Fire Classes, Extinguishing Agents and Portable Response

Key Takeaways

The fire tetrahedron includes fuel, oxidizer, heat and chain reaction.
Select an extinguisher rated for the actual fire class and hazards.
PASS means pull, aim at the base, squeeze and sweep.
Raise the alarm and retain a safe escape route before any trained portable response.

Last updated: October 2026

Science of Combustion: Fire Triangle & Fire Tetrahedron

Combustion is an exothermic chemical oxidation reaction releasing heat and light.

Fire Triangle vs. Fire Tetrahedron

Fire Triangle: The classical combustion model consisting of three components: Fuel (reducing agent), Oxygen (oxidizer, required concentration depends on the fuel and conditions), and Heat (thermal energy to reach auto-ignition temperature). Extinguishment occurs by cooling (removing heat), smothering (excluding oxygen), or starving (removing fuel).
Fire Tetrahedron: Modern science adds the fourth vital element: the uninhibited chemical chain reaction. During high-temperature combustion, fuel breaks down into highly reactive free radicals ( $H^\bullet, OH^\bullet, O^\bullet$ ) that propagate the flame front. Extinguishing agents like dry chemicals and halocarbon clean agents extinguish fire primarily by chemically neutralizing these free radicals.

Classification of Fires & Extinguishing Agents

Under NFPA 10 and Canadian standards, fires are divided into five classes based on the fuel:

Fire Class	Fuel Type & Plant Examples	Extinguishing Mechanism	Approved Extinguishing Agents	Prohibited Practices & Hazards
Class A	Ordinary solid combustibles (wood, paper, textiles, coal dust, rubber).	Cooling and quenching below ignition temperature.	Water (spray, fog, stream); Multi-purpose dry chemical (monoammonium phosphate).	Do not rely on $CO_2$ outdoors on deep-seated embers (re-ignition risk).
Class B	Flammable/combustible liquids and gases (fuel oil, diesel, turbine lube oil, natural gas, propane).	Smothering oxygen barrier; Chain reaction interruption.	Foam specifically rated and approved for the fuel and installation; Carbon Dioxide ( $CO_2$ ); Dry chemical (Purple-K potassium bicarbonate, sodium bicarbonate).	NEVER apply solid water streams to Class B. Water flashes to steam ( $1,700\times$ expansion), violently ejecting burning oil (boilover / splatter).
Class C	Energized electrical equipment (transformers, switchgear, motors, cable trays, DCS panels).	Non-conductive extinguishment without electrocuting the operator.	Carbon Dioxide ( $CO_2$ ); Dry chemical; Clean agents (Novec 1230, FM-200, Inergen).	NEVER use water or conductive foam on energized Class C equipment (fatal electrocution hazard). De-energizing converts it to Class A or B.
Class D	Combustible metals (magnesium, sodium, potassium, titanium, zirconium).	Smothering and crusting using specialized dry flux powders.	Class D Dry Powder (granular sodium chloride based Met-L-X, copper powder for lithium).	NEVER use water, $CO_2$ , or halocarbons on Class D. Molten metal strips oxygen from $H_2O$ and $CO_2$ , creating explosive hydrogen gas ( $H_2$ ).
Class K	Commercial cooking oils and animal fats (cafeteria deep-fat fryers).	Saponification: Alkaline agent reacts with hot fats to form a thick, soapy foam blanket.	Wet chemical extinguishing agents (potassium acetate, potassium carbonate solutions).	Never use plain water or dry chemical; violent splattering and re-flash occur due to extreme oil heat ( $> 300^\circ\text{C}$ ).

Portable Fire Extinguishers & Rating Nomenclature

The PASS Operating Sequence

Operating portable extinguishers follows the PASS protocol:

P - Pull: Pull the safety pin, breaking the plastic inspection seal.
A - Aim: Aim the nozzle at the base of the fire (burning fuel), not the upper flames.
S - Squeeze: Squeeze the operating trigger to discharge extinguishing agent.
S - Sweep: Sweep the nozzle side to side across the base of the fire until extinguished.

Extinguisher Numerical Ratings

Extinguisher nameplates carry alphanumeric ratings (e.g., $4\text{-A}:20\text{-B:C}$ ):

Class A Rating (Water Equivalency): Each unit of $1\text{-A}$ equals $1.25\text{ U.S. gallons}$ ( $4.73\text{ L}$ ) of water effectiveness. A $4\text{-A}$ rating equals $4 \times 1.25 = 5.0\text{ gallons}$ ( $18.9\text{ L}$ ) of water extinguishing capacity.
Class B Rating (Area Coverage): The number indicates the square footage of deep-layer flammable liquid fire the standard rating test can extinguish ( $20\text{-B} = 20\text{ sq ft}$ ).
Class C Rating: The letter "C" indicates the agent is electrically non-conductive; it has no numerical prefix.

Decide whether a portable response is appropriate

Raise the alarm, keep an escape route and follow the site emergency plan. Use a portable extinguisher only if trained, the fire is small and within your capability, the correct rated equipment is available and conditions remain safe. Smoke, rapid growth, pressurized fuel, hazardous chemicals or a threatened exit require withdrawal and emergency response. A rating is not a promise that any worker can extinguish that area under any conditions.

For a gas fire, extinguishing the visible flame without controlling the fuel can create an unburned explosive release. Isolation must follow the emergency plan and be possible without entering danger. Do not approach to close a valve through flame or a vapour cloud. For energized electrical equipment, an electrically suitable agent addresses one hazard but does not remove shock, arc or stored-energy risks. Qualified isolation and responder coordination remain essential. After apparent extinguishment, protect against re-ignition and wait for the required assessment before restarting equipment.

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2; Reference 3.

Test Your Knowledge

Why is a solid water jet unsuitable as a general method for extinguishing a flammable hydrocarbon pool fire?

Water chemically dissociates into hydrogen gas and accelerates metal combustion

It can spread burning fuel and, with sufficiently hot liquid, cause violent steam-related splashing

Water converts the liquid hydrocarbon into an electrically conductive Class C hazard

Water permanently neutralizes the chemical chain reaction and creates toxic chlorine fumes

Test Your Knowledge

An industrial dry chemical fire extinguisher displays the rating 4-A:20-B:C. What does the numerical prefix '4-A' specifically signify?

The extinguisher is certified to operate at 4,000 kPa internal cylinder pressure

The extinguisher will discharge its complete agent capacity in exactly 4 seconds

The extinguisher can suppress a metal Class D fire covering 4 square meters

The extinguisher has an extinguishing effectiveness equivalent to 5.0 U.S. gallons of water on Class A combustibles

Sections you finish are checked off in the contents.

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