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Article 506

Zone 20, 21, and 22 Locations for Combustible Dusts or Ignitible Fibers/ Flyings

Informational Note: Text that is followed by a reference in brackets has been extracted from NFPA 499-2013, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installation in Chemical Process Areas. Only editorial changes were made to the extracted text to make it consistent with this Code.

  1. Scope. This article covers the requirements for the zone classification system as an alternative to the division classi‐ fication system covered in Article 500, Article 502, and Arti‐ cle 503 for electrical and electronic equipment and wiring for all voltages in Zone 20, Zone 21, and Zone 22 hazardous (classi‐ fied) locations where fire and explosion hazards may exist due to combustible dusts or ignitible fibers/flyings.Informational Note No. 1: For the requirements for electrical and electronic equipment and wiring for all voltages in Class I, Division 1 or Division 2; Class II, Division 1 or Division 2; Class III, Division 1 or Division 2; and Class I, Zone 0 or Zone 1 or Zone 2 hazardous (classified) locations where fire or explo‐ sion hazards may exist due to flammable gases or vapors, flam‐ mable liquids, or combustible dusts or fibers, refer to Articles 500 through 505.Informational Note No. 2: Zone 20, Zone 21, and Zone 22 area classifications are based on the modified IEC area classification system as defined in ANSI/ISA 60079– 10–2 (12.10.05)–2013, Explosive Atmospheres — Part 10–2: Classification of Areas — Combustible Dust Atmospheres.Informational Note No. 3: The unique hazards associated with explosives, pyrotechnics, and blasting agents are not addressed in this article.
  2. Definitions. For purposes of this article, the following definitions apply.•Protection by Encapsulation “m”. Type of protection whereelectrical parts that could cause ignition of a mixture of combustible dust or fibers/flyings in air are protected by enclosing them in a compound in such a way that the explosive atmosphere cannot be ignited.Informational Note No. 1: For additional information, see ANSI/ISA-60079-18 (12.23.01)-2012, Explosive atmospheres — Part 18: Equipment protection by encapsulation “m”; ANSI/ UL 60079-18-2009, Explosive atmospheres — Part 18: Equipment protection by encapsulation “m”; and ANSI/ISA-61241-18 (12.10.07)-2011, Electrical Apparatus for Use in Zone 20, Zone 21 and Zone 22 Hazardous (Classified) Locations — Protection by Encap‐ sulation “m.”Informational Note No. 2: Encapsulation is designated level of protection “maD” or “ma” for use in Zone 20 locations. Encap‐ sulation is designated level of protection “mbD” or “mb” for use in Zone 21 locations. Encapsulation is designated type of protec‐ tion “mc” for use in Zone 22 locations.Protection by Enclosure “t”. Type of protection for explosive dust atmospheres where electrical apparatus is provided with an enclosure providing dust ingress protection and a means to limit surface temperatures.Informational Note No. 1: For additional information, see ANSI/UL 60079-31-2015, Explosive Atmospheres — Part 31: Equip‐ ment Dust Ignition Protection by Enclosure “t” ; and ANSI/ ISA-61241-1 (12.10.03)-2006, Electrical Apparatus for Use in Zone21 and Zone 22 Hazardous (Classified) Locations — Protection by Enclosure “tD”.Informational Note No. 2: Protection by enclosure is designated level of protection “ta” for use in Zone 20 locations. Protection by enclosure is designated level of protection “tb” or “tD” for use in Zone 21 locations. Protection by enclosure is designated level of protection “tc” or “tD” for use in Zone 22 locations.Protection by Intrinsic Safety “i”. Type of protection where any spark or thermal effect is incapable of causing ignition of a mixture of combustible dust, fibers, or flyings in air under prescribed test conditions.Informational Note No. 1: For additional information, see ANSI/ISA-60079-11 (12.01.01)-2014, Electrical Apparatus for Explo‐ sive Gas Atmospheres — Part 11: intrinsic safety “i”; ANSI/ UL 60079-11-2013, Electrical Apparatus for Explosive Gas Atmos‐ pheres — Part 11: Intrinsic safety “i”; and ANSI/ISA- 61241-11 (12.10.04)-2006, Electrical Apparatus for Use in Zone 20, Zone 21 and Zone 22 Hazardous (Classified) Locations — Protection by Intrin‐ sic Safety “iD.”Informational Note No. 2: Intrinsic safety is designated level of protection “iaD” or “ia” for use in Zone 20 locations. Intrinsic safety is designated level of protection “ibD” or “ib” for use in Zone 21 locations. Intrinsic safety is designated type of protec‐ tion “ic” for use in Zone 22 locations.Protection by Pressurization “p”. Type of protection that guards against the ingress of a mixture of combustible dust or fibers/flyings in air into an enclosure containing electrical equipment by providing and maintaining a protective gas atmosphere inside the enclosure at a pressure above that of the external atmosphere.Informational Note: For additional information, see ANSI/ ISA-61241-2 (12.10.06)-2006, Electrical Apparatus for Use in Zone 21 and Zone 22 Hazardous (Classified) Locations — Protection by Pres‐ surization “pD.”•
  3. Other Articles. All other applicable rules contained inthis Code shall apply to electrical equipment and wiring instal‐ led in hazardous (classified) locations.Exception: As modified by Article 504 and this article.
