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reliable fire detection

BUILDING SERVICESdesigning

Martin Carpentere: martin.carpenter@pmg.ie

The purpose of any firedetection system should be toprevent a fire breaking out asthe damage from smoke andfire can be costly in terms of lostbusiness associated withdowntime. The cost associatedalso with the suppression orcontainment of a fire can beconsiderable, hence anunderlying principle with firedetection should be to providethe most reliable smokedetection possible (see Figure 1).

It will never be possible toeliminate the possibility of fireentirely but it is possible to avoidloss of life and minimise thedamage to property byfollowing the standards as laidout in the code of practice forfire alarms and detection

systems, IS 3218:1989. This code iscurrently in review and isexpected to be re-issued laterthis year as part of EN 54. Atpresent all fire detection andalarm equipment must complywith the European StandardEN54 Parts 1 to 14.

It is common for the local fireauthority, insurance company orclient to determine the level offire detection coveragerequired, and whether it shouldbe life protection or propertyprotection. L1/M, the mostcomplete coverage whichprotects persons and property, isusual when starting any design.Other factors which will drive thedesign of a system include:— Type and speed of fire

/smoke detection;— Cost of equipment;

— System performance and reliability relating to anticipated/expected fire type;

— Aesthetics of any proposed system;

— Expected maintenance of proposed system.

With the advance of electronics,manufacturers have developedaddressable fire detectionsystems providing the exact firelocation, thereby reducingsubstantially the response timeto a fire and consequently theextent of fire or smoke damage.Analogue addressable systemsare very flexible as they providethe exact fire location and areduced false fire alarm ratedue to the filtering out ofunwanted alarms. Detectorscan be grouped into zonescorresponding to the buildinglayout and not the wiring pathtaken by the electricalcontractor.

The fire alarm system will usuallyhave programmable outputs todrive external equipment suchas voice evacuation systems,bells, sirens, klaxons, buildingmanagement system signal, etc.All analogue addressabledetectors are continuallymonitored with specific faultindication. Pre-warning of firemay be given for any areathereby eliminating fullevacuation, a feature which isparticularly useful for hotels andhospitals which may require aphased evacuation.

Figure 1

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reliable fire detectionThe types of fire detectorscommonly found in today’smarket include:—— Ionization smoke-type point

detector;— Optical smoke-type point

detector;— Fixed temperature/rate of

rise, heat detector;— Multisensor point type of

detector (a combination of smoke and heat);

— Break glass unit;— Beam detector;— Aspirating smoke detection;— High-sensitivity aspirating

smoke detection;— Linear heat detection cable;— Ultraviolet/Infrared (UV/IR)

flame detection;— Radio-linked fire detectors,

ionisation, optical.

All smoke detectors generate afire signal when a suitable levelof smoke particles pass across abeam generated by an LED orradioactive Isotope. Ambientsmoke levels are analysed andprocessed and compared withset fire patterns or algorithms.Suitable software protocols, suchas pre-warning or fire are set inmotion and may instruct the firealarm panel to operate in anevacuate mode. The systemmay lay inactive, therebyavoiding a nuisance alarm, ormay set an evacuationprocedure as already definedby the client and fire officer.

Point-type smoke detector’s arevery common and are generallyhighly reliable. With poormaintenance these items willbecome ineffective afterapproximately six years.

Ionization or optical smokedetectors are suitable in areaswhose aesthetic content will not

be spoiled. Maximum coveragearea of a smoke detector is 10metres square as defined in theIrish Code of Practice (IS3218:1989). It must be statedthat an ionization detectordetects a non-visible smokeplume, such as that found froman over-heating PVC cable,quicker than an opticaldetector, but is not suitable forhumid areas. If in doubt of thesource of fire, a combination ofionization and optical smokedetectors gives maximumprotection against a fire.

Heat detectors are suitable onlyin areas that will not allow thesiting of a smoke detector onaccount of potential nuisancealarms, typically in areas such askitchens, truck delivery lobbies,stables and flour mills. Heatdetectors operate by means ofa fixed temperature level and/orrate of rise sensing element,typically a non-linear resistorelement which provides a signalif the detection element sensesa fire by a rise of airtemperature. Heat detectorsprovide a signal too late forprotection of life, as fire hasalready occurred and thesource of the fire may beincreasing.

A combination of the smokeand heat detection elements inone unit yields a “multisensordetector” which requires bothheat and smoke before analarm signal is activated.Multisensor detectors have lessfalse alarms. However, theyoperate slightly slower than apoint-type optical smokedetector.

The most common means ofdetection and still the best is the

human nose. IS3218:1989 callsfor the installation of manual callpoints at all final exits and onescape routes. By the nature oftheir operation they are meantto be obvious and so they mustbe sited as recommended inIS3218:1989.

Standard point-type detectorsare commonly found to operateeffectively in most cases.However, when the area is non-standard then other detectionunits are required and are listedas follows:

Beam DetectionLarge open-plan areas such aswarehouse or shopping centreconcourses may be protectedusing beam detection. Beamdetectors operate by measuringthe concentration of smokethrough the attention of a lightbeam transmitted from onepoint and received throughanother. These units are ideal forhigh open-plan clean areas butthey do have their limitedapplications as access must bereadily available, not always aneasy task. Beams may beaffected easily by dust/mist orlarger flying objects such asballoons, bats/birds.

