Fires begin when a flammable /combustible material, in combination with a sufficient quantity of an oxidizer i.e. oxygen gas or another oxygen-rich compound is exposed to heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. Fires can’t exist without existence all of these elements in right proportions. Once ignited, a chain reaction must take place whereby fires can sustain their own heat by the further release of heat energy in the process of combustion. This and may propagate, provided there is a continuous supply of oxygen and fuel.
Classes of Fire:
Fires fall within five broad classifications:
- Class 'A' Fire involving ordinary combustible materials
- Class 'B' Fire involving flammable liquids/liquid flammable solids
- Class 'C' Fire involving gases
- Class 'D' Fire involving burning metals
- Class 'F' Fire involving flammable liquids
The 5 Classes of Fire
- Class 'A' Fire involve ordinary combustible materials like wood, cloth and paper. Most fires are of this class.
- Class 'B' Fires involving flammable liquids or liquid flammable solids such as petrol, paraffin, paints, oils, greases and fat.
- Class 'C' Fires involving gases. Gaseous fires should be extinguished only by isolating the supply. Extinguishing a gas fire before the supply is off may cause an explosion.
- Class 'D' Fires involving burning metals. These should only be dealt with, by using special extinguishers, by personnel trained in the handling of combustible metals.
- Class 'F' Fires involving flammable liquids (Deep Fat Fryers)
4 Stages of Fire Growth
Incipient – This first stage begins when heat, oxygen and a fuel source combine and have a chemical reaction resulting in fire. This is also known as “ignition” and is usually represented by a very small fire which often (and hopefully) goes out on its own, before the following stages are reached. Recognizing a fire in this stage provides your best chance at extinguishing it or escaping from the venue of fire break out.
Growth – The growth stage is where the structures fire load and oxygen are used as fuel for the fire. There are numerous factors affecting the growth stage including where the fire started, what combustibles are near it, ceiling height and the potential for “thermal layering”. It is during this shortest of the 4 stages when a deadly “flashover” can occur; potentially trapping, injuring or killing firefighters.
Fully Developed – When the growth stage has reached its max and all combustible materials have been ignited, a fire is considered fully developed. This is the hottest phase of a fire and the most dangerous for anybody trapped within.
Decay – Usually the longest stage of a fire, the decay stage is characterized a significant decrease in oxygen or fuel, putting an end to the fire. Two common dangers during this stage are first – the existence of non-flaming combustibles, which can potentially start a new fire if not fully extinguished. Second, there is the danger of a backdraft when oxygen is reintroduced to a volatile, confined space.