Foam Fire Extinguishers, also called AFFF FOAM (Aqueous Film Forming Foam) create a smothering film of foam over the fire, which starves the fire of oxygen.
The foam also penetrates porous materials and cools the fire through evaporation of the water content in the foam. As the foam creates a foam carpet on burning liquids like petrol, foam extinguishers are also suitable for flammable liquids and areas where man-made fibres in soft furnishings and carpets might liquidise under the influence of heat.
Foam extinguishers are safe for use with electrical equipment if they had a successful dielectric test and are used at a safety distance of 1 meter. The electrical equipment will of course be damaged by the liquid.