Not all fires are the same, and not all fire extinguishers work on all fires. Using the wrong extinguishing agent on the wrong fire can be ineffective — or it can make the fire worse. In a live event environment, where the types of potential fires range from burning fabric and paper trash to electrical panels and commercial cooking oil, understanding how fires are classified and what that classification means for fire response is practically important knowledge for every member of the event safety team.

Fire classification is the foundation of extinguisher selection, fire risk assessment, and fire-fighting equipment provision. It is also the foundation for understanding how fires start and how they can be prevented — which is always preferable to fighting them.

The Fire Triangle

The fire triangle is a simple conceptual model that describes the three conditions required for a fire to start and sustain itself: fuel, heat (an ignition source), and oxygen (an oxidizing agent). Remove any one of the three, and the fire cannot ignite — or, if it has already ignited, it will be extinguished.

This model is the conceptual foundation of fire prevention. Fire safety planning for live events is, at its core, an exercise in identifying potential fuels, identifying potential ignition sources, and putting in place measures to prevent them from combining. The fire triangle makes this systematic: a fuel-focused review (what materials are present that can burn?) combined with an ignition-source review (what could ignite those materials?) identifies the primary fire risks at any specific event site (National Fire Protection Association [NFPA], 2024a).

• Fuel: anything that can burn, from paper and fabric to generator diesel, propane used for cooking, wooden staging, polyurethane foam, and dry vegetation. Fuels can be eliminated, substituted (diesel rather than gasoline, fire-retardant treated wood rather than untreated), stored away from ignition sources, or protected with approved flame-resistant finishes.
• Heat: the ignition source. Sources at live events include electrical faults, poorly maintained equipment, vehicle exhaust, pyrotechnic sparks, cooking appliances, smoking, friction, and intentionally set fires. Each requires its own specific control measures.
• Oxygen: present in the atmosphere at concentrations sufficient to support combustion. Special precautions apply in areas where enriched oxygen is in use (gas cutting equipment, medical oxygen) or where materials contain their own oxygen source (pyrotechnics, certain combustible metals).

The Fire Tetrahedron

The fire tetrahedron adds a fourth element to the fire triangle: the chemical chain reaction. Once a fire has started and crossed the threshold of self-sustaining combustion, the exothermic chain reaction sustains the fire even if the original heat source is removed. Certain extinguishing agents — particularly halon and its modern replacements — work by interrupting this chain reaction at the molecular level, rather than by cooling the fuel or excluding oxygen. This explains why some fires can be extinguished by very small quantities of certain agents that have no meaningful cooling or smothering effect (NFPA, 2022a).

Understanding the tetrahedron model helps explain the extinguishing mechanism of different agent types and why the wrong agent fails — or backfires. Water disrupts the heat element and can cool fuel below its ignition temperature. Carbon dioxide displaces the oxygen element. Dry chemical and halon-replacement clean agents interrupt the chain reaction. Sand and specific dry powders smother metal fires by excluding oxygen without reacting violently with the burning metal (NFPA, 2022a).

Fire Classification Under NFPA 10

NFPA 10, Standard for Portable Fire Extinguishers, classifies fires into five categories based on the fuel type involved. Each classification designates the most appropriate extinguishing agents and corresponds to specific extinguisher ratings (NFPA, 2022a).

Class A Fires: Ordinary Combustibles

Class A fires involve ordinary combustible materials: paper, wood, cloth, rubber, and many plastics. The designation symbol is a green triangle containing the letter A. Class A fires are the most common fire type in most live event environments — they involve the same materials found throughout event staging, production infrastructure, audience seating, and temporary structures (NFPA, 2022a).

Effective agents for Class A fires include water, foam, and dry chemical. Water and foam are generally preferred because they cool the fuel below its ignition temperature and penetrate smoldering materials. ABC-rated dry chemical extinguishers are also effective on Class A fires and are the most common multipurpose extinguisher type found in event venues.

Common Class A fuel sources at live events: wooden stage decks and platforms, fabric drapes and backdrops, cardboard packing materials, paper trash, fabric-covered seating, temporary wooden structures, and any untreated or insufficiently flame-retarded decorative material.

Class B Fires: Flammable and Combustible Liquids and Gases

Class B fires involve flammable and combustible liquids — gasoline, diesel fuel, paints, tars, oils, and greases — as well as flammable gases such as propane and natural gas. The designation symbol is a red square containing the letter B. Class B fires represent a significant risk at events where generators are in use, cooking with open flames or gas appliances is occurring, or vehicles are operated near event structures (NFPA, 2022a).

Effective agents for Class B fires include dry chemical, carbon dioxide, and foam (AFFF, FFFP). Water must never be applied to a liquid fuel fire as a solid stream — it scatters the burning fuel and spreads the fire. A fine water mist may be used in some circumstances, but only by trained responders. Carbon dioxide extinguishers have an upper rating limit for Class B fires due to their limited range and cooling capacity; in windy outdoor conditions, CO2 may be ineffective (NFPA, 2022a).

