A Class A fire is the most common type of fire. This fire is fueled by common materials such as wood, paper, rubber, plastics or cloth. This type of fire leaves an ash behind after burning and has an ember flame. To extinguish this type of fire, the temperature has to be brought down using water or a general-use fire extinguisher.
A Class B fire is fueled by flammable and combustible liquids. A flammable liquid must be burning at room temperature in order to be a Class B fire, and a combustible liquid must be heated to its flammable point to be classified as Class B. Some flammable and combustible liquids included in this classification are petroleum, tars, oil-based paints, oil, alcohols, lacquers, solvents and flammable gases. Water may not always extinguish these types of fires, and it is best to use a layer of foam from a Class B fire extinguisher.
A Class C fire is a fire that can be classified as an A or B fire, but that involves electrical equipment as well. These types of fires cannot be extinguished with water or Class A/B fire extinguishers. Carbon dioxide or a dry chemical agent fire extinguisher must be used on this type of fire.
A Class D fire is characterized by combustible metals. These metals can include potassium, magnesium, titanium or lithium. Automobiles contain most of these types of combustible metals. A Class D fire burns at an exceptionally high temperature, and when water is used on these types of fires it can actually break down into hydrogen and oxygen and fuel the fire further. Special powders such as sodium chloride, dry sand or salt can extinguish this type of fire.
Class K is the newest NFPA classification of fire. This classification includes appliances involved in cooking that ignite using a combustible medium such as cooking oil, other fats and alcohols.