The temperature at which a fuel will continue
auto-ignition point is the temperature at which
to burn after it has been ignited is known by
point is usually at a much higher temperature than the
fire point. The range between the smallest and the
largest amounts of vapor in a given quantity of air that
will burn or explode when ignited is called the
FLAMMABLE RANGE or the EXPLOSIVE RANGE.
For example, let's say that a substance has a flammable
or explosive range of 1 to 12 percent. This means that
either a fire or an explosion can occur if the atmosphere
contains more than 1 percent but less than 12 percent of
the vapor of this substance. In general, the percentages
referred to in connection with flammable or explosive
L e a rn i n g O b j e c t ive : R e c a l l t h e d i ff e r e n t
ranges are percentages by volume.
classifications of fires.
Fires are classified according to the nature of the
Fuels take on a wide variety of characteristics. A
combustibles (or fuels) involved, as shown in table 4-1.
fuel may be a solid, liquid, or even a vapor. Some of the
The classification of any particular fire is of great
fuels you will come into contact with are rags, paper,
importance since it determines the manner in which the
wood, oil, paint, solvents, and magnesium metals. This
fire must be extinguished. Fires are classified as being
is by no means a complete list, but only examples.
either class ALPHA, class BRAVO, class CHARLIE,
or class DELTA fires as follows:
· Class ALPHA (A) fires are those that occur in
The oxygen side of the fire triangle refers to the
oxygen content of the surrounding air. Ordinarily, a
such ordinary combustible materials as wood, cloth,
minimum concentration of 15 percent oxygen in the air
paper, upholstery, and similar materials. Class A fires
are usually extinguished with water, using high or low
s m o l d e r i n g c o m bu s t i o n c a n t a ke p l a c e i n a n
velocity fog or solid streams. Class A fires leave embers
atmosphere with as little as 3 percent oxygen. Air
or ashes and must always be overhauled.
normally contains about 21 percent oxygen, 78 percent
· Class BRAVO (B) fires are those that occur in the
nitrogen, and 1 percent other gases, principally argon.
vapor air mixture over the surface of flammable liquids,
such as gasoline, jet fuels, diesel oil, fuel oil, paints,
thinners, solvents, lubricating oils, and greases.
Aqueous film-forming foam (AFFF), Halon 1211,
Halon 1301, or dry chemical Purple-K-Powder (PKP)
can be used to extinguish class B fires. The agent used
A rapid chemical reaction that releases energy
will depend upon the circumstances of the fire.
in the form of light and noticeable heat is
· Class CHARLIE (C) fires are those which occur
known as combustion.
in electrical equipment. Nonconducting extinguishing
agents, such as PKP, Carbon dioxide, and Halon 1211,
are used to extinguish class C fires. CO2 and Halon
1211 are preferred because they leave no residue.
The lowest temperature at which a flammable
· Class DELTA (D) fires occur in combustible
substance gives off vapors that will burn when
a flame or spark is applied is known by what
metals, such as magnesium, titanium, and sodium.
Special techniques have been developed to control this
type of fire. If possible, you should jettison the burning
material overboard. Most class D fires are fought by
applying large amounts of water on the burning material
to cool it down below its ignition temperature. However,
a magnesium fire can be smothered by covering it with a
large volume of dry sand.