Industrial hygienists often need to work in or around unfamiliar locations that may present electrical hazards. These locations fall under OSHA regulation 29 CFR 1910.307, , of Subpart S, Electrical. This article provides basic information about the classification of these hazardous spaces, commonly referenced standards, and other sources of information. Hazard classification by 29 CFR 1910.307 is intended for managing the electrical equipment used in locations that contain, or may contain, material that is flammable or combustible in the air. The classification of the area involves a separate process from the classification of the flammable or combustible material itself. Per 29 CFR 1910.307, OSHA mandates that classification and selection of equipment be performed “under the supervision of a qualified registered professional engineer.” The regulation explains that classification of the space “depend[s] on the properties of the flammable vapors, liquids or gases, or combustible dusts or fibers that may be present therein and the likelihood that a flammable or combustible concentration or quantity is present.” It is required that “each room, section, or area” be considered individually.

Classification is based on the type of hazard and its severity. The terms associated with classification are defined in Subpart S, 29 CFR 1910.399. OSHA allows two classification systems: the class/division system and the class/zone system. There are three classes based on the general type of hazard. Flammable gases and vapors are designated Class I, combustible dusts are Class II, and ignitable fibers and “flyings” are Class III. No definition of flyings appears in the OSHA regulation, but the 2019 version of NFPA 499,, describes flyings as solids, such as fibers, that can suspend in the air and settle under their own weight, and are larger than 500 µm nominal. In the class/division system, the three classes are further separated into Division 1 and Division 2 to indicate modes for hazardous conditions. Division 1 designates areas where the hazard is present normally. Division 2 areas are those where the hazard is present only under abnormal conditions.  The class/zone system is an alternative to the class/division system. While OSHA currently recognizes only three zones in 29 CFR 1910.307 (Zone 0, 1, and 2), an additional three zones (Zone 20, 21, and 22) are recognized by many organizations IHs tend to be familiar with, including Underwriters Laboratories, ANSI, the International Society of Automation, and NFPA.  OSHA defers to NFPA 70, (NEC), for guidelines regarding the type and design of electrical equipment and installations. OSHA does not reference a particular revision of NFPA 70 in 29 CFR 1910.307. The current edition is 2020.  Article 500 of NFPA 70 mirrors OSHA in requiring classification based on properties of materials that could be present in the area and “the likelihood that a flammable or combustible concentration or quantity is present.” Independent classification of each room, section, or area is also required. Sections of the standard specific to situations or industries will provide details about which electrical area classifications can abut. Information pertaining to dual classification—the use of both the class/zone system and  he class/division system in a single facility—is included. Overlapping of classification is generally not allowed. NFPA 70 makes use of the “qualified person” concept. For example, when a zone classification system is implemented, according to NFPA 70, “classification of areas, engineering and design, selection of equipment and wiring methods, installation, and inspection” must be conducted by qualified persons. Thus, if the organization is following this standard requirement, even inspection elements must be performed by a person recognized as qualified.  The 2020 edition of NFPA 70 incorporates several new references. In particular, IHs may be interested in reviewing the “Combustible Gas Detection System” section. The 2020 NFPA 70 references the 2018 revision of the American Petroleum Institute’s (API RP 505). NFPA 70 is therefore more up to date than the OSHA regulation, which still refers to the 1997 version of the API standard. NFPA 70 references two important documents IHs should be familiar with if involved with locations classified as hazardous. These documents are NFPA 497, , and NFPA 499, The current revision of NFPA 497 and NFPA 499 (2017) both recommend implementing the guidance “with sound engineering judgment.” NFPA 497 and NFPA 499 have sections dedicated to procedures for classifying that make use of engineering judgment.  While NFPA provides guidelines for classification, a process is desired in order to meet the challenge of classification. The Process Industry Practices organization provides a process to approach classification in the 2016 revision of PIP ELEHA01, , which “describes an engineering work process for gathering the necessary information and applying the applicable codes and standards to develop a well-documented area classification” for “areas containing flammable liquids, gases, or vapors and combustible dust.” This guide is useful because it suggests which industry standard should be used for assessing area classification—for example, it explains when to apply NFPA 497, NFPA 499, or API RP 505.  OSHA recommends several references within 29 CFR 1910.399 as guides for determining when flammable gases or vapors are present continuously or for long periods of time. Unfortunately, the references are woefully out of date. Table 1 can help readers find the modern equivalents. The table is especially useful for IHs who need expertise to work closely with engineers. The recommended references are for flammable gases and vapors; additional standards exist for dusts and fibers—see ISA12.10-1988, , and ANSI/ISA-12.00.02-2009 (R2014), . 

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