Alertness and Warning systems –fire Safety

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MIRROR: G K Bhat
The first aid firefighting systems are capable of handling the fire at its early stage only. In industries or any building where all the locations are not occupied by human being , it is difficult to know if fire takes place. The locations where there is no people or if the location is covered (Like ducts,cable trays ect) or isolated locations where there is possibility of fire, the detection system needs to be installed. Basically the fire detection system is nothing but the sensors which will sense the indications which will cause fire, like smoke, heat or spark ect.  Further the same will be communicated to the control room by the system. We will be discussing fire detection system in our next few topics.
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Fire detection system is basically the system which contains various types of fire detectors which detects the circumstances or indications which will cause fire. Also it indicates if a fire has already taken place but at its early stage.As shown above the fire detection system is consisting of various types of devices which can detect the fire and act according to the situation. These devices are connected at places, where possibility of fire ignition is visualised. An expert person in fire safety has to carry out the fire risk assessment and design the system for its efficiency and effectiveness.
Fire detectors: Few of the popularly known and used detectors are mentioned below.

  1. Smoke detectors
  2. Heat detectors
  3. Duct detectors
  4. Photoelectric type detectors
  5. IR type detectors
  6. Flame detectors
  7. Gas detectors
  8. Multi sensor detectors
  9. Beam detectors

The two primary types of residential smoke detectors contain either ionization or photoelectric sensors. Smoke detectors consist of two basic parts:
A sensor to sense smoke and a loud electronic alarm to alert the people. They can run off of a 9-volt battery or be hardwired into a 120-volt house current. Generally the smoke detectors are connected through a hardwire system in a loop to the main fire control panel. The control panel supplies the power required to the detectors. When the detector detects the indication of the fire or possible fire, it sends the signal to the main control panel and main control panel gets activated and the alarm is raised immediately. The hooter/speaker for the purpose of raising alarm is not necessarily located near the detector. The speaker/hooters are located where the people are available, so that the people gets alerted and take action according to the requirement. The fire alarm systems are always connected and powered through batteries so that there is no power failure. (The batteries, or backup of the system, should be tested on a regular basis and replaced periodically).
Commonly used detectors are:
Ionization
Photoelectric
Dual Sensor
 

Ionization:Ionization smoke detectors contain a very small amount of americium-241 within an ionization chamber. They create an electric current between two metal plates, which sound an alarm when disrupted by smoke entering the chamber. Ionization smoke alarms can quickly detect the small amounts of smoke produced by fast flaming fires, such as cooking fires or fires fuelled by paper or flammable liquids.

This type of smoke detector, which is commonly used in kitchens, is prone to nuisance tripping. For example, we’ve all experienced the loud annoying chirping when we leave a cake in the oven too long or add oil to an extremely hot pan. When this happens, people are more prone to disable the alarms.
Photoelectric: Photoelectric smoke detectors contain a light source in a light-sensitive electric sensor, which are positioned at 90-degree angles to one another. Normally, light from the light source shoots straight across and misses the sensor.  When smoke enters the chamber, it scatters the light, which then hits the sensor and triggers the alarm.
Photoelectric smoke detectors typically respond faster to a fire in its early, smouldering stage – before the source of the fire bursts into flames. These detectors are more sensitive to the large combustion particles that emanate during slow, smouldering fires, which usually occur at night when people are asleep.
Dual Sensor: Dualsensor smoke detectors include both ionization and photoelectric sensors, so they should adequately alert homeowners of a smouldering fire or a fire with active flames. Advantage of these types of detectors is they cover a broad range of fires.
However, there are no industry standards for setting the individual sensor sensitivity in dual sensor alarms. This means that a dual sensor alarm could have a non-functional ionization sensor, but as long as the photoelectric sensor works, it still meets the national standards developed by Underwriters Laboratories (UL).
Will be Continued ………
G K Bhat
[email protected]

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