A fire alarm system is a type of network consisting of a number of devices that are working together for the purpose of detecting and warning people by alarms when fire, smoke, carbon monoxide, or other emergencies are present. The alarms that a fire alarm system generates can be activated automatically from smoke detectors and heat detectors.
The alarms may also be activated via a manual fire alarm activation device, for example, manual call points or pull stations. The alarms used can be motorized bells or wall-mounted sounders or horns. In certain places, speaker strobes that sound an alarm are also used.
This alarm is usually followed by a voice evacuation message that will warn people to not use elevators inside the building. These can be set to certain frequencies and different tones including low, medium, and high based on the country and manufacturer of the devices.
The fire alarm system has a feature that enables a fire alarm at a sufficiently early stage so that people who are at risk can be notified at the earliest. This will help the people at risk to be safe by either escaping from the fire or extinguishing the fire itself. In this way, extensive property damage is also prevented.
These measures are taken when people are alerted, not before that. The fire detection and alarm system’s efficiency depends on the stage of the fire at which it is operated. An important parameter to consider is that the alarm must operate early so that people are able to evacuate the place before the escape routes become smoke logged. If the routes are smoke clogged, the people in the building would find it difficult to find their way out.
Functions of a Fire Alarm System
- The fire alarm system protects life and property.
- It detects a fire at an early stage.
- Alerts people and hence facilitate evacuation.
- The fire alarm system notifies the relevant personnel.
- Activates auxiliary functions like smoke controls
- Help in identifying the source and guiding firefighters.
Components of a Fire Alarm System
For a fire alarm system to function properly, it comprises some components. They are:
Initiating Devices
Initiating devices place the system in an alarm state. Basically, initiating devices can be ionization smoke detectors, photoelectric smoke and heat detectors, in-duct smoke detectors, manually operated pull stations, and sprinkler water flow sensors.
Indicating Appliances
Indicating appliances are used for announcing alerts to building occupants. Indicating appliances announce at a remote location when the system enters an alarm state. Examples are horns, strobe lights, bells, chimes, or combination units. Indicating appliances are weatherproof and suitable for installation in hazardous locations.
Control Panel
A control panel has programming and operating electronics and a user interface. A control panel has inputs through branch circuit wiring and contains a replaceable circuit card for each zone. A control panel has an alphanumeric display that shows the state of a system and provides information for troubleshooting. A control panel also has a touchpad for the onsite person to silence the alarm or trouble signal, reset the system after an event, and for reprogramming.
Batteries
Batteries are sealed batteries similar to emergency light batteries that are provided. Generally, these batteries are of 6 V and are wired in series to make 24V DC in a power-limited system. These batteries can be placed in a control panel or a separate enclosure. When the AC power fails, without any interruption these batteries take over in fire protection. A charger is also there so as to charge the batteries.
Auxiliary Devices
Auxiliary devices consist of remote annunciators with LED. Auxiliary devices show the state of the system, alarm silence switch, and a visual LED indication of the zone from where the fire alarm was initiated. There are electromagnetic door holders which are available. They can be floor or wall mounted. When there is an alarm, the magnet is de-energized and thus the door swing shuts. After the door shuts, it can be opened manually only.
Initiating Devices
Initiating devices are connected to the control panel through a 2 or 4-wire initiating device circuit. When there is a power-limited system, a 24 V DC is applied to the two wires heading to the string of initiating devices that are wired in parallel. The wire is not grounded. The polarity of this voltage is also critical. This voltage is used for powering up the solid-state circuitry within each detector. The control panel uses the Initiating devices for monitoring the state of the initiating devices and zone wiring (i.e. alarm or no alarm states).
Working of a Fire Alarm System
The fire alarm system has three states of operation: normal, alarm, and trouble. The alphanumeric display shows the current state of the system. All the indicating appliances throughout the building will go off when the system goes into alarm.
The alarm can be very loud or a soft chime depending on the type of occupancy. At this time, the control panel continuously monitors all the initiating device’s circuits for short and open wiring via the applied DC voltage. The initiating devices generally remain close. In the event of a fire, these become non-conductive and open.
Types of a Fire Alarm System
The fire alarm system has four types. They are: conventional, addressable, intelligent, and wireless.
Conventional Fire Alarm Systems
Conventional fire alarm system uses physical cabling for interconnecting several call points and detectors. The signals from these are wired back to the main control unit. The call points and detectors are arranged in different zones so as to simplify the process of locating the cause of the alarm.
It is important for both the fire brigade and general building management to locate the cause of the fire. In the fire alarm control panel each zone is indicated with an indicator lamp or a text display. In some cases, both indicator lamps and text displays are present. The more the division of the building into zones, the more it becomes easier to locate the cause of the alarm. The panel has a minimum of two sounder circuits for an audible fire alarm.
Addressable Fire Alarm Systems
Addressable fire alarm system follows the same detection principle as the conventional system, the only change is that here each detector gets a set address. The addressing is done usually by a DIP switch. The control panel thus can determine the exact detector or call point which has initiated an alarm.
The loop of the detection devices can have a maximum of 99 devices connected to each other (this number can increase or decrease depending upon the make and model number). Addressable fire alarm devices are not considered intelligent as they cannot decide whether there is a fire, fault, pre-alarm or other.
Intelligent Fire Alarm Systems
In the intelligent fire alarm system type, each detector has its own computer that is able to evaluate the environment around it and communicate it to the control panel whether there is fire, fault or the detector needs cleaning. Intelligent fire alarm systems are more complex. Intelligent fire alarm systems find use in preventing the occurrence of false alarms. Intelligent fire alarm systems are available in 2, 4, and 8-loop versions.
Wireless Fire Alarm Systems
Wireless fire alarm type of fire system makes use of secure, license-free radio communications for interconnecting the devices and sensors with the controllers.
