Phases and Classification of Fire Study Guide for McGraw-Hill's Firefighter Exams

Phases of Fire

There are three phases of fire: incipient (growth), free burning (fully developed), and smoldering (decay). Each phase has its own unique characteristics and dangers to firefighters and should be understood thoroughly to enhance safety during firefighting operations inside buildings and structures. These phases are part of the standard time/temperature curve, which helps in visualizing the heat energy and temperatures attained during a fire.

Incipient (Growth) Phase

Most fires extinguished by firefighters are in this phase. In this phase, the fire is in the beginning, slow fuel combustion stage, with the oxygen content in the area still within the normal range (21 percent). There is limited heat being generated but high levels of smoke production and flammable carbon monoxide (CO) gas. Physical destruction from fire is limited to the immediate surrounding area. In certain situations, the introduction of fresh air by firefighters entering the area of fire can cause pent up CO gas to react violently and explode (backdraft), leading to serious injury while increasing the intensity of the fire. Also during this phase, there is the possibility of fire gases reaching their ignition temperatures (flashover) causing the entire area's contents to become suddenly engulfed in fire, greatly increasing the temperature of the fire and leading to the next phase of fire, the free-burning phase.

Free-Burning (Fully Developed) Phase

As fire spreads throughout an area, more heat and smoke are generated and travel in an upward direction toward the ceiling. During the free-burning phase, oxygen content in the area drops from 21 percent to approximately 15 percent, causing the volume of flames to eventually decrease, while smoke production continues to increase. When the oxygen level falls below 15 percent, flame generation ceases and the fire enters the next and last phase, the smoldering phase.

Smoldering (Decay) Phase

During this phase, the oxygen content in the area is below 15 percent, causing the rate of heat production and active flaming to decrease rapidly. Combustibles in the room have been largely consumed by the fire and are no longer actively burning. These combustibles, however, are still emitting large amounts of smoke and flammable gases. If fresh air (oxygen) is introduced into the fire area at this time, a backdraft situation is possible, since the influx of oxygen will complete the fire triangle and cause reignition of the flammable gas mixture in the area.