Extinguisher Ratings

Extinguisher ratings are used to determine which classes of fire can be controlled effectively and safely with the extinguisher being tested. Ratings also provide a guide to the size of fire an extinguisher will control. All extinguishers receive a class rating. Extinguishers for class A fires are given a numerical rating from 1A to 40A which is based on a relative scale. A 4A extinguisher, for example, will control a fire approximately twice as large as a 2A extinguisher. Class B extinguishers are rated numerically from 1B to 640B based on the approximate square footage of a spill fire they can control. A 10B extinguisher will extinguish a spill fire of approximately 10 square feet (0.9 square meters). Fire tests are conducted using wood and excelsior for class A extinguishers. Class B extinguishers are tested on n-heptane fires in square pans using a liquid depth of two inches. Extinguishers for class C fires do not undergo fire tests but are tested for nonconductivity of electricity. Special tests using the specific combustible metals are employed for class D extinguishers.

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