How to Stop Combustion: Fire Triangle & Extinguishing Methods

Every fire that has ever burned depends on the same three things working together: fuel, heat and oxygen. Combustion is the chemical reaction between them, and the moment you take any one of those elements away, the reaction stops and the fire goes out. That simple idea — known as the fire triangle — sits behind every method of putting a fire out, from a kitchen fire blanket to a sprinkler system. This guide explains how combustion works, the three ways to break the triangle, and how to choose the right equipment for the type of fire you are facing.

Understanding combustion and the fire triangle

Combustion is a rapid chemical reaction in which a fuel combines with oxygen and releases heat and light. For that reaction to begin and continue, three elements must all be present at the same time. Picture them as the three sides of a triangle: remove any single side and the triangle collapses — the fire cannot start, and an existing fire is extinguished.

• Fuel — anything that will burn: solids such as wood, paper and plastic; liquids such as petrol and oils; gases such as propane and methane; and certain metals such as magnesium.
• Heat — an ignition source that raises the fuel to a high enough temperature for its vapours to ignite and keep the reaction going.
• Oxygen — fires generally need an oxygen concentration of at least around 16%; ordinary air contains roughly 21%, more than enough to sustain combustion.

This is the single most important principle in practical fire-fighting: to extinguish a fire you only need to remove one element of the fire triangle — fuel, heat or oxygen. Every extinguishing method, and every type of fire extinguisher, works by attacking one or more of these three sides.

Three ways to extinguish a fire

There are three fundamental methods of putting a fire out, each named after the side of the triangle it removes: cooling, starvation and smothering. Most extinguishing equipment uses one or a combination of these.

Cooling: removing the heat

Cooling reduces the temperature of the burning material below the point at which it can produce flammable vapours, breaking the heat side of the triangle. Water is the classic cooling agent — it absorbs a large amount of heat energy as it warms and turns to steam. Water mist and sprinkler systems work on the same principle, drawing heat out of the fire quickly while using less water overall.

Cooling is highly effective on ordinary combustible solids such as wood, paper and textiles. It must never be used on electrical fires or burning liquids, where water can conduct electricity or spread the fuel.

Starvation: removing the fuel

Starvation removes or isolates the fuel so the fire has nothing left to burn. In practice this can mean shutting off the gas supply to a leaking appliance, switching off a fuel pump, moving combustible materials away from the fire, or allowing a contained fire to burn out once its fuel is exhausted. Removing the fuel is often the safest and most decisive way to deal with fires fed by a controllable supply, such as a gas line.

Smothering: removing the oxygen

Smothering cuts off the supply of oxygen to the fire. A fire blanket draped over a pan fire, a layer of foam spread across a burning liquid, or carbon dioxide (CO₂) discharged over equipment all displace or exclude the oxygen the fire needs to continue. Once the concentration drops below the level required to sustain combustion, the fire is starved of air and goes out. Smothering is particularly useful where cooling with water would be dangerous, such as cooking-oil and electrical fires.

Selecting and using fire extinguishing equipment

Choosing the right equipment depends entirely on what is burning. Using the wrong extinguisher can be ineffective or actively dangerous — spraying water on a live electrical fire or a chip-pan fire can cause electrocution or a violent flare-up. UK fire extinguishers are colour-coded and classified by the type of fire they are designed to tackle.

When to use a fire blanket

A fire blanket smothers a fire by excluding oxygen and is ideal for small, contained fires — most commonly a pan fire on the hob. Placed carefully over the flames, it seals the fire from the air and is also suitable for wrapping around a person whose clothing has caught alight. Because it works by smothering rather than cooling, it avoids the dangerous splattering that water would cause on burning oil.

Tackling flammable liquids

Burning liquids must never be doused with water, which spreads the fuel and the fire. Foam and dry powder are the appropriate agents: foam forms a blanket over the liquid surface to smother the flames, while dry powder interrupts the chemical reaction. CO₂ can also be effective on smaller liquid fires.

Dealing with electrical fires

If it is safe to do so, the first step with an electrical fire is to isolate the power supply at the mains — this both removes a source of heat and eliminates the electrocution hazard. Never use water or a water-based extinguisher on live electrical equipment. Carbon dioxide (CO₂) extinguishers are the preferred choice because they smother the fire without leaving residue and do not conduct electricity; dry powder is an alternative. Once the equipment is de-energised, any remaining fire can be treated according to the material that is burning.

Fire safety measures and prevention

Stopping combustion is the last line of defence — preventing it is far better. A few measures make the biggest difference in any workplace or home:

• Fire alarms and prompt alerts. Working smoke and heat detectors give people the earliest possible warning, buying vital time to evacuate and to tackle a small fire before it spreads.
• Regular fire risk assessments. Under the Regulatory Reform (Fire Safety) Order 2005, the responsible person for any non-domestic premises must carry out and keep up to date a fire risk assessment, identifying ignition sources, fuel and the people at risk.
• Good housekeeping. Keeping combustible materials away from heat sources, maintaining electrical equipment, storing flammable liquids safely and keeping escape routes clear all reduce the chance of a fire starting and spreading.
• Trained staff. Appointing and training enough fire wardens ensures someone on site knows how to raise the alarm, use the right extinguisher and lead a safe evacuation.

Key takeaways

Combustion needs fuel, heat and oxygen together; remove any one and the fire goes out. The three practical methods are cooling (removing heat, e.g. with water), starvation (removing fuel, e.g. isolating a gas supply) and smothering (removing oxygen, e.g. with a fire blanket, foam or CO₂). Always match the extinguishing agent to the class of fire — and remember that prevention, early warning and trained people are what stop a small fire from becoming a disaster.

Mark McShane

Fire Safety Training Specialist, Online CPD Academy

Mark writes about workplace fire safety, compliance and accredited training for Fire Marshal Training, part of Online CPD Academy.