Four types (or classes) of fire extinguishers exist, and their uses depend on the type of fire you are trying to put out. Each class of fire extinguisher shows a letter that designates its class and a colored geometric shape. Class A extinguishers display a green triangle, and you use them to put out fires involving ordinary combustibles, such as wood and paper. Class B extinguishers are labeled with a red square, and you use them to put out fires involving flammable liquids, such as gasoline and oil. Class C extinguishers show a blue circle and are suitable for use on electrically energized fires. The last type of fire extinguisher is labeled with a yellow star and the letter D, as well as the specific flammable metal you use the extinguisher with. Classes A and B fire extinguishers also display a numerical rating specific for the size and type of extinguisher.
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Realize that fire extinguisher ratings are determined based on tests that laboratories conduct and that ratings are used to estimate the extinguishing potential for each Class A or B fire extinguisher.
Know that the rating that a Class A extinguisher displays refers to the amount of water that the extinguisher holds and, thus, the size of fire it is able to put out. You find Class A extinguishers with ratings such as 1-A, 2-A, 3-A, 4-A, 6-A, 10-A, 20-A, 30-A and 40-A. The 4-A extinguisher holds 4 gallons of water and is, thus, expected to put out twice as much fire as a 2-A extinguisher holding 2 gallons of water.
Understand that the rating that a Class B extinguisher shows states the approximate number of square feet of a flammable liquid fire that a nonexpert person can expect to extinguish. You find Class B extinguishers with ratings such as 1-B, 2-B, 5-B, 10-B, 20-B, 30-B, 40-B and up to 640-B. A 40-B extinguisher, for example, is effective for putting out a flammable liquid fire covering up to 40 square feet.
Know that Class C and D fire extinguishers, as well as some other class combination fire extinguishers, do not display any ratings and should be used for the specific purposes they are intended for.