Class I

DOE-HDBK-1092-98

10. NFPA 496, "Purged and Pressurized Enclosure for Electrical Equipment in Hazardous

Locations."

5.3.1 CLASS I

Class I locations are identified in the NEC as those in which flammable gases or vapors are or may

be present in the air in amounts sufficient to create explosive or ignitable mixtures. Gases or vapors

may be continuously or intermittently present. However, if a gas or vapor is present, there is a

potential that a flammable mixture will be present.

From an engineering standpoint, greater precautions are needed if a particular set of conditions is

likely to occur (e.g., the presence of a flammable mixture within the explosive range) than if it is

unlikely. This is the reason for dividing hazardous locations into two divisions.

5.3.1.1 DIVISION 1

NEC Section 500-5(a) defines Class I Division 1 hazardous locations as those in which:

1. Ignitable concentrations of flammable gases, liquids, or vapors can exist under normal

operating conditions;

2. Ignitable concentrations of such gases or vapors may exist frequently because of repair or

maintenance operations or because of leakage; or

3. Breakdown or faulty operation of equipment or processes might release ignitable concentrations

of flammable gases, liquids, or vapors and might also cause simultaneous failure of electrical

equipment.

Note: In each case, ignitable concentrations are mentioned. This means concentrations between the

lower and upper flammable or explosion limits (see Section 5.3.5 and Table 5-5).

The fine-print note to NEC Section 500-5(a) describes a number of areas and occupancies normally

classified as Class I Division 1 locations.

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 Section 500-5(b) 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

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