Flashover is a key event during a fire indicating the transition of a fire from an event that is a threat to the lives of the occupants to one where life is not tenable and the threat to the structure becomes significant. Analysis of compartment fires usually considers pre flashover and post flashover conditions separately adopting different assumptions and methods for each case.
Flashover can be a dramatic and well defined (almost explosive) event when a fire rapidly grows from a relatively small localised fire into a fire where all the combustible surfaces are burning and the space is filled with flames. However, in some cases while the pre- and post-flashover states can be clearly identified, the moment of the transition may be less well defined.
Estimating the time to flashover and understanding the nature of the flashover event is an important part of most fire engineering analysis as it marks the point where different fire engineering tools need to be used. It is also important for developing a fire safety strategy for a building, although fire safety designs may include measures to prevent, or reduce the likelihood of flashover occurring (e.g. the installation of sprinklers).
Theoretical and experimental investigations involving flashover have been ongoing since the 1950s, initially using small enclosures and scale models to identify the basic mechanisms in the context of domestic buildings, cellular offices and similar sized enclosures using wall linings and building contents typical of the day. While this work is still relevant today for similar enclosures, different mechanisms may be involved, or become dominant, especially in very large and/or highly insulated spaces encountered in modern buildings.
The technical outline of the flashover process and the factors that influence its occurrence and development are summarised in the Understanding the factors affecting flashover of a fire in modern buildings information paper.