  4. General.
    1. Documentation for Industrial Occupancies. Areas desig‐ nated as hazardous (classified) locations shall be properly documented. This documentation shall be available to those authorized to design, install, inspect, maintain, or operate elec‐ trical equipment.
    2. Reference Standards. Important information relating to topics covered in Chapter 5 can be found in other publications.
    Informational Note No. 1: It is important that the authority having jurisdiction be familiar with the recorded industrial expe‐ rience as well as with standards of the National Fire Protection Association (NFPA), the International Society of Automation (ISA), and the International Electrotechnical Commission (IEC) that may be of use in the classification of various loca‐ tions, the determination of adequate ventilation, and the protec‐ tion against static electricity and lightning hazards.Informational Note No. 2: For additional information concern‐ ing the installation of equipment utilizing optical emissions technology (such as laser equipment) that could potentially

    become an ignition source in hazardous (classified) locations,see ANSI/ISA-60079-28 (12.21.02)-2013, Explosive Atmospheres — Part 28: Protection of equipment and transmission systems using optical radiation.
  5. Classification of Locations.
    1. Classifications of Locations. Locations shall be classified on the basis of the properties of the combustible dust or igniti‐ ble fibers/flyings that may be present, and the likelihood that a combustible or combustible concentration or quantity is present. Each room, section, or area shall be considered indi‐ vidually in determining its classification. Where pyrophoric materials are the only materials used or handled, these loca‐ tions are outside of the scope of this article.
    2. Zone 20, Zone 21, and Zone 22 Locations. Zone 20, Zone 21, and Zone 22 locations are those in which combustible dust or ignitible fibers/flyings are or may be present in the air or in layers, in quantities sufficient to produce explosive or ignitible mixtures. Zone 20, Zone 21, and Zone 22 locations shall include those specified in 506.5(B)(1), (B)(2), and (B) (3).Informational Note: Through the exercise of ingenuity in the layout of electrical installations for hazardous (classified) loca‐ tions, it is frequently possible to locate much of the equipment in a reduced level of classification and, thus, to reduce the amount of special equipment required.
      1. Zone 20. A Zone 20 location is a location in which either of the following occur:
    1. Ignitible concentrations of combustible dust or ignitible fibers/flyings are present continuously.
    2. Ignitible concentrations of combustible dust or ignitible fibers/flyings are present for long periods of time.
    Informational Note No. 1: As a guide to classification of Zone 20 locations, refer to ANSI/ISA-60079-10-2 (12.10.05)-2013, Explo‐ sive Atmospheres — Part 10-2: Classification of areas — Combustible dust atmospheres.Informational Note No. 2: Zone 20 classification includes loca‐ tions inside dust containment systems; hoppers, silos, etc., cyclo‐ nes and filters, dust transport systems, except some parts of belt and chain conveyors, etc.; blenders, mills, dryers, bagging equip‐ ment, etc.(2) Zone 21. A Zone 21 location is a location where one of the following apply:
    1. Ignitible concentrations of combustible dust or ignitible fibers/flyings are likely to exist occasionally under normal operating conditions; or
    2. Ignitible concentrations of combustible dust or ignitible fibers/flyings may exist frequently because of repair or maintenance operations or because of leakage; or
    3. Equipment is operated or processes are carried on, of such a nature that equipment breakdown or faulty opera‐ tions could result in the release of ignitible concentra‐ tions of combustible dust or ignitible fibers/flyings and also cause simultaneous failure of electrical equipment in a mode to cause the electrical equipment to become a source of ignition; or
    4. The location is adjacent to a Zone 20 location from which ignitible concentrations of dust or ignitible fibers/flyings could be communicated.
