Division 2

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. ----

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.

MIC: Minimum Igniting Current - Each gas or vapor will ignite at a different level of current when tested in a standard intrinsic safety test apparatus.

Other factors include Explosion Pressure and Time to Peak Pressure.

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Groupings of Gases taken from Article 500 of the NEC
Class I Groups

Class I, Zone 0 - A location in which explosive gas atmospheres are present continuously or for long periods of time.

Class I, Zone 11 - A location in which explosive gas atmospheres are likely to exist in normal operation or may exist frequently because of repairs, maintenance operations, and leakage or where equipment breakdowns could release gases or vapors and also cause simultaneous failure of electrical equipment in a mode to cause the electrical equipment to become a source of ignition.

Class I, Zone 2 - A location in which explosive gas atmospheres are not likely to occur in normal operation and, if they do occur, will exist for a short time only; or where volatile flammable liquids, flammable gas, or flammable vapors are handled, processed, or used, but are normally confined within closed containers or systems from which they can escape only as a result of accidental rupture or breakdown of the containers or system, or as a result of abnormal operation of the equipment with which the liquids or gases are handled, processed, or used; or where ignitable concentrations of flammable gases or vapors are normally prevented by adequate ventilation, but which may occur as a result of failure or abnormal operation of the ventilation system.

Class I Groups
Group I - Atmospheres containing explosive gas in underground coal mines. Electrical apparatus that is intended for use in underground mines.

Group IIC - Atmospheres containing acetylene, hydrogen (H2), or gases of equivalent hazard.

Group IIB - Atmospheres containing acetaldehyde, ethylene, or gases or vapors of equivalent hazard.

Group IIA - Atmospheres containing acetone, ammonia, ethyl alcohol, gasoline, methane, propane, or gases or vapors of equivalent hazard.

Note: There is potential for confusion between the NEC/CE and IEC gas classification systems since the Group letters are reversed and even combined. Care should also be taken to avoid confusing Group II and Class II, since both use Roman numerals. An unintended result of specifying the IEC gas groups, which combine the traditional Groups A and B into Group IIC, is that equipment approved for hydrogen (H2) would also have to be approved for acetylene. Since very little equipment is designed for acetylene, the wording as originally adopted severely limits the availability of equipment for hydrogen applications. As a result, NEC Section 505-7(d) now allows for equipment to be listed for a specific gas or vapor, specific mixtures of gases or vapors, or any specific combination of gases or vapors. One common example is equipment marked for "IIB + H2". At present, the NEC or CE Code does not recognize any CENELEC or IEC dust classifications.

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