CLASS II

CEC/NEC (North American) Area Classifications

Combustible Dusts

CLASS II (Explosive Dusts) Hazardous substances

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. To be considered a “dust”, the combustible material must exist as a finely divided solid of 420 microns (0.420 mm) or less. Such a dust will 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 will be an explosion hazard.

Division 1 A Class II, Division 1 location exists where combustible dust is normally in suspension in the air in sufficient quantities to produce ignitable materials, or

where mechanical failure or abnormal operation of equipment might cause an explosion or ignitable dust-air mixture to be produced, or where combustible dusts of an electrically conductive nature may be present in hazardous quantities (Group E). (Note – the term “hazardous quantities” is intended to mean those locations where the dust may not be in suspension in the air in sufficient quantity to cause an explosion, but might have settled on electrical equipment so that the electrically conductive particles can penetrate the openings in the equipment and cause an electrical failure.)

Division 2
A Class II, Division location exists where combustible dust is not normally in the air in sufficient quantity to produce an explosion, and dust accumulations are not normally sufficient to interfere with the normal operation of electrical equipment. It includes locations where combustible dust may be in suspension in the air only as a result of infrequent malfunctioning of handling or processing equipment, and those locations where dust accumulation may be on or in the vicinity of the electrical equipment and may be sufficient to interfere with the safe dissipation of heat from the equipment, or may be ignitable by abnormal operation or failure of the electrical equipment.

Groups.
Division into three groups in Class II locations is for the same reasons Class I locations are divided into Groups A, B, C and D: equipment design and area classification. However the three Class II groups (Groups E, F & G) are based on different characteristics than the four Class I groups because the design characteristics are different. In Class II locations, the ignition temperature of the dust, the electrical conductivity of the dust, and the thermal blanketing effect the dust can have on heat-producing equipment such as lighting fixtures and motors are the deciding factors in determining the Class II groups...

Group E dusts include the metal dusts, such as aluminum and magnesium. In addition to being highly abrasive, and thus likely to cause overheating of motor bearings if the dust gets into the bearing. Group E dusts are electrically conductive and if they are allowed to enter an enclosure, they can cause electrical failure of the equipment

Group F dusts are carbonaceous, the primary dust in this group being coal dust. (*also coal dust and carbon black). These dusts have somewhat lower ignition temperatures than the Group E dusts and a layer of Group F dust has a higher thermal insulating value than the layer of Group E dust, thus requiring a more careful control of the temperature on the surface of the material. Such dusts are semi-conductive but this is not usually a factor for equipment rated 600 volts and less.

Group G dusts include plastic dusts, most chemical dusts and food and grain dusts. They are not electrically conductive. These dusts, in general, have the highest thermal insulating characteristics and the lowest ignition temperatures. Thus dust-ignition proof equipment for use in Group G atmospheres must have the lowest surface temperatures to prevent ignition of a layer by the heat generated by the equipment.

CLASS II (Explosive Dusts) [temperatures are in ° C]

Temperature Codes (T-Codes)
The ignition temperature or auto-ignition temperature (AIT) is the minimum temperature required to initiate or cause self-sustained combustion in a substance without any apparent source of ignition. The lowest published ignition temperature should be the one used to determine the acceptability of equipment. This is of particular concern when selecting heat producing equipment such as lighting fixtures or motors which could generate sufficient heat to ignite the surrounding atmosphere.

Class I and Class II areas use T-Codes or are subject to maximum temperature limitations as shown in the following chart. North America and the IEC are consistent in their temperature or T-Codes. However, unlike the IEC, North America includes incremental values as shown. Equipment tested must have nameplates and marked showing class, group and operating temperature based on operation in a 40 ° C ambient (see NEC 500-3 for exceptions). Non-heat producing equipment does not have this requirement.