A05-0095

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Application Guide

Projected BeamSmoke Detector

P R O J E C T E D B E A M S M O K E D E T E C T O R

Section Contents

1 Glossary 2

2 Principles of Operation 4

Accessories 5Heat Detection Feature 5Proper Application

3 Beam Smoke Detectors vs. Spot-Type Smoke Detectors 6

Coverage 6Ceiling Height 6High Air Velocity 7Stratification 8Hostile Environments 8

4 Design Requirements 9

Sensitivity 9Location and Spacing 9Mounting 10

5 Conclusion 12

Foreword

The purpose of this guide is to provide information on the proper utilization of beam smoke detectors in life-safety and property protection applications. Thisguide briefly summarizes the principles of operation of projected beam smokedetectors, their design requirements, and practical applications for their use as acomponent of an automatic fire alarm system.

Beam smoke detectors can be important components of a well designed automaticfire alarm system. Because of their unique capabilities, beam smoke detectors can overcome many of the problems and limitations of spot type smoke detectorsin some applications. This guide was developed to help the fire alarm designergain an understanding of the beam smoke detector’s capabilities and limitations,and how they differ from spot-type smoke detectors.

Since equipment from different manufacturers has varying specifications and listings, the information in this guide is general in nature and should not be usedas a substitute for the manufacturer’s recommendations or code requirements.

Projected Beam Smoke Detector

2

.

Section 1

AnnunciatorA device which gives a visible or audible indication of the condition or status,such as normal, trouble, or alarm, of a smoke detector or system.

Automatic Gain Control (AGC)The ability of a beam smoke detector to compensate for light signal degradationdue to dust or dirt. Rate of compensation is limited to insure that the detector is still sensitive to slow, smoldering fires.

Beam Smoke Detector (Projected Beam Smoke Detector)A device which senses smoke or smoke and heat by projecting a light beam froma transmitter across the protected area to a receiver that monitors the light signal. Smoke and/or heat entering the beam path will decrease the light signalcausing an alarm.

Beam RangeThe distance between the transmitter and receiver. Where mirrors are used todirect the beam, the total beam range includes the distance between the mirrorsand the transmitter and receiver.

Detector CoverageThe area in which a smoke detector or heat detector is considered to effectivelysense smoke and/or heat. This area is limited by applicable listings and codes.

ListedThe inclusion of a device in a list published by a recognized testing organization,indicating that the device has been successfully tested to meet the accepted standards.

Obscuration (Cumulative Obscuration)The reduction of the ability of light to travel from one point to another due tothe presence of solids, liquids, gases, or aerosols. CUMULATIVE OBSCURATIONis a combination of the density of these light blocking particles per foot and the linear distance which these particles occupy, i.e., smoke density times the linear distance of the smoke field.

ReceiverThe device, in a projected beam smoke detector system, which monitors the signal level of the light which is sent by the transmitter.

SensitivityThe ability of a smoke detector to respond to a given level of smoke.

SmokeThe solid and gaseous airborne products of combustion.

Smoke ColorThe relative lightness or darkness of smoke, ranging from invisible to white togray to black.

Glossary of Terms

S Y S T E M S E N S O R

3Section 1 Glossary of Terms

Smoke DensityThe relative quantity of solid and gaseous airborne products of combustion in a given volume.

Spot-Type DetectorA device which senses smoke and/or heat at its location only. Spot-type detectorshave a defined area of coverage.

StratificationThe effect which occurs when smoke, which is hotter than the surrounding air, rises until equal to the temperature of the surrounding air, causing the smoketo stop rising.

Transmitter (Projector)The device in a projected beam smoke detector which projects the light acrossthe protected area.

Transparencies (Filters)A panel of glass or plastic having a known level of obscuration, which can beused to test the proper sensitivity level of a beam smoke detector.

Trouble ConditionThe status of a device or system which impairs its proper operation, i.e., open circuit on an initiation loop. The notification of a trouble condition indicatedon a control panel or annunciator is a “TROUBLE” SIGNAL.


4 Section 2 Principles of Operation

Projected beam smoke detectors consist of a transmitter that projectsan infrared beam across the protectedarea to a receiver containing a photosensitive cell, which monitorsthe signal strength of the light beam.

