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SMOKE DETECTION AND ALARM SYSTEMS
INTRODUCTION:
Fire is detected by having sensors that detect the by-products of fire, typically heat
and smoke but also ultraviolet & infra-red radiation. Detection of heat has long been
the only method of automatically detecting a fire. This is because it could be
implemented relatively easily using mechanical detection means, and the effects of
heating and heat transfer were fairly well known. However since the electronic
advancements in the 1940s, smoke detection has also been available. However it was
not until the last couple of decades that smoke detection has become reliable and cost
effective to be used widely, yet this may not always be the most appropriate detection
method.
Smoke detectors are a much newer technology, having gained wide usage
during the 1970's and 1980's in residential and life safety applications. As the name
implies, these devices are designed to identify a fire while in its smoldering or early
flame stages, replicating the human sense of smell. The most common smoke
detectors are spot type units, that are placed along ceilings or high on walls in a
manner similar to spot thermal units. They operate on either an ionization or
photoelectric principle, with each type having advantages in different applications.
For large open spaces such as galleries and atria, a frequently used smoke detector is a
projected beam unit. This detector consists of two components, a light transmitter and
a receiver, that are mounted at some distance (up to 300 ft/100m) apart. As smoke
migrates between the two components, the transmitted light beam becomes obstructed
and the receiver is no longer able to see the full beam intensity. This is interpreted as a
smoke condition, and the alarm activation signal is transmitted to the fire alarm panel.
1
Importance of smoke detectors:
Smoke detectors are one of the most important safety devices one can have
in the home. They should be installed in the hallways and in the bedrooms. Smoke
detectors provide early warning in the event of a fire, and enable emergency action in
the event of a fire. They are inexpensive, easy to install, unobtrusive, and require very
little maintenance, and no home should be without them.
Most experts agree that smoke is responsible for approximately 80 percent of the
fatalities caused by fires. This fact of life has triggered the phenomenal growth in
popularity of residential-type smoke detectors. In many communities, legislation has
been passed requiring the installation of smoke detectors in houses and apartments.
And the dramatic rise in residential smoke detector popularity is spilling over into
industry not only as life-saving devices, but also as a key part of fire protection
systems for buildings and equipment.
The National Electrical Manufacturers Association (NEMA) in its Standards
Publication SB 9 defines a smoke detector as "a device which detects visible or
invisible particles of combustion." The publication points out that the real value of
such a device lies in its ability to detect a fire even before flame and large quantities
of heat develop . All smoke detectors depend on various combinations of minute
liquid or solid particles suspended in a gaseous dispersion agent for their operation.
Particle characteristics that affect detection thresholds include diameter, shape,
internal structure, optical properties, and concentration.
2
How Good Are Smoke Detectors?
The common home smoke detector cost around $10 and will detect smoke in very
small concentrations in the home. Smoke emission occurs during the early stages of a
fire, so smoke detectors in the ceilings of the home will provide plenty of early
warning in the event of fire.
The key advantage of smoke detectors is their ability to identify a fire while it
is still in its incipient. As such, they provide added opportunity for emergency
personnel to respond and control the developing fire before severe damage occurs.
They are usually the preferred detection method in life safety and high content value
applications.
Evolution of smoke from fire:
When a nice fire is going, and it has burned down to the point where a collection of
hot "glowing embers is seen." The fire is still producing a lot of heat, but it is
producing no smoke at all. If a piece of wood is tossed, or even a sheet of paper, onto
this fire, it is noticed that the new fuel produces a lot of smoke as it heats up. Then,
all of a sudden (often with a small pop), it bursts into flame and the smoke disappears.
3
When a fresh piece of wood or paper is put on a hot fire, the smoke that is seen
are volatile hydrocarbons evaporating from the wood. They start vaporizing at a
temperature of about 300 degrees F (149 degrees Celsius). If the temperature gets
high enough, these compounds burst into flame. Once they start burning, there is no
smoke because the hydrocarbons are turned into carbon dioxide and water (both
invisible) when they burn.
. Coke is coal that has been heated in the absence of oxygen to drive off the
organics. The smoke that this process produces is actually very valuable .It contains
coal tar, coal gas, alcohols, formaldehyde and ammonia, among other things. And all
of these compounds can be distilled out of the smoke for use. Methanol (a form of
alcohol) referred to as "wood alcohol.” is used to be produced by distilling out of
wood.
Detector operating times - Smoke detectors
At present there are no models or calculations to predict the operation time of smoke
detectors. There are, however, a number of methods used to predict the operation of
the detector. The first method is to generate an "equivalent" thermal detector that has
an activation temperature of 10-200C above the ambient and a low RTI value. This is
then used in the thermal detector calculations as an indication of the response of the
smoke detector. This method however is only an estimation and generally considered
as a "rule of thumb" rather than an accurate measure. The other method of
determining the detector response is by using the smoke transport models.
