Electrical equipment in hazardous (classified) locations present a serious threat for workers due to the existence of explosive dust and vapors. Examples of such facilities range from grain mills and gasoline stations to aircraft hangars and chemical manufacturing plants.
To prevent devastating explosions that may occur from the ignition of combustible particles in the atmosphere, the National Electric Code (NEC) published guidelines on the proper classification of hazardous locations.
Classes, Divisions and Groups
In the NEC, hazardous locations are categorized using a classification/division system. Such details are classified under Class, Division and Group. Class refers to the broad nature of the hazardous material in the air that may be explosive in sufficient amounts. Division establishes the probability of the hazardous material producing an explosion or ignition due to its presence. Lastly, Group refers to the specific types of hazardous elements in the atmosphere, where groups A, B, C and D are for gases (Class I) and groups E, F and G are for dusts and flyings (Class II or Class III).
Class I (Article 501)
Class I hazardous locations include areas where flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors may be present in amounts that could produce an explosion or ignition. Examples of such locations include gas stations, petroleum refineries, dyeing plants, paint spray booths and other public facilities that fit the description of the classification. Workers in Class I hazardous locations are required to use special equipment that minimizes the creation of spark or provide sources of ignition. Electrical devices may either be intrinsically safe or explosion proof, while hand tools must be manufactured from non-sparking material, such as wood, rubber or plastic.
Class II (Article 502)
Class II deals with hazardous locations where combustible dust may be present. Such locations are not typically found in the oil and gas sector, where primary concerns involve explosive gases or liquids. Flour and cornstarch are common examples of elements that are filed under this classification. When present in the air, such components may become explosive despite only burning or smoldering in compact mass form. Metallic dust, such as aluminum and magnesium, also fit in this category due to their electrically conductive properties. Such components are extremely explosive, especially when small amounts become airborne. In compact mass form, metallic dust is still dangerously explosive.
Class III (Article 503)
Hazardous locations that contain explosive fibers or flyings, but are not suspended in the atmosphere in concentrations that may lead to explosions, meet the requirements of Class III. Textile mills, fiber processing plants, cloth manufacturing facilities and woodworking plants are examples of establishments that may be required to adhere to this NEC code. Specific materials that are categorized in this classification includes the following: cotton, hemp, cocoa fiber, Spanish moss and oakum.
Published by the National Fire Protection Association (NFPA), best practices for industrial equipment, electrical wirings, sealings and protection techniques are covered extensively in each section of the respective classification articles to ensure safety and compliance in the workplace. It is important to consider that devices or equipment approved for Division 1 hazardous locations may be used in a Division 2 hazardous locations, as long as it is in the same class and group. However, devices or equipment that has been cleared for a Class I hazardous location may not be used in a Class II hazardous location.