User Manual

21
Siemens Switzerland Ltd
Building Technologies Division
3. Fire detection
3.1. Basic considerations
The earlier a fire is detected, the more time there is for evacuation and firefighting, and the less damage can
occur. Earliest possible detection is thus the key to minimizing damage and gaining precious intervention time.
This is particularly true for historical buildings where fires can spread very rapidly, making fire suppression
considerably more difficult.
To ensure reliable, early detection of 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 fire phenomena propagate differently, depending on the room size and the room conditions such as a high air
circulation, which must be taken into account when planning a fire detection system.
Figure 8 Propagation characteristics of fire phenomena
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) decreases 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 importance in historical
buildings and museums, which generally have a large number of rooms with high ceilings.
To allow time to extinguish a fire before it has time get a 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.