Fire classes

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.

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.

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.

Purpose and limits of classification

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.

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.

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.

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.

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.

Ordinary combustible materials

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.

US fire classes group hazards to support agent selection. Based on U.S. Nuclear Regulatory Commission (2005), pp. B-15–B-16.

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.

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.

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.

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.

Flammable liquids and gases

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.

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.

Liquid-fire geometry affects the usefulness of a foam blanket. Based on Scheffey (2002), p. 4-88 and Fleming (2002), p. 4-72.

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.

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.

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.

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.

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.

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.

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.

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.

Combustible metals

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.

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.

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.

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.

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.

Cooking media and kitchen fires

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.

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.

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.

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.

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.

Agents and mechanisms

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.

Class selection is only the first step in choosing an extinguishing agent. Based on U.S. Nuclear Regulatory Commission (2005), pp. B-15–B-16 and Scheffey (2002), p. 4-88.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

See also

• Aqueous film-forming foam (AFFF)
• Extinguishing agent
• Fire extinguisher

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.
• 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.
• 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.
• 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.
• 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.