    Exception: When communication from an adjacent Zone 20 location is minimized by adequate positive pressure ventilation from a source ofclean air, and effective safeguards against ventilation failure are provi‐ ded.Informational Note No. 1: As a guide to classification of Zone 21 locations, refer to ANSI/ISA-60079-10-2 (12.10.05)-2013, Explo‐ sive Atmospheres — Part 10-2: Classification of areas — Combustible dust atmospheres.Informational Note No. 2: This classification usually includes locations outside dust containment and in the immediate vicin‐ ity of access doors subject to frequent removal or opening for operation purposes when internal combustible mixtures are present; locations outside dust containment in the proximity of filling and emptying points, feed belts, sampling points, truck dump stations, belt dump over points, etc., where no measures are employed to prevent the formation of combustible mixtures; locations outside dust containment where dust accumulates and where due to process operations the dust layer is likely to be disturbed and form combustible mixtures; locations inside dust containment where explosive dust clouds are likely to occur (but neither continuously, nor for long periods, nor frequently) as, for example, silos (if filled and/or emptied only occasionally) and the dirty side of filters if large self-cleaning intervals are occurring.(3) Zone 22. A Zone 22 location is a location where one of the following apply:
    1. Ignitible concentrations of combustible dust or ignitible fibers/flyings are not likely to occur in normal operation and, if they do occur, will only persist for a short period; or
    2. Combustible dust or fibers/flyings are handled, processed, or used but in which the dust or fibers/flyings are normally confined within closed containers of closed systems from which they can escape only as a result of the abnormal operation of the equipment with which the dust or fibers/flyings are handled, processed, or used; or
    3. The location is adjacent to a Zone 21 location, from which ignitible concentrations of dust or fibers/flyings could be communicated.
    Exception: When communication from an adjacent Zone 21 location is minimized by adequate positive pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provi‐ ded.Informational Note No. 1: As a guide to classification of Zone 22 locations, refer to ANSI/ISA-60079-10-2 (12.10.05)-2013, Explo‐ sive Atmospheres — Part 10-2: Classification of areas — Combustible dust atmospheres.Informational Note No. 2: Zone 22 locations usually include outlets from bag filter vents, because in the event of a malfunc‐ tion there can be emission of combustible mixtures; locations near equipment that has to be opened at infrequent intervals or equipment that from experience can easily form leaks where, due to pressure above atmospheric, dust will blow out; pneu‐ matic equipment, flexible connections that can become damaged, etc.; storage locations for bags containing dusty prod‐ uct, since failure of bags can occur during handling, causing dust leakage; and locations where controllable dust layers are formed that are likely to be raised into explosive dust–air mixtures. Only if the layer is removed by cleaning before hazard‐ ous dust–air mixtures can be formed is the area designated unclassified.Informational Note No. 3: Locations that normally are classified as Zone 21 can fall into Zone 22 when measures are employed to prevent the formation of explosive dust–air mixtures. Such measures include exhaust ventilation. The measures should be used in the vicinity of (bag) filling and emptying points, feed
    belts, sampling points, truck dump stations, belt dump over points, etc.
  6. Material Groups. For the purposes of testing, approval, and area classification, various air mixtures (not oxygen enriched) shall be grouped as required in 506.6(A), (B), and (C).
    1. Group IIIC. Combustible metal dust. Group IIIC shall be considered to be equivalent to Class II, Group E.
    2. Group IIIB. Combustible dust other than combustible metal dust. Group IIIB shall be considered to be equivalent to Class II, Groups F and G.
