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 in case of accidental rupture or breakdown of such containers or systems or in case of abnormal equipment operation.

Table 5-5. Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres

Table 5-5. Class I Division 1 and Class I Division 2 summary of selected hazardous atmospheres (continued).

Notes:

1 See NFPA 325 and 497M.

2 See "Handbook of Fire Protection Engineering", Society of Fire Protection Engineers.

3 See NFPA 325.

4 See NEMA 250, "Enclosures for Electrical Equipment."

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.