Page 45 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

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30 Automobile mechanical and electrical systems

The fi re triangle or combustion triangle is a simple model for understanding the
ingredients necessary for most fi res ( Fig. 1.42 ). The triangle illustrates that a fi re
requires three elements: heat, fuel and an oxidizing agent (usually oxygen from
the air). The fi re is prevented or extinguished by removing any one of them. A fi re
naturally occurs when the elements are combined in the right mixture.
Without suffi cient heat, a fi re cannot start or continue. Heat can be removed by
the application of a substance that reduces the amount of heat available to the
fi re reaction. This is often water, which requires heat to change from water to
steam. Introducing suffi cient quantities and types of powder or gas in the fl ame
also reduces the amount of heat available for the fi re reaction. Turning off the
electricity in an electrical fi re removes the ignition source.

Figure 1.42 Fire triangle. (Source: Without fuel, a fi re will stop. Fuel can be removed naturally, as where the fi re
Wikimedia) has consumed all the burnable fuel, or manually, by mechanically or chemically
removing the fuel from the fi re. The fi re goes out because a lower concentration
of fuel vapour in the fl ame leads to a decrease in energy release and a lower
Safety fi rst temperature. Removing the fuel therefore decreases the heat.

A fi re is prevented or extinguished Without enough oxygen, a fi re cannot start or continue. With a decreased oxygen
by removing any side of the fi re concentration, the combustion process slows. In most cases, there is plenty of
triangle. air left when the fi re goes out, so this is commonly not a major factor.
The fi re tetrahedron is an addition to the fi re triangle ( Fig. 1.43 ). It adds the
requirement for the presence of the chemical reaction which is the process of
fi re. For example, the suppression effect of a Halon extinguisher is due to its
interference in the fi re chemical inhibition. Note that Halon extinguishers are only
now allowed in certain situations and are illegal for normal use.
Combustion is the chemical reaction that feeds a fi re more heat and allows it
to continue. When the fi re involves burning metals like magnesium (known as
a class D fi re), it becomes even more important to consider the energy release.
The metals react faster with water than with oxygen and thereby more energy
)
is released. Putting water on such a fi re makes it worse. Carbon dioxide (CO 2
extinguishers are ineffective against certain metals such as titanium. Therefore,

inert agents (e.g. dry sand) must be used to break the chain reaction of metallic
combustion. In the same way, as soon as we remove one out of the three
elements of the triangle, the fi re stops.
If a fi re does happen your workplace should have a set procedure so you will
know:
Figure 1.43 Fire tetrahedron. (Source: how the alarm is raised
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Wikimedia)
what the alarm sounds like
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what to do when you hear the alarm
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your escape route from the building
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where to go to assemble
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who is responsible for calling the fi re brigade.
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There are a number of different types of fi re, as shown in Table 1.9 .
If it is safe to do so you should try to put out a small fi re. Extinguishers and a
fi re blanket should be provided. Remember, if you remove one side of the fi re
triangle, the fi re will go out. If you put enough water on a fi re it will cool down
and go out. However, spraying water on an electrical circuit could kill you.
Spraying water on a petroleum fi re could spread it about and make the problem
far worse. This means that a number of different fi re extinguishers are needed.
Internationally there are several accepted classifi cation methods for hand-held
fi re extinguishers. Each classifi cation is useful in fi ghting fi res with a particular