In firefighting, fires are identified according to one or more
fire classes. Each class designates the fuel involved in the fire, and thus the most appropriate
extinguishing agentFire control consists of depriving a fire of fuel, oxygen or heat to prevent it from spreading or to put it out entirely.- Class-A fires :...
. The classifications allow selection of extinguishing agents along lines of effectiveness at putting the type of fire out, as well as avoiding unwanted side-effects. For example,
non-conductiveIn science and engineering, an electrical conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminum, the movable charged particles are electrons...
extinguishing agents are rated for electrical fires, so to avoid electrocuting the
firefighterFirefighters, or firemen, are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car accidents, collapsed and burning buildings and other such situations...
.
Multiple classification systems exist, with different designations for the various classes of fire.
The
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
uses the
NFPAThe National Fire Protection Association is a U.S. organization charged with creating and maintaining minimum standards and requirements for fire prevention and suppression activities, training, and equipment, as well as other life-safety codes and standards...
system.
Comparison of fire classes
| American |
European/Australiasian |
Fuel/Heat source |
| Class A |
Class A |
Ordinary combustibles |
| Class B |
Class B |
Flammable liquids |
| Class C |
Flammable gases |
| Class C |
Class E |
Electrical equipment |
| Class D |
Class D |
Combustible metals |
| Class K |
Class F |
Cooking oil or fat |
In firefighting, fires are identified according to one or more
fire classes. Each class designates the fuel involved in the fire, and thus the most appropriate
extinguishing agentFire control consists of depriving a fire of fuel, oxygen or heat to prevent it from spreading or to put it out entirely.- Class-A fires :...
. The classifications allow selection of extinguishing agents along lines of effectiveness at putting the type of fire out, as well as avoiding unwanted side-effects. For example,
non-conductiveIn science and engineering, an electrical conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminum, the movable charged particles are electrons...
extinguishing agents are rated for electrical fires, so to avoid electrocuting the
firefighterFirefighters, or firemen, are rescuers extensively trained primarily to put out hazardous fires that threaten civilian populations and property, to rescue people from car accidents, collapsed and burning buildings and other such situations...
.
Multiple classification systems exist, with different designations for the various classes of fire.
The
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
uses the
NFPAThe National Fire Protection Association is a U.S. organization charged with creating and maintaining minimum standards and requirements for fire prevention and suppression activities, training, and equipment, as well as other life-safety codes and standards...
system.
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally divided from Asia to its east by the water divide of the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains , and the Black Sea to the southeast...
and
AustralasiaAustralasia is a region of Oceania: Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes . He derived it from the Latin for "south of Asia" and differentiated the...
use another.
Ordinary combustibles
"Ordinary combustible" fires are the most common type of fire, and are designated
Class A under both systems. These occur when a solid, organic material such as
woodWood is an organic material; in the strict sense wood is produced as secondary xylem in the stems of trees . In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, enabling woody plants to reach large sizes or to stand up for themselves...
, cloth,
rubberNatural rubber is an elastomer that was originally derived from a milky colloidal suspension, or latex, found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically...
, or some
plasticPlastic is the general common term for a wide range of synthetic or semisynthetic organic amorphous solid materials suitable for the manufacture of industrial products...
s become heated to their
flash pointThe flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a liquid's flashpoint requires an ignition source. This is not to be confused with the auto-ignition temperature, which requires no ignition source. At the flash...
and
igniteIgnite is a program for gifted and talented young people in South Australia, created in 1998 and formerly known as SHIP . It is administered by the South Australian Department of Education and Children's Services...
. At this point the material undergoes
combustionCombustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering.Direct combustion by atmospheric oxygen is a reaction...
and will continue burning as long as the four components of the fire tetrahedron (heat, fuel, oxygen, and the sustaining chemical reaction) are available.
This class of fire is commonly used in controlled circumstances, such as a
campfireA campfire is a fire lit at a campsite, usually in a fire ring. Campfires are a popular feature of camping, particularly among organized campers such as Scouts or Guides. Without proper precautions they are also potentially dangerous. A certain degree of skill is needed to properly build a...
,
matchA match is a consumable tool for lighting a fire in controlled circumstances on demand. Matches are readily available, being sold by tobacconists and many other kinds of shops. Matches are rarely sold singly; they are sold in multiples, packaged in match boxes or matchbooks...
or wood-burning
stoveA stove is an enclosed heated space. The term is commonly taken to mean an enclosed space in which fuel is burned to provide heating, either to heat the space in which the stove is situated or to heat the stove itself, and items placed on it...
. 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
woodWood is an organic material; in the strict sense wood is produced as secondary xylem in the stems of trees . In a living tree it transfers water and nutrients to the leaves and other growing tissues, and has a support function, enabling woody plants to reach large sizes or to stand up for themselves...
, 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 and become a
wildfireA wildfire is any uncontrolled fire that occurs in the countryside or a wilderness area. Reflecting the type of vegetation or fuel, other names such as brush fire, bushfire, forest fire, grass fire, hill fire, peat fire, vegetation fire, and wildland fire may be used to describe the same phenomenon...
. This is the case when firefighting and
fire controlFire control consists of depriving a fire of fuel, oxygen or heat to prevent it from spreading or to put it out entirely.- Class-A fires :...
techniques are required.