There are two types of models available, the zone model and the field
models. The field models are considered to give a more accurate picture of what
actually happens though they aren't combined with simple detector operating time
functions and take much longer to run. Hence, generally the zone models are used
when considering detector operation. The zone model typically divides the fire
compartment into two zones, the upper layer and the lower layer. The upper layer is
the location of the smoke and gases and the detector. The zone model calculates the
volume, smoke density, temperature size and other factors for the upper layer at
discrete time intervals from the fire initiation. As the optical density of the smoke is
4
calculated at various points in time the fire engineer can make an evaluation as to
when the detector will activate based on the sensitivity of the detector.
Smoke detection:
The future of performance based design lies largely with smoke detection as the
production of smoke is usually the first combustion product to be released, hence if it
can be detected then we will achieve a faster detection time and therefore longer
escape time.
The general modeling procedure was to run the zone model and work out
values for the optical smoke density at the upper layer at various points in time. When
the optical density reached the rating of the detector then one could assume that the
detector would operate. However in real life there are a number of factors why this is
an invalid scenario.
One of the main reasons is the same as for thermal detectors in that an exact
activation optical density is not know, rather a range. Smoke detectors are grouped
into 3 sensitivity groups Normal, High and Very High. Each of these groups has a
nominated sensitivity range that the detector must fall in. However the foremost
reason why a direct relation between the expected optical density of smoke produced
and the actual detector performance cannot be produced is due to the actual operation
of the detector. In general smoke detectors do not measure smoke obscuration
directly, rather some other factor related to smoke obscuration. Thus the operation of
each of the different types of smoke detectors needs to be considered to see where the
problems lie in the performance of the various types.
Different types of smoke detectors and their operation:
Not all fires are alike. Some are slow burning and smoky, some are fast burning,
producing high heat but less smoke. Each type of fire requires an appropriate
technology.
5
The various types of smoke detectors are:
Photoelectric smoke detectors.
Ionization smoke detectors.
Laser technology smoke detectors.
Detectors based on air sampling technology.
Infrared beam smoke detectors.
Video smoke detectors
Photoelectric Detectors:
Photoelectric detectors are better at sensing smoky fires, such as a smoldering
fires .Occasionally, when we walk into a store and a bell will go off as we cross the
threshold. If we look, we will often notice that a photo beam detector is being used.
Near the door on one side of the store is a light (either a white light and a lens or a low-
power laser), and on the other side is a photodetector that can "see" the light.
When we cross the beam of light, we block it. The photodetector senses the lack
of light and triggers a bell. In a similar way, this same type of sensor could act as a
smoke detector. This is known Photo-optical detectors obviously use a light source to
measure smoke density but rather than measure how much the intensity of the light is
reduced due to the smoke they measure how much light is reflected by the smoke.This
is known as the light scatter principle.
6
In one type of photoelectric device, smoke can block a light beam. In this case, the
reduction in light reaching a photocell sets off the alarm. In the most common type of
photoelectric unit, however, light is scattered by smoke particles onto a photocell,
initiating an alarm. In this type of detector there is a T-shaped chamber with a light-
emitting diode (LED) that shoots a beam of light across the horizontal bar of the T. A
photocell, positioned at the bottom of the vertical base of the T, generates a current
when it is exposed to light. Under smoke-free conditions, the light beam crosses the
top of the T in an uninterrupted straight line, not striking the photocell positioned at a
right angle below the beam. When smoke is present, the light is scattered by smoke
particles, and some of the light is directed down the vertical part of the T to strike the
photocell. When sufficient light hits the cell, the current triggers the alarm. The sensor
then sets off the horn in the smoke detector.
. The diagram below illustrates how the technology works. Under normal,
smoke-free conditions, the LED beam moves in a straight line, through the chamber
without striking the photo cell. When smoke enters the chamber, smoke particles
deflect some of the light rays, scattering them in all directions. Some of it reaches the
photocell. When enough light rays hit the photocell, they activate it. The activated
photocell generates a current. The current powers the alarm, and the smoke alarm has
done its job.
7
Photoelectric Smoke Alarm Technology
smoke free chamber
light beam travels straight through
smoke particles in chamber
deflect some light rays
Light
emitting
diode
activated
photocell
powers alarm
no light reaches photoelectric
cell
deflected light rays
activate photocell
But there are two problems here: 1) It's a pretty big smoke detector, and 2) it is not
very sensitive. The main problem with this is that the detector is only generally
sensitive to particle sizes around the size of the wavelength of the light used. Thus it
would be possible for smoke, with an optical density greater than the rated activation
sensitivity, that consisted of small unreflective particles to be present and not cause
activation of the detector. Photoelectric detectors look for the presence of visible by-
products of combustion in the detection chamber. When a sufficient density of visible
combustibles fill the detection chamber, the detector sounds an alarm condition.
Ionization smoke detectors:
Ionising smoke detectors are approved by the Swedish Institute of Radiation
Protection and contain radioactive sources. The advantage of ionization detector is
that the smoke can be invisible to the human eye, while remaining very much visible
to the ionization detector.
