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'''Fire classes''' are categories used to describe fires by fuel type or by a particular hazard, such as energised electrical equipment. They support the selection of extinguishing agents and equipment by grouping situations with similar suppression concerns. A class label does not describe fire size, development or the suitability of every agent within that category.<ref name="NUREG-B16">U.S. Nuclear Regulatory Commission (2005), pp. B-15–B-16.</ref><ref name="Fleming-4-72">Fleming (2002), p. 4-72.</ref>
'''Fire classes''' are letter categories that sort fires by the kind of material that is burning, such as ordinary combustibles, flammable liquids and gases, energised electrical equipment and reactive metals.<ref name="G153">Grant (2002), p. 4-153.</ref> A report of the US Nuclear Regulatory Commission, NUREG-1805, adds that the letter gives a general indication of the type and severity of the hazard.<ref name="ISB13">Iqbal and Salley (2004), p. B-13.</ref> The classes are used to match a fire with a suitable [[Extinguishing agent]] and [[Fire extinguisher]]. Water, foam, dry chemical powders and gases work in different ways, and some of them make certain fires worse.<ref name="ISB15">Iqbal and Salley (2004), p. B-15.</ref><ref name="ISB16">Iqbal and Salley (2004), p. B-16.</ref>


Classification systems vary by country and document. The examples in this article use the US classes reproduced in NUREG-1805: A, B, C, D and K. They must not be treated as identical to every European or other national system.<ref name="NUREG-B16" />
Class letters are set by standards, and they are not the same in every document. The SFPE Handbook of Fire Protection Engineering (third edition, 2002) shows four classes, A to D, taken from a 1991 handbook of the US National Fire Protection Association (NFPA).<ref name="G171">Grant (2002), pp. 4-153 and 4-171.</ref> The 2002 edition of the US standard NFPA 10, as summarised in NUREG-1805, adds Class K for cooking oils and fats.<ref name="NFPA10">National Fire Protection Association (2002), as cited in Iqbal and Salley (2004), p. B-13.</ref>


Correct selection also requires knowledge of fuel form, electrical state, enclosure and agent compatibility. Water, foam, dry powder and gaseous agents act through different mechanisms and can introduce their own hazards. An extinguisher marked for one class is not automatically suitable for every fire involving the same broad fuel name.<ref name="NUREG-B15">U.S. Nuclear Regulatory Commission (2005), p. B-15.</ref><ref name="NUREG-B57">U.S. Nuclear Regulatory Commission (2005), p. B-57.</ref>
The wrong agent can increase the hazard. NUREG-1805 states that water should never be used on burning metals, that water under very hot cooking oil can throw out burning oil, and that water-based agents on live equipment can cause electric shock.<ref name="ISB16" /><ref name="ISB58">Iqbal and Salley (2004), p. B-58.</ref><ref name="ISB60">Iqbal and Salley (2004), p. B-60.</ref> Even an agent listed for the right class can fail on smouldering deep inside a solid or on a running liquid fire.<ref name="ISB43">Iqbal and Salley (2004), p. B-43.</ref><ref name="SC88">Scheffey (2002), p. 4-88.</ref>


== Purpose and limits of classification ==
== How fire classes are defined ==


Classification provides a common language for choosing an initial suppression approach. Ordinary solid fuels often benefit from cooling and wetting, while liquid pools may require a surface blanket. Energised equipment and reactive metals require particular attention to conductivity and chemical compatibility.<ref name="NUREG-B16" />
[[Combustion]] is a set of mainly heat-releasing chemical reactions between a fuel and an oxidiser, usually the oxygen in air, which produce heat and light as a flame or a glow.<ref name="VR41">Van Rossum (2027), §4.1.</ref> Solids and liquids do not burn directly but only once they have turned into gas. A liquid first evaporates, as in a pool fire, while a solid first breaks down by [[Pyrolysis]] into smaller molecules that escape as gas and burn in the flames.<ref name="VR41" /> The class system groups fires according to the type of fuel and the possible spread of the fire.<ref name="ISB13" />


The system does not predict extinguishing time or required agent quantity. Those depend on the fuel arrangement, fire area, application rate, agent delivery and losses. A label should therefore support, rather than replace, an assessment of the actual fire.<ref name="Scheffey-4-88">Scheffey (2002), p. 4-88.</ref><ref name="NUREG-B59">U.S. Nuclear Regulatory Commission (2005), pp. B-58–B-59.</ref>
The [[Fire triangle]] names the three factors that combustion needs: fuel, oxygen and a high enough temperature.<ref name="VR42">Van Rossum (2027), §4.2.</ref> Air contains about 21 percent oxygen by volume. When the oxygen level falls, a fire may stop spreading and eventually go out, depending on the exact concentration.<ref name="VR42" /> Typical extinguishing removes one side of the triangle by taking away the fuel, limiting the oxygen (smothering) or removing the heat (quenching).<ref name="G152">Grant (2002), pp. 4-152–4-153.</ref> Halons instead break up the chain reaction of the flame, so the triangle was extended into a "tetrahedron of fire" with the chain reaction as a fourth element.<ref name="G152" />


Fire classes also do not establish whether suppression is safe for occupants. Powder can obscure visibility and irritate breathing, and gaseous agents can alter the enclosure atmosphere. Water-based agents can create electrical and reactive-material hazards. Those effects require separate evaluation.<ref name="NUREG-B57" /><ref name="NUREG-B60">U.S. Nuclear Regulatory Commission (2005), p. B-60.</ref>
NUREG-1805 lists four ways to put out a fire: separating the fuel from the flame, removing or diluting the oxygen, cooling the fuel or the flame, and adding chemicals that change the combustion chemistry.<ref name="ISB15" /> More often several of these act at once, and their relative importance changes with the circumstances.<ref name="ISB15" /><ref name="ISB16" /> Water on a burning solid cools the solid, so that it gives off less flammable gas, and cools the flame, so that less heat returns to the fuel. In a confined space the steam may also keep oxygen away, and a fine water fog may block radiant heat.<ref name="ISB15" /> Dry chemical powder may interfere with the flame chemistry, coat the fuel surface, cool the flame and block radiation.<ref name="ISB15" />


[[Combustion]] involves the interaction of fuel, oxidant, heat and chemical reactions. Extinguishing agents can cool, separate fuel, dilute oxygen or interfere with flame chemistry. Class selection helps identify which of those actions is most useful, but several usually act together.<ref name="NUREG-B15" />
== Classes in United States documents ==


Classification should be revisited when the situation changes. Electrical isolation changes the electrical hazard, while fuel melting, spreading or becoming enclosed changes delivery requirements. A single label assigned at ignition may not describe every later stage.<ref name="NUREG-B16" /><ref name="Scheffey-4-88" />
In the United States, NFPA 10, Standard for Portable Fire Extinguishers (2002 edition), as summarised in NUREG-1805, divides fires into five classes according to the fuel involved.<ref name="NFPA10" /> Class A covers ordinary combustible materials such as wood, cloth, paper, rubber and many plastics. Class B covers flammable or combustible liquids, petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols and flammable gases. Class C covers fires in energised electrical equipment, where it matters that the extinguishing agent does not conduct electricity. Class D covers combustible metals such as magnesium, titanium, zirconium, sodium, lithium and potassium. Class K covers cooking appliances that use vegetable or animal oils and fats.<ref name="NFPA10" />
[[File:fse-fire-classes-v1.svg|thumb|upright=1.5|alt=Five US classes cover ordinary solids, liquids, energised equipment, metals and cooking media.|The five US fire classes of NFPA 10 (2002 edition), as summarised in NUREG-1805. Based on Iqbal and Salley (2004), pp. B-13 and B-16.]]