Line Heat DetectionA bi-metallic strip cable (up toapprox 500m in Length) willprovide a noticeable change inresistance to an abrupttemperature differential. Thisprovides the basis for a detectorthat may detect a fire by itsheat. This system is ideal forcovered car parks, tunnels,conveyor belts, or cellars but asthe detection is heat, firedamage will already haveoccurred.

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reliable fire detectionDuct DetectionA duct detector operates bytaking a sample of air from aprobe placed in the airflowpath. The sample air flows overa smoke detector placed in anenclosure. This type of detectionis common for high airflow ductsor air conditioning ducts. It isnecessary in all types of buildingand is commonly overlooked asan early means of detection.

Flame DetectionUltra-violet/infra red-flamedetectors operate by detectingthe natural UV/IR energy givenoff by the flame from a fire. Theranges of these units aresomewhat limited as they mustsee the flame. Their cost andlimited line of sight means thatthey are not commonly used.The unit is fantastic forprotecting lube oil tanks forinstance.

Line heat detection, flamedetection and duct detectionoperate by detecting the heat,flame and smoke, respectively,coming from a fire. Obviously, afire is developing rapidly at thisstage and damage is alreadydone. These forms of detectionare slightly more advanced thanthe first three mentioned,ionization, optical and heat, butthey do have their limitations.

The following two systemsprovide us with a means ofprotecting difficult access areassuch as domes, or inaccessibleroof voids commonly foundwithin buildings with highentrance lobby/atriums.

Radio-Controlled Fire DetectorsAs the name suggests, thesedetectors communicate bymeans of a radio signal and this

is a field with great potential.They are ideal for installing inplaces where cables areunacceptable or too difficult toinstall.

Panel-to-panel communicationis about two kilometres. Theability of this equipment tochange with use provides greatflexibility and a site test willcheck the system’s suitability.Remember, detectors can onlybe placed in accessible areas,as the batteries will requirereplacing at regular intervals.

High-Sensitivity Smoke DetectionSystemThe damage limit for differingdetection systems diminisheswith response, naturally enough.Very early detection systemyields least damage andprovides us with a very effectivedetection system withoutcompromising the aesthetics ofthe premises to be protected.The sensitivity of detectors andsystems are expressed asnormal, enhanced or very high.Typical very high sensitive

detectors detect by lightscattering obstruction per metre,particle counts per unit volume,or a combination of the two.

A high-sensitivity smoke detectorconsists of a central, very earlywarning, detector whichanalyses samples of air takenfrom a network of pipes. Thepipes are routed along ceilingswith holes drilled in positionswhere a normal detector wouldbe sited. The decorative ceilingsfound in great buildings, forexample, are areas where wemay discreetly place air samplecapillaries.

Most aspirating detectionsystems employ a range ofsensitivities depending on siteconditions and provide an idealfire detection system. Air fromeach area is drawn through thepipes back to the detector. Thegraph in Figure 2 indicates thereduction in fire damage bydetecting an incipient fire at theearliest possible moment.This now provides us with a veryeffective unique way of

Figure 2

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reliable fire detectionprotecting previouslydifficult/sensitive spaces such asheritage buildings, churches,highly decorated rooms, domes(Figure 3).

Future TrendsIt is impossible to predict thefuture of detection principles.However, technology advancesall the time and onemanufacturer has developed asystem whereby the detectorperforms a self-check or “selfverify” process that actually teststhe detector fully each day andwill print out a log of faulty unitson a site. This increase in

reliability will further increase theprotection of people andproperty from a fire.

Another manufacturer hasdeveloped a unit that detectsthe presence of a person withina room and provides the firebrigade with the room locationat the fire panel to aid inevacuation. Yet another systemsupplier has developed a “videodetection” system that “sees” aflame and will yield a fire signalin the event of a fire.

Disregarding future detectiondevelopments, it is clear that the

most important factor inproviding a reliable detectionsystem is to install a suitabledetection device into the areaof protection. The fire detectioncoverage within any premisesshould be ”complete“, meaningthat there should be nopossibility of a fire causing deathor building damage.

So, it follows that reliability iswhat is required above all elsefrom any fire detection andalarm system. A reliable firedetection system may beachieved by applying the mostsuitable detector for each areaand by applying a goodmaintenance regime whichregularly tests the detectors,sounders and associatedancillary items.

Martin Carpenter, PM Group is aSenior Electrical Engineerspecialising in fire safety. He is aFellow of the Institute ofEngineers in Ireland (FIEI) and hasover 18 years experience insystems design, construction,project management andengineering across a broadrange of projects. These includepower generation, hospitals,hotels, and internet hosting anddata centres, in Ireland, the UKand US. Martin’s expertiseincludes high-technologyinstallations, power systems anddesign of life safety systems inengineering design andmanagement, projectmanagement, contract andInstallation management,procurement and projectplanning, and businessdevelopment.

Figure 3