Foam Type fire extinguisher is designed to be most effective on class ' B ' fires, although they can also be used in class ' A ' fires. Foam is a powerful flame knockdown for spill fires involving flammable and volatile liquid like petrol, paints and alcohols. These Extinguishers form a thick blanket of foam over the burning surface, effectively cutting off the oxygen supply, thereby smothering the fire and preventing re-ignition.