The detector works on the principle oflight obscuration. The photosensitiveelement of the beam smoke detectorsees light produced by the receiver in anormal condition. The receiver is cali-brated to a preset sensitivity levelbased on a percentage of total obscura-tion. This sensitivity level is determinedby the manufacturer based on thelength of the beam (the distancebetween the transmitter and receiver).Typically, more than one setting isavailable for selection by the installerbased on the length of the beam usedin a given application. For UL listeddetectors the sensitivity setting mustcomply with UL Standard 268,“Smoke Detectors for Fire ProtectiveSignaling Systems”.


The transmitter on some units may bepowered independently from thereceiver, which can greatly reducewiring runs and, therefore, installationcost. Since battery back up is requiredfor fire alarm systems, battery back up would be required for the transmit-ter whether it is powered from thepanel or independently.

Unlike spot type photoelectronic smokedetectors, beam smoke detectors aregenerally less response sensitive to thecolor of smoke. Therefore, a beamsmoke detector may be well suited toapplications unsuitable for spot-typephotoelectronic detectors, such asapplications where the anticipated firewould produce black smoke. Beamsmoke detectors do require visiblesmoke and therefore may not be assensitive as ion detectors in someapplications.

Beam smoke detectors are sensitive tothe cumulative obscuration presentedby a smoke field. This cumulativeobscuration is created by a combina-tion of smoke density and the lineardistance of the smoke field across theprojected light beam. Cumulativeobscuration, then, is a measure of thepercentage of light blockage.

Since the sudden and total obscurationof the light beam is not a typicalsmoke signature, the detector will seethis as a trouble condition, not analarm. This threshold is typically setby the manufacturer at a sensitivitylevel which exceeds 90 to 95% totalobscuration. This minimizes the possi-bility of an unwanted alarm due to theblockage of the beam by a solid object, such as a sign or ladder, beinginadvertently placed in the beam path.

5Section 2 Principles of Operation

Very small, slow changes in the qualityof the light source also are not typicalof a smoke signature. These changesmay occur because of environmentalconditions such as dust and dirt accu-mulation on the transmitter and/orreceiver’s optical assemblies. Thesechanges are typically compensated forby an automatic gain control (AGC).When the detector is first turned onand put through its setup program, itassumes the light signal level at thattime as a reference point for a normalcondition. As the quality of the lightsignal degrades over time, perhaps dueto dust, the AGC will compensate forthis change. The rate of compensationis limited to insure that the detectorwill still be sensitive to slow or smol-dering fires. When the AGC can nolonger compensate for the loss of sig-nal (as with an excessive accumulationof dirt) the detector will signal a trou-ble condition.

Receiver

Figure 1

Accessories to the beam smoke detec-tor may include remote annunciators,remote test stations which allow forthe periodic electronic testing of thedetector, and transparencies (filters)used as a “go/no go” test of the detec-tor’s proper calibration. Some manu-facturers provide for the use of mirrorsto direct the beam. Intelligent firealarm systems can give the beamsmoke detector a discrete address toprovide better annunciation of the firelocation. Conventional systems mayalso remotely annunciate through theuse of relays.

Accessories


Beam path or beam range

Some beam smoke detectors incorpo-rate a heat sensing element in thereceiver which monitors the frequencyof the beam pulse. Heat attenuates ordeflects the pulsed beam which can bereceived by the receiver, causing it toalarm. This deflection is generallygreater when the fire is closer to thetransmitter than the receiver. Careshould be taken that frequency modu-lation from fluorescent lights does notinterfere with this heat sensing feature.Follow the manufacturer’s recommen-dations.

Proper Application

Like spot-type smoke detectors, beamsmoke detectors are inappropriate foroutdoor applications. Environmentalconditions such as temperatureextremes, rain, snow, sleet, fog, anddew can interfere with the properoperation of the detector. Outdoorconditions make smoke behaviorimpossible to predict.

Heat Detection Feature

Transmitter or Projector

6 Section 3 Beam Smoke Detectors vs. Spot-type Smoke Detectors

Even though beam and spot-typesmoke detectors are governed by thesame UL and NFPA standards, therequirements under these standardsdiffer because the principle of theiroperation differs. It is important thatthe designer understand and give fullconsideration to these differenceswhen selecting and applying smokedetectors to fire alarm systems.

