Fire Scales and Occupancy

8/11/2019 Fire Scales and Occupancy

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ORDINARY COMBUSTIBLES

"Ordinary combustible" fires are the most common type of fire, and are designatedClass A under both systems. These occur when a solid, organic material such aswood, cloth, rubber, or some plastics become heated to their ignition point. At thispoint the material undergoes combustion and will continue burning as long as thefour components of the fire tetrahedron (heat, fuel, oxygen, and the sustainingchemical reaction) are available.

This class of fire is commonly used in controlled circumstances, such as a campfire,match or wood-burning stove. To use the campfire as an example, it has a firetetrahedronthe heat is provided by another fire (such as a match or lighter), thefuel is the wood, the oxygen is naturally available in the open-air environment of aforest, and the chemical reaction links the three other facets. This fire is notdangerous, because the fire is contained to the wood alone and is usually isolatedfrom other flammable materials, for example by bare ground and rocks. However,when a class-A fire burns in a less-restricted environment the fire can quickly growout of control and become a wildfire. This is the case when firefighting and firecontrol techniques are required.


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This class of fire is fairly simple to fight and containby simply removing the

heat, oxygen, or fuel, or by suppressing the underlying chemical reaction, thefire tetrahedron collapses and the fire dies out. The most common way to do

this is by removing heat by spraying the burning material with water; oxygen

can be removed by smothering the fire with foam from a fire extinguisher;

forest fires are often fought by removing fuel by backburning; and an

ammonium phosphate dry chemical powder fire extinguisher (but not sodium

bicarbonate or potassium bicarbonate both of which are rated for B-class[2]

fires) breaks the fire's underlying chemical reaction.

As these fires are the most commonly encountered, most fire departments have

equipment to handle them specifically. While this is acceptable for most

ordinary conditions, most firefighters find themselves having to call for special

equipment such as foam in the case of other fire.


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FLAMMABLE LIQUID AND GAS

A CO2fire extinguisher rated for flammable liquids and gasses

These are fires whose fuel is flammable or combustible liquid or gas. The US systemdesignates all such fires "Class B". In the European/Australian system, flammableliquids are designated "Class B", while burning gases are separately designated"Class C". These fires follow the same basic fire tetrahedron (heat, fuel, oxygen,chemical reaction) as ordinary combustible fires, except that the fuel in question isa flammable liquid such as gasoline, or gas such as natural gas. A solid stream ofwater should never be used to extinguish this type because it can cause the fuel toscatter, spreading the flames. The most effective way to extinguish a liquid or gas

fueled fire is by inhibiting the chemical chain reaction of the fire, which is done bydry chemical and Halon extinguishing agents, although smothering with CO2or, forliquids, foam is also effective. Halon has fallen out of favor in recent times becauseit is an ozone-depleting material; the Montreal Protocol declares that Halon shouldno longer be used. Chemicals such as FM-200 are now the recommendedhalogenated suppressant. Some newer clean agents designed to replace halonwork by cooling the liquid below its flash point, but these have limited classB effect.


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ELECTRICAL

Electrical fires are fires involving potentially energized electrical equipment. TheUS system designates these "Class C"; the Australian system designatesthem "Class E". This sort of fire may be caused by short-circuiting machineryor overloaded electrical cables. These fires can be a severe hazard tofirefighters using water or other conductive agents: Electricity may beconducted from the fire, through water, the firefighter's body, andthen earth. Electrical shocks have caused many firefighter deaths.

Electrical fire may be fought in the same way as an ordinary combustible fire, but

water, foam, and other conductive agents are not to be used. While the fire isor possibly could be electrically energized, it can be fought with anyextinguishing agent rated for electrical fire. Carbon dioxide CO2, FM-200 anddry chemical powder extinguishers such as PKP and even baking soda areespecially suited to extinguishing this sort of fire. PKP should be a last resortsolution to extinguishing the fire due to its corrosive tendencies. Onceelectricity is shut off to the equipment involved, it will generally become anordinary combustible fire.


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METALS

Certain metals are flammable or combustible. Fires involving such are designated "Class D"in both systems. Examples of such metals

include sodium, titanium, magnesium,potassium, uranium, lithium, plutonium,and calcium. Magnesium and titanium fires are common. When one of these combustiblemetals ignites, it can easily and rapidly spread to surrounding ordinary combustiblematerials.

With the exception of the metals that burn in contact with air or water (for example, sodium),masses of combustible metals do not represent unusual fire risks because they have theability to conduct heat away from hot spots so efficiently that the heat of combustioncannot be maintainedthis means that it will require a lot of heat to ignite a mass of

combustible metal. Generally, metal fire risks exist when sawdust, machine shavings andother metal 'fines' are present. Generally, these fires can be ignited by the same types ofignition sources that would start other common fires.

Water and other common firefighting materials can excite metal fires and make them worse.The NFPA recommends that metal fires be fought with "dry powder" extinguishingagents. Dry powder agents work by smothering and heat absorption. The most commonof these agents are sodium chloride granules and graphite powder. In recent yearspowdered copper has also come into use.


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Some extinguishers are labeled as containing dry chemicalextinguishing agents. This maybe confused with dry powder. The two are not the same. Using one of these

extinguishers in error, in place of dry powder, can be ineffective or actually increase theintensity of a metal fire.

Metal fires represent a unique hazard because people are often not aware of thecharacteristics of these fires and are not properly prepared to fight them. Therefore, evena small metal fire can spread and become a larger fire in the surrounding ordinarycombustible materials.

Cooking oils and fats (kitchen fires)

Laboratory simulation of a chip pan fire: a beaker containing wax is heated until it catches

fire. A small amount of water is then poured into the beaker. The water sinks to thebottom and vaporizes instantly, ejecting a plume of burning liquid wax into the air.

Fires that involve cooking oils or fats are designated "Class K" under the American system,and "Class F" under the European/Australasian systems. Though such fires aretechnically a subclass of the flammable liquid/gas class, the special characteristics ofthese types of fires are considered important enough to recognizeseparately. Saponification can be used to extinguish such fires. Appropriate fireextinguishers may also have hoods over them that help extinguish the fire


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TYPES OF OCCUPANCY DEPENDING ON THE PURPOSE


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Fire Scales and Occupancy