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Basics of Fire Alarm Systems

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Be sure to use Annex A in the National Fire Alarm and Signaling Code. Many of the spacing requirements and details for installation are found there. 
Always read the code definitions so you understand what the word means in the Code. 
Advanced Smoke Detection

Line detection techniques use a projected beam of light that covers an elon- gated path. Therefore, all line type smoke detectors are of the photoelectric projected beam design. The typical projected beam detector consists of a light source (transmitter), a light beam receiver and a re- ceiver control which may or may not be combined in the same housing with the receiver. The latest beam detectors now in- corporate both transmitter and receiver in the same housing, with a mirror or reflector placed at the far end of the path to be covered. The majority of units on the market are of the four-wire configura- tion. That is, the receiver control requires power from the control panel or power supply, usually contains both alarm and trouble dry contacts and is wired to an initiating circuit in the same manner as a four-wire spot detector except that an end-of-line relay may not be required, since  the  receiver/control contains its own trouble contacts.

The light source projects a beam across a protected area. The light beam is typically infrared, and in some cases is modulated, to eliminate the possibil- ity of extraneous infrared radiation interfering with the operation. If a smoke build-up causes a gradual obscuration of the beam over a period of several sec- onds, the receiver control causes the alarm contacts to transfer. An abrupt in- terruption of the beam will cause a trouble indication.

Thus the receiver can also perform the function of an end-of-line relay so a trans- mitter power failure will be immediately detected as an abrupt interruption.

At present, the projected beam detector protects areas of up to 19,680 square feet (which is approximately the coverage of 21 spot type detectors) as opposed to the 900 square foot area typically protected by spot detectors. Even though the width of the beam might seem relatively narrow es- pecially at extreme ranges, (typically up to 100 meters), the standard recognizes the tendency of smoke to billow or “mushroom” as it rises, hence the large area of protection. Thus the detector is primarily concerned with visibility of the beam over a long path, rather than with the amount of smoke entering a spot detector smoke chamber.

Sensitivity of line detectors is expressed in terms of percentage obscuration. One such detector will go into alarm upon a 40 to 90% obscuration of the beam for a period of 5 or more seconds. In comparison to “spot” detector sensitivities of 2-3% per foot, this does not sound sensitive at all. However, remember this obscuration applies over an elongated path which could be over 300 feet long. Therefore when expressed in terms of obscuration per foot, the detector has a sen- sitivity which the standard lists as 0.2 to 2.3% per foot depending on the length of the protected area. Because the projected beam detector works  on  an obscuration basis and not on light scat- ter, the detector is “color blind” and responds readily to black smoke.
Since line detectors must project a light beam, it stands to reason thatcurrent consumption will be higher than that of a spot detector. Attempts to alleviate this situation have met with mixed success. One such method is to “flash” both the transmitter and receiver simultaneously and thus drastically reduce current consumption much in the same manner as spot detec- tors. Advanced technology using microprocessors have largely overcome this prob- lem, so both two-wire and “intelligent addressable” configurations are now becoming common.

Since projected beam detec- tors are not dependent on smoke chambers, most of the “spot” detector limita- tions do notapply. Depend- ing on the manufacturer, ambient temperatures can range from -22oF to 131oF, and some units are listed as waterproof, so the trans- mitters and receivers may be hosed down to remove contaminants. Most units are unaffected by presence of low levels of contami- nants in the surrounding atmosphere.

Virtually all projected beam detectors contain AGC (Au- tomatic Gain Control) cir- cuitry of one sort or another which makes compensat- ing sensitivity adjustments in the event of build-up of contaminants on the lenses. One such unit is capable of making a number of periodic adjustments and then will cause an intermit- tent trouble signal after the final adjustment. Various manufacturers have shown different preferences in regard to the length of the adjustment period, which may range from minutes up to 8 hours. The author feels that a longer period is pref- erable, since in one known instance, a detector with a very short adjustment period (in the vicinity of 10 minutes) kept compensating during a smoldering fire and failed to alarm.

Line detectors are ideally suited for protection of large, open areas. Because of the 100 meter range ca- pability, and their ability to function in the presence of low-level contaminants these detectors are ide- ally suited for use in ware- houses, atriums, tunnels, aircraft hangars, garages, stables, trash transfer sta- tions, freight terminals, cathedrals, museums, man- ufacturing facilities, etc. Correctional facilities with large open areas can also be protected.

