Hazardous Areas and Classification by NEC

NEC articles 500-516 inform about hazardous area classifications and standards application in united states. Article 505 tells the use of zone system for flammable gasses, vapors and liquids. Zones does not apply to dust, fiber or flyings


Table 1 - Class location definitions

 

Table 2 – Area location comparison, Class I NEC® Zone versus Division locations

 
Hazardous area locations are classified by the type of combustible material present, the extent of time it is present, and the physical construction of the area where such material is present.
 
The presence of flammable gasses or vapors in sufficient quantities to produce an explosive or ignitable mixture constitutes a Class I location. A Class II location is characterized by combustible dust. Class III locations have easily ignitable fibers, but not suspended in the air in quantities sufficient to produce an ignitable airborne mixture.

Division 1 locations, in general, are those areas where ignitable or flammable concentrations of combustible materials exist continuously or repeatedly during normal operations. Division 2 locations, in general, are those areas where such materials exist in ignitable or flammable concentrations only during periods of abnormal operating conditions.

Zone 0 locations, in general, are those areas where ignitable or flammable concentrations of combustible materials exist continuously or for long periods of time. Zone 1 locations, in general, are those areas where ignitable or flammable concentrations of combustible materials are likely to or frequently exist during normal operations. Zone 2 locations, in general, are those areas where ignitable or flammable concentrations of combustible materials are not likely to occur during normal operations or will exist for only a brief period of time.

For all of these locations there are also various groups of gasses, vapors, dusts or fibers. The groups have been established based on the ignition energy required for each of the constituents within that group. Table 3 identifies typical materials within each group and the group identification

All flammable materials have an ignition temperature. Even if the material is not exposed to an open flame or spark, they will ignite if they are exposed to an object whose temperature exceeds the ignition temperature for that material. All apparatus designed for installation in hazardous areas are rated for their maximum surface temperature. Consequently, all apparatus have a temperature code associated with their hazardous area classification. Table 4 shows the temperature code/class for apparatus.



selected equipment shall have site casing operational temperature below the above temperature.