Per the National Electrical Code (NEC) there are (3) classifications for areas that are defined as hazardous. They are Class I (gases & vapors), Class II (flammable dusts) & Class III (fibers), the focus of today’s Blog is on Class II locations.
Class II locations are those that are hazardous because of the presence of combustible dust. Note that the dust must be present in sufficient quantities for a fire or explosion hazard to exist. The fact that there is some combustible dust present does not mean a Class II hazardous location exists. Dust is defined as a combustible material that must exist as a finely divided solid of 420 microns (0.420 mm) or less. This will allow the dust to pass through a No. 40 sieve. Just as in Class I, Division 1 and 2, the subdivision of Class II into Divisions 1 and 2 identifies the likelihood that there is an explosion hazard.
Division 1 locations are defined as an area where the amount of combustible dust is either suspended in the air or accumulated on surfaces in a sufficient concentration to allow for ignition. The ignition could be caused by a failure or malfunction of the equipment in the classified area. Group E & F dust (see chart below) are considered conductive and could penetrate into electrical equipment such as electric motors, control panels, electrical panels, etc., and cause an electrical failure.
Group E dusts are metal dusts, such as aluminum and magnesium. In addition to being highly abrasive, and likely to cause overheating of motor bearings if it gets into them. Group E dusts are also electrically conductive and if they are allowed to enter an enclosure can cause an electrical failure.
Group F dusts are carbonaceous, the primary dust in this group is coal dust. Coal dust has a lower ignition temperatures than those in Group E. While Group F dust has a higher thermal insulating value than the layer of Group E. Therefore Group F requires more control of the temperature on the surfaces that the dust settles on. Group E dusts are semi-conductive, however if the voltages are 600 volts or less it is not generally considered a factor.
Group G dusts include plastic dusts, most chemical dusts and food-grain dusts. They are not electrically conductive. Generally these dusts have the highest thermal insulating characteristics and the lowest ignition temperatures. Therefore, the equipment used in Group G areas must have the lowest surface temperatures to prevent ignition of a layer.
Lastly, equipment rated for use in Classified Environments have a rating called the Temperature Code or “T-Code”. This is the temperature or temperature range that the rated device will operate normally and/or in a failed or failing state. Consider something as common as a light fixture, electric motors, etc., as they could become hot enough to cause ignition depending on the type of dust in the area. So be sure to check the “T-Codes” for every piece of equipment that will be used within a Classified Environment.