Which type of foam cannot be used to suppress flammable or combustible liquid fires?

Class A foam is specifically designed for use on ordinary combustibles such as wood, paper, and textiles, making it ineffective for suppressing flammable or combustible liquid fires. It works by forming a blanket over burning materials, cooling and insulating them, which is suitable for the solid materials of Class A fires but does not provide adequate performance when dealing with liquids that are flammable or combustible.

In contrast, Class B foam is formulated to tackle flammable liquids and works by suppressing vapors and forming a barrier on the surface of the liquid, while Class C foam is used for fires involving energized electrical equipment. Finally, Class D foam is intended for metal fires and also serves a different purpose altogether. Understanding these different foam types and their applications is crucial for effective fire suppression.