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DOE-HDBK-1092-2004
NEC Article 100 defines a flammable liquid as one that has a flashpoint below 38C (100F) or
one whose temperature is raised above its flashpoint. Flashpoint is the lowest temperature to
which a combustible or flammable liquid may be heated before sufficient vapors are driven off
and the liquid will flash when brought into contact with a flame, arc, spark, or another ignition
source. (See Section 1-3 of NFPA 497M for more details.)
5.3.1.2 DIVISION 2
NEC 500.5(B)(2) defines Class I Division 2 locations as those:
1. In which flammable liquids or gases are handled, processed, or used, but where such
materials are normally confined in closed containers or closed systems from which they can
escape only 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 500.5 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 500 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 11 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.)
5-21


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