== Ordinary combustible materials ==
Other US documents use fewer letters. The SFPE Handbook chapter on halon design calculations shows four classes of fire: ordinary combustibles (Class A), flammable liquids and gases (Class B), electrical equipment (Class C) and reactive metals (Class D).<ref name="G153" /> This grouping, which the chapter takes from the seventeenth edition of the NFPA Fire Protection Handbook (1991), has no separate class for cooking oils and fats.<ref name="G171" /> A class letter therefore only has a clear meaning together with the name and edition of the document that defines it.


The US Class A category in NUREG-1805 covers ordinary combustibles such as wood, cloth, paper, rubber and many plastics. The table lists water, dry chemicals, foam and some Halon among agent types. Those entries are examples rather than a claim of equal performance or current acceptability.<ref name="NUREG-B16" />
NUREG-1805 also lists extinguishing agents for each class. It names water, dry chemicals, foam and some halon for Class A, and carbon dioxide, dry chemical agents, halon and foam for Class B.<ref name="ISB16" /> For Class C it names carbon dioxide, dry chemical agents and halon, for Class D only dry chemical agents, and for Class K dry chemical agents, carbon dioxide and wet chemical agents.<ref name="ISB16" /> The halon entries need care, because the same report calls halon obsolete for fire protection owing to its possible effect on the ozone layer.<ref name="ISF35">Iqbal and Salley (2004), p. F-35.</ref> Under the Montreal Protocol, the production of halons in the developed countries ended on 31 December 1993.<ref name="DN173">DiNenno (2002), p. 4-173.</ref>


Water absorbs heat and cools the solid, reducing the rate at which gaseous fuel is produced by pyrolysis. Cooling the flame also reduces heat feedback to the fuel surface. In confined conditions, steam and water fog can contribute to oxygen displacement and radiation attenuation.<ref name="NUREG-B15" />
== Class A: ordinary combustibles ==


Deep-seated combustion can remain after visible flame is removed. Some dry chemical formulations form a coating that helps isolate the solid surface, while other agents mainly suppress the flame. The risk of renewed burning must therefore be considered after initial knockdown.<ref name="NUREG-B56">U.S. Nuclear Regulatory Commission (2005), pp. B-55–B-56.</ref><ref name="NUREG-B44">U.S. Nuclear Regulatory Commission (2005), pp. B-43–B-44.</ref>
Class A fires burn ordinary solid materials such as wood, cloth, paper, rubber and many plastics.<ref name="NFPA10" /> Water is the most commonly used extinguishing agent, mainly because it is widely available and inexpensive. It also absorbs a large amount of heat as it warms up and turns into steam.<ref name="FL72">Fleming (2002), p. 4-72.</ref> Fleming gives 9280 Btu for one US gallon of water heated from a room temperature of 21 °C to steam at 100 °C, which is almost 2.6 MJ for every litre.<ref name="FL72c">Fleming (2002), p. 4-72 (9280 Btu per US gallon, converted to SI).</ref>


Water is widely available and has high specific and latent heat, but it is not universally suitable. Reactive materials, sensitive equipment and certain hot fuels can make it inappropriate. Class A describes the fuel group, not the safety of every water application within it.<ref name="Fleming-4-72" />
Two kinds of burning can occur in Class A materials. In flaming combustion the hot fuel surface gives off gases that burn in the flame, while in [[Smouldering]] or glowing combustion the material reacts with oxygen at its surface or inside the fuel mass.<ref name="ISB39">Iqbal and Salley (2004), p. B-39.</ref> Smouldering cannot be put out as quickly as flaming, and it can continue for weeks in bales of cotton or in heaps of sawdust. It stops only when the oxygen or the fuel runs out or the fuel becomes too cool to react. It is usually extinguished by cooling with water or by covering the fuel with an inert gas.<ref name="ISB39" />


The fire's geometry influences whether water reaches the burning region. Shielded material and deep fuel beds can remain hot after the surface is wetted. Inspection for remaining combustion is necessary before regarding the fire as extinguished.<ref name="NUREG-B44" /><ref name="NUREG-B56" />
A deep-seated fire is smouldering below the surface that may continue for some time after the surface flames have been suppressed.<ref name="ISB39" /> Halon 1301 at the normal concentration of 5 to 6 percent puts out the visible flames at once. Glowing combustion inside the material may continue, however, and re-ignite when the space is ventilated again.<ref name="ISB43" /> Grant notes that complete extinguishment needs far higher concentrations of 18 to 30 percent, which are seldom economical.<ref name="G153" /> NUREG-1805 states that the interior of the material must be investigated once the surface fire has been put out.<ref name="ISB39" />


== Flammable liquids and gases ==
Dry chemical powders show the same limit. All seven common types suppress flames, but only monoammonium phosphate, known as "ABC" or multipurpose powder, is considered effective against deep-seated fires. It forms a glassy coating over the burning surface.<ref name="ISB55">Iqbal and Salley (2004), pp. B-55–B-56.</ref> The class can also hide differences between materials. Many plastics belong to Class A, yet Grant notes that most plastics behave like flammable liquids under halon 1301 and are put out rapidly at 4 to 6 percent. Cellulosic (plant-fibre) products, in contrast, can in certain forms develop deep-seated fires.<ref name="G153" />


The US Class B category in NUREG-1805 includes flammable liquids, gases and liquid-derived solids. Its examples include gasoline, oils, liquefied petroleum gas, paraffin and heavy lubricants. Agent selection must address whether the material forms a pool, spray, running fuel or pressurised release.<ref name="NUREG-B16" /><ref name="Scheffey-4-88" />
== Class B: flammable liquids and gases ==


Plain water is denser than many hydrocarbon fuels and does not mix with them. The fuel can float on the water and continue to burn or spread. A suitable foam can instead float on the liquid and restrict vapour release and radiant feedback.<ref name="Fleming-4-72" /><ref name="Scheffey-4-88" />
Class B fires involve flammable or combustible liquids and flammable gases.<ref name="NFPA10" /> The NUREG-1805 table describes the class as flammable liquids, gases and liquid-derived solids, with examples such as gasoline (petrol), oils, liquefied petroleum gas, heavy lubricants and grease.<ref name="ISB16" /> Water is a poor agent for many of these fires. It is denser than most hydrocarbon fuels and does not mix with them, so the burning fuel floats on top of the water, keeps burning and may spread.<ref name="FL72" />


