Enclosure fires

This means that it is diffi cult to make an exact assessment as
to whether a fl ashover is imminent or not. The rate of a fi re’s
development obviously is largely dependent on the type of fuel
involved and the fuel arrangement. It often takes time for the
walls to heat up and the fi re often develops fairly slowly up
until that point. Fire service staff need to be very observant.
4.4.3 Course of action
It is vitally important to try to extinguish the fi re before a
fl ashover occurs, as the chances of saving lives, property and
the surroundings are much better in this case. Obviously, the
risks involved must be assessed in relation to what can be
achieved by the operation.
As we discussed earlier on, a fi re’s development can sometimes
accelerate quite unexpectedly, which can lead to problems,
especially in the case of long access routes. Alarm points
can only withstand a fl ashover for a small number of seconds.
This means that communication between the fi refi ghter commander
and their crews is extremely vital.
It is also important to remember that a normal nozzle cannot
cope with fi res with a heat release rate in excess of 15–20
MW. 30 This fi gure is larger than the amount of heat released in
a normal apartment fi re, but apartment fi res where a lot of
window panes have been broken get close to this level. It is
possible to estimate the heat release rate for ventilation-controlled
fi res, using the size of the openings.
4.5 Summary
The following defi nition of fl ashover is used in Sweden:
During a compartment fi re a stage may be reached where the
thermal radiation from the fi re, the hot gases and the hot enclosure
surfaces cause all combustible surfaces in the fi re room to
pyrolyse. This sudden and sustained transition of a growing fi re
to a fully developed fi re is fl ashover.
Flashovers occur in premises where there is good access to a
supply of air during a particular stage of development. The
room can have any kind of opening. The defi nition makes it
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