Hazardous Classes and Zones - Standard Definitions

A standard titled ANSI/ISA-12.01.01-1999, Definitions and Information Pertaining to Electrical Apparatus in I Hazardous (Classified) Locations, defines terminology to help those involved with the design, manufacture, installation, and maintenance of apparatus used in hazardous (classified) locations. It is also intended to promote uniformity. The standard uses definitions and explanations as published by several accepted standards organizations.

For hazardous location apparatus, atmospheric conditions are generally considered to be:

1. an ambient temperature range of ?20 C ( ?4 F) to 40 C (104 F);

2. an oxygen concentration of not greater than 21% by volume;

3. a pressure of 86 kPa (12.5 psia) to 108 kPa (15.7 psia); and

4. a relative humidity of 5% to 95%.

Class, Division and Zone Definitions

Class I location: a location 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 location: a location (1) in which ignitable concentrations of flammable gases or vapors can exist under normal operating conditions; (2) in which ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage; or (3) in which breakdown or faulty operation of equipment or processes might release ignitable concentrations of flammable gases or vapors and might also cause simultaneous failure of electrical equipment that could act as a source of ignition.

Class I, Division 2 location: a location (1) in which volatile flammable...