[[Firefighting foam]] creates a blanket that limits air contact and evaporation from the surface. Film-forming products can add a vapour-sealing aqueous layer on suitable hydrocarbons. The blanket must remain coherent despite heat, drainage and mechanical disruption.<ref name="Scheffey-4-89">Scheffey (2002), p. 4-89.</ref><ref name="NUREG-B59" />
[[Firefighting foam]] consists of air-filled bubbles made from a solution of foam concentrate in water, typically at 1, 3 or 6 percent concentrate.<ref name="SC88" /> The foam is lighter than the fuel and floats on it. It forms an air-excluding layer that stops the fuel from evaporating at its surface, and when the whole surface is covered the fire goes out.<ref name="SC88" /> Film-forming foams can also spread a vapour-sealing film of water solution over most hydrocarbon fuels.<ref name="SC88" /> Foam breaks down under heat and flames, so it must be applied at a sufficient rate and in sufficient volume to keep a layer over the fuel.<ref name="ISB59">Iqbal and Salley (2004), p. B-59.</ref>


Water-soluble fuels require a different assessment. Sufficient water can dilute them, but a deep pool may require substantial dilution and time. Alcohol-resistant foam is designed to resist breakdown by polar solvents, so a generic hydrocarbon foam cannot be assumed suitable.<ref name="Scheffey-4-89" /><ref name="NUREG-B58">U.S. Nuclear Regulatory Commission (2005), pp. B-57–B-58.</ref>
Liquids that mix with water, such as alcohols, need a different approach. Enough water can dilute them until they no longer burn, but for a deep pool of alcohol this can take so long that a foam is the better choice.<ref name="ISB58" /> Foams for alcohol fires must be alcohol resistant. Their concentrates form a floating polymer skin that protects the foam from breakdown by polar solvents such as acetone, methanol and ethanol.<ref name="ISB59" /><ref name="SC89">Scheffey (2002), p. 4-89.</ref>


Three-dimensional liquid fires are not the same as a pool fire. Running fuel and pressurised spray do not present a continuous horizontal surface for low-expansion foam. An auxiliary agent or another application approach may be required, subject to compatibility.<ref name="Scheffey-4-88" />
The shape of a liquid fire matters as much as the fuel. Low-expansion foams have at most ten times the volume of the solution they are made from. They work well on flat pool fires but are not particularly effective on three-dimensional fires, such as running fuel or a pressurised spray.<ref name="SC88" /> This is particularly true when the fuel has a low flash point, and for such fires an auxiliary agent such as dry chemical powder is typically used with the foam.<ref name="SC88" /> Dry chemical agents are themselves used most often on relatively small flammable liquid fires.<ref name="ISB56">Iqbal and Salley (2004), p. B-56.</ref> Halon 1301 works well on liquid fires inside a closed space, but on a large outdoor running pool fire it dissipates without reaching the flames.<ref name="G153" />
[[File:fse-fire-classes-v2.svg|thumb|upright=1.5|alt=Foam blankets a pool, while running fuel and a pressurised spray lack a continuous cover.|Liquid-fire geometry affects the usefulness of a foam blanket. Based on Scheffey (2002), p. 4-88 and Fleming (2002), p. 4-72.]]


== Energised electrical equipment ==
== Class C: energised electrical equipment ==


The US Class C category concerns Class A and B fuels together with energised electrical equipment. The classification reflects the electrical condition and the need for a nonconductive extinguishing agent. It does not imply that electricity is the only fuel.<ref name="NUREG-B16" />
Class C differs from the other classes because it does not add a new fuel. NUREG-1805 describes it as the same fuels as Class A and B fires together with energised electrical equipment, such as household appliances.<ref name="ISB16" /> The class exists because the agent must not conduct electricity, and extinguishers for Class C fires are rated by the non-conductive properties of their agent.<ref name="NFPA10" /><ref name="ISB16" /> Once the equipment has been de-energised, extinguishers designed for Class A or Class B fires can be used safely.<ref name="NFPA10" />


NUREG-1805 lists carbon dioxide, dry chemical agents and Halon in its Class C table. Those entries are tied to its historical US nuclear-industry guidance. They do not make every formulation suitable for every voltage, enclosure or modern installation.<ref name="NUREG-B16" />
Water and water-based agents, including foam, carry a risk of electric shock when they reach live equipment.<ref name="ISB60" /> Carbon dioxide is an electrically non-conductive gas that is suitable for Class B and Class C fires. It puts out a fire by lowering the concentration of oxygen, of fuel vapour or of both until combustion stops.<ref name="ISF12">Iqbal and Salley (2004), p. F-12.</ref> Halon 1301 is particularly suited to Class B and Class C fires, and electrical and electronic equipment is typically protected with a concentration of 5 percent.<ref name="G153" /> Dry chemical powder is safe on electrical fires from the point of view of electric shock. The powders can, however, damage delicate electrical equipment, especially monoammonium phosphate, which is notably acidic and corrodes more readily than the others.<ref name="ISB56" />


Water-based agents can create a shock hazard when applied to energised equipment. Foam shares that concern because it is principally water based. Selection must consider both the fire and the electrical exposure created by the discharge.<ref name="NUREG-B16" /><ref name="NUREG-B60" />
NUREG-1805 reviews US nuclear power plant fires in which crews avoided water on live equipment. At Waterford 3 in 1995, a switchgear fire was attacked with portable carbon dioxide, halon and dry chemical extinguishers without success. Water was only permitted about 90 minutes after the fire started, and firefighters then brought the fire under control within 4 minutes.<ref name="ISH21">Iqbal and Salley (2004), p. H-21.</ref> The report notes that the fire was allowed to burn far longer, and the damage was perhaps made worse, than with prompt and effective suppression.<ref name="ISH23">Iqbal and Salley (2004), p. H-23.</ref>


Dry chemical application can avoid an electrical-shock hazard in the cited guidance, but residue can damage delicate equipment. Monoammonium phosphate is described as particularly corrosive. This illustrates why extinguishing effectiveness and equipment damage must be evaluated together.<ref name="NUREG-B56" /><ref name="NUREG-H29">U.S. Nuclear Regulatory Commission (2005), p. H-29.</ref>
At Prairie Island 1 in 2001, a first attempt to de-energise a burning breaker failed, as arcing showed, and the decision not to use water on the live equipment complicated the attack.<ref name="ISH30">Iqbal and Salley (2004), pp. H-30–H-31.</ref> The fire took about 1.5 hours and more than 20 portable carbon dioxide extinguishers to put out. The report concludes that operating experience with fires in energised electrical equipment shows a relatively small quantity of water to have been effective.<ref name="ISH30" />


When equipment is de-energised, the electrical condition changes but the remaining fuels do not disappear. The fire may then be assessed using the ordinary combustible or liquid-fuel categories. The isolation condition and residual energy should be established rather than assumed.<ref name="NUREG-B16" />
== Classes D and K: metals and cooking oils ==


== Combustible metals ==
Class D covers fires in combustible metals or in metal alloys with combustible metal components.<ref name="ISB16" /> Metals such as magnesium, titanium, sodium, potassium, lithium and zirconium ignite easily as thin sections, fine particles or molten metal. As a solid block, however, they are comparatively hard to ignite.<ref name="ISB29">Iqbal and Salley (2004), p. B-29.</ref> Even aluminium, iron and steel can burn when finely divided, and clean fine steel wool may ignite. Burning metals can be hotter than burning flammable liquids, and some keep burning in carbon dioxide, nitrogen, water or steam.<ref name="ISB29" />


