Published In November 2000
Article 502 in the National Electrical Code (NEC) covers the installation of electrical equipment in a combustible dust location. Determining the extent of a Class II dusty environment and the related area classification can be difficult, and a thorough understanding of these concepts is critical to the proper classification and installation of electrical equipment in Class II locations. Combustible dust is present in many locations, but may not be in sufficient quantity to become a hazard. Combustible dusts are often either organic or metal dusts that are finely ground into very small particles of 420 microns or less in diameter. A U.S. No. 40 Sieve is used to measure the particle size of the dust. If a sufficient quantity or percentage of dust, based upon the amount used for the test, falls through the sieve, the area where the dust could occur should be considered to be a Class II location. The actual quantity of dust that may accumulate in the affected area could vary, depending upon air movement, size of the leak in the system, or any number of other factors. Section 500-8 of the 1999 NEC provides definitions for Class II locations. To determine the area classification, dust off a horizontal surface of a piece of electrical equipment in the area where the dust occurs, and then examine the surface after a 24-hour time lapse. The general rule of thumb is that, if the accumulated dust is 1/8 of an inch or greater in depth, the area should be considered a Class II, Division 1 area. If, after the 24-hour period, the accumulation is less than 1/8 inch and the color of the surface of the equipment cannot be discerned, the area is probably a Class II, Division 2 area. If the accumulation is less than 1/8 inch in depth and the color of the surface is discernable, the area is probably nonclassified. The quantity of combustible dust that may be present and the adequacy of dust removal systems are factors that merit consideration in determining the classification and may result in an unclassified area. Where products such as grain in bags are handled in a manner that produces low quantities of dust, the amount of dust deposited on surfaces or that forms a dust cloud in the area may not warrant classification. Also, improved housekeeping practices in an area may permit reclassification or the declassification of the area. Combustible dusts may be electrically nonconductive, such as dust produced in the handling and processing of grain and grain products, handling of dried egg and milk powders, pulverizing sugar and cocoa, and other organic materials. Organic dusts are not generally considered conductive unless some material has been added that makes them conductive. Only Group E dusts are considered to be electrically conductive for classification purposes. Dusts containing magnesium or aluminum are particularly hazardous, and the use of extreme precaution is necessary to avoid ignition and explosion. A Class II, Division 1 location is one in which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitable mixtures. An example of this would be a grain elevator where large quantities of grain dust are likely to be in the air during handling of the grain. The grain dust may also accumulate on the electrical equipment and other horizontal surfaces in the general area of the grain handling process. A Class II, Division 1 location may also be a location where mechanical failure or abnormal operation of machinery or equipment could produce explosive or ignitable dust mixtures. Such abnormal operation of equipment may also provide a source of ignition through simultaneous failure of electric equipment or the operation of the electrical equipment due to the failure. For example, an overheated control circuit could fail, causing an unanticipated release of material, and the associated cloud of dust, upon reaching the overheated circuit, could result in an explosion. A Class II, Division 1 location can also be a location in which combustible dusts of an electrically conductive nature may be present in sufficient quantities to become hazardous. Any finely ground metal dust that could enter the electrical equipment may produce a short circuit or arcing between uninsulated parts that may cause a fire within the equipment, ignition of the dust, or other hazardous condition. A Class II, Division 2 location is a location where combustible dust is not normally in the air in quantities sufficient to produce explosive or ignitable mixtures, and dust accumulations are insufficient to interfere with the normal operation of electrical equipment or other apparatus. Combustible dust is permitted to be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment. A Class II, Division 2 location may also be a location where combustible dust may accumulate on, in, or in the vicinity of the electrical equipment and may be sufficient to interfere with the safe dissipation of heat from the electrical equipment. It may also be a location where combustible dust may be ignited by abnormal operation or failure of electrical equipment. ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at email@example.com.