The Fire Triangle

The Fire Triangle

A basic knowledge of the nature of fire is needed to assist in the understanding of how explosion-protection techniques make electrical equipment safe for use in hazardous areas. For an explosion or fire to occur three conditions must exist:

  • there must be a flammable material
  • there must be enough oxygen to enable the flammable material to burn
  • there must be a source of ignition

This is most commonly referred to as the ‘Fire Triangle’.



The first side of the triangle is oxygen. The oxygen for most fires is obtained from the air, which contains approximately 21% oxygen. If air can be excluded from the flammable material and ignition source, combustion cannot be started.

When air and a flammable gas/vapour (or combustible dust) are mixed there is a limited range of air to fuel ratios that can burn. Too much air causes the mixture to be too lean, and it will not burn. Not enough air, and the mixture will be too rich, and it will not burn. 

Thus, for each flammable gas, dust, or liquid vapour, there exists a lower and an upper air to fuel ratio for which combustion can take place. These limits are known as the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL) respectively.

The LEL and UEL vary significantly for different gases. For example, Nonane has a UEL of only 5.6%, whereas Hydrogen has a UEL of 77%.


Flammable material

The second side of the triangle is the flammable material. Flammable materials that could cause an explosion include:

  • flammable liquids;
  • combustible liquids;
  • combustible dusts; and
  • flammable gases.


Ignition source

The third side of the triangle is the ignition source. Electrical equipment can ignite a flammable mixture by two main processes.  Firstly, the equipment can create an arc or spark under normal or abnormal conditions with enough energy to ignite the mixture.  Secondly, the equipment could develop a surface temperature under normal or abnormal conditions, which is enough to ignite the mixture.  

If electrical equipment is designed so it cannot create a spark or surface temperature capable of causing ignition, or if the electrical equipment is located where there is no flammable mixture, a fire or an explosion (due to the electrical equipment) cannot occur.

By Brad Guy,

Senior Hazardous Area Specialist