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Equipment, tools and wiring utilized in potentially hazardous (explosive) areas must be designed as inherently safe. In other words, this equipment must be designed so that in and of itself, it cannot cause an explosion. The terminology used for this application is “intrinsically safe”.

Intrinsically safe equipment, tools, and wiring is designed to prevent the release of sufficient electrical or thermal energy that could ignite specific hazardous atmospheric mixtures in their most easily ignited concentrations.

Equipment such as combustible gas detectors and monitors used in potentially hazardous areas should be tested and certified as “Intrinsically Safe” by independent third party entities such as Factory Mutual (FM), Underwriters Labs (UL), and the Mine Safety and Health Administration (MSHA).

Potentially hazardous areas can be defined as any area that contains gases, dusts, or fibers in concentrations that could be ignited. This could include for example, manholes or vaults. In North America, a system has been developed and utilized as part of the National Electric Code (NEC) to designate potentially hazardous locations by Classes , Divisions , and Groups , based on the characteristics of these various flammable substances.

NEC classes include:

Class I Locations – areas where flammable gases could be present in quantities sufficient enough to produce explosive or flammable mixtures.

Class II Locations – areas where combustible dusts may be present in sufficient quantities to explode.

Class III Locations – areas where easily ignitable fibers or flyings may be present.

NEC Divisions include:

Division 1 – an environment where flammable gases, vapors, liquids, combustible dusts, fibers or flyings are likely to exist under normal conditions.

Division 2 – an environment where the materials described and listed as part of Division 1 are not likely to exist under normal conditions.

NEC Groups under Class I, Division1, include atmospheres that contain:

Group A – acetylene.

Group B – gases such as hydrogen, ethylene oxide, butadiene.

Group C – ethyl-ether vapors, ethylene, hydrogen sulfide.

Group D – gases such as methane, butane, gasoline, hexane, naptha, benzene, propane, alcohol, acetone, benzol, lacquer solvent vapors, natural gas.


NOTE 1: not all vapors or gases are listed; these are only examples what are included in each NEC group.

NOTE 2: Additional information for the other Classes, Divisions and groups may be found in the National Electric Code.

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