclosure with a protective gas, with or without continuous flow, at sufficient pressure to prevent the entrance of a material. NFPA 496, Standard for Purged and Pressurized Enclosures for Electrical Equipment, provides requirements for these enclosures in Class I and Class II hazardous locations. In Class I locations, purged and pressurized enclosures are used to eliminate or reduce, within the enclosure, the Class I hazardous location classification. In Class II locations, pressurized enclosures prevent the entrance of dusts into an enclosure. Purged and pressurized enclosures make it possible for equipment that is not otherwise acceptable for Class I and Class II locations to be used in those locations. The types of pressurizing are as follows: Type X reduces the classification within a protected enclosure from Division 1 or Zone 1 to unclassified. Type Y reduces the classification within a protected enclosure from Division 1 to Division 2 or from Zone 1 to Zone 2. Type Z reduces the classification within a protected enclosure from Division 2 or Zone 2 to unclassified. A panel that uses the Type X purge and pressurization technique is pictured below. (Courtesy of Pepperl+Fuchs, Inc.) Intrinsic Safety. This protection technique shall be permitted for equipment in Class I, Division 1 or Division 2; Class II, Division 1 or Division 2; or Class III, Division 1 or Division 2 locations. Nonincendive Circuit. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations. Nonincendive Equipment. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations. Nonincendive Component. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations. Oil Immersion. This protection technique shall be permitted for current-interrupting contacts in Class I, Division 2 locations as described in 501.115(B)(1)(2). Hermetically Sealed. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2 locations. Detection System for Flammable Gases. A detection system for flammable gases shall be permitted as a means of protection in restricted industrial establishments. General. Any gas detection system used as a protection technique shall meet all of the requirements in 500.7(K)(1)(a) through (K)(1)(e). The gas detection equipment used shall be listed for Class I, Division 1 and listed for the detection of the specific gas or vapor to be encountered. The gas detection system shall not use portable or transportable equipment or temporary wiring methods. The gas detection system shall only use point-type sensors. The system shall be permitted to be augmented with open-path (line-of-sight)–type sensors, but open-path–type sensors shall not be the basis for this protection technique. The type of detection equipment and its listing, installation location(s), alarm and shutdown criteria, and calibration frequency shall be documented where gas detectors are used as a protection technique. The applications for the use of gas detection systems as a protection technique shall be limited to 500.7(K)(2), (K)(3), or (K)(4). Informational Note No. 1: See ANSI/UL 121303, Guide for Use of Detectors for Flammable Gases, or ANSI/FM 121303, Guide for Use of Detectors for Flammable Gases, for additional information. Informational Note No. 2: See ANSI/UL 60079-29-1, Explosive Atmospheres — Part 29-1: Gas Detectors — Performance Requirements of Detectors for Flammable Gases, or ANSI/FM 60079-29-1, Explosive Atmospheres — Part 29-1: Gas Detectors — Performance Requirements of Detectors for Flammable Gases, for additional information. Informational Note No. 3: See 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 additional information. Informational Note No. 4: See ANSI/UL 60079-29-2, Explosive Atmospheres — Part 29-2: Gas Detectors — Selection, Installation, Use and Maintenance of Detectors for Flammable Gases and Oxygen, or ANSI/FM-60079-29-2, Explosive atmospheres — Part 29-2: Gas Detectors — Selection, Installation, Use and Maintenance of Detectors for Flammable Gases and Oxygen, for additional information. ENHANCED CONTENT Collapse The gas detection system must be suitable for the original division classification of the area, even though the remainder of installed equipment is permitted to be suitable for the next lower division. Inadequate Ventilation. A location, enclosed space, or building that is classified as a Class I, Division 1 location due to inadequate ventilation and is provided with a detection system for flammable gases shall be permitted to use electrical equipment, installation methods, and wiring practices suitable for Class I, Division 2 installations. Sensing a gas concentration of not more than 40 percent of the lower flammable limit or a gas detector system malfunction shall activate an alarm (audible or visual, or both, as most appropriate for the area). ENHANCED CONTENT Collapse Section 17.11 of NFPA 30, Flammable and Combustible Liquids Code, and 3.3.1 of 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, provide information on what is considered adequate ventilation. Interior of a Building or Enclosed Space. Any building or enclosed space that does not contain a source of flammable gases or vapors that is located in, or has an opening into, a Class I, Division 2 hazardous (classified) location and is provided with a detection system for flammable gases shall be permitted to use electrical equipment, installation