Fire classification

Fire classification is a system of categorizing fires with regard to the type(s) of combustible material(s) involved, and the form(s) of suitable extinguishing agent(s).^[1] Classes are often assigned letter designations,^[2] which can differ somewhat between territories.

• International (ISO): ISO3941 Classification of fires
• Australia: AS/NZS 1850
• Europe: DIN EN2 Classification of fires
• United States: NFPA 10 Chapter 5.2.1-5.2.5

Fires involving ordinary flammable solids fall under Class A. This includes wood, paper, fabric, rubber, and some types of plastics. Such fires may be extinguished by water, wet chemical suppression, or dry chemical powder.^[3][4]

Fires involving flammable liquids or liquefiable solids fall under Class B^{[note 1]}. Examples may include petrol/gasoline, oil, paint, some waxes & plastics, though cooking fats and oils are explicitly excluded (discussed and categorised separately below).

A solid stream of water should never be used to extinguish this type of fire because it can cause the fuel to scatter, spreading the flames.^[6] The most effective way to extinguish a liquid fire is by inhibiting the chemical chain reaction of the fire, which can be done by dry chemical or Halon extinguishing agents.^[7] Smothering with CO₂ or foam is also effective. Halon has fallen out of favor in recent times (except for aircraft fire extinguishing systems) because it is an ozone-depleting material (the Montreal Protocol declares that Halon should no longer be used). Chemicals such as FM-200 are now the recommended halogenated suppressant.^[8]

Fires involving flammable gases fall under Class C in the European/Australian system, and Class B (along with flammable liquids) in the US system.^[9][7][10] This can include natural gas, hydrogen, propane, and butane.

Due to the nature of the fuel, these fires can be difficult to extinguish. The most effective techniques for the control of a flammable gas fire are to stop the flow of fuel (by turning off any gas taps or valves) or to displace the supply of oxygen.^[4] Control of fires involving flammable gases where the gas source cannot be controlled must be carefully managed. If the flames are extinguished, but the gas continues to leak, an explosive atmosphere may be created, and the gas may find a source for reignition outside of the originally affected area. Strategies employed to manage these fires may include trying to direct or contain the fire to prevent the ignition of other fuels whilst work is done to control the fuel supply.^[1]

Fires involving combustible metals fall under Class D. This especially concerns alkali metals like lithium, potassium and sodium, alkaline earth metals such as magnesium, and group 4 elements such as titanium and zirconium.^[3]

Metal fires represent a unique hazard because people are often unaware of the characteristics of these fires and are not properly prepared to fight them.^[3] It is also not always clear what type of metal is burning. Certain metals catch fire in contact with air or water (for example, sodium), which exacerbates this risk.^[10] Monolithic masses of combustible metals do not usually represent great fire risks because heat is conducted away from hot spots so efficiently that the heat of combustion cannot be maintained. In consequence, significant heat energy is required to ignite a contiguous mass of combustible metal. Generally, metal fires are a hazard when the metal is in the form of swarf which combusts more rapidly than larger blocks due to the increased surface-area-to-volume ratio.^[7][10] Metal fires can be ignited by the same ignition sources that would start other common fires.^[7]

Special care must be taken when extinguishing metal fires. Water and other common firefighting agents can exacerbate them.^[7] The National Fire Protection Association recommends that metal fires be fought with dry powder extinguishing agents that work by smothering and heat absorption.^[7] Different metals require different extinguishing agents and for a particular metal, agents cannot necessarily be substituted for one another. The most common agents are sodium chloride granules and graphite powder.^[10] In recent years, powdered copper has also come into use.^[10] These dry powder extinguishers should not be confused with those that contain dry chemical agents. The two are not the same, and only dry powder should be used to extinguish a metal fire. Using a dry chemical extinguisher in error, in place of dry powder, can be ineffective or actually increase the intensity of a metal fire.^[7] Lacking a specialist extinguisher, sand may in some cases provide a sufficient alternative, though notably sand may contain moisture which could result in a rapid, hazardous release of steam.^[11]

Fires involving the danger of electrical hazards fall under Class E under the Australian system, and Class C under the US system.^[4] The European system previously assigned this Class E, but no longer has a specific classification for electrical fires, instead focussing on the ignited fuel type.

Electrical fires can result from the likes of faulty electrical equipment or wiring^[7] (for instance loose or corroded connections; deteriorated insulation; electrical arcing), overheating electrical components (possibly due to improper system or product design), or unintentional contact of electrical or electronic components with flammable materials. These fires can be a severe hazard to firefighters using water or other conductive agents, which upon contact with electrical sources can establish a conductive path through the firefighter to Earth. Such electric shocks have caused many firefighter deaths.^[7]

Electrical fires may be fought in the same way as an ordinary combustible fire, but water, foam, and other conductive agents should not be used due to the dangers just described. Instead an extinguishing agent specifically rated for electrical fires should be used. Carbon dioxide (CO₂), NOVEC 1230, FM-200, dry chemical powder extinguishers, and even baking soda are especially suited to extinguishing this sort of fire.^[6] PKP should be a last resort solution due to its corrosive tendencies.

Once electricity is shut off to the equipment involved, it will generally become an ordinary combustible fire.^[6]

Fires involving cooking oils and fats (greases) fall under Class F under the European and Australian systems, and Class K under the US system.^[2][4][9] Though such fires are technically a subclass of the flammable liquid/gas class(es), the special characteristics of these types of fires, namely the higher flash point, are considered important enough to recognize them separately.

Some special extinguishers designed for this use smother the fire by turning the oil into a foam. As with Class B fires, a solid stream of water should never be used to extinguish this type because it can cause the fuel to scatter, spreading the flames.^[7] Commercial kitchens may have a fire suppression system built into exhaust hoods to release an extinguishing agent onto stove tops, releasing automatically with a fusible link or manually using a pull station.^[9]

If a fire blanket is used to extinguish burning liquids the liquid can soak into the blanket. A hot liquid might then lead to the self-ignition of the blanket.^[4] A blanket must therefore only be used if it does not act as a wick or if it is heavy enough to cool the liquid down below autoignition temperature. Preferably a lid or a similar solid item should be used to seal the burning container off from the surrounding atmosphere.^[5]

Comparison of letter codes in different regional standards.

• Fire control
• Fire extinguisher

• Classification of Portable Fire Extinguishers, Occupational Safety and Health Administration
• Evacuation Plans and Procedures eTool, Occupational Safety and Health Administration
• Information on Fire Extinguishers, The Fire Safety Advice Centre
• Australian Fire Classes, Futura Fire – Australia
• For Fire Extinguishers, Wollongong Extinguisher Service-Australia