Principles of fire spread

Once a fire has started and there is sufficient fuel and oxygen to sustain it, there are three recognised ways in which it can spread within the building convection, conduction and radiation. There are also the effects of direct burning or heat transfer to take into account. 

Class F - fires that involve cooking mediums such as vegetable or animal oil and fats in cooking appliances. Such fires are particularly difficult to extinguish as they retain considerable heat allowing the chemical reaction to restart. 

It is worthy of note that there is no classification for electrical fires this is due to the fact that electricity does not actually involve any fuels which can be extinguished. Fires that involve electrical circuits and appliances, cables, etc. can either be extinguished with a non-conductive extinguishing medium, or the supply can be isolated and the actual material (given its own class ) can be extinguished. 

Due to its properties hot air rises this can be seen graphically when smoke from a bonfire rises and disperses within the atmosphere or a fire is started within a grate and rises up through the chimney. 

The convection process begins when combustible materials are subject to excessive levels of heat and they give off a vapour which in turn ignites. When these vapours are heated they expand and become less dense than air. As they rise they leave an area of low pressure which is replaced instantly by cooler unheated air. This fresh air is then mixed with the vapour and heated, assisting in the development of greater temperatures. The process is cyclic, continuing to support the fire process. 

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