Phases of a Fire

Phases of a Fire

Incipient Phase

  • The Incipient Phase, also known as ignition phase, is the first stage of fire.
  • At this stage, a fire’s heat energy is less than the cooling energy of its surroundings.
  • Combustible materials start to heat up and pyrolyse, breaking down into various components.
  • Some parts of these materials vaporise and mix with oxygen, forming a flammable mix.
  • In this phase, the fire is usually controllable and can be extinguished with water or a fire extinguisher.

Growth Phase

  • During the Growth Phase, heat production exceeds the cooling capacity of the environment, and the fire starts to spread.
  • Further pyrolysis occurs, and the fire spreads by heat transfer.
  • This phase can be swift or slow, based on the availability and characteristics of fuel, existing ventilation conditions and the methods of heat transfer present.
  • The fire may create an updraft, dragging fresh air into the fire and pushing hot gases up and out.
  • In closed environments, while the oxygen levels decrease, the presence of hot gases and smoke increases proportionately, creating a thick, dark smoke overhead.

Fully Developed Phase

  • Reaching the Fully Developed Phase, emissions are intense and the fire is at its peak heat release rate.
  • The term flashover refers to the moment thoroughly heated gases and materials ignite almost simultaneously, causing the fire to become fully developed.
  • The heat and flames are emitted through available openings, including windows and doorways.
  • The fire consumes most of the available fuel and oxygen in the enclosed space.

Decay Phase

  • Finally, the Decay Phase sees the fire start to diminish, primarily due to the depletion of available fuel or oxygen.
  • However, very hot gases and harmful substances will still be present.
  • Secondary fires may occur if sufficient fuel and oxygen are reintroduced to hot materials.
  • A phenomenon known as backdraft can occur if fresh oxygen is suddenly introduced into the environment.

In Forensic Fire Investigation, understanding the four phases of a fire is critical. It supports the investigators in understanding the fire science, identifying the point of origin, observing the fire patterns and investigating the causes of fire.