Spot-Type Detector900sq.ft. (30 ft. x 30 ft.)

Figure 2 Beam vs. Spot-Type Detec

Beam Detector19,800sq.ft. (330 ft. x 60

Theoretical Maximum Are

Spot-type smoke detectors are consid-ered to have a maximum coverage of900 sq. ft. or 30’x30’. The maximumlength between detectors is 41 feetwhen the width of the area being pro-tected does not exceed 10 feet, as in a hallway.

Beam smoke detectors generally have amaximum range of 330 feet and amaximum distance between detectorsof 60 feet. This gives the beam smokedetector theoretical coverage of 19,800sq. ft. (Figure 2). Manufacturer’s rec-ommendations and other factors, suchas room geometry, may impose practi-cal reductions of this maximum cover-age. Even with these reductions beamsmoke detectors can cover an areawhich would require a dozen or morespot-type detectors. Fewer devicesmean lower installation and mainte-nance cost.

Coverage


tors

ft.)

a Coverage

A spot type smoke detector’s responseis generally decreased as its distancefrom the fire increases. When ceilingheights exceed 16 feet the designershould consider whether the spacing ofspot-type detectors should be decreased.

This is not necessarily the case withbeam smoke detectors, which are ideallysuited for high ceiling applications.Some manufacturers allow increasedcoverage as the ceiling height increases.This is because of the anticipatedbehavior of a plume of smoke.

While not all fires start at the lowerelevations of the hazard, at or near thefloor level, this is a typical fire sce-nario. When this is the case the smokeproduced by the fire will rise to,or near the ceiling. Typically the col-umn of smoke begins to spread out asit travels from its point of origin,

Ceiling Height

7Section 3 Beam Smoke Detectors vs. Spot-type Smoke Detectors

Figure 3 Typical smoke field in the shape of an inverted cone

12 to 18 inches

Smoke Field

Ceiling

Inverted Cone Smoke Field

forming a smoke field in the shape ofan inverted cone. The density of thesmoke field can be affected by the rateof growth of the fire. Fast fires tend toproduce more uniform densitythroughout the smoke field than slowburning fires where there may be dilu-tion at the upper elevations of thesmoke field.In some applications, especially wherehigh ceilings are present, beam smokedetectors may be more responsive toslow or smoldering fires than spot-typedetectors because they are looking acrossthe entire smoke field intersecting thebeam. Spot-type detectors can only sam-ple smoke at their particular “spot”. Thesmoke which enters the chamber maybe diluted below the alarm threshold(level of smoke needed for an alarm).

The major limitation of the projectedbeam smoke detector is that it is aline-of-sight device and is thereforesubject to interference from any objector person which might enter the beam path. This makes its use imprac-tical in most occupied areas with normal ceiling heights.

However, many facilities have areaswhere beam smoke detectors are notonly acceptable, but are the detector ofchoice. High ceiling areas such as atri-ums, lobbies, gymnasiums, sports arenas,museums, church sanctuaries, as wellas factories and warehouses might becandidates for beam smoke detectors.Many of these applications presentspecial problems for the installation ofspot-type detectors, and even greaterproblems for their proper mainte-nance. The use of beam smoke detec-tors in many of these areas may reducethese problems since fewer devices may be required, and the devices canbe mounted on walls, which are moreaccessible than ceilings.

High Air Velocity

High air movement areas present aspecial problem for detecting smoke forboth spot-type and beam smoke detectors because the propagation ofsmoke developing under normal condi-tions may not occur. High air velocitymay blow smoke out of the sensingchamber of a spot-type detector. Careful


consideration should be given to thespot-type detector’s performance whereair velocities exceed 300 feet per minute(fpm) or when air changes in the protected area exceed 7.5 changes perhour. See NFPA 72-1999, 2-3.6.6.3.Beam smoke detectors are not testedfor listing purposes for stability inhigh air flow because high air move-ment does not have as great an effecton a beam smoke detector. A beamsmoke detector’s sensing range can beas long as a football field (maximumbeam range is typically 330’) not theone or two inch dimension of a spot-type sensing chamber. It is thereforeless likely that smoke will be blown outof the beam smoke detector’s sensingrange. Although reduced spacing is notrequired in high air flow areas, atten-tion should be given to the anticipatedbehavior of smoke in these applica-tions.