methods, and wiring practices suitable for unclassified installations under all of the following conditions: An alarm (audible or visual, or both) shall be sounded at not more than 20 percent of the lower flammable limit. Sensing a gas concentration of not more than 40 percent of the lower flammable limit or a gas detector system malfunction shall activate an alarm (audible or visual, or both, as most appropriate for the area) and initiate automatic disconnection of power from all electrical devices in the area that are not suitable for Class I, Division 2. The power disconnecting device(s) shall be suitable for Class I, Division 1 if located inside the building or enclosed space. If the disconnecting device(s) is located outside the building or enclosed space, it shall be suitable for the location in which it is installed. Redundant or duplicate equipment (such as sensors) shall be permitted to be installed to avoid disconnecting electrical power when equipment malfunctions are indicated. When automatic shutdown could introduce additional or increased hazard, this technique shall not be permitted. Interior of a Control Panel. Inside the interior of a control panel containing instrumentation or other equipment using or measuring flammable liquids, gases, or vapors which is provided with a detection system for flammable gases shall be permitted to use electrical equipment, installation methods, and wiring practices suitable for Class I, Division 2 installations. An alarm (audible or visual, or both) shall be sounded at not more than 40 percent of the lower flammable limit. Inherently Safe Optical Radiation “op is.” This protection technique shall be permitted for equipment in Class I or II, Division 1 or 2 locations for which the equipment is identified. Informational Note: The identified class and division depends on the intended explosive atmosphere and the number of faults applied as part of the protection technique evaluation. (M) Protected Optical Radiation “op pr.” This protection technique shall be permitted for equipment in Class I or II, Division 2 locations for which the equipment is identified. Informational Note: The identified class and division depends on the intended explosive atmosphere as part of the protection technique evaluation. (N) Optical System With Interlock “op sh.” This protection technique shall be permitted for equipment in Class I or II, Division 1 or 2 locations for which the equipment is identified. Informational Note: The identified class and division depends on the intended explosive atmosphere and the number of faults applied as part of the protection technique evaluation. (O) Protection by Skin Effect Trace Heating “IEEE 844.1”. This protection technique shall be permitted for skin effect trace heating equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 2 locations for which it is listed. Protection by Electrical Resistance Trace Heating “60079-30-1". This protection technique shall be permitted for electrical resistance trace heating equipment in Class I, Division 1; Class I, Division 2; Class II, Division 1; Class II, Division 2; Class III, Division 1; or Class III, Division 2 locations for which it is listed. Protection by Impedance Heating “IEEE 844.3”. This protection technique shall be permitted for impedance heating equipment in Class I, Division 2; Class II, Division 2; or Class III, Division 2 locations for which it is listed. Enclosed-Break. This protection technique shall be permitted for equipment in Class I, Division 2 locations. Nonsparking. This protection technique shall be permitted for equipment in Class I, Division 2 locations. Sealed. This protection technique shall be permitted for equipment in Class I, Division 2; Class II, Division 2; Class III, Division 1; or Class III, Division 2 locations. Special Protection Techniques. Protection techniques not specified in 500.7(A) through (T) shall be permitted for use in equipment listed for use in hazardous (classified) locations. Informational Note: See ANSI/UL 60079-33, Explosive Atmospheres — Part 33: Equipment Protection by Special Protection “s”, for additional information. 500.8 Equipment. Explosionproof or dust-ignitionproof equipment shall not be permitted for use at temperatures lower than -25°C (-13°F) unless they are identified for low-temperature service. Informational Note: At low ambient temperatures, flammable concentrations of vapors might not exist in a location classified as Class I, Division 1 at normal ambient temperature. Suitability. Suitability of identified equipment shall be determined by one of the following: Equipment listing or labeling Evidence of equipment evaluation from a qualified testing laboratory or inspection agency concerned with product evaluation Evidence acceptable to the authority having jurisdiction such as a manufacturer’s self-evaluation or an owner’s engineering judgment Informational Note: Additional documentation for equipment might include certificates demonstrating compliance with applicable equipment standards, indicating special conditions of use, and providing other pertinent information. ENHANCED CONTENT Collapse Several testing and product evaluation agencies list electrical equipment that is suitable for use in hazardous locations. Testing laboratories outside the United States provide listings of equipment for use in