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DOE-HDBK-1092-98
in case of accidental rupture or breakdown of such containers or systems or in case of abnormal
equipment operation.
2. In which gases or vapors are normally prevented, by positive mechanical ventilation, from
forming ignitable concentrations and which might become hazardous through failure or
abnormal operation of the ventilating equipment
3. That are adjacent to a Class I Division 1 location and to which ignitable concentrations of
gases or vapors might occasionally be transmitted unless such transmittal is prevented by
adequate positive-pressure ventilation from a source of clean air, and effective safeguards
against ventilation failure are provided.
The fine-print note #2 to NEC Section 500-5(b) describes a number of areas and occupancies
normally classified as Class I Division 2 locations. For example, piping systems without valves,
meters, and devices do not usually cause a hazardous condition, even though they carry flammable
liquids, because they are considered a contained system. Therefore, the surrounding area can be
classified as a Class I Division 2 location.
5.3.2 CLASS II
A Class II location is defined in NEC Section 500-6 as an area where combustible dust presents a fire
or explosion hazard. Class II locations are divided into two divisions based on the normal presence
or absence of dust.
5.3.2.1 CLASS II DIVISION 1
A Class II Division 1 location is one:
1. In which combustible dust is in the air under normal operating conditions in quantities
sufficient to produce explosive or ignitable mixtures;
2. Where mechanical failure or abnormal operation of machinery or equipment might cause such
explosive or ignitable mixtures to be produced and might also provide a source of ignition
through simultaneous failure of electrical equipment, operation of protective devices, or other
causes; or
3. In which combustible dusts of an electrically conductive nature may be present in hazardous
quantities. (See Table 5-6.)
5.3.2.2 CLASS II DIVISION 2
A Class II Division 2 location is one where:
1. Combustible dust is not normally in the air in quantities sufficient to produce explosive or
ignitable mixtures;
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