    3. Group IIIA. Solid particles, including fibers, greater than 500 µm in nominal size, which could be suspended in air and could settle out of the atmosphere under their own weight. Group IIIA shall be considered to be equivalent to Class III.
    Informational Note: Examples of flyings include rayon, cotton (including cotton linters and cotton waste), sisal, jute, hemp, cocoa fiber, oakum, and baled waste kapok.
  7. Special Precaution. Article 506 requires equipment construction and installation that ensures safe performance under conditions of proper use and maintenance.Informational Note: It is important that inspection authorities and users exercise more than ordinary care with regard to the installation and maintenance of electrical equipment in hazard‐ ous (classified) locations.
    1. Implementation of Zone Classification System. Classifica‐ tion of areas, engineering and design, selection of equipment and wiring methods, installation, and inspection shall be performed by qualified persons.
    2. Dual Classification. In instances of areas within the same facility classified separately, Zone 22 locations shall be permit‐ ted to abut, but not overlap, Class II or Class III, Division 2 locations. Zone 20 or Zone 21 locations shall not abut Class II or Class III, Division 1 or Division 2 locations.
    3. Reclassification Permitted. A Class II or Class III, Division 1 or Division 2 location shall be permitted to be reclassified as a Zone 20, Zone 21, or Zone 22 location, provided that all of the space that is classified because of a single combustible dust or ignitible fiber/flying source is reclassified under the require‐ ments of this article.
    4. Simultaneous Presence of Flammable Gases and Combus‐ tible Dusts or Fibers/Flyings. Where flammable gases, combustible dusts, or fibers/flyings are or may be present at the same time, the simultaneous presence shall be considered during the selection and installation of the electrical equip‐ ment and the wiring methods, including the determination of the safe operating temperature of the electrical equipment.
  8. Protection Techniques. Acceptable protection techni‐ ques for electrical and electronic equipment in hazardous (classified) locations shall be as described in 506.8(A) through (I).
    1. Dust Ignitionproof. This protection technique shall be permitted for equipment in Zone 20, Zone 21, and Zone 22 locations for which it is identified.
    2. Pressurized. This protection technique shall be permitted for equipment in Zone 21 and Zone 22 locations for which it is identified.
    3. Intrinsic Safety. This protection technique shall be permitted for equipment in Zone 20, Zone 21, and Zone 22 locations for which it is identified.
    4. Dusttight. This protection technique shall be permitted for equipment in Zone 22 locations for which it is identified.
    5. Protection by Encapsulation “m”. This protection techni‐ que shall be permitted for equipment in Zone 20, Zone 21, and Zone 22 locations for which it is identified.Informational Note: See Table 506.9(C)(2)(3) for the descrip‐ tions of subdivisions for encapsulation.
    6. Nonincendive Equipment. This protection technique shall be permitted for equipment in Zone 22 locations for which it is identified.
    7. Protection by Enclosure “t”. This protection technique shall be permitted for equipment in Zone 20, Zone 21, and Zone 22 locations for which it is identified.Informational Note: See Table 506.9(C)(2)(3) for the descrip‐ tions of subdivisions for protection by enclosure “t.”
    8. Protection by Pressurization “pD”. This protection tech‐ nique shall be permitted for equipment in Zone 21 and Zone 22 locations for which it is identified.
    9. Protection by Intrinsic Safety “iD”. This protection techni‐ que shall be permitted for equipment in Zone 20, Zone 21, and Zone 22 locations for which it is listed.
  9. Equipment Requirements.
  1. Suitability. Suitability of identified equipment shall be determined by one of the following:
    1. Equipment listing or labeling
    2. Evidence of equipment evaluation from a qualified test‐ ing laboratory or inspection agency concerned with prod‐ uct evaluation
    3. 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 may include certificates demonstrating compliance with applica‐ ble equipment standards, indicating special conditions of use, and other pertinent information.