The transition from diesel to gasoline should be understood as a fire risk reduction: diesel is a combustible liquid (flash point above 100°F / 38°C) while gasoline is a flammable liquid (flash point below 73°F / 23°C). Flammable liquids present a significantly higher ignition risk, particularly in warm weather. Wherever possible, generator fuel selection should prefer diesel over gasoline.

Class C Fires: Energized Electrical Equipment

Class C fires involve energized electrical equipment: motors, generators, electrical panels, stage lighting equipment, audio amplifiers, and any other energized device. The designation symbol is a blue circle containing the letter C. The defining characteristic of a Class C fire — and the reason for its separate classification — is the electrocution hazard it presents to firefighters (NFPA, 2022a).

Water, foam, and any conductive extinguishing agent must never be used on a Class C fire while the equipment is energized. The conductive path created by a water stream between the fire and the person holding the extinguisher can deliver a lethal electrical shock. Carbon dioxide, dry chemical, and clean agent extinguishers are appropriate because their agents are non-conductive (NFPA, 2022a).

Once power has been cut to the affected equipment, the fire typically becomes a Class A fire involving the structural materials that have been ignited, and water may be used. Knowing the location of the main electrical shutoff for each section of the event site is therefore important information for anyone who might respond to an electrical fire.

Live events involve extensive electrical infrastructure: temporary distribution equipment, dimmer racks, power feeder cables, and large quantities of energized production equipment. Electrical fires are a persistent risk and one of the primary reasons that appropriately rated extinguishers must be positioned near electrical intake rooms, generator locations, and production areas (NFPA, 2022a).

Class D Fires: Combustible Metals

Class D fires involve combustible metals — magnesium, titanium, sodium, lithium, potassium, and certain others. The designation symbol is a yellow star (decagon) containing the letter D. While Class D fires are uncommon in most live event settings, they are worth understanding because some special effects, automotive vehicles, and pyrotechnic compounds involve materials that can react unexpectedly when ignited (NFPA, 2022a).

The critical point about Class D fires: water and common extinguishing agents can make them significantly worse. When water contacts burning metal at the high temperatures involved in a metal fire, the water is converted to steam so rapidly that it can cause an explosion. Carbon dioxide is ineffective against certain burning metals such as titanium. Only Class D-specific dry powder agents — sodium chloride-based, copper-based, or graphite-based — should be applied to metal fires, and they should be applied gently to avoid disrupting the burning material (NFPA, 2022a).

Class K Fires: Cooking Oils and Fats

Class K fires involve combustible cooking media — vegetable and animal oils and fats used in commercial cooking equipment. The designation symbol is a black K. Class K is a subcategory of Class B, recognized separately because the extremely high temperatures involved in commercial cooking fires, and the re-flash potential of hot cooking oil, require a specific extinguishing approach that differs from other liquid fuel fires (NFPA, 2022a).

Wet chemical extinguishers containing potassium acetate-based agents are required for Class K fires. The wet chemical agent discharges as a fine mist, cools the cooking medium and appliance, and reacts with the oil to form a soap-like layer (saponification) that seals the surface and prevents re-ignition. Standard dry chemical extinguishers, while rated for Class B fires, are not appropriate for Class K fires in commercial cooking equipment (NFPA, 2022a).

Events with commercial food vendors, catering operations, or food trucks must ensure that Class K extinguishers are present at each cooking location. This is a specific and frequently overlooked requirement. A standard ABC extinguisher near a food vendor’s fryer does not meet the Class K requirement — and a grease fire suppressed with the wrong agent may re-ignite minutes later (NFPA, 2022a).

What Fire Classification Means for Event Planning

For event organizers, fire classification drives two related decisions: what types of fires are most likely at the specific event site, and what types of extinguishers are required in each location. The fire risk assessment should identify the dominant fuel types in each area of the event and specify the correct extinguisher class for each location.

A typical large outdoor event might have Class A risks throughout most of the site, Class B risks near generators and fuel storage areas, Class C risks at electrical distribution and production areas, and Class K risks at each food vendor location. This distribution means that a single extinguisher type — even a multipurpose ABC — is not appropriate for every location. Class K extinguishers must be at food stations; Class D materials require Class D-specific agents; ABC dry chemical placed near a grease cooking area does not satisfy the Class K requirement (NFPA, 2022a; FEMA, 2010).

References

Federal Emergency Management Agency. (2010). Special events contingency planning job aids manual. U.S. Department of Homeland Security.

National Fire Protection Association. (2022a). NFPA 10: Standard for portable fire extinguishers. NFPA.

National Fire Protection Association. (2024a). NFPA 1: Fire code. NFPA.