Automatic Alarm System (Detectors)

7/28/2019 Automatic Alarm System (Detectors)

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FT 35AUTOMATIC ALARM SYSTEM

AUTOMATIC FIRE DETECTORS

OBJECTIVE

1. To explain the fundamentals of detection, types of detectors, types of detectors theirfunction and operation.

REFERENCE

2. a. Manual of Fireman ship Book 9.b. Fire College Notes.

CONTENTS

FIRE ALARM SYSTEMS

3. Introduction. What do you understand by fires alarm systems? It is a means ofinforming people of the situation at or around their area of a fire outbreak, smoke or both andits spreading effect. By informing we can do many things or get many things done viz.

a. Evacuate people from the danger area.b. Reduce the spread fire/smoke or both.c. Reduce damage or further damage.d. Stop the fire at its initial stage.e. Call for help i.e. fire brigade, police, ambulance, salvage team and etc.

4. Means of Activating Alarm:

a. By shouting fire, fire, fire.

b. By blowing whistle.

c. Telephone.

d. Ring or sound bells siren, gongs etc.

e. By its automatic function if an automatic fire alarm system is installed using acombination of detectors, call points and also, if a more sophisticated system,suppression.

5. Automatic Fire Alarm System. Most of the modern complexes civilian andmilitary and especially high rise building and high risk areas these automatic systems areemployed. Its usually a combination of manual break glass call points, bells, detector heads(smoke, heat and f lame etc) and suppression component (if included).

6 Automatic Detection consists of various types of detection components/elements:

a. Smoke detectors.b. Heat detectors.c. Radiation (Flame) detectors.d. Laser beam detectors.

7. Prime function of fire detector is to detect one or more changes in the protectedenvironment, indicative of the development of a fire condition. Detectors are normally ceilingmounted or cavity, floor or ducts mounted to monitor:


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a. After ignition has occurred and the invisible products of combustion are beingreleased.

b. When visible smoke is being produced.

c. When fire is producing flame and certain degree of a predetermined figure,(RATE OF RISE OR FIXED TEMPERATURE).

8. Automatic Fire Detectors. Automatic fire detection is desirable because fires whichoccur in premises normally only occupied in daytime have usually reached seriousproportions before discovery if they occur at night or weekend. The times included are:

a. Outbreak to discovery.b. Discovery to call.c. Call to arrival of Fire Brigade.

9. With today modern appliances and location of fire station the time taken for is at aminimum so ways to reduce the times (a) and (b) have to be found and one of these ways into install automatic fire detector equipment. The value of this equipment in recognized byinsurance companies and they will grant reductions in premiums depending upon the grade

of equipment installed.

10. The British Standard Code of Practice No 327/104 lays down specification for designand installation.

11. Automatic fire detectors will be considered under the following heading:

a. Requirements of a good detector and system.b. The working principles of those detectors.c. The types of circuits in which they can be installed.

12. The Detector and System:

a. Must be able to detect fire quickly.

b. Must sound audible alarm.

c. Indicates location of fire.

d. Should not give frequent false alarms.

e. Must not be susceptible to easy damage.

f. Should be reliable throughout its working life.

g. Must not be affected by changes in its surrounding atmosphere and mustcompensate for ordinary temperature increases.

13. The Working Principles Employed in Detectors. They can break down into threemain classes:

a. Those which detect heat.b. Those which detect smoke or visible vapors.c. Those which detect flame.

14. All the detected signals by the various forms of detector and sensors will be fed to acentral monitor panel called control and indicating panel; or depending on the system designand the vastness of the complexes or the number of building involved, these panels may beknown as Master Control Panel, Control Panel, Sub panel or Zone panel.15. From these panel, alarm will be activated and also if a suppression system is

included, the extinguishing agent will be released to put off the fire.