The US Class D category covers combustible metals and alloys. NUREG-1805 lists magnesium, sodium, potassium, titanium, zirconium and lithium as examples. The agent must suppress the fire without causing a harmful physical or chemical reaction with the metal.<ref name="NUREG-B16" />
Water is not usually recommended, because several metals react with water to release hydrogen, which burns rapidly. Water entering molten metal can also cause violent steam explosions.<ref name="ISB31">Iqbal and Salley (2004), p. B-31.</ref> The class table in NUREG-1805 states that water and water-based extinguishers should never be used on Class D fires.<ref name="ISB16" /> Its text nevertheless reports that large quantities of water have put out small quantities of burning magnesium when no pools of molten metal were present.<ref name="ISB31" /> Fleming likewise notes that enough water can overcome the reaction of minor amounts of water-reactive materials.<ref name="FL72" />


The table warns against water and water-based extinguishers for these fires. Reactive materials can produce heat, flammable or toxic gases, or explosions when water is applied. Fleming also explains why water is not a universal extinguishing agent for reactive substances.<ref name="NUREG-B16" /><ref name="Fleming-4-72" />
To work on a Class D fire, an extinguisher must suppress the fire without reacting physically or chemically with the metal.<ref name="ISB16" /> NUREG-1805 lists special powders, each for particular metals, and notes that only argon and helium are acceptable inert gases for metals that react with nitrogen or carbon dioxide.<ref name="ISB31" /> Halons should not be used on metal fires. Grant notes that reactive metals burn so intensely that they overpower halon 1301.<ref name="ISB31" /><ref name="G152" />


Dry chemical agents are listed for Class D, but that does not imply one powder is appropriate for every metal. The actual metal, particle form and reaction must be understood. Product-specific evidence is required for that compatibility.<ref name="NUREG-B16" />
Class K covers cooking appliances that use vegetable or animal oils and fats.<ref name="NFPA10" /> When burning oil is much hotter than the boiling point of water, water applied to it penetrates the oil and turns into steam below the surface. The resulting eruption of oil, a Boilover, speeds up the burning and may spread the fire.<ref name="ISB58" /> Foam carries a similar risk, because on a liquid at 100 °C or above the steam can expand the foam fourfold and cause boilover.<ref name="ISB60" />


Metals also show why class selection cannot rely only on flame appearance. Cooling may be useful for one material but dangerous for another. A nonreactive and tested agent is a fundamental requirement, not an optional refinement.<ref name="NUREG-B16" /><ref name="Fleming-4-72" />
For this class NUREG-1805 lists dry chemical agents, carbon dioxide and wet chemical agents.<ref name="ISB16" /> A wet chemical agent is a solution of water with potassium carbonate, potassium acetate or potassium citrate. Wet chemical systems are used mainly for cooking range hoods, ducts and the associated appliances.<ref name="ISF70">Iqbal and Salley (2004), p. F-70.</ref> Among the powders, monoammonium phosphate is not recommended for kitchen fires involving hot fat because it is acidic, and an alkaline powder such as potassium bicarbonate is preferred.<ref name="ISB56" />


A small quantity of reactive material does not automatically justify water use. Fleming notes that sufficient water can sometimes overcome reactions involving minor amounts, but that is conditional and not a general selection rule. The nature and quantity of the material remain decisive.<ref name="Fleming-4-72" />
== How the classes are used ==


== Cooking media and kitchen fires ==
In the United States, the class letters appear in the rating marked on an extinguisher. NUREG-1805 describes the NFPA 10 rating as a number, such as 5, 20 or 70, that shows relative effectiveness. The number is followed by one or more letters for the classes of fire on which the extinguisher was found to be effective.<ref name="ISF25">Iqbal and Salley (2004), p. F-25.</ref> The rating is based on standardised tests developed by Underwriters Laboratories.<ref name="ISF23">Iqbal and Salley (2004), p. F-23.</ref> One extinguisher can carry ratings for more than one class. Hand-portable extinguishers holding 9 litres of aqueous film-forming foam are rated 3A:20B, and wheeled units holding 125 litres are rated 20A:160B.<ref name="SH126">Scheffey and Hickey (2002), p. 4-126.</ref>


The US Class K category concerns cooking appliances using combustible vegetable or animal oils and fats. NUREG-1805 lists dry chemical agents, carbon dioxide and wet chemical agents in its table. The intended application and formulation must match this particular hot-fuel hazard.<ref name="NUREG-B16" />
The classes are also linked to standard test fires. Extinguishing concentrations of gaseous agents for Class A fuels were traditionally developed with burning wood cribs, and tests with sheets of plastics have more recently been required as well.<ref name="DN177">DiNenno (2002), p. 4-177.</ref> For Class B, the US foam test standard UL 162 burns commercial n-heptane in a square pan of at least 4.6 m².<ref name="SC95">Scheffey (2002), p. 4-95.</ref>


Water introduced beneath very hot oil or fat can turn to steam and eject burning material. NUREG-1805 describes this as an eruption that can accelerate burning and spread the fire. A class-appropriate agent must therefore avoid increasing the hazard during application.<ref name="NUREG-B58" />
The letters can describe the hazard of a whole space. Citing NFPA 10 and a handbook chapter on extinguisher use, NUREG-1805 describes light hazard occupancies such as offices and classrooms. These hold only a minor quantity of Class A materials and small amounts of Class B liquids in closed containers.<ref name="ISB14">Conroy (1997) and National Fire Protection Association (2002), as cited in Iqbal and Salley (2004), p. B-14.</ref> Ordinary hazard occupancies, such as dining areas, shops and parking garages, hold larger amounts of Class A and B materials. Extra hazard occupancies, such as woodworking, vehicle repair and cooking areas, hold more than that.<ref name="ISB14" />


Monoammonium phosphate is not recommended for kitchen fires involving hot fat in the cited NUREG discussion because of its acidic nature. An alkaline dry chemical is preferred in that source. The agent's chemistry matters in addition to the broad dry-powder label.<ref name="NUREG-B56" />
== Limits and common misconceptions ==


Foam also carries water-related hazards and can expand strongly when applied to sufficiently hot liquid. Steam formation can damage the blanket and contribute to boilover. A cooking-media fire should not be treated as an ordinary hydrocarbon pool by default.<ref name="NUREG-B60" />
A class letter describes the fuel, not the whole fire. One fire can involve several classes at once, as Class C itself shows by combining ordinary and liquid fuels with live equipment.<ref name="ISB16" /> The same class can also cover very different fires, such as a flat pool and a pressurised spray of the same liquid, or burning plastic and a smouldering bale of cotton.<ref name="SC88" /><ref name="G153" /><ref name="ISB39" /> The choice of agent therefore depends on the form of the fuel and on the hazards of the agent itself, not only on the letter.
[[File:fse-fire-classes-v3.svg|thumb|upright=1.5|alt=Class and fuel form lead to compatibility, delivery, persistence and secondary-effect checks.|Class selection is only the first step in choosing an extinguishing agent. Based on Iqbal and Salley (2004), pp. B-15–B-16 and Scheffey (2002), p. 4-88.]]


