Prerequisite of following article to start reading this article Hazardous Area Classification NEC & Principles of Class I Areas – Hazardous Area Classification NEC & Principles of Class II Areas – Hazardous Area Classification NEC.
Principles of Class III Areas – Hazardous Area Classification NEC
Class III Areas
These areas are considered hazardous due to the presence of easily ignitable fibers or flyings, for example rayon, cotton, henequen, jute, hemp, cocoa fiber or other materials of a similar nature. Sufficient quantities of these fibers or flyings are not likely to be suspended in air to produce an ignitable mixture.
Class III Divisions
Class III has two Divisions: Division 1 and Division 2.
Division 1
In Division 1 locations, easily ignitable fibers, or materials producing combustible flyings, are handled, manufactured, or used.
Division 2
In Division 2 locations, easily ignitable fibers, or materials producing combustible flyings, are handled or stored.
Conditions Necessary for Ignition
Excessively high surface temperatures of electrical equipment may dehydrate or carbonize accumulated fibers or flyings, causing these materials to reach their ignition temperature, or making them susceptible to spontaneous ignition.
Location Specific Hazardous Areas
- Gasoline dispensing stations and service stations are locations where gasoline or other volatile flammable liquids or liquefied flammable gases are transferred to the fuel tanks of self-propelled vehicles or approved containers. These areas shall be classified in accordance with NFPA 70, Article 514.
- Bulk storage plants are locations where flammable liquids are received by tank vessel, pipeline, tank car, or tank vehicle, and are stored or blended in bulk, for the purpose of distributing the liquids by tank vessel, pipeline, tank car, tank vehicle, portable tank, or container. These areas shall be classified in accordance with NFPA 70, Article 515, and the applicable sections of NFPA 30 and of API RP 500.
- Spray painting application locations are areas where regular or frequent application of flammable liquids, combustible liquids and combustible powders by spray operations are performed. These areas shall be classified in accordance with NFPA 70, Article 516.
Intrinsic Safety
- Electrical systems shall be considered intrinsically safe when the circuits supplying them are incapable of releasing sufficient energy to ignite an explosive atmosphere, either when operating normally, or under specified failure conditions.
- The application of intrinsically safe apparatus and wiring shall be permitted in any hazardous (classified) locations for which it is approved in the NFPA 70, Article 504. Its use eliminates the need for explosion-proof apparatus and wiring systems when applied in accordance with NFPA 70, Article 504.
Hazardous Area Classification Drawings
- After area classification evaluation of facilities where volatile liquids or combustible gases or vapors are present, electrical area classification drawings shall be developed or revised.
- Area classification drawings shall be developed or revised for all applicable proposal/concept designs, final designs, or as-builts, where hazardous areas have been created or changed.
- The area classification drawings shall show the extent of the electrically classified areas, using plans, elevations and sections.
- The minimum area classification drawing presentation shall show:
a. The locations and boundaries of the Division 1, Division 2 and unclassified locations on the plot
b. The locations and boundaries of the different Groups on the plot
c. The minimum ignition temperatures for each flammable or combustible material represented on the area classification drawings
d. Materials List. A listing of flammable and combustible materials considered in the classification process, with flash points, autoignition temperatures, LFL, UFL, vapor densities, Group, and ‘T’ identification number.
e. Area List or Equipment List. If classification is done on an equipment basis, an equipment list should be used. The list shall contain equipment by tag number, name, location, flammable or combustible material involved, operating temperature and pressure, volume and flow rates, area ventilation, source of release, containment description, extent of classified area, standard or code reference, and any notes. If classification is done on a room or area basis, an area/room list can be used. The area/room list shall include the location, chemical, description of containment (open or closed), sources of release, ventilation class, autoignition temperature, standards reference, and notes.
f. Design Basis. A description of the process, and the referenced standards related to hazardous locations and classifications used in preparing designs.
g. Appendices. Any reports used to reach decisions on classification.
h. References. Standards used in the decision-making process. - Table IV to VII and Figures 1 to 21 (Check this articles Hazardous Area Classification NEC, Principles of Class I Areas, Principles of Class II Areas) shall be used as a basis for developing the area classification drawings. For a more comprehensive presentation of figures to use in the development of the area classification evaluation see API RP 500, and NFPA 497 and NFPA 499.
Classification and Comparison of Maximum Surface Temperature (T-Rating)
All apparatus approved for use in hazardous area shall be marked with Class, Group and maximum operating temperature. The T-Rating or T-Code designates the maximum operating temperature on the surface of apparatus approved for use in hazardous locations.
Table IV lists the classifications of maximum surface temperature as defined in the NFPA 70 and the T-Ratings of the IEC for comparison. IEC T-Rating designations are identical to those found in the NFPA 70, however the NFPA 70 has many intermediate codes for applications where more precise information is required.