In the conventional devices , the material is nearly destroyed before
alarm comes as they are triggered only when sufficient smoke or heat is evolved .On
8
the other hand , the Ionization detectors sense these particles at the incipient stage by
monitoring the electrical change which occurs when the particles reach the charged
space in the detector.
Ionization technology is faster at reacting to fast flaming fires that give off
little smoke. . Ionization smoke detectors feature a harmless radioactive source within
a dual detection chamber. They operate by sensing for a change in the electrical
conductivity across the detection chamber. . This type of smoke detector is more
common because it is inexpensive and better at detecting the smaller amounts of
smoke produced by flaming fires.
Ionization chamber is very simple. It consists of two plates with a voltage
across them, along with a radioactive source of ionizing radiation , a small amount
(perhaps 1/5000th of a gram) of americium-241 to detect the smoke. The radioactive
element americium has a half-life of 432 years, and is a good source of alpha
particles. . Typical detector contains 0.9 microcurie of americium-241. A curie is a
unit of measure for nuclear material. The amount of radiation in a smoke detector is
extremely small. It is also predominantly alpha radiation. Alpha radiation cannot
penetrate a sheet of paper, and it is blocked by several centimeters of air. The
americium in the smoke detector could only pose a danger if you were to inhale it.
In this chamber, the americium is embedded in a gold foil matrix within an
ionization chamber. The matrix is made by rolling gold and americium oxide gets
together to form a foil approximately one micrometer thick. This thin gold-americium
foil is then sandwiched between a thicker (~0.25 millimeter) silver backing and a 2
9
micron thick palladium laminate. This is thick enough to completely retain the
radioactive material, but thin enough to allow the alpha particles to pass.
Americium: The vital ingredient of household smoke detectors is a very small
quantity (<35 kBq) of americium-241 (Am-241). This element was discovered in
1945 during the Manhattan Project in USA. The first sample of americium was
produced by bombarding plutonium with neutrons in a nuclear reactor at the
University of Chicago.
Americium is a silvery metal, which tarnishes slowly in air and is soluble in
acid. Its atomic number is 95. Its most stable isotope, Am-243, has a half-life of over
7500 years, although Am-241, with a half-life of 432 years, was the first isotope to be
isolated.Americium oxide, AmO2, was first offered for sale by the US Atomic Energy
Commission in 1962 and the price of US$ 1500 per gram has remained virtually
unchanged since. One gram of americium oxide provides enough active material for
more than 5000 household smoke detectors.
Americium (in combination with beryllium) is also used as a neutron source in
non-destructive testing of machinery and equipment, and as a thickness gauge in the
glass industry. However, it’s most common application is as an ionisation source in
smoke detectors, and most of the several kilograms of americium made each year is
used in this way.
Formation of Americium
Plutonium-241, which is about 12% of the one percent content of plutonium in typical
spent fuel from a power reactor, has a half life of only 14 years, decaying to Am-241
through emission of beta particles. Am-241 has a half life of 432 years, emitting alpha
particles (see above) to become neptunium-237. The plutonium 241 is formed in any
nuclear reactor by neutron capture ultimately from uranium (actually U-238), such as
supplied on the world market for electricity generation. The detailed steps are:
U-238 + neutron => U-239,
U-239 by beta decay => Np-239,
Np-239 by beta decay =>Pu-239,
Pu-239 + neutron => Pu-240,
10
Pu-240 + neutron => Pu-241.
This will decay (emitting a beta particle) both in the reactor and subsequently.
Principle of operation:
The alpha particles generated by the americium have the following property: They
ionize the oxygen and nitrogen atoms of the air in the chamber. To "ionize" means to
"knock an electron off of." When you knock an electron off of an atom, you end up
with a free electron (with a negative charge) and an atom missing one electron (with a
positive charge).. The positive atoms flow toward the negative plate, as the negative
electrons flow toward the positive plate. The movement of the electrons registers as a
small but steady flow of current. When smoke enters the ionization chamber, the
current is disrupted as the smoke particles attach to the charged ions and restore them
to a neutral electrical state. This reduces the flow of electricity between the two plates
in the ionization chamber hat these electrons and ions moving toward the plates
represent. The electronics in the smoke detector sense the small amount of electrical
current. . When the electric current drops below a certain threshold, the alarm is
triggered.
.
11
Chemical reactions in the Ionisation smoke detector:
It is more a physical reaction than a chemical reaction. the americium in the smoke
detector is high speed alpha particles (helium nuclei).the particles hit molecules in air
and knock off electron.
o2+he(+2)o2(+1)+e(-1)+he(+2).
Working:
The working of the fire alert ionization chamber detector is shown in the
fig:
FIG (1)
The detector is basically a simple series resistance , capacitance (RC)
Circuit . The charging current to charge the capacitor is supplied by the ionised
air in the sample chamber , which constitutes the resistance R. The charge carriers
12
are a mixture of electrons and positive ions produced by high velocity alpha
particles.The positive ions enter the sample chamber .
