Understanding how fires start and what maintains a fire is essential knowledge especially if you get in a situation where you may have to use fire extinguishers and other fire safety equipment.

It is also a key focus of a Fire Risk Assessor during a Fire Risk Assessment.  They will look for ignition sources and fire load (combustible materials) and how they can be kept apart. In essence removing an element of the fire triangle and fire tetrhedron.

The Fire Triangle

The Fire Triangle, also referred to as the Combustion Triangle, refers to the three components required to be present for a fire to start and to keep burning. These are: fuel, heat, and oxygen. Taking out just one of these components will extinguish a fire.

Fuel

Fuel is the material that fire burns and which keeps it burning. Highly combustible materials are found in all residential and commercial space such as oils, paper, textiles, and wood. This highlights the need to have appropriate storage for materials that pose fire risks.

Heat

A fire is ignited in the presence of a heat source. As the fire burns, it produces more heat, which in turn increases the temperature of the burning material. For most types of fires, applying water removes this element of the fire triangle. However, other fire classes require foam, powders, or chemical substances to reduce the heat and suffocate the fire.

Oxygen

When both fuel and heat are present, just 16% oxygen is enough to both trigger and sustain a fire. As the air contains 21% oxygen, there is more than enough to ignite and keep a fire going. Removing oxygen from the combustion triangle is possible with the use of fire blankets and fire extinguishers that work by displacing oxygen in the atmosphere.

The Fire Tetrahedron

Over the years, advances in research determined that it was necessary to add a fourth element to the existing three in the Fire Triangle. Thus, for a fire to occur, there has to be:

• Fuel (the combustible material),
• Heat (that raises the combustible material’s temperature to its flash point),
• Oxygen (which sustains the combustion process), and
• the Chemical Chain Reaction in the combustible material.

When the combustible material’s temperature reaches the right temperature, the chemical reaction becomes self-sustaining, making the consequent chemical reactions escalate that an external ignition source is no longer needed for the fire to keep going.

With this added fourth element, the Fire Triangle was changed to the Fire Tetrahedron to adequately symbolise these elements. A tetrahedron is a pyramid with four faces and six straight edges, in which all the faces are triangles. In this model, Fuel, Heat, Oxygen and Chemical Reaction are represented by the four triangular faces of the tetrahedron.

As with the Fire Triangle, taking out at least one of these elements extinguishes the fire. 

Some fire extinguishers work by removing one or more of these elements. For example, water extinguishers have a cooling effect thereby taking out the Heat element. Meanwhile, foam extinguishers create a barrier around the Fuel as well as take out Oxygen. Finally, other extinguishers contain a mix of substances which impedes the Chemical Chain Reaction to suppress the fire.

For an explanation of the various Fire Classes and their corresponding Fire Extinguishers, please refer to our Guide To The Different Types Of Fire Extinguishers In The UK.

For more information about fire risk assessments, including the bigger picture scenario of your building’s fire safety strategy, don’t hesitate to contact us.