Perimeter Solutions Phos-Chek WD881A Class A Foam 20 Litre Pail. PHOS-CHEK WD881A is a foam-forming water additive designed for use on Class A fires – those defined by the National Fire Protection Association (NFPA) as fires in ordinary combustible materials such as wood, cloth, paper, rubber, and many plastics. It is composed of a mixture of surfactants and other performance components dissolved in a water miscible system. When the concentrate is diluted with water, it produces an excellent “wet water” which then forms a stable foam when mixed with air.