  2. Listing. Equipment that is listed for Zone 20 shall be permitted in a Zone 21 or Zone 22 location of the same dust or ignitible fiber/flying. Equipment that is listed for Zone 21 may be used in a Zone 22 location of the same dust or ignitible fiber/flying.•
  3. Marking.
    1. Division Equipment. Equipment identified for Class II, Division 1 or Class II, Division 2 shall, in addition to being marked in accordance with 500.8(C), be permitted to be marked with all of the following:
      1. Zone 20, 21, or 22 (as applicable)
      2. Material group in accordance with 506.6
      3. Maximum surface temperature in accordance with 506.9(D), marked as a temperature value in degrees C, preceded by “T” and followed by the symbol “°C”
      4. Zone Equipment. Equipment meeting one or more of the protection techniques described in 506.8 shall be marked with the following in the order shown:
        1. Zone
        2. Symbol “AEx”
        3. Protection technique(s) in accordance with Table 506.9(C)(2)(3)
        4. Material group in accordance with 506.6
        5. Maximum surface temperature in accordance with 506.9(D), marked as a temperature value in degrees C, preceded by “T” and followed by the symbol “°C”
        6. Ambient temperature marking in accordance with 506.9(D)
      Informational Note: The EPL (or equipment protection level) can appear in the product marking. EPLs are designated as G for gas, D for dust, or M for mining, and are then followed by a letter (a, b, or c) to give the user a better understanding as to whether the equipment provides (a) a “very high,” (b) a “high,” or (c) an “enhanced” level of protection against ignition of an explosive atmosphere. For example, a Zone 21 AEx pb IIIB T165°C motor can additionally be marked with an EPL of “Db”, Zone 21 AEx p IIIB T165°C Db.Exception: Associated apparatus NOT suitable for installation in a hazardous (classified) location shall be required to be marked only with 506.9(C)(2)(2) and (3), and where applicable (4), but BOTH the symbol AEx in 506.9(C)(2)(2) and the symbol for the type of protection in 506.9(C)(2)(3) shall be enclosed within the same square brackets; for example, [AEx iaD] or [AEx ia] IIIC.
      Table 506.9(C)(2)(3) Types of Protection Designation
      imageDesignation Technique Zone*iaDProtection by intrinsic safety20iaProtection by intrinsic safety20ibDProtection by intrinsic safety21ibProtection by intrinsic safety21icProtection by intrinsic safety22[iaD]Associated apparatusUnclassified**[ia]Associated apparatusUnclassified**[ibD]Associated apparatusUnclassified**[ib]Associated apparatusUnclassified**[ic]Associated apparatusUnclassified**maDProtection by encapsulation20maProtection by encapsulation20mbDProtection by encapsulation21mbProtection by encapsulation21mcProtection by encapsulation22pDProtection by pressurization21pProtection by pressurization21pbProtection by pressurization21tDProtection by enclosures21taProtection by enclosures20tbProtection by enclosures21tcProtection by enclosures22*Does not address use where a combination of techniques is used.**Associated apparatus is permitted to be installed in a hazardous (clas‐ sified) location if suitably protected using another type of protection.
      Informational Note: The “D” suffix on the type of protection designation was employed prior to the introduction of Group IIIA, IIIB, and IIIC; which is now used to distinguish between the type of protection employed for Group II (Gases) or Group III (Dusts).
  4. Temperature Classifications. Equipment shall be marked to show the maximum surface temperature referenced to a 40°C ambient, or at the higher marked ambient temperature ifthe equipment is rated and marked for an ambient tempera‐ ture of greater than 40°C. For equipment installed in a Zone 20 or Zone 21 location, the operating temperature shall be based on operation of the equipment when blanketed with the maxi‐ mum amount of dust (or with dust-simulating fibers/flyings) that can accumulate on the equipment. Electrical equipment designed for use in the ambient temperature range between-20°C and +40°C shall require no additional ambient tempera‐ ture marking. Electrical equipment that is designed for use in a range of ambient temperatures other than -20°C and +40°C is considered to be special; and the ambient temperature range shall then be marked on the equipment, including either the symbol “Ta” or “Tamb” together with the special range of ambi‐ ent temperatures.Informational Note: As an example, such a marking might be “–30°C ≤ Ta ≤ +40°C.”Exception No. 1: Equipment of the non–heat-producing type, such as conduit fittings, shall not be required to have a marked operating temperature.Exception No. 2: Equipment identified for Class II, Division 1 or Class II, Division 2 locations as permitted by 506.20(B) and (C) shall be permitted to be marked in accordance with 500.8(C) and Table 500.8(C).