The circuit works in a cycle with the voltage(E+) on the
capacitor(C) being an exponential function of time . Under normal conditions the
capacitor (C) will be allowed to build up a charge ( CE) determined by the integration
timer (T).If at the integration cycle time the capacitor C has received adequate
charge to trigger the charge detector (Q) it will set for (RC+2) seconds.
When this occurs both timers (T) and (T+2) will repeat the above cycle. If the
positive ions of the products of combustion are introduced into the sample
chamber , the resistance of the chamber is increased and the capacitance (C) does not
fully charge in the time constant established by the integration timer(T). Under this
condition the charge detector (Q) will not reset both timers .If the alarm timer (T+2)
is not reset it will complete its cycle and cause the alarm relay to “lock in” indicating
the alarm condition.
The circuit is infinitely adjustable for cycling timer by the presetter
and the sensitivity variable resistors .The maximum allowable cycle time represents
‘minimum sensitivity’and the minimum allowable cycle time represents the
maximum sensitivity .When set for maximum sensitivity, it exhibits extreme
sensitivity with the ability to detect even the electrical short circuit or overload
conditions .When set for minimum sensitivity it provides an alarm with test fires with
an established sensitivity to monitor 360 sq .m. of area .The detector is stable at any
sensitivity and is not adversely affected by air velocity ,humidity or temperature from
-10C to 65C.
13
This principle of working of this detector could be better understood by the actual
relay circuitry shown in the fig (2):
In the fig , TDR1 is a time delay relay which closes a normally open contact 1
TDR1 aft r some time delay . This time delay is set such that after this time, under
normal atmospheric conditions the charge built up across the capacitor C will be
sufficient to energise the relay Q , which otherwise will not pick up before this
voltage . 1Q&2Q are the normally closed contacts of the relay Q and these contacts
open out when the relay Q is energized . TDR2 is another timer which I set at time
equal to 2seconds more than the time for which TDR1 is set and ITDR2 is its
normally open contact which initiates the alarm . Under normal atmospheric
conditions i.e when there is no fire , the charge built up across capacitor after time T
is equal to E which can energise relay Q .Also after time T i.e , for which relay
TDR1has been set the contact 1 TDR1 makes and the relay Q is thus energized. Due
to its energisation the timers TDR1&TDR2 cease to get supply and are reset .
14
Incase fire is there , then the resistance (R)of the circuit is increased and the
exponential curve is modified as shown in the fig 3.
Under this condition , after time T , the contact ITDR1 makes but relay Q can’t be
energized because voltage across the capacitor is only E’’ which is less than E’ and
hence not sufficient to energise relay Q . As a result the timer TDR1 &TDR2 are not
reset and after time (T+2)seconds the contact ITDR2 makes and initiates the alarm.
Special features:
1> This device operates at 24 V DC(full wave rectified )thus giving a safe and simple
operation and consumes only 3 to 4 watts.
2> It is capable of operating in fast moving air and is impervious to moisture and
corrosion .
3> A lamp is incorporated in the unit to give visual indication when detector operates
and a separate lamp can be provided to give remote indication also.
4> No expensive equipment is required to provide stand by supply in the event of
mains failure .
5> Its operation is not effected by transients in supply or normal voltage fluctuations.
6> I t is capable of operating at sub zero temperatures and elevated temperatures with
no change in operations.
15
Inside a ionisation Smoke Detector
. Here is the smoke detector:
When we take off the cover we find that a smoke detector is pretty simple. This one
consists of a printed circuit board ,an ionization chamber (the silver cylinder toward the top
right in the following picture) and an electronic horn (the brass cylinder toward the bottom
right in the following picture):
16
Here is a close-up of the board:
and the underside of the board:
17
Amount of radiation in Ionisation smoke detectors:
The radiation source in an ionization chamber detector is a very small disc, about 3 to
5 millimeters in diameter, weighing about 0.5 gram. It is a composite of americium-
241 in a gold matrix. The average activity in a smoke detector source is about one
microcurie, 1 millionth of a curie.
18
Americium emits alpha particles and low energy gamma rays. It has a half-life
of about 432 years. The long half-life means that americium decays very slowly,
emitting very little radiation. At the end of the 10 year useful life of the smoke
detector, it retains essentially all its original activity.
How much radiation exposure will be got from a smoke detector?
As long as the radiation source stays in the detector, exposures would be negligible
(less than about 1/100 of a million per year), since alpha particles cannot travel very
far or penetrate even a single sheet of paper, and the gamma rays emitted by
americium are relatively weak.
