User Manual

14
3 Fire detection
3.1 Basic considerations
The earlier a fire is detected, the more time there is for
evacuation and extinguishing the fire, and the less
damage can occur. Earliest possible detection is thus the
key to minimizing damage and gaining precious
intervention time. To ensure reliable detection at the
earliest possible stage of a fire, it is important to be
familiar with the different fire phenomena, fire
propagation and possible deceptive phenomena.
3.1.1 Fire phenomena
Fire phenomena are physical values that are subject to
measurable changes in the development of a fire (e.g.
smoke, heat, radiation and gas). Large volumes of volatile
fire aerosols are produced with almost all hostile fires.
Smoke has thus become the most important fire
phenomenon for the early detection of fire.
Liquid fires directly develop flames which immediately
create a temperature increase and heat radiation.
3.1.2 Fire propagation
The various fire phenomena propagate differently,
depending on room size and room conditions such as
high air circulation, which must be taken into account
when planning a fire detection system.
In general, the higher the room, the greater the distance
between the seat of a fire and the fire detectors on the
ceiling. Consequently the intensity of the fire
phenomenon to be detected (e.g. smoke density,
temperature increase or radiation intensity) diminishes
with increasing ceiling height.
It must also be taken into consideration that with
increasing ceiling height (and consequent larger room
volume) an incipient stage fire can become larger without
necessarily increasing the danger of rapid fire
propagation. This is of particular relevance in college and
university buildings, which generally have numerous
rooms with high ceilings.
To be able to extinguish a fire before it has time to take
hold and to minimize any damage that may occur, it is
imperative that incipient fires are discovered as early as
possible: highly sensitive smoke detection systems are
essential in this type of application.
3.1.3 Deceptive phenomena
Propagation characteristics of fire phenomena
A fire detector has the task of detecting fire from aerosols,
heat and radiation at an early stage and triggering an
alarm. Aerosols, heat and radiation, however, are also
generated by production processes, by electrical
equipment such as motors, or by environmental factors
such as sunlight. If these deceptive phenomena are
sufficiently intense and exist over a certain period of time,
they may influence fire detectors enough to trigger an
unwanted alarm. State-of-the-art fire detectors are largely
capable of distinguishing between deceptive phenomena
and genuine fires. If, however, massive deceptive
phenomena are to be expected, it is often necessary to
combine different fire detection principles to provide the
best possible solution for that particular application.