The type of protective technique selected and the level of protection it must provide depend upon the potential hazard caused by using electrical apparatus in a location where a combustible, flammable, or ignitible substance may be present.
Area classification schemes and systems of material classification have been developed to provide a succinct description of the hazard so that appropriate safeguards may be selected. All useful area classification systems specify the kind of flammable material that may be present and the probability that it will be present in ignitible concentrations.
1 North American methods
2 In the United States, area classification principles are stated in Article 500 of the National Electrical Code, ANSI/NFPA 70. Similar requirements in Canada are given in the Canadian Electrical Code, Part 1, Section 18, (CSA C22.1).
Various organizations have developed numerous guides and standards that have substantial acceptance by industry and governmental bodies for area classification.
Area classification descriptions used in the United States and Canada include the following:
Locations are classified as
(1) by CLASS—the generic form of the flammable materials in the atmosphere (gas or vapor, dusts, or easily ignitible fibers or flyings);
(2) by DIVISION—an indication of the probability of the presence of the flammable material in ignitible concentration;
(3) by GROUP—the exact nature of the flammable material.
Class I locations are those in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitible mixtures.
Class II locations are those that are hazardous because of the presence of combustible dusts.
Class III locations are those in which easily ignitible fibers or flyings may be present but not likely to be in suspension in sufficient quantities to produce ignitible mixtures.
Class 1, Division 1 locations are those in which:
a) hazardous concentrations exist continuously, intermittently, or periodically under normal operating conditions;
b) hazardous conditions may exist frequently because of repair or maintenance operations or because of leakage; or
c) breakage or faulty operation of process or other nonelectrical equipment or processes might release flammable concentrations or gases or vapors and might also cause simultaneous failure of electrical equipment that causes a source of ignition.
Class I, Division 2 locations are those in which:
a) hazardous volatile liquids, vapors, or gases are normally confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such containers or systems, or in case of abnormal operation of equipment;
b) flammable concentrations are normally prevented by positive mechanical ventilation but might become hazardous through failure or abnormal operation of the ventilating system; or
c) areas adjacent to Division 1 locations to which hazardous concentrations of gases or vapors might occasionally be communicated.
Class II, Division 1 locations are those in which:
a) combustible dust is, or may be, in suspension in the air continuously, intermittently, or periodically under normal operating conditions, in quantities sufficient to produce explosive or ignitible mixtures;
b) breakage or faulty operation of a process or machinery may produce combustible
concentrations of dusts and might also cause simultaneous failure of electrical equipment, which, in turn, may act as a source of ignition; or c) electrically conductive combustible dusts may be present.
Class II, Division 2 locations are those in which:
a) combustible concentrations of suspended dust are not likely, but where deposits or
accumulations of dust may interfere with the safe dissipation of heat from electrical
equipment or apparatus; or
b) combustible concentrations of suspended dust are not likely, but where deposits or
accumulations of dust on, in, or in the vicinity of electrical equipment might be ignited
by arcs, sparks, or burning material from such equipment.
Class III, Division 1 locations are those in which easily ignitible fibers or materials producing combustible flyings are handled, manufactured, or used.
Class III, Division 2 locations are those in which easily ignitible fibers may be stored or handled (except in the process of manufacture).
NOTE: Some plant areas in the manufacture, handling, and storage of explosives or ammunition and nitrocellulose products (such as celluloid photographic films), etc., involve conditions that are not covered by
NEC Classifications. This is particularly true where black powders, smokeless powder, dust from TNT, and other explosives are present. See NFPA 495, Reference Code for Explosive Materials, for guidance.
The United States and Canadian standards recognize seven groups: Groups A, B, C, D, E, F, and G. Groups A, B, C, and D apply to Class I locations; Groups E, F, and G apply to Class II Locations. These groups include the following:
Group A—Atmospheres containing acetylene.
Group B—Atmospheres such as butadiene*, ethylene oxide*, propylene oxide*, acrolein*, or hydrogen (or gases or vapors equivalent in hazard to hydrogen, such as manufactured gas).
Group C—Atmospheres such as cyclopropane, ethyl ether, ethylene, hydrogen sulfide, or gases or vapors of equivalent hazard.
Group D—Atmospheres such as acetone, alcohol, ammonia*, benzene, benzol, butane,
gasoline, hexane, lacquer solvent vapors, methane, naphtha, natural gas, propane, or gases or vapors of equivalent hazard.
In the United States:
Group E—Atmospheres containing combustible metal dusts regardless of resistivity or other combustible dusts of similarly hazardous characteristics having resistivity of less than 102 ohmcentimeter (magnesium, aluminum, bronze powder, etc.)
Group F—Atmospheres containing carbon black, charcoal, coal, or coke dusts that have more than 8 percent total volatile material (coal and coke dusts per ASTM 3175-82) or atmospheres containing these dusts sensitized by other materials so that they present an explosion hazard and having resistivity greater than 102 ohm-centimeter but equal to or less than 108 ohmcentimeter.
Group G—Atmospheres containing combustible dusts (flour, starch, pulverized sugar and cocoa, dairy powders, dried hay, etc.) having resistivity of 108 ohm-centimeter or greater.
NOTE: See NFPA 497M for a more detailed treatment of dusts. In Canada, the ranges of resistivities are not specified.