Fire Triangle vs Fire Tetrahedron: What’s the Difference?

The fire triangle is a simple model to understand fires. It shows the three elements needed for a fire to start and burn: heat, fuel and oxygen.

• Heat: The energy needed to raise a combustible material to its ignition point. It can come from many sources in the workplace, including electrical equipment, machinery, heaters and lighting.
• Fuel: The combustible material that can burn, like paper, fabric or chemicals.
• Oxygen: Supports the chemical reaction that is fire. It’s in the air around us.

While this might be interesting, the fire triangle is mostly used to explain the fundamentals of fire prevention and fighting:

• Stop the three elements from meeting and a fire won’t start.
• Take any one of the elements away and a fire will go out.

The fire tetrahedron is an updated model of the fire triangle. It includes heat, fuel, oxygen and a fourth element: the chemical chain reaction.

Fire is a combustion reaction between fuel and an oxidant, typically oxygen in the air. If these two elements combine and are heated to a high enough temperature, a fire ignites – as shown in the fire triangle.

The heat generated by this fire causes the fuel to release gases. These gases mix with oxygen in the air, creating a flammable mixture. This mixture also ignites and triggers a series of quick chemical reactions. During these reactions, fuel and oxygen molecules break apart and form new compounds, releasing energy in the form of heat.

This heat causes more fuel to release gases, which mix with oxygen and continue the combustion reaction known as fire. This ongoing cycle is the chemical chain reaction that keeps a fire burning.

Certain fires can keep burning even after one of the three elements is removed. This shows that the fire triangle isn’t always enough to explain fire behaviour.

The fire tetrahedron was developed as a better model. It adds the chemical chain reaction as the fourth element. This development helps us understand fire better and shows an additional way to put out fires: breaking the chemical chain reaction.

The fire triangle explains how to extinguish most fires. For example, a water extinguisher works well with the fire triangle. Water absorbs the heat from the fire and reduces the temperature of the fuel below its ignition point. This stops the fuel from releasing more gases, which are necessary for the fire to keep burning.

However, a water extinguisher is not suitable for all types of fires. For instance, water can make the situation worse in burning metal fires, known as Class D fires. Class D fires involve materials like magnesium or sodium that can react violently with water.

In these situations, we need to use the fire tetrahedron. Class D fires require breaking the chemical chain reaction to stop the fire. Special extinguishing agents like dry powder are used. These agents smother the fire, interrupt the chemical chain reaction and prevent the fire from continuing to burn.

Understanding the fire tetrahedron can significantly improve workplace fire safety, particularly in choosing the right fire extinguishers. Recognising all four elements – heat, fuel, oxygen and the chemical chain reaction – helps you select the right extinguishers for different types of fires that may break out in your workplace.

Here’s how understanding the fire tetrahedron aids in choosing the right fire extinguisher.

Class A fires involve organic solids like paper, wood and fabric and can be put out using water or foam Class A extinguishers. These extinguishers remove heat and cover the fuel, effectively breaking the fire triangle.

Class B fires involve flammable liquids, such as gasoline, oil and solvents. Workshops and storage facilities might face these risks. Class B extinguishers are designed to handle these fires by interrupting the chemical chain reaction, preventing the fire from spreading.

These fires involve flammable gases, such as propane, butane and methane. They’re a risk in industrial settings and areas where gas cylinders are used. Class C extinguishers work by smothering the fire and stopping the chemical chain reaction.

Class D fires involve combustible metals such as magnesium, aluminium and sodium. Manufacturing plants and laboratories dealing with these metals need Class D extinguishers. These extinguishers use dry powder to smother the fire and break the chemical chain reaction, preventing the fire from reigniting.

Class F fires involve cooking oils and fats, which are common in commercial kitchens. Water can make these fires worse, so Class F extinguishers use special agents to break the chemical chain reaction and safely extinguish the fire. They also discharge at a lower force than other extinguishers to minimise the risk of burning oils spreading.

Fire blankets are another option for tackling burning oils and fats. They can smother a fire and cut off the oxygen supply.

Electrical fires involve electrical equipment, such as computers, machinery and wiring. Water extinguishers are unsuitable for this type as water conducts electricity.

Instead, non-conductive extinguishers, such as CO² or dry powder extinguishers, are used to safely put out these fires by removing heat and interrupting the chemical chain reaction.

While the fire tetrahedron provides a deeper insight into how fires sustain themselves through the chemical chain reaction, most people don’t need to understand fire on a molecular level to maintain a safe environment.

Instead, workers need to know how to prevent fires in the first place and what to do should one happen.

Our online Fire Safety Training course covers these essentials. It teaches fire prevention strategies, evacuation principles and how to choose and use the correct extinguishers in an emergency.

By investing in Fire Safety Training, you ensure that everyone in the workplace is equipped with the knowledge and skills to prevent fires and handle emergencies effectively.