Alertness and Warning systems –fire Safety

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MIRROR: G K BHAT
As of now we are clear that the “principle of working” of the beam smoke detectors.Therefore the signal between the transmitter and receiver is most important. Even the single ended reflective type of beam detector is also need highly stable mounting since the shaking of either the trans-receiver or the reflective unit will cause disruption is line of sight of the infrared light beam .
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Because beam smoke detectors are line-of-sight devices which go into trouble on sudden and total loss of signal, care must be taken that all opaque obstacles be kept clear of the beam path at all times. (See NFPA 72- 2002, 5.7.3.4.8) This requirement could make the use of beam smoke detectors impractical in factory applications where overhead cranes and hoists are present and in warehouses where high fork lifts may block the beam. This factor should also be considered in occupied areas where normal ceiling heights exist. Location and spacing limitations are also outlined in NFPA 72 as follows:

 

 
On smooth ceilings, a space of not more than 60 ft. (18.3m) between projected beams, and one-half of the maximum spacing between a projected beam and a sidewall (wall parallel to the beam travel) may be used as a guide. Other spacing may be determined depending on the ceiling height, airflow characteristics, and response requirements. In some cases, the light beam projector (same as transmitter/receiver) is mounted on one end wall, with the light beam receiver (same as reflector) mounted on the opposite wall. However, it is also permitted to suspend the projector and receiver from the ceiling at a distance from the end walls not exceeding one-quarter the selected spacing.” NFPA 72-2002, A-5.7.3.4 It should be noted that smoke originating behind the transmitter/receiver or reflector cannot be sensed by the detector unless and until the smoke migrates into the beam path. Therefore, consideration should be given to keeping this dimension to a minimum where possible. Although the above example allows a maximum of 60 foot spacing between detectors, manufacturer’s recommendations may limit this criterion. Other design factors also need to be considered when spacing detectors. Consideration must also be given to the need for a rapid response due to life safety factors, or the high value of the hazard. Spacing should be reduced where these factors apply, or where the anticipated fire will produce limited smoke, especially in its early stages. Ceiling mounted detectors in a very high atrium of a hotel, for instance, may need to be supplemented by additional detectors at lower elevations. In applications where reduced spacing is required, care should be taken to keep two parallel beams at a minimum distance so that the receiver from one detector cannot see the light source from another detector. Where two or more detectors are installed with their respective beams at angles, care should be taken that the receiver of each detector can sense only the light from its own transmitter. Follow the manufacturer’s testing procedures in the manual to ensure that this does not occur.
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Additional Mounting Considerations for Reflective Beam Detectors: There must be a permanent clear line of vision between the detector and the reflector. Reflective objects must not be near the line of vision between the detector and reflector. Reflective objects too near to the line of sight can reflect the light beam from the transmitter to the receiver. If this occurs, the detector will not be able to distinguish these reflections from those of the reflector and the protected space will be compromised. Reflective objects should be a minimum of 4 feet (1.2m) from the line of sight between the detector and reflector. Light sources of extreme intensity such as sunlight and halogen lamps, if directed at the receiver, can cause a dramatic signal change resulting in fault and alarm signals. To prevent this problem, direct sunlight into the transmitter/receiver unit should be avoided. There should be a minimum of 10° between the pathway of the light source and detector and the line of sight between detector and reflector. Operation of the detector through panes of glass should be avoided. Since single ended beam detectors operate on a reflection principle, a pane of glass perpendicular to the line of sight between the detector and the reflector can reflect the light beam from the transmitter to the receiver. If this occurs, the detector will not be able to distinguish these reflections from those of the reflector and the protected space will be compromised.
 
 
 
Single End Reflective beam Smoke detector : Panes of glass will also absorb some of the light as it passes through it. This absorption of light will reduce the acceptable installed distance between the detector and the reflector. In cases where operation through panes of glass cannot be avoided some specific installation practices can help to minimize the effects of the glass. These practices include: avoid penetration of multiple panes of glass, position the glass so that it is not perpendicular to the line of sight between the detector and the reflector, (A minimum of 10° off perpendicular should be considered), and make certain that the glass is smooth, clear and mounted securely. The complete reflector blockage test can be used to determine if the installation is acceptable. Where high ceilings (in excess of 30 feet or 9.1 meters) are present additional beams may be required to detect smoke at lower level
Note: During our discussion of the topic we will be coming across with the words like NFPA and UL standards. These are some standards accepted globally. We will discuss the same in our future topics separately
We will continue with our discussion related to fire detection systems in our next few topics
G K Bhat
[email protected]
 

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