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This means that it is diffi cult to make an exact assessment as<br />
to whether a fl ashover is imminent or not. The rate of a fi re’s<br />
development obviously is largely dependent on the type of fuel<br />
involved and the fuel arrangement. It often takes time for the<br />
walls to heat up and the fi re often develops fairly slowly up<br />
until that point. Fire service staff need to be very observant.<br />
4.4.3 Course of action<br />
It is vitally important to try to extinguish the fi re before a<br />
fl ashover occurs, as the chances of saving lives, property and<br />
the surroundings are much better in this case. Obviously, the<br />
risks involved must be assessed in relation to what can be<br />
achieved by the operation.<br />
As we discussed earlier on, a fi re’s development can sometimes<br />
accelerate quite unexpectedly, which can lead to problems,<br />
especially in the case of long access routes. Alarm points<br />
can only withstand a fl ashover for a small number of seconds.<br />
This means that communication between the fi refi ghter commander<br />
and their crews is extremely vital.<br />
It is also important to remember that a normal nozzle cannot<br />
cope with fi res with a heat release rate in excess of 15–20<br />
MW. 30 This fi gure is larger than the amount of heat released in<br />
a normal apartment fi re, but apartment fi res where a lot of<br />
window panes have been broken get close to this level. It is<br />
possible to estimate the heat release rate for ventilation-controlled<br />
fi res, using the size of the openings.<br />
4.5 Summary<br />
The following defi nition of fl ashover is used in Sweden:<br />
During a compartment fi re a stage may be reached where the<br />
thermal radiation from the fi re, the hot gases and the hot enclosure<br />
surfaces cause all combustible surfaces in the fi re room to<br />
pyrolyse. This sudden and sustained transition of a growing fi re<br />
to a fully developed fi re is fl ashover.<br />
Flashovers occur in premises where there is good access to a<br />
supply of air during a particular stage of development. The<br />
room can have any kind of opening. The defi nition makes it<br />
105