The immunity of these de- tectors to air velocity also suits them for use in large plenums or areas of high air turbulence. They also have the ability to function through windows (except for single ended reflective units) with only a minor reduction in range, thus allowing them to protect explosive atmospheres

without actually being located in the atmosphere. Older editions of NFPA 72, the National Fire Alarm Code, contain an exception to the conven- tional installation methods stating that “Beams may be installed verti- cally, or at any angle needed to afford protection of the hazard involved. (Example: vertical beams through the open shaft area of a stairwell where there is a clear vertical space inside the handrails.)”. However, consult your local codes regarding installa- tion as some jurisdictions may adopt different versions of the National Fire Alarm Code.

Limitations, Advantages and Disadvantages
Beam detectors do have limitations, notably that they are limited to areas
no less than approximately 30 feet in length. Cost is considerably higher than that of “spot” detectors, making them economical only for the larger areas or hostile environments. Be- cause of their higher current require- ments, power supplies and standby battery size must also be taken into consideration.

On the other hand, their huge area of coverage (approximately the cover- age of 21 spot type detectors) makes installation costs much lower in large areas, and cost of periodic testing as required by the authorities is ex- tremely low in comparison to spot de- tectors, as the greatest problem with spot detectors in high ceiling areas is their inaccessibility. However, be- cause of the ability of many projected beam detectors to operate in hostile environments, they can protect areas that previously have been classed as unprotectable.

Laser Detectors
The latest Laser detectors operate on the photoelectronic principle but use a laser source for detection. The Laser detector is much more sophisticated than the conventional photo detector and therefore is used in specialized applications. Due to its nature and use of microprocessors, it is capable of operating with unusually high sen- sitivity, such as .02%/ft, which is 100 times more sensitive than that of the standard photoelectric detector!

Multicriteria Sensors
A new generation of advance multicriteria smoke detectors is also now on the market, making full use of mi- croprocessor circuitry and advanced detection sensors. The multicriteria sensor uses more than one element of fire to report an alarm. Some of these multi technologies are photo/ thermal, others are photo/CO, while others are photo/thermal/IR and CO all in one device. The photo/ thermal 135°F detector uses advanced on-board software to combine the signals from the photo and thermal elements to create a true multicriteria detector.

It’s capable of rejecting nuisance sources, but still responds quickly to real fires. These multicriteria detec- tors have the capability of adjusting their sensitivity according to their environment. Certain multicriteria photo/CO detectors contain both smoke and fire sensors which mea- sure both carbon monoxide (CO) and infrared that are common to most fires. Therefore, the processor reviews all the elements and decides that if input from all the elements is enough to constitute a legitimate warning of fire. Equation of all these factors virtually eliminates the possibility of nuisance alarms. These sensors are used in analog addressable fire alarm systems, although the technology is spreading to conventional detectors. In one instance, a conventional detec- tor operates at reduced sensitivity in order to minimize nuisance alarms. When CO is detected, the sensitivity is increased greatly, so the detector is ready to alarm in the event the pho- toelectric chamber sees smoke.

Air Sampling Detectors
Air sampling smoke detectors fall into an area that can neither be considered as “spot” detection or “line” detection. These detection systems are quite sophisticated and do not readily belong in a basic article as this. However they are becoming more and more common, and bear mention here.

The air sampling system consists of a network of pipes connected to a control unit. An aspirating pump sucks air through these pipes, passes

it through a filter to remove any large particles and transports it into the chamber of a laboratory-quality smoke detector (usually a laser detec- tor) that actually measures the smoke obscuration of the air. The detector is programmable so it can indicate various thresholds, such as “Alert”, “Action” or “Fire” levels. These thresholds are adjustable, depending on the environment and sensitiv- ity desired. Some of the detectors in these units have a sensitivity of 0.03% per foot! These detectors are also quite sophisticated as they can furnish continuous bar-graph readout of their status at any stage, and are adjustable over wide ranges with all sorts of provisions for compensation, indicate low or high air flow fault, contain verification features, etc.

Since these detectors can detect smoke during the earliest incipient stages of a fire, they are most suit- able for clean rooms, laboratories, museums, libraries, or any structure housing high value material.

At any rate, these systems can be con- nected to a conventional fire alarm initiating circuit in the same manner as any other smoke detector, and may be connected to a monitor module that will furnish an address to an ad- dressable fire alarm system.

Installation Guide
NFPA Standard 72 contains data governing installation of smoke and fire detectors and should be consulted for further information regarding spacing of detectors, etc. Keep in mind that, although the NFPA 72 2017 Edition has now been issued, some jurisdictions may not have adopted it in conjunction with their Fire Alarm Codes and older editions of the Standard may still be in force

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The majority of this module is reprinted from ECM Magazine and was originally authored by:
Anthony J. Shalna © 2009 Principal IMSA Representative to the Automatic Fire Alarm Association
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