In firefighting, fires are organized into several fire classes that describe what kind of fuel or heat source it has, and by extension what methods will be necessary to contain it or put it out.

This article primarily deals with the United States system of classifying fires. See the section below for the European and Australasian classifications, and note the table to the right which indicates the relationship between the two sets.

Fire classifications

In order to facilitate consistent extinguishment approaches, and maximize occupant and fire fighter safety, fires are classified using code letters in many countries. Below is a table showing the standard operated in Europe and Australasia against the system used in the United States.

Flammable gas Packaging label

Class-A fires

A campfire is an example of a class-A fire.

Class-A fires are the most common type of fire, that occurs when a material such as wood becomes sufficiently hot, and has oxygen available to it, causing combustion. (See fire tetrahedron) At this point the material bursts into flame, and will continue burning as long as the fire tetrahedron (heat, fuel, oxygen, and chemical reaction) continues to be available to it.

Class-A fires are used all around buildings and everywhere in the world in controlled circumstances, such as a campfire, lighter, match, or candle. This makes an example easy to come by. For example, a campfire has a fire triangle - the heat is provided by another fire (such as a match or lighter), the fuel is the wood, and the oxygen is naturally available in the open-air environment of a forest. This fire is not dangerous, because the fire is contained to the wood alone and is usually isolated from the ground by rocks. However, when a class-A fire burns in an environment where fuel and oxygen are in accessible positions, the fire can quickly grow out of control; this is the case where firefighting and fire control techniques are required.

Class-A fires are fairly simple to fight and contain - by simply removing the heat or oxygen (or in some cases fuel), the fire triangle collapses and the fire dies out. The most common way to do this is by removing heat and oxygen by spraying the burning material with water. Other means of control or containment would be to "smother" the fire with carbon dioxide or nitrogen from a fire extinguisher, cutting off its oxygen and causing the fire to die.

Class-A fires are the most commonly encountered fires, and as such most fire departments have equipment to handle them specifically. While this is acceptable for most ordinary conditions, most firefighters find themselves having to call for special equipment such as foam in the case of other fires.

Class-B fires

A firefighting plane drops a chemical retardant.

Class-B fires involve flammable or combustible fuels. These fires follow the same basic fire triangle (heat, fuel, and oxygen) as class-A fires, except that the fuel in question is a flammable liquid such as gasoline. A solid stream of water should never be used to extinguish this type because it can cause the fuel to scatter, spreading the flames. The most effective way to extinguish a class B fire is by inhibiting the chain reaction of the fire, which is done by dry chemical and halon extinguishing agents, although smothering with CO2 or foam is also effective. Some newer clean agents designed to replace halon work by cooling the liquid below its flash point, but these have limited class B effectiveness.

A CO2 fire extinguisher rated for class B, and C fires.

Class-C fires

Class-C fires are electrical fires, where the heat side of the fire triangle is caused by, for example, short-circuiting machinery or overloaded electrical outlets. These fires can be a severe hazard to firefighters using water: when the solid stream of water hits the electrical fire, the electricity is conducted through it and into the hose, then into the firefighter's body. Electrical shocks have caused many firefighter deaths.

There are two main ways of fighting a class-C fire: cutting off its oxygen, or simply turning off the electricity to the fire from a breaker. A class-C fire can be put out with a fire extinguisher rated for class-C fires, or with protein foam, but the primary approach is to simply turn off the power as noted above. This causes the fire to become an ordinary class-A fire, or perhaps to die out entirely. The agents used to extinguish fire of class-c are sodium bicarbonate, carbon dioxide, halons, potassium bicarbonate and PKP.

Class-D fires

Class-D fires are metal fires. Certain metals, such as sodium, titanium, magnesium, potassium, uranium, lithium, plutonium, calcium and others are flammable. Magnesium and titanium fires are common. When one of these combustible metals ignites, it can easily and rapidly spread to surrounding class-A materials.

Generally, masses of combustible metals do not represent unusual fire risks because they have the ability to conduct heat away from hot spots so efficiently that the heat of combustion cannot be maintained - this means that it will require a lot of heat to ignite a mass of combustible metal. Generally, metal fire risks exist when sawdust, machine shavings and other metal 'fines' are present. Generally, these fines can be ignited by the same types of ignition sources that would start other common fires.

Water and other common firefighting materials can excite metal fires and make them worse. The NFPA recommends that class D fires be fought with 'dry powder' extinguishing agents. Dry Powder agents work by smothering and heat absorption. The most common of these agents are sodium chloride granules and graphite powder. In recent years powdered copper has also come into use.

