What are the Stages of a Fire?

Updated: Feb 16

The more we know about how fires spread, and the life cycle of a fire, the more of a chance we have of stopping the fire from continuing to develop, or indeed returning.

We can use the example of a cigarette end dropped into a wastepaper basket to help us understand the stages of a fire, how it can spread so easily and become deadly. [1]

Incipient This first stage of the fire occurs immediately after ignition and before the spread of the fire develops. Although some people could be able to put out as fire in this stage, using a fire blanket or extinguisher for example, it is still advisable to leave the building and not attempt to fight the fire unless you are trained to do so. The rate of fire growth will depend on the combustibility of the item first ignited and other materials in the vicinity. For example, if a fire is started in a wastepaper basket by a cigarette end, then combustion will be fast and rapid, as paper is extremely flammable.

At this stage, most fire or smoke alarms will sound[2], and fire curtains will be deployed: allowing for increased protection for users during egress and compartmentation for the building. Growth The second stage of a fire is growth. As this stage continues, the fire growth will be relatively quick and hard to control. In this stage, a flashover becomes highly likely as the room’s temperature increases. Fire can spread from out wastepaper basket in multiple ways, including:

Direct Contact, when the flames have direct contact with another item, for example, growing flames touching a sofa that is near the wastepaper basket.

Conduction relies on the spread of flames via heat transfer. Good thermal conductors, like metals, can absorb and transfer heat, allowing the heating up of flammable materials around the metal that is absorbing and transferring heat: causing the fire to spread. This could happen as the metal skeleton of the sofa heats up and the sofa padding and lining catch fire, passing it on through the room.

Convection. Remember the message around escaping while staying low to the ground? Well, that advice is to directly stop you becoming victim to the risk of fire spread through convection, and to avoid the inhalation of dangerous, toxic gases. As hot gas and smoke rise, they become trapped by the ceiling, and with enough oxygen, fuel, and heat are in the area, fires can develop secondary to the original fire, in our example in the wastepaper basket. This dangerously hot gas and smoke is also very harmful in breathed in and can cause serious damage to the trachea and lungs.

Radiation describes the spread of heat via electro-magnetic waves; recognisable to us as the feeling of warmth on our skin on a hot sunny day. As the heat transfers through these electromagnetic waves, it can cause nearby objects to combust if enough radiated heat meets it.

If the temperature in the room reaches between 500-600°C, a Flashover is like to occur.

Flashovers cause all other combustibles in the space to ignite and release energy through combustion, along with smoke and potentially toxic gasses. This will cause the temperature in the room to spike and will cause very serious damage to the body if you are exposed to it. Lay people are very unlikely to survive a flashover, as it has even claimed the lives of some experienced firefighters. [3]

Fully Developed As explained by the Fire Triangle/ Tetrahedron model, if a fire has continued access to a good oxygen source, for example from being in a well-ventilated space, and has plenty of combustible material, then the fire will continue. It will peak in this fully developed stage and begin to die back, traditionally due to a lack of fuel or oxygen; in our example the fire will have consumed much, if not all, of the furniture and bric-a-brac in the room and is starting to die back.

If the fire has consumed most of the oxygen in the space and does not receive any from another source then at this level the fire will reduce back to a smouldering level, filling the space with soot. This smouldering would continue until the final, decay, stage.

Decay This final stage of the fire’s life cycle and will result in the fire ‘dying’ after it has consumed all the fuel and oxygen available. These fires will inevitably either burn themselves out or be fully extinguished by the fire brigade, putting them to an end. A serious concern here is a backdraught. If a window were to break, or a door be opened by the fire brigade for example, this could cause a backdraught, as oxygen is reintroduced to the ‘dying’ fire.

A Backdaught A backdraught [4] is the explosive reignition of soot and other fuel sources because of the introduction of fresh oxygen supplies. While lacking oxygen, fuel sources are gradually becoming hotter and hotter, and the reintroduction of oxygen will cause instantaneous ignition. This incredibly dangerous phenomenon can allow for a fire to return to the fully developed stage, to begin its descent into decay once more.

Some fires, especially if they are particularly large, (for example wildfires) can take weeks to die down, sustaining incredibly high temperatures and carries the possibility for reignition all the while.

In Summary The best cure against serious damage to your building and loves ones is fire prevention, and the use of active and passive fire protection in your building can ensure that users are protected and the damage to the building is minimised.

Fire curtains allow for compartmentation of open-plan spaces, easy egress routes for inhabitants, and access for the emergency services: all essential for the conservation of life and minimising the damage to property. If you would like any help with your fire protection needs, please do not hesitate to contact us here at Adexon®. _____________________________________________________________________________ [1] Image taken from ‘Fire Development and Fire Behavior Indicators,’ by Battalion Chief Ed Hartin, MS, EFO, MIFireE, CFO, found online at <http://cfbt-us.com/pdfs/FBIandFireDevelopment.pdf > 11.10.2021 [2] Paraphrased from ‘Stages of a Fire,’ taken from Fire Fighter Garage, <https://firefightergarage.com/stages-of-fire/> 11.10.2021 [3] Images taken from Debra Thornton, ‘Basic Firefighting’ slide show, <https://www.slideserve.com/debra/basic-firefighting> 11.10.2021

[4] Image from FireEngineering.com, 'USFA: Backdraft and Reading Smoke,' published 19.05.2020, <https://www.fireengineering.com/firefighting/usfa-backdraft-and-reading-smoke/#gref> 14.02.2022

Cover image from Pexels.com, courtesy of Pixabay

1 view0 comments

Recent Posts

See All