Question:
After searching through various fire extinguisher websites, I have noticed that, even though many of them explain the definition of AB and C class fires, I haven't been able to find one that explains the meaning of the numbers (ie. 1-A 10-BC). I think that the 10-B rating, for example, corresponds to the number of square feet of burning flammable liquid that can be extinguished, but I'm not sure. I was wondering if anyone could tell me what these numbers mean.

Answer: IFSTA "Essentials of Fire Fighting" 4th Ed., pp 134-135 note: excerpts from the book Class A portable fire extinguishers are rated from 1-A through 40-A. The Class A rating of water extinguishers is primarily based on the amount of extinguishing agent and the duration and range of the discharge used in extinguishing test fires. For a 1-A rating, 1 1/4 gallons (5L) of water are required. A 2-A rating requires 2 1/2 gallons (10L) or twice the 1-A capacity Extinguishers suitable for use on Class B f ires are classified with numerical ratings ranging from 1-B through 640-B The rating is based on the approximate square foot (square meter) area of a flammable liquid fire that a nonexpert operator can extinguish. There are no fire extinguishing capability tests specifically conducted for Class C ratings. Extinguishers for use on Class C f ires receive only the letter rating because Class C fires are essentially Class A or Class B fires involving energized electrical equipment. Test fires for establishing Class D ratings vary with the type of combustible metal being tested. The following factors are considered during each test: - Reactions between the metal and the agent - Toxicity of the agent - Toxicity of the fumes produced and the products of combustion - Time to allow metal to burn out without fire suppression efforts versus time to extingush ... although no numberical rating is given. Class D agents cannot be given a multipurpose rating for use on other classes of fire. MULTIPLE MARKINGS The ratings for each separate class of extinguisher are independent and do not effect each other. ... such as the multipurpose extingusher rated 4-A 20-B:C, can be reviewed. This extinguisher should extinguish a Class A fire that is 4 times larger than a 1-A fire, extinguish approximately 20 times as much Class B fire as a 1-B extinguisher, and extinguish a deep-layer flammable liquid fire of a 20 square-foot area. It is also safe to use on fires involving energized electrical equipment. See NFPA 10 for more information. I hope it has helped.