Deciding to classify a location as hazardous is not only a safety issue but an economic one. Classifying a location is not as simple as it first appears. For instance, because No. 1 fuel oil is present at a location, it does not automatically mean the entire area should be classified as a Class I, Division 1, Group D hazardous location. Although classifying the entire area to the most stringent level is not prohibited by the NEC®, doing so could require the use of expensive and unnecessary protection techniques or even prevent the installation of necessary process equipment. Determining the appropriate classification requires an understanding of the type of material present, of the potential for that presence under normal and abnormal conditions, and of the entire process that the material will or might undergo in given circumstances.
For example, No. 1 fuel oil blends have a flash point at or above 100°F. Below 100°F they will not give off enough vapor to form an ignitible concentration with the surrounding air. Therefore, classifying an area where the fuel is present in the Arctic might not be the same as classifying one in a desert. Fuel handling and processing also affects the classification. Vented storage, open transfer systems, or a process that involves pressurization or heating could alter a classification. Consideration must be given to a potential release, including the rate and volume involved, under normal, abnormal, or maintenance conditions. One part of a process area may be unclassified, while another part of the same area might require a Division 1 classification.
Who Classifies the Location?
The NEC neither classifies specific hazardous locations nor assigns that responsibility to the authority having jurisdiction. For an individual facility, classification could require a team of process, mechanical, and safety engineers; chemists; and operations personnel, and could also include insurance companies, the Coast Guard, or other interested parties or regulatory agencies. Those responsible must not only determine whether a hazard exists but to what extent. Even an apparently simple storage facility could require considerable evaluation before classification.
These teams, as well as the several NFPA committees and other organizations (such as the American Petroleum Institute for example), that make the documents the teams ultimately will use for guidance, have the experience working with various gases, liquids, vapors, dusts, and flyings necessary to both understand the hazards inherent in a process under operation (normal and abnormal) and determine the extent or degree of those hazards.
Criteria for Classification
Established processes such as motor vehicle fuel dispensing units or spray paint booths, or locations such as motor vehicle repair garages or aircraft hangers, have predetermined area classifications, compliant with specific NFPA codes, standards, or recommended practices. NEC Articles 511 through 517 contain information extracted from several of these documents. Because the classification is predicated upon the installation meeting all the provisions of the document from which the information has been extracted, that originating document should be used in order to correctly apply the classification. Even with the use of a predetermined classification, the team must assure that the designation is applicable to their specific location. Conditions like those mentioned earlier—temperature for instance, or the presence of fuels—could alter the classification.
Other locations require a more analytical approach wherein the team must develop a classification scheme through the use of documents such as NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas and NFPA 499, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. Both of these documents contain valuable information that can be used when classifying locations.
Classifying a Location
In both NFPA 497 and NFPA 499, the procedure for classifying an area is summarized in four steps.
1. Determining a need to classify the area. If a combustible or flammable material is handled, processed, or stored in the location, the area should be classified.
2. Gathering information for the facility. This often includes a plot plan showing the location of ditches, building structures, and process and storage vessels, and even prevailing wind direction. A flow diagram for the process showing pressures, temperatures, flow rates, and the like is needed to conduct the evaluation. An existing facility’s experience is useful, since operations and maintenance personnel can provide information not only on normal conditions but also on abnormal conditions or instances and on frequency of leaks or spills.
3. Selecting an appropriate diagram from the applicable Recommended Practice. Both NFPA 497 and NFPA 499 contain general classification diagrams, such as a source at grade level in an indoor location. Care is necessary when selecting and applying the diagram, since each diagram includes qualifying factors. In NFPA 499 for example, the classification of this indoor location at grade level is applicable to dust with a particle density of at least 40 lb/ft³.
4. Determining the extent of the hazardous location. This involves using sound engineering judgment to analyze the collected data—such as the flammability level of the material present, the vapor density of the material, expected air flow, and the location of partitions and floor drains—and the selected diagram. Each source—a potential leak at a flange for example—requires an evaluation and could require the use of a different diagram.
Why Classify an Area?
Section 500.4(A) of the NEC requires that an area classified as hazardous be properly documented as such. After the evaluation, the team often combines the collected information into an area classification document to show the extent of each hazardous location. Using this information, the proper electrical equipment protection techniques can be selected, and the NEC provides the installation requirements.
Properly classifying a location will affect present and future electrical installations. The use of unnecessary and expensive protection techniques can be avoided. Some equipment might be moved to a less hazardous location, or a simple process change or providing adequate ventilation might lower or even remove the classification. It might be determined that the area does not require classification. Having a facility properly classified is the only way to know. Proper classification provides not only a safety benefit but an economic one.
Christopher D. Coache is Senior Electrical Engineer at NFPA. He serves as staff liaison for NFPA 73, Electrical Inspection Code for Existing Dwellings. Prior to joining NFPA in 2008, he was employed in the information technology field and conducted evaluations of hazardous (classified) location equipment and gas detection equipment.