The cooking appliance and fuel temperature influence the appropriate response. A blanket that works on a cooler liquid may not survive the hot cooking medium. Product-specific testing and the fire's thermal condition must be considered.<ref name="NUREG-B60" /><ref name="NUREG-B16" />
Putting out the flames does not mean that the fire is out. Deep-seated fires can re-ignite when air returns, and NUREG-1805 names unexpected re-ignition as a hazard of dry chemical agents.<ref name="ISB43" /><ref name="ISB57">Iqbal and Salley (2004), p. B-57.</ref> At the Point Beach 1 nuclear power plant in 2001, the fire brigade discharged about 32 kg (70 pounds) of dry chemical from three portable extinguishers on a small fire of rags and debris. The fire re-ignited twice and was finally put out with about 57 to 76 litres (15 to 20 gallons) of water. The powder covered large areas of the containment building in white dust.<ref name="ISH29">Iqbal and Salley (2004), pp. H-29–H-30.</ref>


== Agents and mechanisms ==
The class lists in NFPA 10 (2002 edition) and in the SFPE Handbook do not mention batteries.<ref name="NFPA10" /><ref name="G153" /> A [[Lithium-ion battery fire]] can involve [[Thermal runaway]], a failure in which a battery cell heats itself and can release flammable gases. The fire then keeps itself going and re-ignites until the energy of the battery pack has been used up.<ref name="NIPV22">Van der Graaf et al. (2021/2023), p. 22.</ref> In an electric vehicle the battery pack is sealed, so cooling water cannot reach the cells directly, and a Dutch report describes prolonged cooling with large amounts of water.<ref name="NIPV22" /> The same report states that a burning battery pack of an electric vehicle is very difficult or impossible to extinguish, and that water has to reach the inside of the pack.<ref name="NIPV30">Van der Graaf et al. (2021/2023), p. 30.</ref>


An [[Extinguishing agent]] is selected for its action on a particular fire, not for its name alone. Water mainly cools and wets, foam blankets and cools, powders can interrupt chemistry and coat, and gases can inhibit reactions or reduce oxygen. Several mechanisms can occur at once.<ref name="NUREG-B15" /><ref name="Mawhinney-4-313">Mawhinney and Back (2002), pp. 4-313–4-314.</ref>
The word "class" is also used in unrelated systems, which can cause confusion. NUREG-1805 groups [[t-squared fire|t-squared fires]] into slow, medium, fast and ultra-fast growth classes.<ref name="ISB3">Iqbal and Salley (2004), pp. B-3, B-17 and C-11.</ref> Following NFPA 30, it classifies flammable liquids as Class IA, IB and IC by flash point, and following NFPA 256 it rates roof coverings as Class A, B or C by flame spread.<ref name="ISB3" /> None of these is a fire class in the sense used for choosing an extinguishing agent.
 
Water mist changes delivery by producing fine droplets with greater opportunity for heat absorption and evaporation. It can cool flames and surfaces, attenuate radiation and contribute to oxygen dilution. Those actions depend on droplet delivery and enclosure conditions rather than water quantity alone.<ref name="Mawhinney-4-311">Mawhinney and Back (2002), p. 4-311.</ref><ref name="Mawhinney-4-313" />
 
Dry powder can suppress flames rapidly but leave residue, corrosion and reduced visibility. Its performance depends on particle size, chemistry and delivery. A clean-looking flame result can therefore have significant secondary consequences.<ref name="NUREG-B55">U.S. Nuclear Regulatory Commission (2005), pp. B-55–B-56.</ref><ref name="NUREG-H29" />
 
Gaseous agents can reach obstructed regions but require retention of an effective atmosphere. Leakage and ventilation can reduce concentration, while deep-seated combustion can survive initial flame suppression. Re-ignition remains possible when air returns.<ref name="NUREG-B44" />
 
[[Fire extinguisher]] selection should follow the label and tested application. The cylinder, agent and discharge arrangement form one unit with a limited supply. A broad class marking does not guarantee effectiveness for every fuel geometry or enclosure.<ref name="NUREG-B60" /><ref name="NUREG-H29" />
 
A [[Lithium-ion battery fire]] presents processes that are not fully captured by a simple fuel-class label. Thermal runaway can release flammable gases, sustain re-ignition and propagate between cells. Electrical and chemical energy remain relevant until neutralised.<ref name="NIPV-22">Van der Graaf, Hessels, and Rosmuller (2021), p. 22.</ref>
 
Battery packs can be sealed, making direct cooling of cells difficult. NIPV describes prolonged application of substantial cooling water in that context and the possibility of gas-cloud ignition and ejected fragments. These conditions differ from an ordinary solid-material fire or a portable electrical-equipment fire.<ref name="NIPV-22" />
 
The class framework should therefore be used with mechanism-specific evidence. Considering the equipment as merely electrical would overlook cell reactions and propagation, while treating the battery as an ordinary solid would overlook its energy and packaging. Classification must not suppress those details.<ref name="NIPV-22" /><ref name="NUREG-B16" />
 
Complex incidents can also contain several classes at once. A battery installation can involve electrical equipment, plastics and other contents, and a kitchen can involve hot oil and nearby ordinary combustibles. Each hazard and its interactions must be assessed.<ref name="NUREG-B16" /><ref name="NIPV-22" />
 
Classification remains useful only when it leads to a compatible and tested response. It does not justify choosing an agent by habit, nor does it remove the need to evaluate re-ignition and secondary effects. The physical and chemical situation determines the complete suppression problem.<ref name="NUREG-B15" /><ref name="Scheffey-4-88" />
 
== Selection, evidence and common mistakes ==
 
Agent selection should start with the actual fuel, form and exposure conditions. The class can guide the next question, but it cannot answer compatibility, application rate or enclosure retention by itself. The complete scenario must be reviewed.<ref name="NUREG-B16" /><ref name="Scheffey-4-88" />
 
A common error is to treat every water-based agent as interchangeable. Conventional sprays, fine mists and foam solutions distribute water differently and have different compatibility issues. Their performance cannot be inferred from the shared presence of water.<ref name="Fleming-4-72" /><ref name="Mawhinney-4-311" />
 
Another error is to assume that visible flame extinction proves complete safety. Deep-seated combustion, hot fuel and damaged foam blankets can support renewed burning. Post-discharge conditions and re-ignition must be assessed as part of the result.<ref name="NUREG-B44" /><ref name="NUREG-B60" />
 
Historical agent lists also need their context. NUREG-1805 includes Halon in several class entries, reflecting its source period and nuclear guidance. Those entries do not establish present environmental acceptability or permission to install the agent.<ref name="NUREG-B16" /><ref name="NUREG-B43">U.S. Nuclear Regulatory Commission (2005), p. B-43.</ref>
 
The appropriate class description therefore includes jurisdiction and edition as well as the letter. It links the fire to a compatible agent while preserving the fuel geometry, electrical state and secondary hazards. This is more reliable than applying one global letter-to-agent table.<ref name="NUREG-B16" /><ref name="Fleming-4-72" />
 
Where several hazards coexist, the response should be broken down by fuel and exposure rather than by searching for one class that covers everything. The class framework remains a selection aid, not a substitute for that analysis. Each agent should be checked against the part of the incident where it will be applied and against its effects on the other hazards present.<ref name="NUREG-B16" /><ref name="NIPV-22" />