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16. The input into the alarm and suppression system is if could an electrical voltagewhich be amplified in most cases and via relays , series of RC and Diode and Transistors,the required voltage, usually 21 volts is fed to either:

a. Solenoid coil triggering mechanism.b. Explosive devices.c. Hydraulic or pneumatic lever/devices.

d. Other triggering mechanism.

17. All this triggering mechanism will break a disc or open a mechanical valve of somesort to release the suppression extinguishing agent e.g. Foam, Dry Powder, halon Gas,CO2, BCF.

18. These triggering mechanisms are usually situated within the agent cylinders or tanksand normally around the neck area where a small thin but robust metal disc is used to sealoff the extinguishing agent under pressure inside the container.

19. Smoke Detectors. As the name implies, it detects smoke long before flame occurs as most fires will start with smoke or smoldering.

20. Type of Smoke Detectors:

a. Ionization.

b. Optical.

(1) Light sea tier type.(2) Obscuration type.(3) Sampling Detectors.

21. Ionization. What is ionization? An atom made up of protons, electrons and neutrons.

In its normal condition the protons and electrons are balanced i.e. three of each in ions as

shown in diagram. (next page page 5).a. The Theory. If the atom is subjected to radiation from a radioactive sourcesome electrons is knee ked off, resulting in atom becoming positively charge (moreprotons than electrons).

b. The free electrons linkup with another atom and becoming negatively charged(more electrons than protons). These new atom are called ions and the processcalled ionization.

Fig. 11.1 Diagram of an Atom


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Fig 11.4 Diagram of an Ionization Detector (Fire Condition)

Fig.11.2 Illustration of ionization

Fig.11.3. Diagram of an ionization detector (Non Fire Condition)


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c. Thus if the atoms of air in a container is subject to radiation ionization takenplace in the same manner but ions moved about haphazardly. By introducing apositively and negatively charged plates a more orderly and predictable movement ifions taken place. Positive ions attached to negative plates and vice versa. Theforms the basis of ionization detector movement of ions between plates in thechamber reduces the resistance of the air enabling small electric current flows in theexternal circuit and amplified for monitoring.

d. The Practice. Normally a double chamber ionization detector principle isused. One ionization chamber in a semi scaled environment which does not permitentry of smoke, the other chamber in open to atmosphere allowing smoke to enter.

e. In normal condition both the inner and open chambers are free from smokeand formed a balanced electrical circuit. In the cold cathode tube (2) which acts an aswitching circuit controlling the operation of the relay.

f. When the smoke enters the open chamber, it will slow the movement of ionsas explained. As the movement of ions is slowed only in the open chamber thiseffectively in balances the electrical circuit offering higher resistance to flow ofelectricity than the inner chamber. Thus voltage or (1) increases causing the cold

cathode tube to strike, thus allowing the high current path to operate relay and soundalarm.

g. Transistors are actually used as amplifier and switching mechanism in mostionization detectors. Effective sensitivity in early stages of fire where smoke particlesare small Thus care to be taken in sitting of these type of detector heads. Kitchenand garages to be avoided, so is ventilation exhaust and fresh air inlet as current ofclean air inhibit speed of reaction in fire situation.

h. Most types of ionization detector heads are designed to be mounted onceiling and usually provide coverage for 100 sq meter floor area; with somemodification it can be used in air duels and trucking for air celled machinery to give.

Fig 11.5 Illustration of an Ionization Detector


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22. Optical Detectors. An ionization detector responds to invisible products ofcombination. Optical detector reacts to visible products of combustion i.e. particles ofcarbon land other chemical which give smoke its characteristic appearance. Two importantcomponents a light source and photoelectric cells.

23. The critical factor in operation of optical detector is the amount of light falling on thephotoelectric cells. There are two main types of optical detectors:

a. Light Scatter - During fire situation more light is thrown onto thephotoelectric cell.

b. Obstruction - Less light thrown onto photoelectric cells.