  5. Threading. The supply connection entry thread form shall be NPT or metric. Conduit and fittings shall be made wrenchtight to prevent sparking when the fault current flows through the conduit system and to ensure the integrity of the conduit system. Equipment provided with threaded entries for field wiring connections shall be installed in accordance with 506.9(E)(1) or (E)(2) and with (E)(3).
    1. Equipment Provided with Threaded Entries for NPT- Threaded Conduit or Fittings. For equipment provided with threaded entries for NPT-threaded conduit or fittings, listed conduit fittings or listed cable fittings shall be used. All NPT- threaded conduit and fittings shall be threaded with a National (American) Standard Pipe Taper (NPT) thread.Informational Note: Thread specifications for NPT threads are located in ASME B1.20.1-2013, Pipe Threads, General Purpose (Inch).
    2. Equipment Provided with Threaded Entries for Metric- Threaded Fittings. For equipment with metric-threaded entries, listed conduit fittings or listed cable fittings shall be used. Such entries shall be identified as being metric, or listed adapters to permit connection to conduit or NPT-threaded fittings shall be provided with the equipment and shall be used for connection to conduit or NPT-threaded fittings. Metric- threaded fittings installed into equipment entries shall be made up with at least five threads fully engaged.
    3. Unused Openings. All unused openings shall be closed with listed metal close-up plugs. The plug engagement shall comply with 506.9(E)(1) or (E)(2).
  6. Optical Fiber Cables. An optical fiber cable, with or with‐ out current-carrying conductors (composite optical fiber cable), shall be installed to address the associated fire hazard and sealed to address the associated explosion hazard in accordance with the requirements of 506.15 and 506.16.
  1. Wiring Methods. Wiring methods shall maintain the integrity of the protection techniques and shall comply with 506.15(A), (B), or (C).
    1. Zone 20. In Zone 20 locations, the following wiring meth‐ ods shall be permitted:
      1. Threaded rigid metal conduit or threaded steel inter‐ mediate metal conduit.
      2. Type MI cable terminated with fittings listed for the loca‐ tion. Type MI cable shall be installed and supported in a manner to avoid tensile stress at the termination fittings.Exception No. 1: MI cable and fittings listed for Class II, Division 1 locations shall be permitted to be used.Exception No. 2: Equipment identified as intrinsically safe “iaD” or “ia” shall be permitted to be connected using the wiring methods identi‐ fied in 504.20.
      3. In industrial establishments with limited public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation, Type MC-HL cable listed for use in Zone 20 locations, with a continuous corrugated metallic sheath, an overall jacket of suitable polymeric material, and a separate equipment grounding conductor(s) in accordance with 250.122, and terminated with fittings listed for the appli‐ cation, shall be permitted. Type MC-HL cable shall be installed in accordance with the provisions of Article 330, Part II.Exception: Type MC-HL cable and fittings listed for Class II, Division 1 locations shall be permitted to be used.
      4. In industrial establishments with restricted public access, where the conditions of maintenance and supervision ensure that only qualified persons service the installation, and where the cable is not subject to physical damage, Type ITC-HL cable listed for use in Zone 1 or Class I, Division 1 locations, with a gas/vaportight continuous corrugated metallic sheath and an overall jacket of suita‐ ble polymeric material, and terminated with fittings listed for the application. Type ITC-HL cable shall be installed in accordance with the provisions of Article 727.
      5. Fittings and boxes shall be identified for use in Zone 20 locations.Exception: Boxes and fittings listed for Class II, Division 1 locations shall be permitted to be used.
