Fire Alarm System
Introduction To Fire Alarm System
- A fire alarm system is designed to detect the unwanted presence of fire by monitoring environmental changes associated with combustion. (automatically or manually actuated or both) .
- Automatic fire alarm system are intended to notify the building occupants to evacuate in the event of fire a fire or other emergency, report the event to the responsible authorities for emergency services, and to prepare the structure and associated system to control the spread of fire and smoke .
Fire alarm system components :
- Initiating devices .
- Control panel .
- Power Supply .
Initiating device
- This component acts as an input to the fire alarm control unit and are either manually or automatically actuated .
- Examples would be Pull stations,heat detectors, or smoke detectors .
- Heat and smoke detectors have different categories of both kinds
Manual initiating devices
Manually actuated devices, also known as fire alarm boxes, are installed to be readily located (near the exits), identified, and operated .
- Manual pull stations-break glass stations.
- Call points or Buttons .
Manual call points
- A manual call point is advice which enables personnel to raise an alarm in the event of a fire incident by pressing a frangible element to activate the alarm system .
- Manual call point should be installed at a height of (1.2 m) above floor level at easily accessible positions, on exit routes, at the entry floor landings of staircases and at all exits to the open air .
Manual call points should be spaced so that one may always be found within a maximum distance of (45 m ) apart or (25 m) for disabled person .
Fire alarm sounders
- Fire alarm sounders should be installed throughout the building with an even distribution, to generally provide a minimum sound level of (65 db) (A) OR (5 db)(B) above any back ground noise which is likely to persist for more than 30 seconds .
- Where the alarm may have to arouse sleeping persons e.g. Hotel bedrooms , Nursing homes, etc, a minimum sound level of (75 db)(A) is required, at the bed head with all the doors shut .
Automatic initiating devices
Automatically actuated devices can take many forms intended to respond to any number of detectable physical changes associated with fire :
- Heat detector .
- Smoke detectors .
- Flame detector .
- Fire gas detector, and release of extinguishing agents .
- The newest innovations can use cameras and computer algorithms to analyze the visible effects of fire and movement in applications inappropriate for or hostile to other detection methods.
Fire alarm detector
A device suitable for connection to a circuit that has a sensor that responds to a physical stimulus such as heat or smoke .
Types of detectors:
- Smoke detector
- Heat detector
- Beam detector
- Combined detector ( Smoke and Heat)
Smoke detector
- Smoke detectors are preferred since it senses fire existence first (smoke) .
- It includes a light source (Infrared led-light- emitting diode) and a photo diode or other photo electric sensor at an angle to the beam as a light detector. in the absence of smoke , the light detector . in the absence of smoke, the light passes in front of the detector in a straight line . when smoke enters the optical chamber across the path of the light beam , some light is scattered by the smoke .
In open spaces under flat horizontal ceilings, every point should lie within (7.5 m) of a smoke detector .
Smoke detection should be generally avoided in the following areas to avoid unwanted alarms :
- Contamination in dusty areas may cause unwanted alarms and reduce the life of the detector .
- Humid conditions such as showers, bath room and external area should be avoided as the water vapor may cause unwanted alarms and reduce the life of the detector
- Detectors should never be used at low temperature where ice or condensation can affect detector sensitivity .
- Kitchens, garages, welding shops and boiler houses should generally be avoided .
Heat detector
- A heat detector is a fire alarm device designed to respond when the convection thermal energy of a fire increases the temperature of a heat sensitive element .
- The most common fixed temperature point for electrically connected heat detectors is (58^C). Technological developments have enabled the perfection of detectors that activate at a temperature of (47^C),increasing the available reaction time and margin of safety .
- In open spaces under flat horizontal ceilings, every point should lie within (5.3 m) of a heat detector.
- Heat detectors are designed to either detect a rapid rise in temperature or to operate at a fixed temperature. although they provide a slower response time than smoke detectors they do provide a method of protection for areas where smoke detectors can not be used .
Beam detector
- A type of photo electric light obscuration smoke detector wherein the beam spans the protected area .
- Used for places which it is height is higher thane ( 10 m) .
- L=50 m or 100 m and installed each 15 m .
Fire alarm control unit (Panel)
A system component that receives inputs from automatic and manual fire alarm devices and might supply power to detection devices.
Addressable control panel
Addressable controle panel
- Allowed range of fire detectors to be connected to one loop is (126-200) detector.
- Maximum number of loops to be connected to an addressable controle panel is (10 loops) (2-4-6-8-10) .
Conventional control panel
Conventional control panel
- Allowed range of fire detectors to be connected to one loop is (20-30) detectors .
- Maximum number of zones to be connected to a conventional control panel is 8 zones (2 - 4 - 6 - 8 ) .
Power supply source
Primary power supply : Commonly the non-switched 120 or 240 volt alternating current source supplied from a commercial power utility . in non-residential applications, a branch circuit is dedicated applications, branch circuit is dedicated to the fir alarm system and its constituents . " Dedicated branch circuits " should not be confused with "Individual branch circuits " which supply energy to a single appliance .
- Secondary (back up) power supplies : this component .commonly consisting of sealed lead - acid storage batteries or other emergency sources including generators, is used to supply energy in the event of a primary power failure.
Notification appliances
This component uses energy supplied from the fire alarm system or other stored energy source, to inform the proximate persons of the need to take action, usually to evacuate .
This is done by means of :
- Flashing light .
- Strobe light .
- Electromechanical horn .
- Bell .
- Speaker .
Or a combination of these devices which makes a beeping sound and electromechanical sound together .
Exit doors
Hint
This type covers space . R = 7.5 M but R=5 m is used .
To ensure that there are no undress areas
Area = 3.14 * r^2 = 3.14 * 25 =80 m2
0 Comments