24. Light Setter Type. The light source and photoelectric cells are mounted in a lightproof darken housing allowing smoke to flow uninterrupted. In non fire situation light fromthe sources does not fall on to the photoelectric cells.

25. During fire condition, some light will be deflected by the smoke upwards to thephotoelectric cells. With the falling light, the cell will either create an electrical current in thedetector circuit or allowing more current to flow through it (depending on type of cell used).The small increase in current is normally amplified by a transistorized circuit in order toenergize relay controlling alarm. Detector is preset so that alarm is given where smokedensity reaches a predetermined level.

a. Practice. Light scatter are commonly used when smoke scatter the light andon the light and on the photoelectric cell a small electrical charges produced andamplified thus actuating alarm relay which raises the alarm and also switch onindicator lamp on the detector and identify which head is operating.

b. Should there be a failure in the power or light supply in the detector, a specialrelay will signal this at a central point (control panel) and also lithium the indicatorlamp on the head; actual f ire signal is not produced in this condition.

c. The area protected by a detector head will vary depending on risk involved,floor plane and other variables. Nominal area coverage for the detector is 100square meters per head but for specific risks or areas, this figure can decrease ofincrease.

d. It is possible with modifications, to mount some optical detectors in air ductsfor protection of calling, air-cond return and machineries.

An Optical Smoke Detector Light Scatter Non-Fire Condition

26. Obscuration Type:

a. Theory. This type works on the reverse of the light scatter principle i.e. the

light is obscured (blocked) by smoke thus reducing the intensity of light falling onphotoelectric cell causing alarm signal to be raised.


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b. Practice. Optical detector particularly useful for the protection of large areas,one detector head can throw the light beam from 3 to 45 meters with 6 metersensitivity on either side.

c. Usually the light source and less are housed at the end of the protected areaand the photoelectric cell at appetite end (like infra-red beam fire detection system Beam Master).

d. L.E.D (Light emitting dudes) are now widely used as light source in opticaldetector instead of tungsten filament lamp which consume little current and moreefficient and long lasting light source.

An Optical Smoke Detector Light Scatter Type (Fire Condition)

27. Sampling Detectors. Consists of a series of small bore sampling tubes located indire risk zone and are connected to a central monitor unit. The monitor unit continuously

samples the atmosphere in protected zone by drawing air through the small holes in thetubes by mean of motor driver fan. The air is then passed through an ionization chamberwhich is designed to raise the alarm when the level of combustion products reaches apredetermined level a set level.

28. Conclusion. Detection of fire by smoke detectors is dependent on a number offactors e.g. smoke concentration, size and shape of smoke particles i.e. the density ofsmoke level a resultant of various different materials burnt. In the initial stages of most firesthe smoke particles accumulated and combine together forming bigger particles.

27. Ionization detector detects smoke particles and usually more sensitive to smallerinvisible smoke particles. Very useful in early stages of relatively clean burning fires e.g.

wood and paper.

29. Optical detector is more efficient on dense smoke (if larger particles) in earlier thanheat sensitive detector.

30. On the whole, earlier detection can be obtained with a smoke sensitive detector thanheat sensitive detector.

31. Radiation Detector:

a. Infra-red radiation.b. Visible light

c. Ultra-violet radiation.


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32. Fire, apart from producing hot gases and raising the temperature of its surround, italso releases radiant energy traveling in waves. Thus radiation detectors are produce todetect these sources.

33. Visible light or flame detectors were produced initially to detect light or flame, butthese detectors create too many problems as they cannot differentiate between the severalof lights and those particularly produced by fire.

34. Therefore, two new types of radiation detectors were made to see and identify actualsource from fire. They are:

a. Infra-red radiation.b. Ultra-violet radiation.

35. Both these detectors uses photoelectric cell which is sensitive to one of thesesources. But these cells and electrical components attached to it must be protected fromdust or dirts an moistures but at the same time allowing the infra -red or ultra violet lamp topass through a certain type of quarts (a crystal stone material).