HIGH SENSITIVITY SMOKE DETECTOR USING
LASER TECHNOLOGY
Features:
Unique smoke particle counting technology
Fast detection of incipient fire
Smoke detection capability from 0.005% to 0.4% obscuration per meter
Calibrated by using smoke-like particulate
Immune to contamination and dust levels reducing false alarms
Minimal periodic maintenance
Low susceptibility to environmental conditions
Senses smoke even in high air flow design
No filter required
No re-calibration required
Incorporates military proven state-of-the-art laser technology
Established laser design ensures long-term stability and accuracy
Long life under-run key electronic components
Can be fitted into existing fire detection and alarm system
Description:
19
The HART high sensitivity smoke detector assembly consists of a metal fan box
and the HART HSSD detection sensor.
The fan box is a robust airtight enclosure which houses a high efficiency
centrifugal fan, superior to an axial fan, producing a greater static pressure necessary
to draw air into the detector.
The air passes without filtration straight through the detection chamber.
This design ensures elimination of filter maintenance (with the inherent possibility
of filtration of smoke particulate) and no flow restriction to the incoming air.
The detector's unique patented optical system focuses a 100 micron laser beam
through the detector chamber.
The detection device detects the presence of individual particles of smoke as they
traverse the laser beam in the moving air stream.
Smoke concentration is determined by counting these discrete events.
This particle count is converted to an analogue signal whose level is exactly
proportional to the smoke concentration, the output of which is available for
transmission to the control panel.
Changes in flow do not affect the measured value of smoke concentration.
The HART HSSD is essentially a particle counter set up to discriminate between
smoke and other potentially confusing particulate
It requires no filter and is immune to the normal levels of airborne dust and to
contamination of the chamber walls.
In large other very early smoke detectors, particles such as dust give "false" alarm
signals; but the HART detector incorporates an adjustable particle size discriminator
(in addition to the particle counting circuitry) which ensures that the HART HSSD
provides output only to smoke itself.
Its unrestricted airflow design ensures that the sensor truly sees the room air
environment. The laser deployed in the HART HSSD is the same type as that used in
CD players.
Refined by Japanese engineering, this type of laser provides a long-term stable
output.
Any changes in laser intensity due to temperature, component ageing or dirt build-
up are automatically compensated for electronically - in the detector itself without
reliance on control software.
Circuits supervise the operation of critical components in the system.
20
Failure of, or an out of tolerance situation in the illuminating or receiving optics,
will result in a detector FAULT signal being transmitted to the control panel.
Air flow is monitored with a special temperature sensor, cooled by air passing
through the detection chamber.
An analogue signal proportional to flow is made available to the control panel so
that it can initiate a fault condition, should the air flow vary or drop during normal
operation.
The HART detector is available in different versions each with a sensitivity to suit
different applications and area of protected space.
HART detectors are assembled into standard packages for use in a wide range of
applications:
HART 100 - the most sensitive detector available from 0.05% to 0.2% obscuration
per meter at full scale, it can be supplied in the approved HART fan box, UniLaser
100 & 1000, LocaLaser, Smoke Seeker and EExd versions. HART 200 - has a
sensitivity of nominally 0.4% obscuration per meter and is available in the basic
HART fan box and UniLaser 200 & 2000 versions.
The HART packaged options include Display Control Card (DCC) and HART
detector in fan box (available in HART 100 and HART200 versions).
Controls packaged as a single or up to 4 zone unit mounted separate to HART
detector enclosed with integral fan in fan box.
UniLaser 100 and UniLaser 200 - single station HSSD
Controls, detector and fan in a single enclosure.
Functionally identical to the DCC and HART detector.
UniLaser 1000 and UniLaser 2000 - single station HSSD
Controls, detector, fan, and power supply with space for up to 24 hours of stand-
by in a single enclosure.
21
Functionally identical to the DCC and HART detector with the added feature of
an integral power supply SMART HART HSSD in fan box
Direct communication to any analogue addressable panel fitted with suitable
software and operating on the Apollo series 90 protocol.
LocaLaser - multi-zone HSSD
Based on the UniLaser concept but with the ability to identify smoke in up to 4
separate zoned areas of sampling.
Smoke Seeker - multi-point HSSD
Specialized smoke sampling to identify the location of smoke from up to 8
separate points of sampling.
Ideal fo EExd
Fully certified HSSD unit for applications of Zone 1 areas of hazardous
protection.
Used in combination with a Display Control Card, or alternatively as a SMART
version.
Ideal for cabinet detection.
Laser Particle Counting Technology :
Laser Particle Counting technology provides the leap forward. Instead of illuminating
the whole sampling chamber, lasers afford the ability to illuminate just a discrete
volume element in the center of the sampling chamber.In this way, the detector
becomes essentially immune to any contamination of the chamber walls.
More important is the fact that HART High Sensitivity Smoke Detectors are set up as
single particle counters.
HSSD as Single Particle Counters:
As the smoke particulate passes into the sampling element, the sensor electronically
counts each particle. Particle counting HSSD devices are thus much more sensitive to
22
the prevailing concentration of small particulate (the tell tale sign of early
combustion) than are conventional HSSD devices. Although larger particulate such as
dust does not significantly affect the signal recorded (number of particles) from either
a background or a prevailing smoke environment, the HART laser HSSD goes one
step further. Most particulate with a diameter less than 10 micrometers is
electronically recognized and thus not added to the smoke count register.
