National Electric Code
Hazardous (Classified) Location Types
Class I Locations
According to the NEC, there are three types of hazardous locations. The first type of hazard is one which is created by the presence of flammable gases or vapors in the air, such as natural gas or gasoline vapor. When these materials are found in the atmosphere, a potential for explosion exists, which could be ignited if an electrical or other source of ignition is present. The Code writers have referred to this first type of hazard as Class I. So, a Class I Hazardous Location is one in which flammable gases or vapors may be present in the air in sufficient quantities to be explosive or ignitable. Some typical Class I locations are:
- Petroleum refineries, and gasoline storage and dispensing areas;
- Dry cleaning plants where vapors from cleaning fluids can be present;
- Spray finishing areas;
- Aircraft hangars and fuel servicing areas; and
- Utility gas plants and operations involving storage and handling of liquefied petroleum gas or natural gas.
- All of these are Class I . . . gas or vapor . . . hazardous locations. All require special Class I hazardous location equipment.
Class II Locations
The second type of hazard listed by the National Electrical Code are those areas made hazardous by the presence of combustible dust. These are referred to in the Code as “Class II Locations.” Finely pulverized material, suspended in the atmosphere, can cause as powerful an explosion as one occurring at a petroleum refinery. Some typical Class II locations are:
- Grain elevators;
- Flour and feed mills;
- Plants that manufacture, use or store magnesium or aluminum powders;
- Producers of plastics, medicines and fireworks;
- Producers of starch or candies;
- Spice-grinding plants, sugar plants and cocoa plants; and
- Coal preparation plants and other carbon handling or processing areas.
Class III Locations
Class III hazardous locations, according to the NEC, are areas where there are easily-ignitable fibers or flyings present, due to the types of materials being handled, stored, or processed. The fibers and flyings are not likely to be suspended in the air, but can collect around machinery or on lighting fixtures and where heat, a spark or hot metal can ignite them. Some typical Class III locations are:
- Textile mills, cotton gins;
- Cotton seed mills, flax processing plants; and
- Plants that shape, pulverize or cut wood and create sawdust or flyings.
Hazardous Location Conditions
In addition to the types of hazardous locations, the National Electrical Code also concerns itself with the kinds of conditions under which these hazards are present. The Code specifies that hazardous material may exist in several different kinds of conditions which, for simplicity, can be described as, first, normal conditions, and, second, abnormal conditions.
In the normal condition, the hazard would be expected to be present in everyday production operations or during frequent repair and maintenance activity.
When the hazardous material is expected to be confined within closed containers or closed systems and will be present only through accidental rupture, breakage or unusual faulty operation, the situation could be called “abnormal.”
The Code writers have designated these two kinds of conditions very simply, as Division 1 – normal and Division 2 – abnormal. Class I, Class II and Class III hazardous locations can be either Division 1 or Division 2.
Good examples of Class I, Division 1 locations would be the areas near open dome loading facilities or adjacent to relief valves in a petroleum refinery, because the hazardous material would be present during normal plant operations.
Closed storage drums containing flammable liquids in an inside storage room would not normally allow the hazardous vapors to escape into the atmosphere. But, what happens if one of the containers is leaking? You’ve got a Division 2 -abnormal – condition . . . a Class I, Division 2 hazardous location.