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Standards for the
selection, placement, and testing of portable fire extinguishers
are issued by the National Fire Protection Association, a
nonprofit technical and educational organization in Quincy,
Massachusetts. The standards establish the minimum requirements
for all types and sizes of extinguishers that are listed and
rated by testing laboratories against standard test fires of the
types they are designed to control. Each extinguisher is rated
as to both type and size of the fire extinguished. For example,
a 20-B extinguisher should extinguish a flammable-liquids fire
that is 20 times the size of a fire that an extinguisher rated
1-B would extinguish. Extinguishers that cannot extinguish the
minimum size test fires are not listed or rated. Some
extinguishers will put out only one class of fire; others are
used for two or even three classes; none is suitable for all
four classes.
Fire
extinguishers may go unused for many years, but they must be
maintained in a state of readiness. For this reason, periodic
inspection and servicing are required, and that responsibility
rests with the owner. Fire department inspectors check at
periodic intervals to see that extinguishers are present where
required by law and that they have been serviced within the
specified time period.
Extinguishers for Class A Fires
Class A fire
extinguishers are usually water based. Water provides a
heat-absorbing (cooling) effect on the burning material to
extinguish the fire. Stored-pressure extinguishers use air under
pressure to expel water. Pump-tank extinguishers are operated by
a hand pump.
Extinguishers for Class B Fires
Class B fires are
put out by excluding air, by slowing down the release of
flammable vapors, or by interrupting the chain reaction of the
combustion. Three types of extinguishing agents—carbon dioxide
gas, dry chemical, and foam—are used for fires involving
flammable liquids, greases, and oils. Carbon dioxide is a
compressed gas agent that prevents combustion by displacing the
oxygen in the air surrounding the fire. The two types of dry
chemical extinguishers include one that contains ordinary sodium
or potassium bicarbonate, urea potassium bicarbonate, and
potassium chloride base agents; the second, multipurpose, type
contains an ammonium phosphate base. The multipurpose
extinguisher can be used on class A, B, and C fires. Most dry
chemical extinguishers use stored pressure to discharge the
agent, and the fire is extinguished mainly by the interruption
of the combustion chain reaction. Foam extinguishers use an
aqueous film forming foam (AFFF) agent that expels a layer of
foam when it is discharged through a nozzle. It acts as a
barrier to exclude oxygen from the fire.
Extinguishers for Class C Fires
The extinguishing
agent in a class C fire extinguisher must be electrically
non-conductive. Both carbon dioxide and dry chemicals can be
used in electrical fires. An advantage of carbon dioxide is that
it leaves no residue after the fire is extinguished. When
electrical equipment is not energized, extinguishers for class A
or B fires may be used.
Extinguishers for Class D Fires
A heat-absorbing
extinguishing medium is needed for fires in combustible metals.
Also, the extinguishing medium must not react with the burning
metal. The extinguishing agents, known as dry powders, cover the
burning metal and provide a smothering blanket.
The Underwriters'
Laboratories, Inc., has available lists of approved fire
extinguishers that may be purchased from different
manufacturers. The extinguisher label gives operating
instructions and identifies the class, or classes, of fire on
which the extinguisher may be used safely. Approved
extinguishers also carry the labels of the laboratories at which
they were tested.
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