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Hazardous Location Basics


HAZARDOUS LOCATION: An Area where the possibility of
explosion and fire is created by the presence of flammable gases,
vapors, dusts, fibers or flying.


CLASS I (NEC-500-4): Those areas in which flammable
gases or vapors may be present in the air in sufficient quantities
to be explosive or ignitable.

CLASS II (NEC-500-4): Those areas made hazardous by the
presence of combustible dust.

CLASS III (NEC-500-6): Those areas in which there are
easily ignitable fibers or flying present, due to type of material
being handled, stored, or processed.


DIVISION 1 (NEC-500,4,5,6): Division One in the normal
situation, the hazard would be expected to be present in everyday
production operations or during frequent repair and maintenance

DIVISION 2 (NEC-500,4,5,6): Division Two in the abnormal
situation, 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.


GROUPS (NEC-500-2 & 502-1): The gases of vapors of
Class I locations are broken into four groups by the code.
A, B, C, and D. Theses materials are grouped according to the
ignition temperature of the substance, its explosion pressure and
other flammable characteristics.

CLASS II: dust locations – groups E, F, and G. These
groups are classified according to the ignition temperature and the
conductivity of the hazardous substance.


SEALS (NEC-501-5 & 502-5): Special fittings that are
required either to prevent the passage of hot gasses in the case of
an explosion in a Class I area of the passage of combustible dust,
fibers, or flyings in a Class II or III area.

ARTICLES 500 Through 503 (1978 NEC): Explain in detail
the requirements for the installation of wiring of electrical
equipment in hazardous locations. These articles along with other
applicable regulations, local governing inspection authorities,
insurance representatives, and qualified engineering/technical
assistance should be your guides to the installation of wiring or
electrical equipment in any hazardous or potentially hazardous

Typical Class I Locations:

  • Petroleum refineries, and gasoline storage and dispensing
  • Industrial firms that use flammable liquids in dip tanks for
    parts cleaning or other operations.
  • Petrochemical companies that manufacture chemicals from gas and
  • Dry cleaning plants where vapors from cleaning fluids can be
  • Companies that have spraying areas where they coat products
    with paint or plastics.
  • Aircraft hangars and fuel servicing areas.
  • Utility gas plants, and operations involving storage and
    handling of liquefied petroleum gas or natural gas.

Typical Class II Locations:

  • Grain elevators, flour and feed mills.
  • Plants that manufacture, use, or store magnesium or aluminum
  • Plants that have chemical or metallurgical processes or
    plastics, medicines and fireworks, etc.
  • Producers or starch or candies.
  • Spice-grinding plants, sugar plants and cocoa plants.
  • Coal preparation plants and other carbon-handling or processing

Typical Class III Locations:

  • Textile mills, cotton gins, cotton seed mills, and flax
    processing plants.
  • Any plant that shapes, pulverizes, or cuts wood and creates
    sawdust or flyings.

NOTE: 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.: