Which class of fire applies to ordinary combustibles?

• A. Class A
• B. Class B
• C. Class C
• D. Class D

Answer: (A)

Fires involving ordinary combustibles, such as wood, paper, and cloth, are best controlled by cooling the fuel below its ignition temperature. Water absorbs a lot of heat, which slows or stops the chemical reactions happening in the flame. As it boils away, steam also helps shield the fuel from heat and can help smother the flame by lowering the surrounding temperature. This cooling approach is why water-based extinguishers are effective for these fires.

Other fire classes involve different materials and require different tactics. Fires with flammable liquids need agents that block vapors or smother the fire, because water can spread the liquid and make things worse. Fires with energized electrical equipment require nonconductive extinguishing agents and, ideally, the power to the equipment is shut off first. Fires involving combustible metals need specialized powders that react safely with the metal and won’t ignite or react violently with water.