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The ABCs of Fire Alarm Systems - Section IIBy Anthony J. Shalna 2009Principal IMSA Representative to the Automatic Fire Alarm Association

President: Southeastern Signalmen of Massachusetts Approvals Manager: Gamewell-FCI by Honeywell

In our rst installment, we discussed basic re alarm con-trol panels that contain one or more initiating circuits and

notication appliance circuits. We will go into greater detailabout addressable panels in future installments, but nowwant to discuss some of the devices that place the initiatingcircuits in alarm.

INITIATING DEVICES

Initiating devices commonly used to activate the initiatingcircuit of a re alarm control panel are: heat detectors, smokedetectors, water ow switches and manual (pull) stations.

In this installment, we will concern ourselves with heat detec-tors, which, like sprinkler heads are basically intended forproperty protection rather than for life safety.

Heat detectors fall into two basic styles of protection: Line,and Spot detection. Line detection protects areas over anelongated path. Spot detection protects an area resemblingthe area lit by a spotlight.

LINE DETECTION

Line heat detection is less common, but is invaluable forprotecting certain hazards. One of the most common linedetectors in use today consists of a twisted pair of wiresinsulated with a thermal coating that has a specic meltingpoint. If excess heat is applied to the cable, the insulationmelts, the wires short circuit together, and the control panelgoes into alarm. The system is restored by cutting out thedamaged section of cable and splicing in a new section.Figure 1 shows a typical line detection device.

Other types of linedetection makeuse of eutecticsalts or similarinsulation that isnon-conductiveuntil it reachesa specified tem-perature and thenconducts currentfrom one conduc-tor to the other.U n l e s s m a j o rdamage occurs,the insula t ionagain becomes non-conductive when the temperature drops,thus making this type of detection essentially self-restoring.Some older systems use copper tubing installed throughoutthe area, lled with air or gas under pressure. Diaphragmarrangements then respond to increases in pressure caused by heat, and close contacts, creating an alarm.

Line detection is best suited to servicing conveyor belts,escalators, raceways, wire troughs, tunnels, grain elevators,silos, etc. Weatherproof versions of line detection cable are

also available. This line can be stapledunder piers or wharfs, allowing out-

door weatherproof protection whereno other sensors would function properly.

SPOT DETECTION

Spot detectors cover a nite area that varies according to therated sensitivity of the detector and the distance (height) ofthe detector from the oor.

The most commonly used types of spot heat detectors are:Fixed Temperature, Rate of Rise, and Rate Anticipationdetectors.

Electronic (thermistor/microprocessor) detectors have beenintroduced fairly recently and may be used only with com-patible control panels, usually addressable panels. Withthe exception of the electronic versions, heat detectors aremechanical in nature, and contain contacts that close whenthe detector is in alarm, making them compatible with anyconventional control panel.

FIXED TEMPERATURE DETECTORS

The xed temperature detector goes into alarm ideally whenthe ambient temperature reaches a certain setpoint. The mostcommonly used xed temperature detectors operate on twodifferent principles: fusible alloy and bimetallic strip.

FUSIBLE ALLOY FIXED TEMPERATURE DETECTORS

The fusible alloy unit uses an alloy physically resembling sol-der, but with a much lower melting point. The most commontemperature melting points are in the vicinity of 135 o F and190-200o F, depending on the manufacturer. The alloy holdsa spring type mechanism in place. This mechanism holdsa spring in an extended position keeping a set of contactsopen. When the alloy reaches its melting point, the spring isreleased, allowing the contacts to close, placing the detectorin alarm. The detector is usually non-resettable, and eitherthe detector or fusible unit must be replaced after actuation.Figure 2 shows a popular xed temperature detector withreplaceable element.

Figure 1

Figure 2

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BIMETALLIC STRIP FIXED TEMPERATURE DETECTORS

The bimetallic strip unit contains a strip of metal, plated oneach side with a different metal, each of which has a different

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Figure 3A

Figure 3B

Figure 4 shows a typical rate compensation heat detector

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coefcient of expansion. This means that, when heat is ap-plied, the metals expand at different rates, causing the stripto bend, or warp. When the strip bends enough, it touchesa contact, completing the circuit. Bimetallic strip detectorsare mostly used in household applications, since their listedarea of coverage is usually insufcient to meet Code require-ments for larger buildings. Advantages and disadvantages

of the various types of heat detectors will be summarizedat the end of the article.

