Fire classes

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.

The US uses a different classification system to Europe and Australasia, details of the differences are given below.

US and European Class-A fires


Class-A fires are the most common type of fire. They occur when an organic solid 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 four components of the fire tetrahedron (heat, fuel, oxygen, and the sustaining chemical reaction) continue to be available to it.

Class-A fires are commonly used in controlled circumstances, such as a campfire, match or wood burning stove. To use the campfire as an example, it has a fire tetrahedron - the heat is provided by another fire (such as a match or lighter), the fuel is the wood, the oxygen is naturally available in the open-air environment of a forest, and the chemical reaction links the three other facets. This fire is not dangerous, because the fire is contained to the wood alone and is usually isolated from other flammable materials, for example by bare ground and rocks. However, when a class-A fire burns in a less restricted environment 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, oxygen, or fuel, or by suppressing the underlying chemical reaction, the fire tetrahedron collapses and the fire dies out. The most common way to do this is by removing heat by spraying the burning material with water; oxygen can be removed by smothering the fire with carbon dioxide, nitrogen or foam from a fire extinguisher; forest fires are often fought by removing fuel by backburning; and a Potassium bicarbonate fire extinguisher (along with several other kinds) breaks the fire's underlying chemical reaction.

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.

US Class-B, European class-B and C fires
US Class-B fires involve flammable or combustible liquid or gaseous fuels. In the European/Australasian system class B fires involve flammable liquids, and class C involve burning gases. These fires follow the same basic fire tetrahedron (heat, fuel, oxygen, chemical reaction) as class-A fires, except that the fuel in question is a flammable liquid such as gasoline, or gas such as natural gas. 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 liquid or gas fueled fire is by inhibiting the chemical chain reaction of the fire, which is done by dry chemical and halon extinguishing agents, although smothering with CO2 or, for liquids, 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.

US Class-C, European class-E fires
Class-C/E fires are fires involving, and probably started by, 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: if an unbroken stream of water hits the electrical fire, the electricity may be conducted through it to earth through the firefighter's body. Electrical shocks have caused many firefighter deaths.

Class-C/E fire are fought in the same way as a class A fire, but water and foam are not to be used, in fact when the electricity is shut off to the electrical device(s) involved in the fire, it would generally become a class A fire. While the fire is, or could possibly be electrically energised, it can be fought with any fire extinguisher rated for class C (in the US) or class E (elsewhere). Carbon dioxide and dry chemical powder extinguishers are especially suited to extinguishing this sort of fire.

US and European 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, and the [exploding laptop batteries] are caused by the lithium in their batteries burning in contact with air. When one of these combustible metals ignites, it can easily and rapidly spread to surrounding class-A materials.

With the exception of the metals that burn in contact with air or water, 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 are labled as containing dry chemical extinguishing agents. This can be confused with dry powder. And, the two are not the same. Using one of these extinguishers in error, in place of dry powder, can be ineffective or actually increase the intensity of a class D fire.

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 and become a class-A fire in the surrounding combustible materials.

US Class-K, European class-F fires
Class-K fires are fires that involve cooking oils.

Though by definition, Class-K/F 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.