AIR SAMPLING SMOKE DETECTORS
Aspirating Smoke Detectors
Introduction :
Aspirating smoke detection is a system that uses an aspirating fan to draw air from the
protected area via a network of sampling pipes and sampling holes.The sampled air is
then passed through a high sensitivity precision detector that analyses the air and
generates warning signals when appropriate. This system has a number of benefits,
particularly in the areas of performance, installation cost and routine maintenance.
The two main types of system are:
Primary Sampling System:
The system is designed to work in conjunction with any air handling systems and will
not provide optimum performance when these are inoperative. The major advantage is
the detection of cool smoke from a minor problem that does not rise to the ceiling,
which would be the ‘conventional’ location.
Secondary sampling system: : The system is designed with sampling holes in the
same positions as normal point detectors to an appropriate standard.These sampling
pipes may be designed and installed to achieve one of three levels of sensitivity:
_ Normal Sensitivity: the same sensitivity as normal point detectors typically at 3% -
5% obscuration per metre.
_ Enhanced Sensitivity:responding to smoke at concentrations of between 2%
23
and 0.8% obscuration per metre.
_ High Sensitivity: responding to smoke at concentrations of less than 0.8%
obscuration per metre.
It is important to note that the detector sensitivity is shared over the network of
sampling points associated with it. In other words, if a system having a detector
registering a ‘Fire’ signal when the smoke density within it reached 0.05%
obscuration per metre was connected to a pipe network with 20 sampling holes the
mean system sensitivity at each hole would be 1.0% (0.05% x 20). This sensitivity is
calculated on the basis smoke only enters one of the twenty holes. If the same density
of smoke entered two holes the mean sensitivity would double. Normally, smoke will
enter from the majority of sampling holes, in which case system sensitivity can be
very high indeed.
Types of Detectors:
There are currently three types of technology used in commercially available
aspirating smoke detectors:
Light Scatter: A stream of sampled air is continually passed through a detection
chamber in which a high-energy light source is pulsed. This light would be scattered
by any smoke particles in the sample and the quantity of scattered light is analysed by
a solid state light receiver. The quantity of scattered light is proportional to the level
of smoke pollution. Light scatter systems are sensitive to smouldering fires and
particles given off by overloaded electrical cables and are therefore particularly useful
where early warning is required. They can be vulnerable to dust however, which is
why most detectors incorporate sophisticated filters and/or electronic dust rejection.
Cloud Chamber: A stream of sampled air is continually passed through a detection
chamber that contains water vapour.Any very small particles cause the vapour to
condense around them to form larger droplets of equal size. The number of these
droplets is regularly measured optically using a pulsed LED. Because cloud chambers
24
consume water they require regular maintenance. Cloud chamber detectors are
resistant to dust.
In comparative field tests, cloud chamber detectors have shown very good
response in detecting the particles released by flaming fires, but poor response in
detecting the particles common to smouldering fires and are therefore of limited use
for early warning.
Particle Counting: A stream of sampled air is continually drawn through a focused
laser beam and light scattered from each particle is measured. This provides an output
relative to the number of particles that have traversed the laser beam. Particle
counting systems are sensitive to smouldering fires and overloaded cables but need to
have their air flow vigorously regulated as their output is proportional to the flow rate.
Particle counting systems are generally resistant to dust but fibres seen ‘end on’ or
large volumes of dust have been known to cause unwanted alarms.
Signal processing:
How signals are processed is fundamental to the reliability of an aspirating detection
system.Provision should be made to accommodate changes resulting from a drift in
detector calibration, contamination of filters or changing environmental conditions
within the protected area, thus ensuring a consistent level of protection.Early
aspirating detection systems were of fixed sensitivity where the detector was
calibrated to a known value and the alarm thresholds fixed at pre-determined points
depending on the site conditions measured during commissioning. These systems
were unable to accommodate fluctuation in site conditions and this rapidly led to a
perception that high sensitivity automatically meant a high incidence of unwanted
(false) alarms. All fixed sensitivity detectors require annual recalibrating in addition
to normal maintenance test procedures.To overcome this problem, a modern
aspirating system uses Artificial Intelligence (AI) to maintain a known probability of
alarm by varying the sensitivity of the detector to match variations in site conditions.
This type of system also automatically compensates for component drift or detector
contamination, thus ensuring optimum performance is always achieved.
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General Design Aspirating smoke detectors are often used where early
warning is required and higher than normal sensitivity is needed. They are also
suitable for many other applications where there are problems using conventional
forms of detection. This could be because:
_ there is an access or maintenance problem
_ the protected area is too high and/or may suffer from smoke stratification problems
_ an invisible installation is required
_ the environmental conditions are extreme (hot, cold, dirty, etc)
When specifying or designing an aspirating smoke detection system it is essential to
define the performance required from the system.