RATE OF RISE HEAT DETECTORS

Another type of widely used heat detector is the Rate of Risedetector. This detector contains a chamber with a calibratedvent hole and diaphragm at the top. An actuator strip islocated above the diaphragm, just below a set of contacts.When the air outside the detector rises in temperature, the airinside the chamber likewise gets warmer, and as we all know,it expands. If the air expands gradually, it escapes through thecalibrated vent. If the air heats rapidly and expands too fast to be vented, pressure is exerted on the diaphragm, causing it to bulge, pushing the contacts closed and placing the detector in

alarm. The rate of temperature rise required to place a detectorin alarm is 15 o F in one minute, or equivalent, such as ve de-grees in 20 seconds. Therefore, this detector does not dependupon high temperatures to go into alarm, but senses a rapidrise in temperature. The rate of rise detector is self-restoring,since the diaphragm returns to normal as the ambient air cools.The ROR detector often has a xed temperature feature as aback-up in the event high temperatures are reached, whilethe temperature rises too slowly to activate the rate of risefeature. This detector is referred to as a combination FixedTemperature and Rate of Rise detector. Figures 3A and 3Bshow typical rate-of-rise/xed temperature detectors. Figure3A shows a high prole detector while Figure 3B shows a lowprole version developed for use in nished interiors. Both

operate identically.

RATE ANTICIPATION DETECTORS

The xed temperature detector depends upon heat absorp-tion to activate it, and in some instances, a rapidly increasingtemperature could conceivably reach a hundred or moredegrees higher than the setpoint of the detector before the

fusible alloy could absorb enough heat to melt it. This isreferred to as thermal lag.

The rate anticipation detector was designed to eliminate ther-mal lag. The rate anticipation detector is cylindrical (cigarshaped), sealed, and contains a pair of bowed struts eachcontaining a contact. In normal operation, the struts are

bowed away from each other, separating the contacts. Thecylindrical case is made from a special alloy with a coefcientof expansion that allows it to expand rapidly. When the am- bient temperature rises, the detector case expands, (actuallystretching or elongating) until the internal struts are likewisestretched, causing their contacts to close, placing the detectorin alarm. This detector has very little thermal lag, and willgo into alarm as soon as the ambient temperature reaches thesetpoint of the detector regardless of the rate of rise. Sincethis detector is sealed, it is a simple matter to weatherproofor make it explosion proof. See Figure 4.

ELECTRONIC HEAT DETECTORS

The electronic detector depends upon thermistors or similarcomponents that change value when exposed to heat. Thisdetector requires operating voltage in order to measure thechange in value, and must also be listed by a NationallyRecognized Testing Laboratory (NRTL) as being compatiblewith the particular control panel initiating circuit, since itdoes not contain dry contacts, but alters the characteristicsof the circuit, placing it into alarm. This will be discussedin greater detail in a future installment.

APPLICATIONS

The rate of rise detector (ROR) responds to rapid increasesin temperature. Therefore, these detectors can be used inany normal ambient and are rated for greater spacing thanthe xed temperature detectors. They are especially suitedfor cooler ambients where they can detect a developing relong before a xed temperature unit can actuate. These de-tectors commonly incorporate a xed temperature elementfor reliability and are referred to as combination FT/RORdetectors.

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The ABCs of Fire AlarmSystems - Section II . . .

The xed temperature detector (FT) alarmswhen the ambient temperature reaches acertain setpoint, commonly 135 or 200 o F,and ignores any uctuations or sudden

increases in temperature. Therefore, thexed temperature detector is best suitedfor applications where rapid uctuationsin temperature can be encountered, suchas attics, boiler rooms, kitchens, bathroomsand loading platforms with forced hot airheaters. The 200 o F version should al-ways be employed in attics, boiler rooms,garages or kitchens. The most commonxed temperature detectors are usuallydestroyed upon activation and must bereplaced after the alarm.

The rate anticipation detector outperforms

the xed temperature detector since it willalarm faster than the FT detector in theevent of a rapid temperature increase upto the setpoint. It is costlier than the FT de-tector, but is also self-restoring, so doesntrequire replacement after actuation. Thisis a great advantage in installations wherestaging or scaffolding would be requiredto replace the detector.

ROR and FT detectors are available inweatherproof and explosion proof ver-sions, but require fairly costly housingsfor these versions. The rate anticipationdetector, being sealed, lends itself toweatherproof or explosion proof applica-tions at a moderate cost.

The rated spacings of these detectorsvary. The ROR detector mounted onlow ceilings can be spaced up to 50 feeton centers, depending on its listing,while the fusible element FT detector istypically rated at only 15 feet on centers.The bimetallic strip is rated for even less.Electronic/thermistor detectors may berated for greater spacings. For spacinginformation, refer to NFPA Standard 72,National Fire Alarm Code. This publica-tion contains all types of informationregarding installation, layout and spacingof detectors on all types of ceilings andelevations. Most state codes are now based on this standard.

Now that we have a basic understand-ing of the devices that place a re alarmcontrol panel into alarm, our next install-ment will concern itself with addressable(microprocessor based) re alarm controlpanels.