Fire depends on four things being present together, FUEL, HEAT, OXYGEN and A CHAIN REACTION. Fire will not start or continue if one is absent. FUEL is a combustible solid or liquid which gives off a vapor that will burn when raised to ignition temperature. It may also be a combustible gas (some examples are paint, paper, wood, linoleum, textiles, oils, and gases) Fire is a chemical reaction of substance raised to its ignition temperature with the presence of air. Combustion is a chemical reaction accompanied by the evolution of light and heat.

HEAT raises the temperature of a combustible substance to a point where vapor is produced. Flash point is at when enough vapor is formed to support a flame. Ignition temperature is at when the vapor is further heated ignites combined with the oxygen. Radiation is from an electrical radiator, match flame, explosion etc. Conduction through a suitable material such as steel bulkhead or deck (wood is a poor conductor of heat). Convection is the transfer of heat, through the motion of heated matter, that is through the motion of smoke, hot air, heated gases produced by the liquids and flying embers.

OXYGEN is present in the atmosphere with 21% oxygen. Ordinarily, a minimum concentration of 16% oxygen in the air is needed to support flaming combustion. However, smoldering combustion can take place in about 3% oxygen. CHAIN REACTION is the 4 th side of the fire-square and can be describe as a series of events, each of which causes or influences its succeeding event. For example, the burning vapor from a fire produces heat which releases and ignites more vapor, the additional vapor burns, producing more heat, which releases and ignites still more vapor, and so forth.

Class A fires involving common (ash producing) combustible solid materials which can be extinguished by the use of water solution. Class B fires involving flammable or combustible liquids, oil, greases paints and similar products. Extinguishment is accomplished by cutting off the supply of oxygen or by preventing flammable vapors from being given off, or by breaking the chain reaction. Class C fires involving gases, e.g. cooking gas, LPG, Butaine, Welding gas which can be extinguished by dry powder or CO2. Class D fires involving combustible metals, such as sodium, potassium, magnesium, titanium and aluminum. Extinguishment is effected through the use of heat-absorbing extinguishing agents such as certain special dry powder, that do not react with burning metal.