System sensitivity:
System sensitivity should be appropriate and realistic.High sensitivity and rapid
response can be achieved from a single detector in a small computer room. Normal
sensitivity and response would be more appropriate when protecting 2000m2 of
warehouse space where height and volume dissipate and dilute the smoke sample.
The total allowable number of sampling points varies for each manufacturer and the
sensitivity at each hole is a function of 'detector' sensitivity and the number of
sampling holes. The more sensitive the detector, the more sampling holes can be
drilled in the pipe network. These systems are based on the assumption that 'any
smoke in the protected area will end up going through the air handling system'.
Sampling points are therefore arranged across the inlet grilles to the air handling units.
As high sensitivity is often required in a high airflow and therefore high
dilution area, a reasonable guideline is to allow for one detector per 1500m3.It is good
practice to use a separate sampling pipe for each air handling unit to balance out as
many pressure variations as possible.If it is necessary to mount the aspirating smoke
detector outside of the protected area the detector exhaust should always be piped
back to the protected area. This will prevent the sampled air, and perhaps smoke,
26
contaminating other areas and balance any pressure variations between the protected
area and the detector location
Because many air handling units add a percentage of fresh air, a Reference
Detector should be considered to prevent false alarms caused by external pollution
entering with the ‘fresh’ air. The Reference Detector monitors the incoming fresh air
and ‘offsets’ the alarm thresholds of other Detectors if polluted air is detected.
This will help prevent false alarms occurring from this source.
Secondary sampling systems generally have sampling points positioned in
the same locations and using the same design criteria as normal detectors . Where
enhanced or high sensitivity systems are required the normal area coverage per
detector (sampling point) should be reduced .Because more heat is required to lift
smoke to great heights the amount of smoke that can reach high level areas can be
minimal if the fire is small .Where protection is provided to high level racking it may
be necessary to install multiple levels of sampling points to achieve best performance
as smouldering fires produce a relatively small amount of heat.
Where the aspirating detection system is the sole form of protection in any
given area, it is inappropriate to use a sequential sampling system (as adjacent areas
would lose their protection while the system sequences through its cycle). Modern
intelligent aspirating detection systems are often used in adverse environments where
site conditions cause unusual effects. Many diverse areas can be protected as hot air
can be cooled down, cold air warmed up, dusty air filtered, dirty air recognised as part
of normal operating conditions and contaminated air returned back to where it was
sampled from. In such applications it is important to site the detector in a more
environmentally friendly area and ensure that the sampling pipe network is
constructed from a suitable material.
Aspirating smoke detection is a very effective method of smoke detection. It
may be useful to note the following points when protecting unusual areas:
_ Atria / High Areas:
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Atria have stratification layers that vary with seasonal temperatures, making it
difficult to predict the optimum level(s) for best detection.
This type of area often benefits from a 3D approach with vertical sampling in
addition to the normal area coverage. This vertical sampling should be at 3m or 2°C
intervals. At each vertical sampling position an attempt should also be made to
maintain coverage in the horizontal plane.
_ Restricted Access / Containment Areas:
Some areas may be difficult to gain normal access to, either because of high security
or because the protected area is a health hazard. Such areas can often be protected
with aspirating systems where the detector is sited outside the problem area. This
minimises any required maintenance access into the protected area. It is important to
pipe the detector exhaust back to the protected area to prevent contamination
_ Dusty Areas:
These areas can be protected by detectors that contain dust recognition systems and/or
filter out dust particles. Contaminated filters reduce the performance of the system
and provision should be made to ensure a consistent level of protection
_ Hot Areas:
Most aspirating smoke detectors are designed to operate below 60°C.If the air sample
is above this temperature, the detector may be sited remotely and the air sample
cooled by either extending the pipe length or running it through a heat exchanger
(water jacket).
Size of the sampling holes:
The size of sampling holes will vary for each system and can be optimised using the
manufacturers computer software modelling packages. The sensitivity of each
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sampling point is proportional to the amount of air-flow through it and there are two
approaches to take with regard to sampling point hole sizing:
_ Identical sampling hole sizes –
This makes it easy for site engineers as all holes are the same, but means that each
sampling point will have a different sensitivity.This is because the air-flow through
each sampling hole will be affected by the pressure gradient down the main pipe
run(s). The holes closest to the detector will draw most air and therefore be more
sensitive.
_ Varying sampling hole sizes –
This requires additional care from site engineers, but also means that each sampling
point will draw similar quantities of air and therefore have a similar sensitivity.
Most aspirating smoke detectors have multiple alarm levels and consideration
should be given to best utilizing them.
VESDA Air Sampling System:
The VESDA system uses a Xenon tube as the light source to bounce light, off fire
byproducts in the detection chamber.
The VESDA air sampling fire detection system detects the invisible byproducts
of materials as they degrade during the pre-combustion stages of an incipient fire.
And, by actively and continuously sampling air, the system operates independently of
air movements.