36. These quartz less is supported by means of special filter/amplifier which allows onlyone type of source, either an ultraviolet or infra-red radiation.

37. However, flame is not the only producer in infra-red radiation there may be a limitednumber of other producers e.g. sunlight or heaters. But flame of fire has a distinctive flickeri.e. a frequency or vibration and normally in the range of 4 Hz - 15 Hz. So you can see thefunction of the filter/amplifier therefore it is range. This signal is then fed to aninterrogations/timer which will activate the alarm circuit only if the signal persists for a presetperiod. (usually 2 15 secs).

38. This small delay may a lightly off set the quick response time of the detector, butnecessary to reduce false alarm to a minimum. Once any signal is rejected the detector

goes back on standby/monitoring modes.

39. As an infra-red detector must see a flame before it with raise an alarm, the diagramabove is useful where the risk is divided into compartments or congested area in whichvisibility night be impaired. Individual detector head can protect each compartment or beplaces in strategic position in congested area.

40. For longer areas, free of congestion land with a more open plan, a scanning infra-reddetector is available. The deflector continually scan the protected area (approx every 20secs) in a 360 horizontal cell is stuck by deflected infra -red radiation and the characteristicflicker is identified by the filter/amplifier, the integrator/timers directly and allows the

radiation to fall continuously on the photoelectric cell.

Diagram of an Infra Scan Radiation Detector


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41. The diagram below shows how these components are fitted into an actual detectorwhich also has a neon Flanders to indicate.

Schematic Diagram of an Infra-Red Radiation Detector

Non-Fire Condition

Optical Detector Obscuration Type

42. The timer than check and ensure the flame flicker persists for 2 5 cells. It theflickers do persist beyond this period than the alarm is raised if not than the integrator

restarts the deflector motor thus putting the detector on standby/monitor.


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43. This detector has an amber fault light which light up a few second after a fault isdetected, the red alarm light illuminates once the integrator activate the alarm. The lendcover is thermally insulated and the cell and deflector are shielded by quartz globe.44. Theoretically there is no limit to the range of the infra scan detector but for betterefficiency and effect evenness especially in the early stages of the fire, the radius ofdetection should be limited to about 90 meters.

45. Wider area of coverage can be obtained from the scanning type than from the statictype. But any particular types used depend on the area to be protected and the internalplan.

46. Infra-red detectors can provide rapid detection in risk areas where flame is likely todevelop at an early stage of combustion because of the almost instantaneous transmissionof radiation. And unlike smoke or heat detector infra-red detectors are equally efficientinside or outside of buildings. As it only need to see the flame, where as heat or smokedetectors have to rely on ceilings or walls to direct combustion products to the sensing head.

47. Thus this make the infra-red detector especially scanning type useful for protection of

open storage areas e.g. timber yards and fuel depots.

48. The Practice. The principle of operation is very similar to that of the ionizationsmoke detector. When the ultra violet radiation/rays strikes the gas filled tube it ionizes thegas in the tube, producing a small current between the two elect roles and the tube becomesa conductor of electricity, when the current flow increased beyond the set point of theamplifier the alarm relay closes to sound alarm. The circuit also has a built in delay for 10-15 secs to suppress false alarms from lightning, arching sparks etc.

49. The detectors are not sensitive to sun light or artificial light, thus it helps inminimizing false alarm but are sensitive to welding areas, furnaces etc. Usually used forprotecting aircraft engine, fuel storage, oil drilling rig, ware house, paint spray booths etc.

50. Ultra Violet Radiation:

a. The Theory. Like the infra-red detector, this type also needs to see the flamebefore it will operate, but since actual sources of ultra violet radiation is very limited,flicker identification is not needed. Basically, the ultra violet detector consists of anamplifier and a photoelectric cell or gas filled tube sensitive to ultra violet radiation.

Diagrammatic Illustration of an Ultra-Violet Detector

Automatic Alarm System (Detectors)

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