Fire classes are categorized by fuel or heat source and how they can be extinguished. Different types of fires require specific extinguishing agents, such as water for ordinary combustibles and carbon dioxide for electrical fires. Fire classification systems vary slightly between regions.
Fire classes, or categories, are usually defined by the type of fuel or heat source responsible for igniting the fire, although, in some cases, classification is also made on the basis of how the fire can be extinguished. The classification also varies slightly between regions of the world. Fire classification is to help people fight fires by quickly matching the type of fire to the most appropriate type of extinguisher. The different classes of fire need to be addressed in distinct ways.
There are several sources that can fuel fires. Ordinary combustibles are physical objects, such as wood, rubber, or cloth, that ignite and burn when heated sufficiently. Some types of metal can also be set on fire, but this will normally require special circumstances and more extreme levels of heat. Non-solid fire classes include those involving flammable liquids or gases, electricity, and cooking oils or fats.
Normal combustible fires, which are often started on purpose in confined spaces, such as in a fireplace, can usually be extinguished by using water to cool the heat that is causing the combustion. This class of fire can also be smothered by foam, depriving the fire of the necessary oxygen supply. Other classes of fire may require more specialized extinguishing agents.
An electrical fire is commonly fought with carbon dioxide, as foam or water could act as a dangerous conductor of electricity. Sometimes flammable liquid fires can also be fought with foam, but gas fires usually require the use of carbon dioxide, halon, or dry chemicals to disrupt the chemical reactions causing the fire. Metal fires are usually fought using dry powders such as graphite, sodium chloride or copper powder, while large fires involving cooking oils or fats require a wet chemical extinguisher. Small fires in the kitchen, such as a tiny flame in a frying pan, can usually be stopped by placing a metal lid over the fire, thus limiting the available oxygen.
There is some overlap in the ways different types of fires can be extinguished. Largely because of this, regions of the world have developed subtly distinct fire classification systems. For example, in the American system, liquids and gases are classified in the same class because they are often fought in a similar way. They are classified as two separate classes in both the Australian and European systems. In all systems, the fire classes are identified by a given letter; for example, a “class A fire” classifies the ordinary fuel class in all regions. However, the particular letters assigned to each fire type vary somewhat between regions.
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