Observable characteristics of flashover

Definitions of flashover are multifarious but may be categorized as three types: mathematical, visual and those relying on diction. There are at least two arguments to be had with regard to the definition of flashover:(i) Is rapidity of transition to a state of total surface involvement an essential criterion for flashover?(ii) Is instability in the heat balance an essential criterion of flashover?This article postulates a one-dimensional scale with three major types of compartment fire (that evolve beyond smoldering but rely on unpressurized contained fuel); and two of those fire types can be split into two sub-categories. In all, five sub-categories exist ranging from fuel-restricted pre-flashover fires at one extreme, to vent-restricted pre-flashover fires at the other, via post-flashover fires in the middle where there is neither fuel nor vent restriction sufficient to prevent flashover; and with borderline (or weak flashover) fires lying between them on the scale.Each sub-category has a tendency to produce its own particular pattern of fire growth (in the form of hot gas temperature rise), which is different from one another and for which some explanation can be offered. By examining the temperature–time plots of 53 fixed-vent, single-source scaled laboratory fires it is concluded that rapidity of fire growth is a common but not an essential characteristic of flashover. Furthermore, conditions considered to be post-flashover can be attained without much/any evidence of thermal instability.It is suggested that the most convenient definition of ‘flashover’ for experimentalists is a visual one, based on observation of flame projecting from the vent; and that any dictionary style definition should be based on this observation.

► Definitions of flashover are examined and categorized.
► Laboratory tests show distinctive growth patterns for five types of flaming fire.
► Growth patterns and flashover indicators are compared and related to real fires.
► Rapidity of fire development is not essential to the definition of flashover.
► Flashover definition should rely on observable characteristics/measureable quantum.