Which action best describes the Class D extinguishing agents' mechanism?

Which action best describes the Class D extinguishing agents' mechanism?

• A. They create a water film
• B. They displace oxygen
• C. They absorb heat rapidly
• D. They form a crust over the fuel

Answer: (D)

Class D extinguishing agents suppress metal fires by forming a crust over the fuel, isolating it from air and stopping the oxidation reaction that keeps the fire going. The dry powder coats the molten metal, creating a barrier that prevents oxygen from reaching the surface. While some cooling can occur as the powder absorbs heat, the defining action is the protective crust that smothers the fire. Water-based methods aren’t suitable for metal fires because many metals react violently with water, and displacing oxygen isn’t how these fires are effectively controlled.