8 Section 3 Beam Smoke Detectors vs. Spot-type Smoke Detectors

12 to 18 inches

Beam Detector

Ceiling Mounted Spot-Type Detector

Smoke Field

Ceiling

Fresh Air (hot)

Stratification
Stratification occurs when smoke isheated by smoldering or burning mate-rials and becomes less dense than sur-rounding cooler air. The smoke risesuntil there is no longer a difference intemperature between the smoke andthe surrounding air. See NFPA 72-1999, A-2-3.6.1.4. Therefore, stratifi-cation may occur in areas where airtemperature may be elevated at theceiling level, but especially where thereis a lack of ventilation. On smoothceilings, beam smoke detectors shouldgenerally be mounted between 12 and18 inches from the ceiling. In manycases, however, the location and sensi-tivity of the detectors shall be theresult of an engineering evaluation thatincludes the following: structural fea-tures, size and shape of the room and
Stratification

bays, occupancy and uses of the area,ceiling height, ceiling shape, surfaceand obstructions, ventilation, ambientenvironment, burning characteristics ofthe combustible materials present, andthe configuration of the contents in thearea to be protected.

Figure 4

One of the major limitations of spot-type smoke detectors is their inability tosurvive in hostile environments, suchas temperature extremes, dirt, humidi-ty, and corrosive gases. Beam smokedetectors may also be subject to someof these debilitating elements. Sincethe beam smoke detector can, insome applications, be placed behindclear glass windows outside the haz-ard, they can overcome these effects.However, windows must be kept cleanand free of obstructions. This featuremay also allow them to be usedin applications where explosion pro-tection is required.

Barns and stables housing animals orequipment are good examples whereearly warning is required but wherespot-type smoke detectors are unsuitedbecause of temperature extremes and

Hostile Environments


dusty, dirty conditions. Beam smokedetectors may be a good alternativebecause their optics can be kept behindwindows which are easily cleaned on aregular basis. They may also have amuch wider operating temperaturerange than spot-type smoke detectors.

9Section 4 Design Requirements

Many factors affect the performance ofsmoke detectors of all types. The typeand amount of combustibles, the rateof fire growth, the proximity of thedetector to the fire, and ventilation fac-tors are all important considerations.

UL listed beam smoke detectors aretested using the 268 Standard, “SmokeDetectors for Fire Protective SignalingSystems” and should be installed andmaintained in accordance with NFPA72, The National Fire Alarm Code.

Sensitivity

Each manufacturer requires that thedetector’s sensitivity be set with referenceto the length of the beam used on agiven application. The detector shouldbe installed within the minimum andmaximum beam length allowed by themanufacturer’s instructions which arelimited by the UL listing.

Location and Spacing

NFPA 72 is the standard for locationand spacing of detectors. The followingdesign criteria are quoted or summarizedfrom NFPA 72, The National FireAlarm Code:

“For location and spacing of projectedbeam smoke detectors, the manufac-turer’s installation instructions shall befollowed.”NFPA 72-1999, 2-3.4.5.2

“Projected beam-type smoke detectorsshall be located with their projectedbeams parallel to the ceiling and inaccordance with the manufacturer’sdocumented instructions. The effectsof stratification shall be evaluatedwhen locating the detectors.”NFPA 72-1999, 2-3.4.4

“Exception: Beams may be installed ver-tically, or at any angle needed to affordprotection of the hazard involved (forexample, vertical beams through theopen shaft area of a stairwell wherethere is a clear vertical space insidethe handrails).”NFPA 72-1999, 2-3.4.4

For solid joist and beam construction(where the joist or beams are less than1 foot deep and the ceiling is 12 feetor lower) no reduction in spacing isrequired if the beam is projected per-pendicular to the joists or beams. (SeeNFPA 72-1999, 2-3.4.6.1)

“For beam depths exceeding 1 foot orfor ceiling heights exceeding 12 feet,detectors shall be located on the ceil-ing in every beam pocket.”NFPA 72-1999, 2-3.4.6.1

10 Section 4 Design Requirements

Beam smoke detectors must be mounted on stable stationary surfacesto prevent movement and subsequentmisalignment.