This class of fire is 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
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
; oxygen can be removed by smothering the fire with foam from a
fire extinguisherA fire extinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations. It is not intended for use on an out-of-control fire, such as one which has reached the ceiling, endangers the user , or otherwise requires the expertise of a fire...
; forest fires are often fought by removing fuel by backburning; and an
ammonium phosphateAmmonium phosphate is the salt of ammonia and phosphoric acid. It has the molecular formula 3PO4 and consists of ammonium cations and phosphate anion...
dry chemical powder fire extinguisher (but not
sodium bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula NaHCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slight alkaline taste resembling that of washing soda . It is a component of the...
or
potassium bicarbonatePotassium bicarbonate , is a colorless, odorless, slightly basic, salty substance...
both of which are rated for B-class fires) breaks the fire's underlying chemical reaction.
As these fires are the most commonly encountered, 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
foamThe most general definition of foam is a substance that is formed by trapping many gas bubbles in a liquid or solid. It can also refer to anything that is analogous to such a phenomenon, such as quantum foam. Often the term is used in reference to polyurethane foam , XPS foam, Polystyrene, or many...
in the case of other fires.
Flammable liquid and gas
Flammable or combustible liquid or gaseous fuels. The US system designates all such fires "Class B". In the European/Australian system, flammable liquids are designated "Class B", while burning gases are separately designated "Class C". These fires follow the same basic fire tetrahedron (heat, fuel, oxygen, chemical reaction) as ordinary combustible fires, except that the fuel in question is a flammable liquid such as gasoline, or gas such as
natural gasNatural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills...
. 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
HalonHalon can refer to:* Haloalkane, or halogenoalkane, a group of chemical compounds consisting of alkanes with linked halogens. In particular, bromine-containing haloalkanes.* Haloalkane fire extinguishing systems...
extinguishing agents, although smothering with CO
2 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.
Electrical
Electrical fires are fires involving potentially energised electrical equipment. The US system designates these "Class C"; the European/Australian system designates them "Class E". 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 or other conductive agents: Electricity may be conducted from the fire, through water, the firefighter's body, and then earth. Electrical shocks have caused many firefighter deaths.
Electrical fire may be fought in the same way as an ordinary combustible fire, but water, foam, and other conductive agents are not to be used. While the fire is, or could possibly be electrically energised, it can be fought with any extinguishing agent rated for electrical fire.
Carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
CO
2,
HalonHalon can refer to:* Haloalkane, or halogenoalkane, a group of chemical compounds consisting of alkanes with linked halogens. In particular, bromine-containing haloalkanes.* Haloalkane fire extinguishing systems...
and dry chemical powder extinguishers such as
PKPPurple-K is a dry chemical fire suppression agent used in some dry powder fire extinguishers. It is the most effective dry chemical in fighting class B fires, and can be used against some energized electrical equipment fires...
and even baking soda are especially suited to extinguishing this sort of fire. Once electricity is shut off to the equipment involved, it will generally become an ordinary combustible fire.
Metal
Certain metals are flammable or combustible. Fires involving such are designated "Class D" in both systems. Examples of such metals include
sodiumSodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
,
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
,
potassiumPotassium is the chemical element with the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash...
,
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
,
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
,
lithiumLithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number three. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
,
plutoniumPlutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...
, and
calciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust...
. Magnesium and titanium fires are common, and 2006-7 saw the recall of laptop computer models containing lithium batteries susceptible to spontaneous ignition. When one of these combustible metals ignites, it can easily and rapidly spread to surrounding ordinary combustible materials.
With the exception of the metals that burn in contact with air or water (for example, sodium), 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 fires 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 metal 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 chlorideSodium chloride, also known as salt, common salt, table salt, or halite, is an ionic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
granules and
graphiteThe mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek γραφειν : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead...
powder. In recent years powdered
copperCopper is a chemical element with the symbol Cu and atomic number 29.It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable and a freshly-exposed surface has a pinkish or peachy color...
has also come into use.
Some extinguishers are labeled as containing
dry chemical extinguishing agents. This may be confused with
dry powder. 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 metal fire.
Metal 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 metal fire can spread and become a larger fire in the surrounding ordinary combustible materials.
Cooking oil
Fires that involve cooking oils or fats are designated "Class K" under the US system, and "Class F" under the European/Australiasian systems. Though such fires are technically a subclass of the flammable liquid/gas class, the special characteristics of these types of fires are considered important enough to recognize separately.
SaponificationSaponification is the hydrolysis of an ester under basic conditions to form an alcohol and the salt of a carboxylic acid . Saponification is commonly used to refer to the reaction of a metallic alkali with a fat or oil to form soap. Saponifiable substances are those that can be converted into...
can be used to extinguish such fires. Appropriate fire extinguishers may also have hoods over them that help extinguish the fire.
External links
- Classification of Portable Fire Extinguishers, Occupational Safety and Health Administration
The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...
- Evacuation Plans and Procedures eTool, Occupational Safety and Health Administration
The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...
- Information on Fire Extinguishers, The Fire Safety Advice Centre
- For Fire Extinguishers, Wollongong Extinguisher Service-Australia