Hazardous locations are areas where fire or explosion hazards exist due to the presence of flammable gases or vapors, flammable liquids, combustible dusts, or ignitable fibers or flyings.

The determination that areas are classified as hazardous locations is based on the following:

The possible presence of an explosive atmosphere such as flammable gases, vapors, or liquids (Class I), combustible dusts (Class II) or ignitable fibers & flyings (Class III).

Identifies the conditions under which the explosive atmosphere is present (is the hazard normally present continuously or for long periods, under normal operating conditions, or under abnormal operating conditions?)

The ignition related properties of the explosive atmosphere that is present (auto ignition temperature, explosive pressure, minimum ignition current, and maximum experimental safe gap.

A location where there is a danger of explosion due to the presence of a flammable gas or vapor.

Examples include petroleum refineries, gasoline storage and dispensing areas, dry cleaning plants where vapors from cleaning fluids can be present, spray finishing areas, aircraft device and fueling areas, utility gas plants and operations involving storage and handling of LPG or natural gas products.

A location where there is a danger of explosion due to the presence of a flammable dust.

Examples include grain elevators, flour and feed mills, plants that manufacture, use or store magnesium or aluminum powders, producers of plastics, medicines or fireworks, coal preparation plants or other carbon handling and processing areas.

A location where there is a danger of explosion due to the presence of flammable fibers or flyings.

Examples include paper mills, textile mills, cotton gins, cotton seed mills, flax processing plants, and plants that shape, pulverize or cut substances such as wood that creates flyings. These fibers and flyings are decidedly dangerous not only because they are easily ignited, but also becasue of the speed at which flames spread through them. Such fires, usually called "flash fires", have been the origin of tremendous disasters.

A location where the hazard is expected to be present during normal operating conditions.

Examples include areas near open dome loading facilities or adjacent to relief valves in a petroleum refinery, because the hazardous material would be present during normal plant operations.

A location where the hazards would only exist as a result of an accident or other abnormal event, such as an accidental rupture of a vessel or container or failure of a ventilating system.

Examples include areas where closed storage drums containing flammable liquids are stored in an inside storage room. While closed drums would not normally allow the hazardous vapors to escape into the atmosphere, what happens if one of the drums were to leak? You would end up with a Division 2 abnormal condition. So the area rating would be designated a Class I, Division 2 location. Note: In the case of Class II or III, Division 2 also includes the possibility of electrical equipment overheating because of the possibility of the enclosure being covered with dust, fibers, or flyings.

The gases and vapors of Class I locations are broken into four groups under Article 500 of the NEC: A, B, C, and D. These materials are grouped according to:

1) The auto ignition temperature (AIT) of the substance (AIT is the temperature at which a gas, vapor, or dust will ignite spontaneously without any source of ignition),

2) The explosive pressure generated by the substance for a given volume,

3) The maximum experimental safe gap (MESG) which is the distance through which an explosion can propagate if the gap between two machined surfaces exceeds a certain value, and

4) The minimum ignition current of the substance (MIC) since each gas or vapor will ignite at a different level of current when tested in a standard intrinsic safety test apparatus.

Atmospheres containing acetylene.

Acetylene makes up only a very small percentage of hazardous locations. Consequently, little equipment is available for this type of location. Acetylene is a gas with extremely high explosion pressures.

Atmospheres containing hydrogen (H2), fuel and combustible process gases containing more than 30% hydrogen by volume, or gases or vapors of equivalent hazard such as butadiene, ethylene oxide, propylene oxide, acrolein and other gases.

Group B gases also represent only a small segment of classified areas.

Atmospheres containing Ethyl Ether, Ethylene, Acetaldehyde, Allyl Alcohol, N-Butyraldehyde, Carbon Monoxide, Crontonaldehyde, Cyclopropane, Diethyl Ether, Diethylamine, Epichlorohydrin, Ethylene, Ethylenimine, Hydrogen Sulfide, Morpholine, 2-Nitropropane Tetrahydrofuran, Isoprene, Unsymmetrical Dimethyl Hydrazine (UDMH) and other gases.

Atmospheres containing Acetic Acid (glacial), Acetone, Acrylonitrite, Ammonia, Benzine, Butane, 1-Butanol (Butyl Alcohol), 2-Butanol (Secondary Butyl Alcohol), N-Butyl Acetate, Isobutyl Acetate, Di-Isobutylene, Ethane, Ethanol (Ethyl Alcohol), Ethyl Acetate, Ethyl Acrylate (Inhibited), Ethylene Diamine (anhydrous), Ethylene Dichloride, Gasoline, Heptanes, Hexanes, Isoprene, Isopropyl, Ether, Mesityl Oxide, Methane (Natural Gas), Methanol, Methyl Amyl Alcohol, Methyl Ethyl Ketone, Methyl, Isobutyl Ketone, 2-Methyl-1-propanol (Isobutyl Alcohol), 2-Methyl-2-Propanol (Tertiary Butyl Alcohol), petroleum Naphtha, Pyridine, Octanes, Pentanes, 1-Pentanol (army Alcohol), Propane, 1-Propanol (Propyl Alcohol), 2-Propanol (Isopropyl Alcohol), Propylene, Styrene, Tolvene, Vinyl Acetate, Vinyl Chloride, Xylenes and other gases.

The dusts of Class II locations are broken into 3 groups under Article 500 of the NEC: E, F, and G. These dusts are grouped according to:

1) The auto ignition temperature of the substance, and

2) The Conductivity of the substance. Conductivity is an important consideration in Class II locations, especially with metal dusts.

Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.

Atmospheres containing combustible carbonaceous dusts including coal, coke, carbon black, and charcoal dust having more than 8% total entrapped volatiles; or dusts that have been sensitized by other materials so that they present an explosion hazard.

Atmospheres containing combustible dusts not included in Group E or F, including agricultural dusts such as flour, cocoa, starch, grain and wood, thermoplastic resins and molding compounds, pharmaceutical drugs, and various chemical powders.

Introduced to North America in 1996 as an attempt to harmonize European and North American standards, the European Committee for Electrotechnical Standardization (CENELEC) and International Electrotechnical Commission (IEC) system of classification of hazardous locations is also permitted to apply to installations in the U.S. and Canada as an alternative in Class I Locations, and is now part of the NEC (Article 505) and CE Code (Section 18).