Hazardous Location Information
Hazardous locations are those areas .where a potential for explosion and fire exist because of flammable gases, vapors or finely pulverized dusts in the atmosphere, or because of the presence of easily ignitable fibers or flyings. Hazardous locations may result from the normal processing of certain volatile chemicals, gases, grains, etc., or they may result from accidental failure of storage systems for these materials. It is also possible that a hazardous location may be created when volatile solvents or fluids, used in a normal maintenance routine, vaporize to form an explosive atmosphere.
Regardless of the cause of a hazardous location, it is necessary that every precaution be take to guard against ignition of the atmosphere. Certainly no open flames would be permitted in these locations, but what about other sources of ignition?
Electrical Sources of Ignition
A source of ignition is simply the energy required to touch off an explosion in a hazardous location atmosphere. Electrical equipment such as lighting fixtures and motors are classified as "heat producing," and they will become a source of ignition if they reach a surface temperature which exceeds the ignition temperature of the particular gas, vapor or dust in the atmosphere.
It is also possible that an abnormality or failure in an electrical system could provide a source of ignition. A loose termination in a splice box or a loose lamp in a socket can be the source of both arcing and heat. The failure of insulation from cuts, nicks or aging can also act as an ignition
source from sparking, arcing and heat.
Hazardous Locations and the National Electrical Code*
The National Electrical Code treats installations in hazardous locations in articles 500 through 517.
Each hazardous location can be classified by the definitions in the NEC. Following are interpretations of these classifications and applications.
CLASS I LOCATIONS
Class I locations are those 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
These are Class I locations where the hazardous atmosphere is expected to be present during normal operations. It may be present continuously, intermittently, periodically or during normal repair or maintenance operations. Division 1 locations are also those locations where a breakdown in the operation of processing equipment results in the release of hazardous vapors and the simultaneous failure of electrical equipment.
Class I, Division 2
These are Class I locations in which volatile flammable liquids or gases are handled, processed or used, but in which they will normally be confined within closed containers or closed systems from which they can escape only in the case of accidental rupture or breakdown of the containers or systems. The hazardous conditions will occur only under abnormal conditions.
CLASS II LOCATIONS
Class II locations are those that are hazardous because of the presence of combustible dust.
Class II, Division 1
These are Class II locations where combustible dust may be in suspension in the air under normal conditions in sufficient quantities to produce explosive or ignitable mixtures. This may occur continuously, intermittently or periodically. Division 1 locations also exist where failure or mal-
function of machinery or equipment might cause a hazardous location to exist while providing a source of ignition with the simultaneous failure of electrical equipment.
Included also are locations in which combustible dust of an electrically conductive nature may be present.
Class II, Division 2
A Class II, Division 2 location is one in which combustible dust will not normally be in suspension in the air and normal operations will not put the dust in suspension, but where accumulation of the dust may interfere with the safe dissipation of heat from electrical equipment or where accumulations near electrical equipment may be ignited by arcs, sparks or burning material from the equipment.
CLASS III LOCATIONS
Class III locations are those that are hazardous because of the presence of easily ignitable fibers or flyings, but in which the fibers or flying are not likely to be in suspension in the air in quantities sufficient to produce ignitable mixtures.
Class III, Division 1
These are locations in which easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used.
Class III, Division 2
These locations are where easily ignitable fibers are stored or handled.
Hazardous Location Equipment
Class I Location Equipment
Devices for Class I locations are housed in enclosures which are designed to be strong enough to contain an explosion if the hazardous vapors enter the enclosure and are ignited. These enclosures then cool and vent the products of combustion in such a ,vay that the surrounding atmosphere is not ignited.
Heat producing equipment for hazardous locations must also be designed to operate with surface temperatures below the ignition temperatures of the hazardous atmosphere.
Since the different vapors and gases making up hazardous atmospheres have varying properties, they have been placed in groups based on common flame propagation characteristics and explosion pressures. These groups are designated A, B, C, and D, and the equipment selected must be suitable for the group of the specific hazardous gas or vapor, with regard to flame propagation, explosion pressures and operating temperatures.
Reference to the National Electrical Code will indicate that most of the equipment used for Class I Division 2 applications is the same as that used for Division 1 applications.
Class II Location Equipment
The enclosures used to house devices in Class II locations are designed to seal out dust. contact between the hazardous atmosphere and the source of ignition has been eliminated and no explosion can occur within the enclosure.
As in Class I equipment, heat producing equipment must be designed to operate below the ignition temperature of the hazardous atmosphere. However, in Class II equipment, additional consideration must be given to the heat buildup which may result from the layer of dust which will settle on the equipment.
Dusts have also been placed in Groups designated E, F, and G, based o their particular hazardous characteristics and the dusts' electrical resistivity. It is important to select equipment suitable for the specific hazardous group.
Class III Location Equipment
Class III locations require equipment which is designed to prevent the entrance of fibers and flyings, prevent the escape of sparks or burning material and operate at a temperature below the point of combustion.
Hazardous Location Equipment Applications
Hazardous location equipment may be required in any area where the presence of flammable gases, vapors or finely pulverized dusts in the atmosphere is sufficient to create a threat of explosion or fire. It may also be required where easily ignitable fibers or flyings are present. The following is a representative (but hardly complete) list of the types of locations and operations requiring hazardous location equipment in at least certain areas.
Class I Locations
• Petroleum refining facilities
• Dip tanks containing flammable or combustible liquids
• Dry cleaning plants
• Plants manufacturing organic coatings
• Spray finishing areas (residue must be considered)
• Petroleum dispensing areas
• Solvent extraction plants
• Plants manufacturing or using pyroxylin (nitro-cellulose) type and other plastics (Class II also)
• Locations where inhalation anesthetics are used
• Utility gas plants, operations involving storage and handling of liquefied petroleum and natural gas
• Aircraft hangers and fuel servicing areas
Class II Locations
• Grain elevators and bulk handling facilities
• Manufacture and storage of magnesium
• Manufacture and storage of starch
• Fireworks manufacture and storage
• Flour and feed mills
• Areas for packaging and handling of pulverized sugar and cocoa
• Facilities for the manufacture of magnesium and aluminum powder
• Some coal preparation plants and coal handling facilities
• Spice grinding plants
• Confectionery manufacturing plants
Class III Locations
• Wood working plants
• Textile mills
• Cotton gins and cotton seed mills
• Flax producing plants