All fires have the potential to get out of control and potentially cause property damage or personal injury. Not all fires, however, work the same way. Differences in the source or nature of a fire can greatly affect the fire suppression technique firefighters employ. To better respond to and suppress fires, they are often divided into classes. In the United States, fires are divided into five classes: A, B, C, D, K. These classes are defined in terms of the source of the fire, and each class requires its own particular techniques.
Class A fires are fueled by "ordinary combustibles," in other words everyday substances like wood, plastic, rubber, and cloth. Class A fires are the most common type and can be suppressed by removing the fire's heat, oxygen, or fuel. Water is effective for suppressing this type of fire, as are most chemical extinguishers. Smaller fires of this type can be smothered easily, for example by pouring dirt onto a campfire.
Class B fires are fueled by flammable liquids and gases. Examples of such fuels include liquids like gasoline and gases like natural gas. These fires should never be extinguished with a spray of water, because this can cause the fuel to spread rather than smothering it. The most effective way of stopping a Class B fire is by shutting down the chemical reaction driving it. This is usually accomplished with dry chemical and halon fire extinguishers, among other means.
Electrical fires fall under Class C. These fires are generally the result of electrical malfunctions like overloaded cables or short circuits. For firefighters, electrical fires are among the most dangerous. If a firefighter attempts to suppress a fire with water or other conductive agents, without realizing the fire is electrical, she can be electrocuted (sometimes fatally) through the stream of water. These fires are made substantially less dangerous by shutting off all electrical current flowing into the system. Agents like halon fire retardants, CO2, and certain dry chemical extinguishers are rated for suppressing Class C fires.
Some metals are combustible, and these fires fall under Class D. Examples of flammable metals include sodium, magnesium, lithium, and sometimes even steel. Titanium and magnesium fires are particularly common in comparison with the others. Metal fires are generally less common than many other types, but they can burn extraordinarily intensely.
Most fires which use metals as fuel are very difficult to initiate, because enormous amounts of heat are required to start them combusting. There are a few noteworthy exceptions to this tendency: most of the sodium and calcium we encounter day-to-day are thankfully bound in compounds like table salt. But the pure metals react violently when exposed to water or (for certain metals) the air.
To suppress a Class D metal fire, water and many other common agents are ineffective. Water, especially with substances like metallic calcium, can intensify metal fires. The best way to extinguish these fires is usually by smothering them with nonreactive 'dry powder' extinguishers. These are different from dry chemical extinguishers, which are usually either ineffective or can make the fire worse. Dry powder extinguishers use substances like graphite powder or sodium chloride (table salt) granules.
The last class of fires is Class K, or cooking oil fires. Cooking oil fires are technically a subset of liquid fuel fires (Class B). However, these deserve their own category because they are very common and have unique characteristics. These tend to burn hotter than many other liquid fuel fires, making many Class B dry chemical fire extinguishers ineffective. Instead, wet chemical extinguishers are recommended. These use strong base chemicals to convert the cooking oils into soap-like compounds which are not flammable, and in the process cool the fire down.
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