What does the "rating" on a fire extinguisher mean?

Underwriters Laboratory approves fire extinguishers upon successful completion of a series of tests. All fire extinguishers are measured against the performance of water as an agent. It’s the reference suppression agent. When dry chemicals came along, they had to prove it was “as good as water”. UL came up with the standardized tests for “rating” fire extinguishers, and you see references to these ratings right on the labels as “1A:10BC”, or a “2A :40BC”. To determine the relative extinguishing effectiveness of hand portable fire extinguishers there are three types of class A fires which must be extinguished. One test consists of a 8' by 8' lattice-like panel of 2" fir strips  positioned upright, sprayed with fuel oil and allowed to burn for about five minutes before the extinguisher is used. It must extinguish the fire in one long “squirt”; the unit must be exhausted before closing the nozzle. The extinguisher that passes this test is given a 1A rating and is said to be the equivalent of one and one-fourth gallons(4.7 L) of water. The 2A has to put out a panel 10´by10', said to be the equivalent of 2 and one-half gallons of water. A 3A put out a 12'by12' panel of 3/4" thick  wooden strips. In addition to the panel test, a “crib” test (it looks like a crate) consists of again stacking 2" wooden strips in alternate layers in a cube design, supported 16" from the floor, a fire is started in a pan underneath. Pre-burn time is 9-10 minutes and the agent must be applied only to the sides not facing the wall away from the operator. Again the size of the crib is determined by the size a 1A could be expected to extinguish- that is one and a fourth gallon of water, or a 2A size crib, and so on. The third type of A fire consists of a “bed” of seasoned wood laid out on the test area floor in a certain amount (pounds) and area. The 1A rated test, for example is six pounds of Excelsior spread out exactly 2' 10" by 5' 8" and a “fuse” of stove and lamp naphtha is laid along the edge and lighted. When the flames reach the centerline of the bed, the extinguisher is applied, again in one long squirt before closing the nozzle. The higher the rating, the more water it would take to put out the fire. The rating on the extinguisher tells all the relative quantity of water which can extinguish the size fire this particular extinguisher can. A dry chemical extinguisher with a 6A rating is equivalent to three 2 1/2 gallon water extinguishers.

Class B fires are tested using pans of heptane in specific sizes (sq. ft.) and quantity (gallons). The number designating the rating represents the square feet of fire area that a non-expert operator can expect to extinguish. It is always 40 percent of what the expert operator at UL must extinguish consistently. An extinguisher with a 40 B:C rating tells all that the expert consistently put out 100 square feet of B type fire with this very fire extinguisher. You may expect to put out 40 square feet of such fire being the novice that you are! (Class C agents are simply non-conductive)

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