Hazardous Locations

The purpose of classification is to provide safe rules for the isolation of ignition sources from areas of ignitable or potentially ignitable materials.

NATIONAL ELECTRICAL CODE and NEC are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02269

Explanations, Rules and descriptions taken from the National Electrical Code (r) Articles 500 thru 516. This page does not completly address the subject of Hazardous locations. For complete rules and regulations you should consult the NEC (NFPA) 70 Articles 500 thru 516 and the FPNs, This article is an attempt to describe hazardous (classified) locations as they are covered by the NEC.

Hazardous (classified) locations as described by NEC article 500-2 and the Fine Print Note. Locations are classified depending on the properties of the flammable vapors, liquids or gases or combustible dusts or fibers that may be present and the likelihood that a flammable or combustible concentration or quantity is present.

By the above statement we see that classifications are broken into three classes (1)Flammable vapors, liquids or gases, (2)combustible dusts, and (3) fibers or flyings. Each Class is broken into two divisions, Division 1 being the highest or most dangerous and Division 2 being the lowest or not as dangerous level.

500-5 says that Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures.

Class I Division 1:

A Class I Division 1 location is one in which ignitable concentrations of flammable gases or vapors can exist under normal operating conditions; or in which ignitible concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage, or in which breakdown or faulty operation of equipment or processes might release ignitible concentrations of flammable gases or vapors, and might also cause simultaneous failure of electric equipment.

Class I Division 2:

A Class I Division 2 location is a location in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of such container or systems, or in case of abnormal operation of equipment; or in which ignitible concentrations of gases or vapors are normally prevented by positive mechanical ventilation, and which might become hazardous through failure or abnormal operation of the ventilationg equipment; or that is adjacent to a Class I Division 1 location, and to which ignitible concentrations of gases or vapors might occasionally be communicated unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air, and effective safeguards against ventilation failure are provided.

500-6 covers class II locations; Class II locations are those that are hazardous because of the presence of combustible dust.

Class II, Division 1:

A Class II, Division 1 location is a location in which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitible mixtures; or where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitible mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, operation of protection devices, or from other causes; or in which combustible dusts of an electrically conductive nature may be present in hazardous quantities.

Class II, Division 2;

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 ignitible mixtures, and dust accumulations are normally insufficient to interfere with the normal operation of electrical equipment or other apparatus, but combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment and where combustible dust accumulations on, in, or in the vicinity of the electrical equipment may be sufficient to interfere with the safe dissipation of heat from electrical equipment or may be ignitible by abnormal operation or failure of electrical equipment.

500-7 covers Class III locations; Class III locations are those that are hazardous because of the presence of easily ignitible fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures.

Class III, Division 1:

A Class III, Division 1 location is a location in which easily ignitible fibers or materials producing combustible flyings are handled, manufactured, or used.

A Class III, Division 2 location is a location in which easily ignitible fibers are stored or handled.

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