Foam Fire Extinguishers are to be used as fire protection against fire types Class A and B. These types of fire hazards typically involve liquids such as petrol, oil, paints and fats. Foam fire extinguishers are not to be used on fires that involve live electrical equipment. Foam fire extinguishers work by forming a layer over the top of the burning substance which stops the fire by restricting access to oxygen. Foam extinguishers are often referred to as Aqueous Film-Forming Foam (AFFF) fire extinguishers.