== See also ==
== See also ==


* [[Aqueous film-forming foam (AFFF)]]
* [[Aqueous film-forming foam]]
* [[Extinguishing agent]]
* [[Firefighting]]
* [[Fire extinguisher]]
* [[Hose reel]]


== Notes ==
== Notes ==
Line 125: Line 96:
== References ==
== References ==


* Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* DiNenno, P. J. (2002). Halon replacement clean agent total flooding systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-173–4-200). National Fire Protection Association; Society of Fire Protection Engineers.
* Mawhinney, J. R., & Back, G. G., III. (2002). Water mist fire suppression systems. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-72–4-87). National Fire Protection Association; Society of Fire Protection Engineers.
* Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Grant, C. C. (2002). Halon design calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-149–4-172). National Fire Protection Association; Society of Fire Protection Engineers.
* U.S. Nuclear Regulatory Commission. (2005). ''Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program'' (NUREG-1805). Author.
* Iqbal, N., & Salley, M. H. (2004). ''Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program'' (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
* Van der Graaf, J., Hessels, T., & Rosmuller, N. (2021). ''Fire safety of indoor car parks accommodating electrically powered vehicles''. Netherlands Institute for Public Safety (NIPV). English translation published 2023.
* Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.
* Scheffey, J. L., & Hickey, H. E. (2002). Foam system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-123–4-148). National Fire Protection Association; Society of Fire Protection Engineers.
* Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). ''Fire safety of indoor car parks accommodating electrically powered vehicles''. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.


[[Category:Firefighting & Fire Service]]
[[Category:Firefighting & Fire Service]]

Latest revision as of 22:32, 2 November 2026

Fire classes are letter categories that sort fires by the kind of material that is burning, such as ordinary combustibles, flammable liquids and gases, energised electrical equipment and reactive metals.[1] A report of the US Nuclear Regulatory Commission, NUREG-1805, adds that the letter gives a general indication of the type and severity of the hazard.[2] The classes are used to match a fire with a suitable Extinguishing agent and Fire extinguisher. Water, foam, dry chemical powders and gases work in different ways, and some of them make certain fires worse.[3][4]

Class letters are set by standards, and they are not the same in every document. The SFPE Handbook of Fire Protection Engineering (third edition, 2002) shows four classes, A to D, taken from a 1991 handbook of the US National Fire Protection Association (NFPA).[5] The 2002 edition of the US standard NFPA 10, as summarised in NUREG-1805, adds Class K for cooking oils and fats.[6]

The wrong agent can increase the hazard. NUREG-1805 states that water should never be used on burning metals, that water under very hot cooking oil can throw out burning oil, and that water-based agents on live equipment can cause electric shock.[4][7][8] Even an agent listed for the right class can fail on smouldering deep inside a solid or on a running liquid fire.[9][10]

How fire classes are defined

Combustion is a set of mainly heat-releasing chemical reactions between a fuel and an oxidiser, usually the oxygen in air, which produce heat and light as a flame or a glow.[11] Solids and liquids do not burn directly but only once they have turned into gas. A liquid first evaporates, as in a pool fire, while a solid first breaks down by Pyrolysis into smaller molecules that escape as gas and burn in the flames.[11] The class system groups fires according to the type of fuel and the possible spread of the fire.[2]

The Fire triangle names the three factors that combustion needs: fuel, oxygen and a high enough temperature.[12] Air contains about 21 percent oxygen by volume. When the oxygen level falls, a fire may stop spreading and eventually go out, depending on the exact concentration.[12] Typical extinguishing removes one side of the triangle by taking away the fuel, limiting the oxygen (smothering) or removing the heat (quenching).[13] Halons instead break up the chain reaction of the flame, so the triangle was extended into a "tetrahedron of fire" with the chain reaction as a fourth element.[13]

NUREG-1805 lists four ways to put out a fire: separating the fuel from the flame, removing or diluting the oxygen, cooling the fuel or the flame, and adding chemicals that change the combustion chemistry.[3] More often several of these act at once, and their relative importance changes with the circumstances.[3][4] Water on a burning solid cools the solid, so that it gives off less flammable gas, and cools the flame, so that less heat returns to the fuel. In a confined space the steam may also keep oxygen away, and a fine water fog may block radiant heat.[3] Dry chemical powder may interfere with the flame chemistry, coat the fuel surface, cool the flame and block radiation.[3]

Classes in United States documents

In the United States, NFPA 10, Standard for Portable Fire Extinguishers (2002 edition), as summarised in NUREG-1805, divides fires into five classes according to the fuel involved.[6] Class A covers ordinary combustible materials such as wood, cloth, paper, rubber and many plastics. Class B covers flammable or combustible liquids, petroleum greases, tars, oils, oil-based paints, solvents, lacquers, alcohols and flammable gases. Class C covers fires in energised electrical equipment, where it matters that the extinguishing agent does not conduct electricity. Class D covers combustible metals such as magnesium, titanium, zirconium, sodium, lithium and potassium. Class K covers cooking appliances that use vegetable or animal oils and fats.[6]

Five US classes cover ordinary solids, liquids, energised equipment, metals and cooking media.
The five US fire classes of NFPA 10 (2002 edition), as summarised in NUREG-1805. Based on Iqbal and Salley (2004), pp. B-13 and B-16.

Other US documents use fewer letters. The SFPE Handbook chapter on halon design calculations shows four classes of fire: ordinary combustibles (Class A), flammable liquids and gases (Class B), electrical equipment (Class C) and reactive metals (Class D).[1] This grouping, which the chapter takes from the seventeenth edition of the NFPA Fire Protection Handbook (1991), has no separate class for cooking oils and fats.[5] A class letter therefore only has a clear meaning together with the name and edition of the document that defines it.

NUREG-1805 also lists extinguishing agents for each class. It names water, dry chemicals, foam and some halon for Class A, and carbon dioxide, dry chemical agents, halon and foam for Class B.[4] For Class C it names carbon dioxide, dry chemical agents and halon, for Class D only dry chemical agents, and for Class K dry chemical agents, carbon dioxide and wet chemical agents.[4] The halon entries need care, because the same report calls halon obsolete for fire protection owing to its possible effect on the ozone layer.[14] Under the Montreal Protocol, the production of halons in the developed countries ended on 31 December 1993.[15]

Class A: ordinary combustibles

Class A fires burn ordinary solid materials such as wood, cloth, paper, rubber and many plastics.[6] Water is the most commonly used extinguishing agent, mainly because it is widely available and inexpensive. It also absorbs a large amount of heat as it warms up and turns into steam.[16] Fleming gives 9280 Btu for one US gallon of water heated from a room temperature of 21 °C to steam at 100 °C, which is almost 2.6 MJ for every litre.[17]

Two kinds of burning can occur in Class A materials. In flaming combustion the hot fuel surface gives off gases that burn in the flame, while in Smouldering or glowing combustion the material reacts with oxygen at its surface or inside the fuel mass.[18] Smouldering cannot be put out as quickly as flaming, and it can continue for weeks in bales of cotton or in heaps of sawdust. It stops only when the oxygen or the fuel runs out or the fuel becomes too cool to react. It is usually extinguished by cooling with water or by covering the fuel with an inert gas.[18]

