Sometimes we need to construct, purchase, or install electrical equipment for a hazardous location where there is a risk of a fire or explosion. The National Electrical Code (NEC) defines hazardous locations as those areas where fire or explosion hazards may exist due to flammable gases or vapors, flammable liquids, combustible dust, or ignitable fibers or flyings. Class I, Division 1, Class II, Division 1 , and Class I, Division 2 are three such classifications for gas explosive atmospheres. What’s the meaning of classes and divisions? Class I Locations According to the NEC, there are three types of hazardous locations. The first type of hazard is one, which is created by the presence of flammable gases or vapors in the air, such as natural gas or gasoline vapor. When these materials are found in the atmosphere, a potential for explosion exists, which could be ignited if an electrical or other source of ignition is present. The Code writers have referred to this first type of hazard as Class I. So, a Class I Hazardous Location is one in which flammable gases or vapors may be present in the air- in-sufficient quantities to be explosive or ignitable. Some typical Class I locations are: • Petroleum refineries, and gasoline storage and dispensing areas • Dry cleaning plants where vapors from cleaning fluids can be present • Spray finishing areas • Aircraft hangars and fuel service areas • Utility gas plants, and operations involving storage and handling of liquefied petroleum gas or natural gas All of these are Class I gas or vapor hazardous locations. Class II Locations The second type of hazard listed by the National Electrical Code is those areas made hazardous by the presence of combustible dust. These are referred to in the Code as Class II Locations. Finely pulverizedm aterial, suspended in the atmosphere, can cause as powerful an explosion as one occurring at a petroleum refinery. Some typical Class II locations are: Grain elevators; Flour and feed mills; Plants that manufacture, use or store magnesium or aluminum powders; Producers of plastics, medicines and fireworks; Producers of starch or candies; Spice-grinding plants, sugar plants and cocoa plants and Coal preparation plants and other carbon handling or processing areas; Division Ratings for Hazardous Locations: There are two different kinds of conditions under which these hazards are present, know as Divisions: Division 1: Explosive Gas or Dust is present during NORMAL conditions. Division 2: Explosive Gas or Dust is present during ABNORMAL conditions. Explosive Materials in each Hazardous Group: Group Hazardous Material in Surrounding Atmosphere Group A Acetylene Group B Hydrogen, fuel and combustible process gases containing more than 30% hydrogen by volume or gases of equivalent hazard such as butadiene, ethylene, oxide, and propylene oxide. Group C Carbon monoxide, ether, hydrogen sulfide, morpholine, cyclopropane, ethyl and ethylene or gases of equivalent hazard. Group D Gasoline, acetone, ammonia, benzene, butane, cyclopropane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard. Group E Combustible metal dusts, including aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment. Group F Carbonaceous dusts, carbon black, coal black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles or dusts that have been sensitized by other material so they present an explosion hazard. Group G Flour dust, grain dust, flour, starch, sugar, wood, plastic and chemicals. Hazardous Locations including Classes, Groups, and Divisions: Summary of Class I, II, III Hazardous Locations Classes Groups Divisions 1 2 I. – Gases, vapors, and liquids A: Acetylene B: Hydrogen, etc. C: Ether, etc. D: Hydrocarbons, fuels, solvents, etc. Normally explosive and hazardous. Not normally present in an explosive concentration (but may accidentally exist). II. – Dusts E: Metal dusts (conductive and explosive) F: Carbon dusts (some are conductive, and all are explosive) G: Flour, starch, grain, combustible plastic or chemical dust (explosive) Ignitable quantities of dust normally are or may be in suspension, or conductive dust may be present. Dust not normally suspended in an ignitable concentration (but may accidentally exist). Dust layers are present. III. – Fibers and flyings Textiles, wood-working, etc. (easily ignitable, but not likely to be explosive) Handled or used in manufacturing. Stored or handled in storage (exclusive of manufacturing).