hazardous locations, but they might not be testing and investigating the equipment for use in hazardous locations as defined in Article 500. These foreign laboratories certify equipment for installation according to an International Electrotechnical Commission (IEC) classification scheme similar to that in Articles 505 and 506. However, equipment certified to a product standard used in another country might not comply with the NEC. Approval for Class and Properties. Equipment Identification. Equipment shall be identified not only for the class of location but also for the explosive, combustible, or ignitible properties of the specific gas, vapor, dust, or fibers/flyings that will be present. In addition, Class I equipment shall not have any exposed surface that operates at a temperature in excess of the autoignition temperature of the specific gas or vapor. Class II equipment shall not have an external temperature higher than that specified in 500.8(D)(2). Class III equipment shall not exceed the maximum surface temperatures specified in 503.5. Exception No. 1: Group D equipment shall be permitted to be used for atmospheres containing butadiene if all conduit runs into explosionproof equipment are provided with explosionproof seals installed within 450 mm (18 in.) of the enclosure. Exception No. 2: Group C equipment shall be permitted to be used for atmospheres containing allyl glycidyl ether, n-butyl glycidyl ether, ethylene oxide, propylene oxide, and acrolein if all conduit runs into explosionproof equipment are provided with explosionproof seals installed within 450 mm (18 in.) of the enclosure. Informational Note: See 500.8(C)(6)(a) regarding general-purpose equipment. Luminaires and other heat-producing apparatus, switches, circuit breakers, and plugs and receptacles are potential sources of ignition and are investigated for suitability in classified locations. Such types of equipment, as well as cable terminations for entry into explosionproof enclosures, are available as listed for Class I, Division 2 locations. Fixed wiring, however, might use wiring methods that are not evaluated with respect to classified locations. Therefore, wiring products such as cable, raceways, boxes, and fittings are not marked as being suitable for Class I, Division 2 locations. ENHANCED CONTENT Collapse Where installed in a Class I or Class II location, equipment must be suitable for the specific group indicated on the classification document (see 500.4). An explosionproof enclosure suitable only for Group D, for example, is not acceptable for Group B. Enclosures are often identified for more than one class or group. Some portable devices — cell phones, multimeters, and flashlights — have the capacity to cause ignition of a hazardous atmosphere. Although such electrical equipment is outside the scope of the NEC, all equipment used should be suitable for the specific hazardous location. Equipment Application. Equipment identified for a Division 1 location shall be permitted in a Division 2 location of the same class, group, and temperature class and shall comply with the requirements of 500.8(B)(2)(a) or (B)(2)(b) as applicable. Intrinsically safe apparatus having a control drawing requiring the installation of associated apparatus for a Division 1 installation shall be permitted to be installed in a Division 2 location if the same associated apparatus is used for the Division 2 installation. Equipment required to be explosionproof shall incorporate seals in accordance with 501.15(A) or (D) when the wiring methods of 501.10(B) are employed. General-Purpose Equipment. Where specifically permitted in Part III of Articles 501, 502, and 503, general-purpose equipment or equipment in general-purpose enclosures shall be permitted to be installed in Division 2 locations if the equipment does not constitute a source of ignition under normal operating conditions. Process Seals. Equipment that depends on a single compression seal, diaphragm, or tube to prevent flammable or combustible fluids from entering the equipment shall be identified for a Class I, Division 2 location even if installed in an unclassified location. Equipment installed in a Class I, Division 1 location shall be identified for the Class I, Division 1 location. Informational Note: Equipment used for flow measurement is an example of equipment having a single compression seal, diaphragm, or tube. (5) Motors. Unless otherwise specified, normal operating conditions for motors shall be assumed to be rated full-load steady conditions. ENHANCED CONTENT Collapse Locked-rotor or other abnormal motor conditions, such as single phasing, are not considered when evaluating motor-operating temperatures (internal and external) in Class I, Division 2 locations. However, such abnormal load conditions must be considered when the external temperatures of explosionproof motors for Class I, Division 1 locations and dust-ignitionproof motors for Class II, Division 1 locations are evaluated. Awareness of increased temperatures in motors controlled by variable-speed drives is important when they are operated at lower speeds and are dependent on the fan for cooling. Simultaneous Classifications. Where flammable gases, flammable liquid–produced vapors, or combustible liquid–produced vapors and combustible dusts are or might be present at the same time, the simu