      6. Where necessary to employ flexible connections, liquid‐ tight flexible metal conduit with listed fittings, liquidtight flexible nonmetallic conduit with listed fittings, or flexi‐ ble cord listed for extra-hard usage and provided with listed fittings shall be used. Where flexible cords are used, they shall also comply with 506.17 and shall be termina‐ ted with a listed cord connector that maintains the type of protection of the terminal compartment. Where flexible connections are subject to oil or other corrosive condi‐ tions, the insulation of the conductors shall be of a type listed for the condition or shall be protected by means of a suitable sheath.Exception No. 1: Flexible conduit and flexible conduit and cord fittings listed for Class II, Division 1 locations shall be permitted to be used.Exception No. 2: For elevator use, an identified elevator cable of Type EO, ETP, or ETT, shown under the “use” column in Table 400.4 for “hazardous (classified) locations,” and terminated with listed connectors that maintain the type of protection of the terminal compart‐ ment shall be permitted.Informational Note No. 1: See 506.25 for grounding require‐ ments where flexible conduit is used.Informational Note No. 2: For further information on construc‐ tion, testing, and marking of cables, cable fittings, and cord connectors, see ANSI/UL 2225-2011, Cables and Cable-Fittings for Use in Hazardous (Classified) Locations.
      7. Optical fiber cable Types OFNP, OFCP, OFNR, OFCR, OFNG, OFCG, OFN, and OFC shall be permitted to be installed in raceways in accordance with 506.15(A). Opti‐ cal fiber cables shall be sealed in accordance with 506.16.
    2. Zone 21. In Zone 21 locations, the wiring methods in (B)
      1. and (B)(2) shall be permitted:
        1. All wiring methods permitted in 506.15(A).
        2. Fittings and boxes that are dusttight, provided with threa‐ ded bosses for connection to conduit, in which taps, joints, or terminal connections are not made, and are not used in locations where metal dust is present, may be used.
        Informational Note: For further information on construction, testing, and marking of cables, cable fittings, and cord connec‐ tors, see ANSI/UL 2225-2011, Cables and Cable-Fittings for Use in Hazardous (Classified) Locations.Exception: Equipment identified as intrinsically safe “ibD” or “ib” shall be permitted to be connected using the wiring methods identified in 504.20.
    3. Zone 22. In Zone 22 locations, the following wiring meth‐ ods shall be permitted:
      1. All wiring methods permitted in 506.15(B).
      2. Rigid metal conduit, intermediate metal conduit, electri‐ cal metallic tubing, dusttight wireways.
      3. Type MC or MI cable with listed termination fittings.
      4. Type PLTC and Type PLTC-ER cable in accordance with the provisions of Article 725, including installation in cable tray systems. The cable shall be terminated with listed fittings.
      5. Type ITC and Type ITC-ER cable as permitted in 727.4 and terminated with listed fittings.
      6. Type MC, MI, MV, TC, or TC-ER cable installed in ladder, ventilated trough, or ventilated channel cable trays in a single layer, with a space not less than the larger cable diameter between two adjacent cables, shall be the wiring method employed. Single-conductor Type MV cables shall be shielded or metallic armored. The cable shall be terminated with listed fittings.
      7. Intrinsic safety type of protection “ic” shall be permitted using any of the wiring methods permitted for unclassi‐ fied locations. Intrinsic safety type of protection “ic” systems shall be installed in accordance with the control drawing(s). Simple apparatus, not shown on the control drawing, shall be permitted in a circuit of intrinsic safety type of protection “ic”, provided that the simple appara‐ tus does not interconnect the intrinsic safety type of protection “ic” circuit to any other circuit.Informational Note: The term Simple Apparatus is defined in 504.2.Separation of circuits of intrinsic safety type of protec‐ tion “ic” shall be in accordance with one of the follow‐ ing:
        1. Be in separate cables
  2. ARTICLE 510 — HAZARDOUS (CLASSIFIED) LOCATIONS — SPECIFIC
  1. Be in multiconductor cables where the conductors of each circuit are within a grounded metal shield
  2. Be in multiconductor cables where the conductors have insulation with a minimum thickness of 0.25 mm (0.01 in.)
  3. Boxes and fittings shall be dusttight.
  4. Optical fiber cable Types OFNP, OFCP, OFNR, OFCR, OFNG, OFCG, OFN, and OFC shall be permitted to be installed in cable trays or any raceway in accordance with 506.15(C). Optical fiber cables shall be sealed in accord‐ ance with 506.16.
  5. Cablebus.
  1. Sealing. Where necessary to protect the ingress of combustible dust or ignitible fibers/flyings, or to maintain the type of protection, seals shall be provided. The seal shall be identified as capable of preventing the ingress of combustible dust or ignitible fibers/flyings and maintaining the type of protection but need not be explosionproof or flameproof.
