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Hazard

Flashover, backdraught and fire gas ignition

Hazard Knowledge

Firefighters need an adequate understanding of the development of fires in ventilated and fuel controlled states, so they can recognise any potential fire development conditions. Tactics such as venting and indirect and direct application of water can then be used more effectively and safely.

A flashover is the stage where the total thermal radiation from the fire plume, hot gases and hot compartment boundaries causes all exposed combustible surfaces to pyrolyse (release flammable gases) and ignite when there is adequate ventilation. This sudden and sustained transition of a growing fire to a fully developed fire is known as a flashover.

All firefighters should be aware of the phenomenon termed backdraught. A backdraught is sudden and fierce and may occur at any stage during enclosed firefighting operations. Tragically, this type of event has killed firefighters in the past.

A backdraught is where limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and unburnt pyrolysis products (pyrolysates). If these accumulate, the admission of air when an opening is made to the compartment can lead to a sudden deflagration. This deflagration moving through the compartment and out of the opening is a backdraught. The force of a backdraught has the potential to damage building elements resulting in an unstable structure.

Fire gas ignitions occur when gases from a compartment fire have leaked into an adjacent compartment and mixed with the air within this additional area. This mixture may then fall within the appropriate flammable limits that, if ignited, will create an increase in pressure either with or without explosive force. Where this process occurs it is not necessary for an opening to be opened for such ignition to take place. If an explosive force is experienced, this is commonly termed a 'smoke explosion'. Where an ignition occurs with much less pressure, the term 'flash fire' is more appropriate.

Fire and rescue service personnel should be aware that the above phenomena are not mutually exclusive and all could be present at the same incident.