A fire is a chemical reaction in which a carbon based material (fuel), mixes with oxygen (usually as a component of air), and is heated to a point where flammable vapors are produced. These vapors can then come in contact with something that is hot enough to cause vapor ignition, and a resulting fire. In simple terms, something that can burn touches something that is hot, and a fire is produced.
A fire alarm system is number of devices working together to detect and warn people through visual and audio appliances when smoke, fire,carbon monoxide or other emergencies are present. These alarms may be activated from smoke detectors, and heat detectors.
Alarms can be either motorised bells or wall mountable sounders or horns. They can also be speaker strobes which sound an alarm, followed by a voice evacuation message which usually state in the lines of “Attention, Attention. A fire emergency has been reported. Please leave the building via the nearest exit and leave the building. DO NOT use elevators!” They may also be activated via Manual fire alarm activation devices such as manual call points or pull stations. Fire alarm sounders can be set to certain frequencies and different tones including low, medium and high depending on the country and manufacturer of the device.
All Fire Alarm Systems essentially operate on the same principle. If a detector detects smoke or heat or someone operates a break glass unit (manual break point), then alarm sounders operate to warn others in the building that there may be a fire and to evacuate. It may also incorporate remote signaling equipment which would alert the fire brigade via a central station.
Libraries, archives, museums, and historic structures frequently contain numerous fuels. These include books, manuscripts, records, artifacts, combustible interior finishes, cabinets, furnishings, and laboratory chemicals. It should be recognized that any item containing wood, plastic, paper, fabric, or combustible liquids is a potential fuel.
When the ignition source contacts the fuel, a fire can start. The characteristic smell of smoke is usually the first indication that an incipient fire is underway. It is during this stage that early detection (either human or automatic), followed by a timely response by qualified fire emergency professionals, can control the fire before significant losses occur.
During the incipient period, a trained person with portable fire extinguishers may be an effective first line of defense. However, should an immediate response fail or the fire grow rapidly, extinguisher capabilities can be surpassed within the first minute. More powerful suppression methods, either fire department hoses or automatic systems, then become essential.A key aspect of fire protection is to identify a developing fire emergency in a timely manner, and to alert the building’s occupants and fire emergency organizations. This is the role of fire detection and alarm systems.
A manually actuated devices can be an initiating device like fire alarm boxes, manual pull stations, or simply pull stations, Break glass stations, call points or Buttons. These devices are installed to be readily located (near the exits), identified, and operated.
An automatically actuated devices can take many forms intended to respond to any number of detectable physical changes associated with fire like convected thermal energy; heat detector, products of combustion; smoke detector, radiant energy; flame detector, combustion gasses; fire gas detector, and release of extinguishing agents; water-flow detector.
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.
The control panel is the “brain” of the fire detection and alarm system. It is responsible for monitoring the various alarm “input” devices such as manual and automatic detection components, and then activating alarm “output” devices such as horns, bells, warning lights, emergency telephone dialers, and building controls. Control panels may range from simple units with a single input and output zone, to complex computer driven systems that monitor several buildings over an entire campus.
There are two main control panel arrangements, conventional and addressable, which will be discussed below:-
1) Convential or Point Wired
The conventional systems are relatively simple for small to intermediate size buildings. The servicing does not require a large amount of specialized training.
2) Addressable or Intelligent Systems
Addressable or “intelligent” systems represent the current state-of-the-art in fire detection and alarm technology. These systems monitor and control the capabilities of each alarm initiating and signaling device through microprocessors and system software. In effect, each intelligent fire alarm system is a small computer overseeing and operating a series of input and output devices.
In an addressable system, each initiating device (automatic detector, manual station, sprinkler waterflow switch, etc.) is given a specific identification or “address”. This address is correspondingly programmed into the control panel’s memory with information such as the type of device, its location, and specific response details such as which alarm devices are to be activated.
Humans can be excellent fire detectors. The person is able to sense multiple aspects of a fire including the heat, flames, smoke, and odors. For this reason, most fire alarm systems are designed with one or more manual alarm activation devices to be used by the person who discovers a fire.
Different ways of fire detections are:-
- Manual fire detection.
- Thermal detectors.
- Flame detectors.
- Fire sprinklers.
Alarm Output Devices
Upon receiving an alarm notification, the fire alarm control panel must now tell someone that an emergency is underway. This is the primary function of the alarm output aspect of a system. Occupant signaling components include various audible and visual alerting components. Bells are the most common and familiar alarm sounding device . Horns are another option, and are especially well suited to areas where a loud signal is needed such as libraries and huge buildings. Speakers are the fourth alarm sounding option, which sound a reproducible signal such as a recorded voice message. They are often ideally suited for large, multi storied . With respect to visual alert, there are a number of strobe and flashing light devices. Visual alerting is required in spaces where ambient noise levels are high enough to preclude hearing sounding equipment, and where hearing impaired occupants may be found.
Other output functions include shutting down electrical equipment such as computers, shutting off air handling fans to prevent smoke migration, and shutting down operations such as chemical movement through piping in the alarmed area. They may also activate fans to extract smoke, which is a common function in large atria spaces. These systems can also activate discharge of gaseous fire extinguishing systems, or use sprinkler systems.