1999, all references to the zone concept of

02:1404:01
In
 NEC 
 1999, all references to the zone concept of hazardous (classified) locations were removed from Article 500 for general hazardous location concepts andfrom most of Article 501 for Class I, Division 1 and 2installations. A fine print note was left in 500-1 to pro- vide the user with direction to Article 505 containingthe bulk of the information on the zone concept of area classification.This single article concept for zone applications pro- vided a more user-friendly method to access pertinentinformation where applying the zone concept. Article505 provides information on equipment and wiringmethods that are permitted in a Zone 0, Zone 1, andZone 2 classified area. All other applicable rules wouldapply to these installations, except as amended by Ar-ticle 505 or amended by those requirements in Article504 dealing with intrinsically safe installations.The zone concept was further enhanced in
 NEC 
 2002by changing the basic layout of Article 505 to fit thenew numbering sequence required by the
 NEC Style Manual 
. All definitions were moved to 505.2; 505.3covers other applicable rules located in other articles;and 505.4 provides general information pertaining tozone installations. This was done to provide consis-tency of section numbering with other similar articles. Further changes to Article 505 made it completely independent of Articles 500 and 501. Section 505.15for wiring methods in a zone area was expanded tomatch its counterpart in 501.4 for wiring methods in aClass I, Division 1 and 2 area. Section 505.16 was in-serted to cover the sealing requirements for zone meth-ods rather than just referring back to 501.5 as was donein the previous
Codes
.
Location and General Requirements for Article 505
In zone applications, hazardous locations are classi-fied in accordance with the properties of flammableliquids, gases, or vapors that may be present in the area where electrical equipment is installed. The liquids,gases, or vapors must be likely to form ignitable con-centrations and the quantities of the material must besufficient to pose a hazard when mixed with a suffi-cient quantity of air. These conditions are similar or often the same as the conditions required for a similar location being designed and installed in the divisionconcept.Each area should be considered individually in de-termining the classification and care should be takento not over-classify, as well as to not under-classify. Elec-trical equipment should be installed and connected inan area that is outside the hazardous (classified) loca-tion; however, where this not possible or practical, thenspecial electrical equipment must be installed in thehazardous (classified) location. All of the factors thatare normally associated with division area classification would apply to a zone classification, such as tempera-ture, density or molecular weight of the substance, air circulation, quantity, pressure, and so forth. Highly oxygenated areas are outside the scope of both the zone classification and the division classifica-tion systems. These areas have had the ignition andburning characteristics of materials changed by exceed-ing the normal volume of oxygen of air that is mixed with the flammable gas or vapor. Where highly oxygen-ated areas are encountered, the user should refer tothe specialized documents that deal with these typesof areas and comply with the requirements for installa-tion of electrical equipment in those areas. Areas where pyrophoric materials are present or handled are also outside the scope of both the zoneclassification and the division classification systems.Pyrophoric materials can be ignited just by introduc-ing the material to air. Where these chemicals are used,installation of special electrical equipment is usually not necessary. However, care should be taken sincethere may be other combustible chemicals in the areathat may require special electrical equipment.Since much of the electrical equipment built for theEuropean and other world markets incorporate metricmeasurement and sizing, it may be necessary to pro- vide adapters to convert from the standard NPT threadsthat are used in the United States to the metric threadedentries or fittings provided with the equipment. If equipment with metric threads is provided for use inthe United States, the
 NEC 
 requires that metric to NPTadapters be included with the equipment or be markedto identify the threaded entries as metric threads.NPT threads provide a ¾ inch per foot taper. Moststandard conduit threading equipment available in theUnited States uses NPT thread. By conforming to a stan-dard threaded system and providing a standard tometric adapter, electrical equipment with metric threadscan be readily and safely adapted. Good electrical andmechanical continuity of conduit connections by theproper threaded adaptor to the enclosure can preventsparking between the fitting, the conduit, and the en-closure during an electrical fault. The proper fittingcan also ensure the integrity of the explosionproof or flameproof path by permitting the venting and subse-quent cooling of the hot flaming gases after an explo-sion occurs within the enclosure.
Definitions of Zone Classification
Zone 0 Locations
 A Zone 0 location involves ignitable concentrationsof flammable gases or vapors that are present con-
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tinuously or present for long periods of time. Thisclassification includes locations inside tanks in whichflammable or combustible liquids, gases, or vaporsare stored or locations such as between the inner or outer roof sections of a floating roof tank containing volatile flammable liquids. If venting is provided toallow the gases or vapors to escape from inside thetank during the process of filling, there may be anarea outside of the tank near the opening of the ventthat is considered to be a Zone 0 location. In somecases, the area around the vent outlet would be con-sidered to be a Class I, Zone 1 location, dependingupon the amount of air circulation and the type of material involved. For Zone 0 locations, it is recommended that elec-trical equipment be located in an area outside the Zone0 location. This is not always feasible since measure-ment equipment and other electrical equipment may be necessary for the function and monitoring of thesystem. In a Zone 0 location, intrinsically safe appara-tus and the associated intrinsically safe circuit are theonly wiring methods and protection technique that arepermitted to be installed.
Zone 1 Locations
 A Zone 1 location, similar to Class I, Division 1 lo-cation, is one in which there may be ignitable concen-trations during normal operating conditions or ignit-able concentrations exist frequently from repair or maintenance of the equipment. A Zone 1 would be alocation where equipment breakdown or faulty op-eration of the equipment could release ignitable con-centrations of gases or vapors and simultaneous fail-ure of electrical equipment. Areas that are adjacent toa Class I, Zone 0 location where ignitable concentra-tions of gases or vapors could migrate are also con-sidered to be Zone 1 locations.The normal classification for Zone 1 locations doesnot take into consideration catastrophic leaks. It does,however, consider small leaks from valves, pump pack-ing glands, and other similar leakage considered com-mon for equipment handling hazardous materials.
Zone 2 Locations
 A Zone 2 location and a Division 2 location are es-sentially the same and have similar definitions. It is alocation where vapors or gases are not likely to occur during normal operation but if a leak does occur, it would normally be for a short duration of time. A Zone 2 location is one where the liquids, gases, or vapors are handled, processed, or used in an area butare normally confined within closed piping or contain-ers that can only leak as a result of a rupture or someother abnormal condition.Positive mechanical ventilation is often used to pre- vent the buildup of ignitable concentrations of gasesor vapors and, thus, to reclassify or declassify the area.Should the ventilation system break down, an ignit-able concentration could require classification as a Zone2 location. An area that is adjacent to a Class I, Zone 1 locationfrom which ignitable concentrations of flammable gasesor vapors could migrate would be a Zone 2 location,unless positive mechanical ventilation has been pro- vided and there are safeguards to ensure against venti-lation failure.