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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). Classes are often assigned letter designations, which can differ somewhat between territories.
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.
Fires involving flammable liquids or liquefiable solids fall under Class B. 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. 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. 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.
Fires involving flammable gases fall under Class C in the European/Australian system, and Class B (along with flammable liquids) in the US system. 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. 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.
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.
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. 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. 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. Metal fires can be ignited by the same ignition sources that would start other common fires.
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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). Classes are often assigned letter designations, which can differ somewhat between territories.
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.
Fires involving flammable liquids or liquefiable solids fall under Class B. 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. 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. 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.
Fires involving flammable gases fall under Class C in the European/Australian system, and Class B (along with flammable liquids) in the US system. 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. 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.
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.
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. 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. 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. Metal fires can be ignited by the same ignition sources that would start other common fires.