  2. Flexible Cords. Flexible cords used in Zone 20, Zone 21, and Zone 22 locations shall comply with all of the following:
  1. Be of a type listed for extra-hard usage
  2. Contain, in addition to the conductors of the circuit, an equipment grounding conductor complying with 400.23
  3. Be connected to terminals or to supply conductors in an approved manner
  4. Be supported by clamps or by other suitable means in such a manner to minimize tension on the terminal connections
  5. Be terminated with a listed cord connector that maintains the protection technique of the terminal compartment

Informational Note: For further information on construction, testing, and marking of cables, cable fittings, and cord connec‐ tors, see ANSI/UL 2225-2011, Cables and Cable-Fittings for Use in Hazardous (Classified) Locations.

506.20 Equipment Installation.

  1. Zone 20. In Zone 20 locations, only equipment listed and marked as suitable for the location shall be permitted.Exception: Equipment listed for use in Class II, Division 1 locations with a suitable temperature class shall be permitted.
  2. Zone 21. In Zone 21 locations, only equipment listed and marked as suitable for the location shall be permitted.Exception No. 1: Apparatus listed for use in Class II, Division 1 loca‐ tions with a suitable temperature class shall be permitted.Exception No. 2: Pressurized equipment identified for Class II, Division 1 shall be permitted.
  3. Zone 22. In Zone 22 locations, only equipment listed and marked as suitable for the location shall be permitted.Exception No. 1: Apparatus listed for use in Class II, Division 1 or Class II, Division 2 locations with a suitable temperature class shall be permitted.Exception No. 2: Pressurized equipment identified for Class II, Division 1 or Division 2 shall be permitted.
  4. Material Group. Equipment marked Group IIIC shall be permitted for applications requiring IIIA or IIIB equipment. Similarly, equipment marked Group IIIB shall be permitted for applications requiring IIIA equipment.
  5. Manufacturer’s Instructions. Electrical equipment instal‐ led in hazardous (classified) locations shall be installed in accordance with the instructions (if any) provided by the manufacturer.
  6. Temperature. The temperature marking specified in 506.9(C)(2)(5) shall comply with (F)(1) or (F)(2):
  1. For combustible dusts, less than the lower of either the layer or cloud ignition temperature of the specific combustible dust. For organic dusts that may dehydrate or carbonize, the temperature marking shall not exceed the lower of either the ignition temperature or 165°C (329°F).
  2. For ignitible fibers/flyings, less than 165°C (329°F) for equipment that is not subject to overloading, or 120°C (248°F) for equipment (such as motors or power trans‐ formers) that may be overloaded.

Informational Note: See NFPA 499-2013, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Processing Areas, for minimum ignition temperatures of specific dusts.

506.25 Grounding and Bonding. Regardless of the voltage of the electrical system, grounding and bonding shall comply with Article 250 and the requirements in 506.25(A) and (B).

  1. Bonding. The locknut-bushing and double-locknut types of contacts shall not be depended on for bonding purposes, but bonding jumpers with proper fittings or other approved means of bonding shall be used. Such means of bonding shall apply to all intervening raceways, fittings, boxes, enclosures, and so forth, between Zone 20, Zone 21, and Zone 22 locations and the point of grounding for service equipment or point of grounding of a separately derived system.Exception: The specific bonding means shall be required only to the nearest point where the grounded circuit conductor and the grounding electrode conductor are connected together on the line side of the build‐ ing or structure disconnecting means as specified in 250.32(B) if the branch side overcurrent protection is located on the load side of the disconnecting means.
  2. Types of Equipment Grounding Conductors. Liquidtight flexible metal conduit shall include an equipment bonding jumper of the wire type in compliance with 250.102.Exception: In Zone 22 locations, the bonding jumper shall be permitted to be deleted where all of the following conditions are met:
    1. Listed liquidtight flexible metal conduit 1.8 m (6 ft) or less in length, with fittings listed for grounding, is used.
    2. Overcurrent protection in the circuit is limited to 10 amperes or less.
    3. The load is not a power utilization load.

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