Reliability Growth Testing of A Smoke Alarm

7

RELIABILITY GROWTH TESTING OF A SMOKE ALARM

ReliabilityGrowth Testing of A Smoke Alarm

ReliabilityGrowth Testing of A Smoke Alarm

TheNational Fire Protection Association (NFA)oftheUnited Statesbetween 2007-2011, smokealarmsrangin 5 out 10homesin which fireoutbreaksoccurred.Italsofurthernotedthat60% of deathsresultingfrom homedeathsoccurredin homesnot possessingsmokealarmsornon-functionalsmokealarms.Theassociationalsoreportedtheabsenceof smokealarmsin morethan 37% of deathsoccurringfrom homefires.Itis, therefore,a knownproblem,which can bereducedby theuseof properfunctioning smokealarms.Thisdocumentpresentsa casestudyof thedesignof a smokealarm.

Casestudy: Casestudy:Consider a timeterminatedreliabilitygrowthtestfora smokedetector.Ifthetestisconductedon timelapseandquantityof smokerequiredto stimulatesignalsfromthesensor. Thelengthof timerequiredfordetectionisdeterminedandanalyzed.Thesensor is theredesigned anda reliabilitygrowthtestcarriedout. Theamountof smokerequiredtriggering signalsfrom thesensor andquantityof smokerequiredwill betabulatedas below.

Table1: Detection time and smoke quantity for a smoke detector

S/N

Design

Detection time

Quantity of smoke

1

2

3

4

  • Fire outbreaks are emergencies that require early detection and so that a quick action should, are taken. It is also significant to note the detection should be done when the fire has just started, and little emission of smoke is taking place, implying that the sensor should be able to detect this little smoke generated.Smoke detectors operate by sensing a decrease in the room current. In ‘an average’ place there is the existence of equal number positively and negatively charged ions. This condition results in the flow of a small ionization current. This current can be measured, by the plates of a smoke detector as ‘normal’. Smoke particle cause a change in this room current and it is this change that that is detected by the smoke detector. However, other factors decrease existing in a room. These include Changes in humidity and atmospheric pressure, dust, small insects. When factors other than smoke cause change in current in the room consequently triggering an alarm, the outcome is that the building occupants will become desensitized and this could be dangerous in case of a real fire outbreak.Reliability growth models are used to filter sensor design in order to meet the target requirements.Modeling is therefore done in the design process to come up with a device that is best suited for its purpose. AS mentioned above, ‘nuisance’ alarm triggered by factors other smoke cause insensitivity of the occupants that may one-time cause negligence hence unnecessary loss of life. It is therefore of paramount importance that the smoke sensor deign puts into consideration all the above factors to optimize the safety of the occupants of a structure. By designing smoke sensors in this way, the loss of lives and property will be reduced.Reliability growth testing of smoke sensors will make their reliability better and can be done through the test-improve-test-improve cycle. The components of the sensor should be redesigned so that it exhibits the following features durability, accuracy, easy maintenance, reliability. The following features should be eliminated by the reliability tests sensing all changes in room current other than those caused by smoke, inaccuracy, high maintenance cost, non-repairable.The sensor components should be designed/redesigned through the following processes

  • Identify the purpose of the room/ building

  • Identify the possible sources of the current disruption in room/ building.

  • Develop systems to cater for the various current disruptions in the building.

  • An alternative design of the sensor is developed tested and improved where necessary.

  • The design is then implemented.

Othertypes of smoke detectors

Photoelectricsmoke detectors

Thesesmoke detectors use the principle of light scattering. These sensors work by directing a beam of light to an area not usuallyseen by aphotosensitiveelement, known a photodiode. When a fire startsthe smoke emitted from, it is struck by the light beam and isreflected onthe photosensitive device consequently causing a response from thedetector.

Smokedetector design considerations

Smokedetectorsshould be designedto minimizetheeffectof unnecessarysignalthat can arisedue to insects,dust,anddrywalldirt.Insensitivity of ionization chambersdue dirtaccumulationshould betakeninto account.Reflectionof lightfrom thesourceof lightoff thewallsof thehousemay trigger an alarmin theabsencesmoke.Nuisancealarmmaybe causedby electrical transientsandradiatedenergyin photoelectric smokedetectorsandionization types,andthisshould beconsideredin design.Firetestsare usedto verifydetectorperformance.Irrespective of their principleof operation,smokedetectorsshould satisfythesamefiretest.

Table2.2:Design considerations for smoke detectors

S/N

Source of nuisance

Type of detector

Ionization

Photoelectric

1

Dust

2

Low humidity

3

Change in atmospheric pressure

4

Insects

Dependingon the area of exposure the prototype each detector is tested if itdoes not meet the desired consideration, improvements are madeon the particular element and the testing repeated to assess theperformance.

Table2.3: Table for implementing the case study

&nbsp

Spread sheet for implementing the case study

S/N

Detector type

&nbsp

Source of nuisance

&nbsp

&nbsp

&nbsp

Ionization

Photoelectric

Dust

Low Humidity

Insects

Atmospheric pressure

1

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

2

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

3

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

4

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

5

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

6

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

7

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

8

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

9

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

10

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

&nbsp

Conclusion

Thisdocumentdescribesthedesignconsiderationsandtestingcarriedout to ascertainthesuitabilityof thetwo typesof smokesensors. Itcan beobservedthatthechoiceanddesignmay depend on thepurposeof thestructure.Itmay befurthernotedthatdetectorlocationis verycrucial.Asan example,theyshould not be fixedwheretheairstreamcarriessmokefrom elsewhereas thiswould starta nuisancealarm.During maintenancework,detectorsshould be protectedfrom dust.Alarmsystemscan alsobe affectedby systemssuchas walkie-talkies,cellphones,elevatorcallsystems,remote-controlledequipmentsuchas doorclosers.Ifchangesaremadein a buildingduring thedesignof thealarmsystem,thedesignplanof thedetectorsshould als be modifiedto matchthechange.Itis a requirementthat sensorsshould be cleanedat leastoncea yearhoweverifnuisancealarmsoccurdue to dirtydetectors,andthecleaningshould be adjustedto eliminatetheproblem.High-poweredvacuumcleanerscan be usedto cleanthesensors.Whenmaintenanceworksare beingcarriedout on thebuilding,a planof thewiringlayoutof thesensorsshould be giventothecontractorsto avoidanyinterferenceon thewiringlayoutof thedetectors.Thesensitivityof eachdetectorshould betestedas specifiedby themanufacturers.