How a fire alarm system works - (PART 1) - Flashover - Detection devices
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You probably have already heard it (at least in the movies): the sound of a bell ringing to warn people of a possible danger inside a building. For example a fire, which if not contained in time can cause great damage and harm.
Thanks to NECS, one of the leading companies in plant engineering for the electrical sector and in information technology, we will have a closer look at fires, including how they spread and how fire alarm systems work.
JAES, which has operated for over 10 years in the industrial supplies sector, has become the partner of choice for some of the most important companies that produce fire alarm systems.
One can define the phenomenon of combustion as an uncontrolled oxidation reaction that occurs quickly and with a strong increase of thermal energy and light. There are 3 necessary elements for the combustion process to start, namely an oxidising material, fuel and energy (or in other words, a heat source). These elements can be schematically represented with the so called fire triangle. (Use NECS image).
Obviously, no fire is the same because there are numerous variables at play, for example the environmental conditions and the type of flammable material. Nevertheless, one can identify four stages of a fire: incipient, growth, fully developed fire and decay.
Let’s have a brief look at the description of the four stages:
1. Incipient, which is the initial stage of the combustion process. An external heat source is applied to the combustible material for some time. If there’s a sufficient level of thermal power to reach the ignition temperature, the combustible mixture will start to burn locally. During this stage the fire can be contained quite easily.
2. Growth. During this stage the fire spreads depending on the surrounding conditions. This causes a slow but steady increase of temperatures as well as the production of smoke
3. Fully developed fire. The third stage starts with the transition from a growing to a fully developed fire which engulfs all locally available fuel sources (also called flashover). Consequently, the combustion process becomes uncontrollable.
4. Decay, during which the fire tends to slow down due to the depletion of fuel sources, therefore causing the temperature to decrease.
In order to minimize the potential damage caused by a fire, any fire-fighting intervention needs to be carried out before the flashover. The flashover, as already mentioned, marks the transition to a fully developed fire, during which all fuel sources are engulfed by the fire.
Fire alarm systems are designed to alert us of an emergency. They are usually found in offices, factories and public buildings. Whatever the detection method, once the alarm gets triggered, the sirens will start ringing in order to warn people of the potential fire threat and give them time to evacuate the
building. Some fire alarm systems also integrate a remote signalling system which is able to alert the fire brigade thanks to a central fire alarm control panel.
The fire alarm control panel represents the “brain” of the system. It is the central hub to which all detector signals are connected, and it provides precise information about which areas are affected by the fire. These detection devices are the most essential component of fire alarm systems, and they can range from very sophisticated smoke detectors to simple break glass units which are operated manually.
Indeed, there are many types of detection devices, such as:
- Heat and smoke detectors
- Carbon monoxide detectors
- Multi-sensor detectors
- Manual call points
In the next video we will dive more deeply into the different types of detection devices and have a closer look at how fire extinguishing systems are designed.
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