A deep-seated fire is smouldering below the surface that may continue for some time after the surface flames have been suppressed.[18] Halon 1301 at the normal concentration of 5 to 6 percent puts out the visible flames at once. Glowing combustion inside the material may continue, however, and re-ignite when the space is ventilated again.[9] Grant notes that complete extinguishment needs far higher concentrations of 18 to 30 percent, which are seldom economical.[1] NUREG-1805 states that the interior of the material must be investigated once the surface fire has been put out.[18]

Dry chemical powders show the same limit. All seven common types suppress flames, but only monoammonium phosphate, known as "ABC" or multipurpose powder, is considered effective against deep-seated fires. It forms a glassy coating over the burning surface.[19] The class can also hide differences between materials. Many plastics belong to Class A, yet Grant notes that most plastics behave like flammable liquids under halon 1301 and are put out rapidly at 4 to 6 percent. Cellulosic (plant-fibre) products, in contrast, can in certain forms develop deep-seated fires.[1]

Class B: flammable liquids and gases

Class B fires involve flammable or combustible liquids and flammable gases.[6] The NUREG-1805 table describes the class as flammable liquids, gases and liquid-derived solids, with examples such as gasoline (petrol), oils, liquefied petroleum gas, heavy lubricants and grease.[4] Water is a poor agent for many of these fires. It is denser than most hydrocarbon fuels and does not mix with them, so the burning fuel floats on top of the water, keeps burning and may spread.[16]

Firefighting foam consists of air-filled bubbles made from a solution of foam concentrate in water, typically at 1, 3 or 6 percent concentrate.[10] The foam is lighter than the fuel and floats on it. It forms an air-excluding layer that stops the fuel from evaporating at its surface, and when the whole surface is covered the fire goes out.[10] Film-forming foams can also spread a vapour-sealing film of water solution over most hydrocarbon fuels.[10] Foam breaks down under heat and flames, so it must be applied at a sufficient rate and in sufficient volume to keep a layer over the fuel.[20]

Liquids that mix with water, such as alcohols, need a different approach. Enough water can dilute them until they no longer burn, but for a deep pool of alcohol this can take so long that a foam is the better choice.[7] Foams for alcohol fires must be alcohol resistant. Their concentrates form a floating polymer skin that protects the foam from breakdown by polar solvents such as acetone, methanol and ethanol.[20][21]

The shape of a liquid fire matters as much as the fuel. Low-expansion foams have at most ten times the volume of the solution they are made from. They work well on flat pool fires but are not particularly effective on three-dimensional fires, such as running fuel or a pressurised spray.[10] This is particularly true when the fuel has a low flash point, and for such fires an auxiliary agent such as dry chemical powder is typically used with the foam.[10] Dry chemical agents are themselves used most often on relatively small flammable liquid fires.[22] Halon 1301 works well on liquid fires inside a closed space, but on a large outdoor running pool fire it dissipates without reaching the flames.[1]

Foam blankets a pool, while running fuel and a pressurised spray lack a continuous cover.
Liquid-fire geometry affects the usefulness of a foam blanket. Based on Scheffey (2002), p. 4-88 and Fleming (2002), p. 4-72.

Class C: energised electrical equipment

Class C differs from the other classes because it does not add a new fuel. NUREG-1805 describes it as the same fuels as Class A and B fires together with energised electrical equipment, such as household appliances.[4] The class exists because the agent must not conduct electricity, and extinguishers for Class C fires are rated by the non-conductive properties of their agent.[6][4] Once the equipment has been de-energised, extinguishers designed for Class A or Class B fires can be used safely.[6]

Water and water-based agents, including foam, carry a risk of electric shock when they reach live equipment.[8] Carbon dioxide is an electrically non-conductive gas that is suitable for Class B and Class C fires. It puts out a fire by lowering the concentration of oxygen, of fuel vapour or of both until combustion stops.[23] Halon 1301 is particularly suited to Class B and Class C fires, and electrical and electronic equipment is typically protected with a concentration of 5 percent.[1] Dry chemical powder is safe on electrical fires from the point of view of electric shock. The powders can, however, damage delicate electrical equipment, especially monoammonium phosphate, which is notably acidic and corrodes more readily than the others.[22]

NUREG-1805 reviews US nuclear power plant fires in which crews avoided water on live equipment. At Waterford 3 in 1995, a switchgear fire was attacked with portable carbon dioxide, halon and dry chemical extinguishers without success. Water was only permitted about 90 minutes after the fire started, and firefighters then brought the fire under control within 4 minutes.[24] The report notes that the fire was allowed to burn far longer, and the damage was perhaps made worse, than with prompt and effective suppression.[25]

At Prairie Island 1 in 2001, a first attempt to de-energise a burning breaker failed, as arcing showed, and the decision not to use water on the live equipment complicated the attack.[26] The fire took about 1.5 hours and more than 20 portable carbon dioxide extinguishers to put out. The report concludes that operating experience with fires in energised electrical equipment shows a relatively small quantity of water to have been effective.[26]

Classes D and K: metals and cooking oils

Class D covers fires in combustible metals or in metal alloys with combustible metal components.[4] Metals such as magnesium, titanium, sodium, potassium, lithium and zirconium ignite easily as thin sections, fine particles or molten metal. As a solid block, however, they are comparatively hard to ignite.[27] Even aluminium, iron and steel can burn when finely divided, and clean fine steel wool may ignite. Burning metals can be hotter than burning flammable liquids, and some keep burning in carbon dioxide, nitrogen, water or steam.[27]

Water is not usually recommended, because several metals react with water to release hydrogen, which burns rapidly. Water entering molten metal can also cause violent steam explosions.[28] The class table in NUREG-1805 states that water and water-based extinguishers should never be used on Class D fires.[4] Its text nevertheless reports that large quantities of water have put out small quantities of burning magnesium when no pools of molten metal were present.[28] Fleming likewise notes that enough water can overcome the reaction of minor amounts of water-reactive materials.[16]

To work on a Class D fire, an extinguisher must suppress the fire without reacting physically or chemically with the metal.[4] NUREG-1805 lists special powders, each for particular metals, and notes that only argon and helium are acceptable inert gases for metals that react with nitrogen or carbon dioxide.[28] Halons should not be used on metal fires. Grant notes that reactive metals burn so intensely that they overpower halon 1301.[28][13]

Class K covers cooking appliances that use vegetable or animal oils and fats.[6] When burning oil is much hotter than the boiling point of water, water applied to it penetrates the oil and turns into steam below the surface. The resulting eruption of oil, a Boilover, speeds up the burning and may spread the fire.[7] Foam carries a similar risk, because on a liquid at 100 °C or above the steam can expand the foam fourfold and cause boilover.[8]

For this class NUREG-1805 lists dry chemical agents, carbon dioxide and wet chemical agents.[4] A wet chemical agent is a solution of water with potassium carbonate, potassium acetate or potassium citrate. Wet chemical systems are used mainly for cooking range hoods, ducts and the associated appliances.[29] Among the powders, monoammonium phosphate is not recommended for kitchen fires involving hot fat because it is acidic, and an alkaline powder such as potassium bicarbonate is preferred.[22]

