Fires are classified based on their primary fuel type, with these classification codes being used in a wide variety of applications including fire extinguishers. Below is a list and description of the more common classes of fires:
"Ordinary combustible" fires are the most common type of fire, and are designated Class A. These occur when a solid, organic material such as wood, cloth, rubber, or some plastics become heated to their flash point and ignite. At this point the material undergoes combustion and will continue burning as long as the four components of the fire tetrahedron (heat, fuel, oxygen, and the sustaining chemical reaction) are available.
Flammable liquids are designated "Class B". These fires follow the same basic fire tetrahedron (heat, fuel, oxygen, chemical reaction) as ordinary combustible fires, except that the fuel in question is a flammable liquid such as petrol. A solid stream of water should never be used to extinguish this type because it can cause the fuel to scatter, spreading the flames.
Electrical fires are fires involving potentially energised electrical equipment. This sort of fire may be caused by, for example, short-circuiting machinery or overloaded electrical cables. These fires can be a severe hazard to firefighters using water or other conductive agents: Electricity may be conducted from the fire, through water, the firefighter's body, and then earth with the resulting electrical shocks potentially causing death.
Fires that involve cooking oils or fats are designated "Class F". Though such fires are technically a subclass of the flammable liquid class, the special characteristics of these types of fires are considered important enough to recognise separately.