Some extinguishers use dry chemical extinguishing agents. This is easily confusable with dry powder. They are quite different, and using one of these extinguishers in error in place of dry powder can actually increase the size of a class D fire much like water.

Class-D fires represent a unique hazard because people are often not aware of the characteristics of these fires and are not properly prepared to fight them. Therefore, even a small class-D fire can spread class-A fires to the surrounding combustible materials. Most fire stations do not have class-D extinguishing agents available to them, making fighting these fires a logistical problem - however, in most places where these materials are found there is a hopper filled with the proper extinguishing agent.

Class-K fires

Laboratory simulation of a Class K (Class F in Europe) chip pan fire: a beaker containing wax is heated until it catches fire. A small amount of water is then poured into the beaker. The water sinks to the bottom and vaporises instantly, ejecting a plume of burning liquid wax into the air.

Class-K fires are fires that involve cooking oils.

Though by definition, Class-K is a subclass of Class-B, the special characteristics of these types of fires are considered important enough to recognize. Saponification can be used to extinguish such fires.

Appropriate fire extinguishers may also have hoods over them that help extinguish the fire.

European and Australasian classifications

In Europe and Australasia, a different classification system is used.

• Class A: Fires that involve flammable solids such as wood, cloth, rubber, paper, and some types of plastics.
• Class B: Fires that involve flammable liquids or liquifiable solids such as petrol/gasoline, oil, paint, some waxes & plastics, but not cooking fats or oils.
• Class C: Fires that involve flammable gases, such as natural gas, hydrogen, propane, butane.
• Class D: Fires that involve combustible metals, such as sodium, magnesium, and potassium.
• Class E: Fires that involve any of the materials found in Class A and B fires, but with the introduction of an electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire, with a resultant electrical shock risk if a conductive agent is used to control the fire.
• Class F: Fires involving cooking fats and oils. The high temperature of the oils when on fire far exceeds that of other flammable liquids making normal extinguishing agents ineffective.

The system is more or less the same as the U.S system, with letter designations shifted around - for instance, Class C fires in the U.S system are known as Class E in Europe.

Class-A fires

Fire control consists of depriving a fire of fuel, oxygen or heat (see fire triangle) to prevent it from spreading or to put it out entirely. The standard and most common way to control a class-A fire (the combustion of a flammable material with oxygen and heat) is to remove heat by spraying the burning solid fuels with water from a fire-hose connected to a pump. Other methods of controlling a class-A fire would be to "smother" the fire with carbon dioxide, such as from a fire extinguisher, cutting off its oxygen. In a forest fire, fire control would usually consist of removing fuel in the fire's path and digging trenches—this prevents the fire from gaining new fuel and spreading.

Class-B fires

Class-B fires (hydrocarbons and fuels on fire) require much different handling than the standard water approach. Many fuels, such as gasoline or oil float on water, and water would actually end up spreading the fire further. Other fuels, such as coal, will not be put out by water, as fire spreads to the inside of the coal and cannot be reached by water—as soon as the water stops, the fire inside of the coal spreads back out to the outside. Fire control of these fires requires specialized methods, and can be problematic to ordinary fire stations because these materials may not always be available.

One way to control a class-B fire would be to dump chemical dust on it—this is also a method for handling class-A fires, and actually tends to be preferable because sprayed water tends to cause property damage. Gasoline fires are more often smothered in a cooling protein foam.

Class-C fires

Class-C fires are electrical fires—fires that are caused by an electrical source and get their heat from electricity. These fires are dangerous because if water is used on them, electrical current will be passed through the stream and back into the firefighter. There are only two ways to deal with this type of fire—take away the oxygen (smother it with foam or a fire extinguisher) or simply turn off the electricity, which will cause the fire to either die out or become a regular class-A fire.

Ventilation

Most fires spread as hot gases move through the structure. Some fires can be controlled or limited by venting these gases to the outside either horizontally through windows and doors or vertically through existing openings or by cutting holes in the structure's roof. This can aggravate a fire if it introduces new oxygen, or permits a draft past fuel or structure, making professional training in how to perform ventilation required to do it safely—a backdraft (the rapid combustion of a room, nearly akin to an explosion) may result if done incorrectly, or at the very least cause the fire to spread further.

See also

• Fire classes
• Fire sand bucket
• Glossary of Terms Used in Forest Fire Control (U.S. Forest Service publication)

See also

• Fire extinguisher