How the classes are used

In the United States, the class letters appear in the rating marked on an extinguisher. NUREG-1805 describes the NFPA 10 rating as a number, such as 5, 20 or 70, that shows relative effectiveness. The number is followed by one or more letters for the classes of fire on which the extinguisher was found to be effective.[30] The rating is based on standardised tests developed by Underwriters Laboratories.[31] One extinguisher can carry ratings for more than one class. Hand-portable extinguishers holding 9 litres of aqueous film-forming foam are rated 3A:20B, and wheeled units holding 125 litres are rated 20A:160B.[32]

The classes are also linked to standard test fires. Extinguishing concentrations of gaseous agents for Class A fuels were traditionally developed with burning wood cribs, and tests with sheets of plastics have more recently been required as well.[33] For Class B, the US foam test standard UL 162 burns commercial n-heptane in a square pan of at least 4.6 m².[34]

The letters can describe the hazard of a whole space. Citing NFPA 10 and a handbook chapter on extinguisher use, NUREG-1805 describes light hazard occupancies such as offices and classrooms. These hold only a minor quantity of Class A materials and small amounts of Class B liquids in closed containers.[35] Ordinary hazard occupancies, such as dining areas, shops and parking garages, hold larger amounts of Class A and B materials. Extra hazard occupancies, such as woodworking, vehicle repair and cooking areas, hold more than that.[35]

Limits and common misconceptions

A class letter describes the fuel, not the whole fire. One fire can involve several classes at once, as Class C itself shows by combining ordinary and liquid fuels with live equipment.[4] The same class can also cover very different fires, such as a flat pool and a pressurised spray of the same liquid, or burning plastic and a smouldering bale of cotton.[10][1][18] The choice of agent therefore depends on the form of the fuel and on the hazards of the agent itself, not only on the letter.

Class and fuel form lead to compatibility, delivery, persistence and secondary-effect checks.
Class selection is only the first step in choosing an extinguishing agent. Based on Iqbal and Salley (2004), pp. B-15–B-16 and Scheffey (2002), p. 4-88.

Putting out the flames does not mean that the fire is out. Deep-seated fires can re-ignite when air returns, and NUREG-1805 names unexpected re-ignition as a hazard of dry chemical agents.[9][36] At the Point Beach 1 nuclear power plant in 2001, the fire brigade discharged about 32 kg (70 pounds) of dry chemical from three portable extinguishers on a small fire of rags and debris. The fire re-ignited twice and was finally put out with about 57 to 76 litres (15 to 20 gallons) of water. The powder covered large areas of the containment building in white dust.[37]

The class lists in NFPA 10 (2002 edition) and in the SFPE Handbook do not mention batteries.[6][1] A Lithium-ion battery fire can involve Thermal runaway, a failure in which a battery cell heats itself and can release flammable gases. The fire then keeps itself going and re-ignites until the energy of the battery pack has been used up.[38] In an electric vehicle the battery pack is sealed, so cooling water cannot reach the cells directly, and a Dutch report describes prolonged cooling with large amounts of water.[38] The same report states that a burning battery pack of an electric vehicle is very difficult or impossible to extinguish, and that water has to reach the inside of the pack.[39]

The word "class" is also used in unrelated systems, which can cause confusion. NUREG-1805 groups t-squared fires into slow, medium, fast and ultra-fast growth classes.[40] Following NFPA 30, it classifies flammable liquids as Class IA, IB and IC by flash point, and following NFPA 256 it rates roof coverings as Class A, B or C by flame spread.[40] None of these is a fire class in the sense used for choosing an extinguishing agent.

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Grant (2002), p. 4-153.
  2. ↑ 2.0 2.1 Iqbal and Salley (2004), p. B-13.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Iqbal and Salley (2004), p. B-15.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 Iqbal and Salley (2004), p. B-16.
  5. ↑ 5.0 5.1 Grant (2002), pp. 4-153 and 4-171.
  6. ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 National Fire Protection Association (2002), as cited in Iqbal and Salley (2004), p. B-13.
  7. ↑ 7.0 7.1 7.2 Iqbal and Salley (2004), p. B-58.
  8. ↑ 8.0 8.1 8.2 Iqbal and Salley (2004), p. B-60.
  9. ↑ 9.0 9.1 9.2 Iqbal and Salley (2004), p. B-43.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 10.5 10.6 Scheffey (2002), p. 4-88.
  11. ↑ 11.0 11.1 Van Rossum (2027), §4.1.
  12. ↑ 12.0 12.1 Van Rossum (2027), §4.2.
  13. ↑ 13.0 13.1 13.2 Grant (2002), pp. 4-152–4-153.
  14. ↑ Iqbal and Salley (2004), p. F-35.
  15. ↑ DiNenno (2002), p. 4-173.
  16. ↑ 16.0 16.1 16.2 Fleming (2002), p. 4-72.
  17. ↑ Fleming (2002), p. 4-72 (9280 Btu per US gallon, converted to SI).
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Iqbal and Salley (2004), p. B-39.
  19. ↑ Iqbal and Salley (2004), pp. B-55–B-56.
  20. ↑ 20.0 20.1 Iqbal and Salley (2004), p. B-59.
  21. ↑ Scheffey (2002), p. 4-89.
  22. ↑ 22.0 22.1 22.2 Iqbal and Salley (2004), p. B-56.
  23. ↑ Iqbal and Salley (2004), p. F-12.
  24. ↑ Iqbal and Salley (2004), p. H-21.
  25. ↑ Iqbal and Salley (2004), p. H-23.
  26. ↑ 26.0 26.1 Iqbal and Salley (2004), pp. H-30–H-31.
  27. ↑ 27.0 27.1 Iqbal and Salley (2004), p. B-29.
  28. ↑ 28.0 28.1 28.2 28.3 Iqbal and Salley (2004), p. B-31.
  29. ↑ Iqbal and Salley (2004), p. F-70.
  30. ↑ Iqbal and Salley (2004), p. F-25.
  31. ↑ Iqbal and Salley (2004), p. F-23.
  32. ↑ Scheffey and Hickey (2002), p. 4-126.
  33. ↑ DiNenno (2002), p. 4-177.
  34. ↑ Scheffey (2002), p. 4-95.
  35. ↑ 35.0 35.1 Conroy (1997) and National Fire Protection Association (2002), as cited in Iqbal and Salley (2004), p. B-14.
  36. ↑ Iqbal and Salley (2004), p. B-57.
  37. ↑ Iqbal and Salley (2004), pp. H-29–H-30.
  38. ↑ 38.0 38.1 Van der Graaf et al. (2021/2023), p. 22.
  39. ↑ Van der Graaf et al. (2021/2023), p. 30.
  40. ↑ 40.0 40.1 Iqbal and Salley (2004), pp. B-3, B-17 and C-11.

References

  • DiNenno, P. J. (2002). Halon replacement clean agent total flooding systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-173–4-200). National Fire Protection Association; Society of Fire Protection Engineers.
  • Fleming, R. P. (2002). Automatic sprinkler system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-72–4-87). National Fire Protection Association; Society of Fire Protection Engineers.
  • Grant, C. C. (2002). Halon design calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-149–4-172). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Scheffey, J. L. (2002). Foam agents and AFFF system design considerations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-88–4-122). National Fire Protection Association; Society of Fire Protection Engineers.
  • Scheffey, J. L., & Hickey, H. E. (2002). Foam system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-123–4-148). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
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