US CODES
No one should trifle with the safety of personnel and equipment deployed in areas where there are or may be flammable or explosive atmospheres. Standards for dealing with such hazardous areas have been established based on caution and experience. But the 1996 National Electrical Code (NEC) was modified to incorporate International Electrotechnical Commission (IEC) standards in addition to the existing code. Comparison of the two methodologies shows the effects and benefits.

Area classification is the methodology used by the NEC to classify areas as to the nature, the likelihood, and the extent of ignitable flammable hazards that could exist where electrical equipment could be installed. The area classification can then be used to select the proper electrical equipment and wiring methods for a safe installation.

The primary intent of area classification is to prevent fires and explosions that could be caused by electrical equipment serving as an ignition source (arc, spark, high temperature, etc). Electrical area classification applies only to areas where electrical ignition sources can exist. Areas where mechanical or process ignition sources exist are a separate consideration. However, if these types of hazards exist all the time, then the area immediately adjacent to the ignition source is not normally electrically classified.

Area classification is defined in National Fire Protection Association (NFPA) - 70, "The National Electrical Code," Chapter 5 (Special Occupancies), Article 500. NEC Articles 501-505 and 511-517 further define the requirements for hazardous (classified) areas. Other articles in the NEC also refer to the articles in Chapter 5 for requirements.

Other NFPA standards such as NFPA-30, "Flammable and Combustible Liquids Code;" NFPA-30A, "Automotive and Marine Service Code;" NFPA-45, "Fire Protection for Laboratories;" NFPA-58, "Storage and Handling of Liquid Petroleum Gases;" etc., also contain requirements in regards to hazardous area installations. Article 500-2(b) lists references related to area classification. The requirements for area classification are not provided in a cookbook manner but rather require the use of technical expertise and sound engineering judgment.

In the United States two methods are now allowed: the traditional method based on Article 500, and the new IEC method allowed for the first time in the 1996 NEC (Article 505).

The Traditional Area Classification Method
The traditional area classification method uses three descriptors—Class, Group, and Division plus extent—to describe an area's classification. In addition, in regards to electrical equipment which may have exposed hot surface temperatures that could serve as an ignition source, the area's temperature class is also specified.

The class descriptor is used to classify the physical nature of the material. There are three classes:

CLASS I - Flammable gases, vapors, and flammable and combustible liquids;
CLASS II - Combustible dusts; and
CLASS III - Ignitible fibers or flyings.

The group descriptor is used to classify the material's flammable and explosive properties. There are seven groups: Groups A-D, which are Class I groups, and Groups E-G, which are Class II groups. Class III does not have any groups. The groups with some of the chemicals in each group are:

GROUP A - Atmospheres containing acetylene;
GROUP B - Atmospheres containing hydrogen, process gases containing more than 30% hydrogen, butadiene, ethylene oxide, or propylene oxide;
GROUP C - Atmospheres containing ethylene or ethyl ether;
GROUP D - Atmospheres containing acetone, ammonia, benzene, methane, ethane, butane, propane, hexane, or natural gas;
GROUP E - Atmospheres containing combustible metal dusts such as aluminum, magnesium, and their alloys;
GROUP F - Atmospheres containing combustible carbonaceous dusts such as carbon black, charcoal, or coal; and
GROUP G - Atmospheres containing combustible dusts not included in E&F, including flour, grain, wood, plastics, or chemicals.

For additional information on material groupings see the NEC Handbook Article 500, NFPA 497M, "Manual for Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous (Classified) Areas;" NFPA-325, "Guide to Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids;" Publication NMAB-4, "Classification of Dusts Relative to Electrical Equipment in Class II Hazardous Areas;" and Publication MNAB-5, "Classification of Gases, Vapors, and Volatile Solids Relative to Electrical Equipment in Class I Hazardous Areas." The NFPA-497 series of recommended practices is currently under revision and will be split into two recommended practices, 497 and 499.

One material property that determines the potential for ignition by hot surface temperatures on electrical equipment is the autoignition temperature (AIT) for gases or vapors. The electrical code states that for a Class I area no exposed surface temperature shall exceed 80% of the AIT in degrees Celsius unless it has been tested and found incapable of igniting the gas or vapor. Class II (minimum cloud or layer ignition temperature) and Class III (NEC Article 503-1) areas also have surface temperature limits. Identification numbers T1-T6 with a number of subclasses given in NEC Table 500-3(d) are used to identify the appropriate temperature class for the materials involved .

The division descriptor defines the likelihood that the hazard will exist in the classified area.

Division 1 is where the hazard is considered to exist under normal conditions, which could include during repair or maintenance activities, during leakage, or where faulty operation of equipment or process could release a flammable mixture and cause a simultaneous failure of electrical equipment.

Division 2 is where the hazard is considered to exist due to abnormal conditions such as accidental rupture, breakdown, or abnormal operation; or due to a failure in positive mechanical ventilation systems. Also, an area is considered Division 2 if it is adjacent to a Division 1 area where the hazard may be occasionally communicated to the Division 2 area.

By definition, any area not classified as hazardous is classified as non-hazardous. An area classified as non-hazardous may, however, still have additional requirements over and above ordinary locations placed on it such as NEC Article 500-3(c) or by the local authority having jurisdiction because of the proximity of the hazardous materials.

The extent of the classification is where the hazard may reasonably exist. The extent must be defined both in the horizontal and the vertical directions, both above and below grade. The extent is many times the hardest to determine. It is based on the material's properties and conditions such as density, temperature, pressure, flow rates, quantities involved, leak mechanism and location, ventilation, experience, and sound engineering judgment. The extent is typically shown on an area classification drawing.

Other documents, such as specifications and project books, are also used to define the area classification. OSHA Process Safety Management (PSM) 1910-119 requires documentation of your facility's area classification. You may be required to provide technical justification for your area classifications under an OSHA compliance audit or in investigations involving a fire or explosion where an electrical device or equipment may have served as the ignition source. 

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