Sid

e W

all

Mounting

Because beam smoke detectors are line-of-sight devices which go into troubleon sudden and total loss of signal, caremust be taken that all opaque obstaclesbe kept clear of the beam path at alltimes. (See NFPA 72-1999, 2-3.6.3.)This requirement could make the useof beam smoke detectors impracticalin factory applications where overheadcranes and hoists are present and inwarehouses where high fork lifts mayblock the beam. This factor shouldalso be considered in occupied areaswhere normal ceiling heights exist.


“

Spacing for Smooth CeilingSide View

TX TX

End

Wal

l

Ceiling

12 to 18 in.

1/2 S S

Figure 5 Spacing for Smooth Ceiling

Location and spacing limitations arealso outlined in NFPA 72 as follows:On smooth ceilings, a space of notmore than 60 ft. (18.3m) between pro-jected beams, and not more than one-half that spacing between a projectedbeam and a sidewall (wall parallel tothe beam travel) may be used as a guide.

Other spacing may be determineddepending on the ceiling height, air-flow characteristics, and responserequirements. (See Figure 5.)

In some cases, the light beam projectoris mounted on one end wall, with thelight beam receiver mounted on theopposite wall. However, it is also per-mitted to suspend the projector andreceiver from the ceiling at a distancefrom the end walls not exceeding one-quarter the selected spacing.”NFPA 72-1999, A-2-3.4.5.2

Spacing for Smooth CeilingTop View

TX

1/2 S Maximum

1/4 S Maximum

S

30 ft. Minimum 330 ft. Maximum

Side Wall

RX

TX RX

111111Section 4 Design Requirements

Sloped CeilingPeaked Type

Sloped CeilingShed Type

1/2 S

1/2 S

S

3 ft. max.

S

3 ft. max.

S

S

1/2 Smax.

Figure 6

It should be noted that smoke originat-ing behind the transmitter or receivercannot be sensed by the detector unlessand until the smoke migrates into thebeam path. Therefore, considerationshould be given to keeping this dimen-sion to a minimum where possible.

Although the above example allowsa maximum of 60 foot spacingbetween detectors, manufacturer’s rec-ommendations may limit this criterion.Other design factors also need to beconsidered when spacing detectors.(See Figures 5 and 6.)

Consideration must also be given tothe need for a rapid response due tolife safety factors, or the high value ofthe hazard. Spacing should be reducedwhere these factors apply, or where theanticipated fire will produce limitedsmoke, especially in its early stages.Ceiling mounted detectors in a veryhigh atrium of a hotel, for instance,may need to be supplemented by addi-tional detectors at lower elevations.

In applications where reduced spacingis required, care should be taken tokeep two parallel beams at a minimumdistance so that the receiver from onedetector cannot see the light sourcefrom another detector. Wheretwo or more detectors are installedwith their respective beams at angles,care should be taken that the receiverof each detector can sense only thelight from its own transmitter.

Some manufacturers may provide forthe use of mirrors to direct the path ofthe beam. There may be requirements,however, to reduce the maximumlength of the beam range.

NFPA 72 states that, where mirrorsare used, they shall be installed inaccordance with the manufacturer’sinstructions. While this technique maybe useful in some applications, specialattention must be given to the stabilityof mounting surfaces, as the possibilityof misalignment increases with thenumber of devices to be aligned.


Beam smoke detector transmitters andreceivers may be placed behind clearglass windows with little effect on per-formance. It is normally recommendedthat the maximum beam range bereduced by 10% for each window.Plastic should not be used in thisapplication. Be sure to follow manu-facturer’s recommendations.

12


Section 5 Projected Beam Smoke Detectors

Notes

While beam smoke detectors are not suited for all applications, theycan be the detector of choice in many applications where spot-typedetectors are not practical.Recognizing the capabilities and limitations of all types of smoke detectors is essential to the properdesign of any automatic fire alarm system.

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