Which condition is most likely to lead to flashover?

The condition that is most likely to lead to flashover is characterized by a mid-level neutral plane with sustained heat. In firefighting, the neutral plane refers to the level in a burning structure where the pressure is equal, typically dividing the smoke above from the fresh air below. When the neutral plane is mid-level, it indicates that hot gases and smoke are accumulating in the upper part of the space while fresh air can still be drawn in from below. If there is sustained heat, meaning that the temperatures in the upper layer of smoke are high enough and sufficient fuel is present, the likelihood of flashover increases significantly.

In this scenario, the sustained heat can preheat surrounding combustibles, making them more likely to ignite almost simultaneously when the heat reaches their ignition temperatures. This rapid ignition of multiple materials can result in a flashover, a phenomenon where the entire space ignites and becomes engulfed in flames.

Other conditions, such as a high or low neutral plane, or a stable neutral plane with good ventilation, do not provide the same conducive environment for flashover. A high neutral plane may indicate that flames are not effectively reaching all areas. A low neutral plane with minimal fuel lacks sufficient combustible material to sustain a rapid transition to flashover. Meanwhile