Operation:
Air samples are continuously drawn from the monitored environment, typically
through a sampling pipe network with the aid of a high efficiency aspirator. On the
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way to the fire detector the air samples pass through a filter assembly to screen out
large airborne dust particles. Once inside the air sampling detector the samples are
exposed to a high-intensity and broad-spectrum light source. The incident light
scattered from smoke particles in the air sample passes through a series of optical
components to a solid state light receiver. The light is converted to an electronic
signal and passed to the control system.
At the control module the signal is processed and presented on an analog bar
graph to visually indicate the level of smoke present in the monitored area. Depending
upon smoke levels and the preprogrammed alarm levels, the appropriate output
signals are generated.
The first of the three staged alarm levels (ALERT) may simply indicate that the
system has detected something out of the ordinary that should be investigated. The
second level (ACTION) indicates that a potential fire exists and that emergency
procedures should begin. The third level (FIRE) signifies an actual fire condition.
AIR SAMPLING SYSTEMS USING LASER TECHNOLOGY:
1. Analaser air sampling system.
2.Stratos-Micra
AnaLASER Air Sampling System
Early detection means catching a fire in its incipient stages. That's enough time to
Analyze the situation
Alert personnel
Shut down equipment
Remove the source of the fire
Control the activation of the fire
suppression system
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ANALASER is used where Early Warning is Critical to Protect High-value Assets
such as:
Telecommunications
Computer Rooms .
Today's sophisticated telecommunications, computer, and business
systems are so vital that even a minor interruption in their operation, or loss of data,
could create serious or even crippling financial losses or life safety dangers. The
electronic hardware found in such installations is highly vulnerable to fire and can be
seriously affected by increases in temperature or contamination from smoke or
corrosive gases. in these and other areas such as museums, electronic manufacturing
facilities, or anechoic chambers where high-value property is present, AnaLASER
Advanced Technology Smoke Detection provides the early warning signs that can
spell the difference between a minor inconvenience and a major catastrophe.
When Conventional Detectors Won't Work Properly in
Clean Rooms
Atriums
High-Bay Warehouses
Nuclear Facilities
In order for a smoke detector to work properly, smoke in a concentration
sufficient to trigger an alarm must get to the detector. In many cases, this may take so
long and involve a fire of such magnitude that conventional detectors are virtually
useless. High-bay warehouses, where detectors are physically far removed from
sources of combustion, clean room with laminar airflow and rapid air changes that
dilute smoke, atriums, and other areas where conventional detectors may be rendered
inoperable by damage or vandalism, require the high-sensitivity and active air
sampling of AnaLASER Advanced Technology Smoke Detection.
Features of Analaser:
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Active System:- Continually Draws Air
Unlike conventional smoke detectors that passively wait for smoke to reach
them, the AnaLASER System continuously draws air from the protected area through
a piping network to the detection chamber. The pipe diameter and hole size in the
piping are optimized using the proprietary Factory Mutual-approved "SNIFF"
computer program to achieve equal sensitivity at all sampling points and minimum air
transport time.
Designed to Eliminate False Alarms:
With conventional, flashing Xenon tube, high-sensitivity devices, large
particles such as dust give "false" alarm signals if a filter is not used. The AnaLASER
Detector incorporates a particle size discriminator which eliminates such false
indications and allows the device to react only to those particles which fall into the
predetermined size ranges of smoke. In addition to this ability to discriminate between
particle sizes, the AnaLASER built-in data logger gathers information on minimum
and maximum ambient particle concentrations for accurate setting of the system's
three independently programmable alarm levels.
Up to 1000X More Sensitive Than Conventional Smoke Detectors:
AnaLASER Advanced Technology Smoke Detection will detect highly
diluted smoke and other overheat by-products at concentrations as low as 0.003%
obscuration per foot (0.01% obscuration per meter) compared to 3.0% obscuration per
foot for conventional smoke detectors. The combination of the ability to detect the
smallest smoke particles and the active airflow sampling network provides the earliest
possible recognition of an incipient fire.
Earliest Detection of Overheat Conditions:
Studies have shown that as electrical or electronic wire begins to overheat, it
releases specific materials as the temperature increases. AnaLASER has been proven
to detect the "plasticizers “ commonly released from PVC wire in the very early
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phases of heat buildup even before hydrogen Chloride (HCl) or pyrolysed
byproducts. It is precisely this ability to detect plasticizers that gives AnaLASER the
capability to provide earliest possible detection of an overheat situation.
Avoid Unnecessary Release of Suppression Agent
The ability to set three independently programmable alarm levels ,
ALARM,ALERT,FIRE allows time for preventative actions . The level three alarm
would indicate an actual fire may exist and initiate any number of actions from
notification of authorities to release of suppression agents. It is this series of alarm
levels that allows preventative action and avoid unnecessary release of agents.
Laser Detection
Laser Particle Counter
No Filter
No Expensive Refurbishing
Discriminates
No Degradation
The Analaser uses a laser beam and a particle discriminator to detect fire and reject
false alarms.
Operation:
The AnaLASER Detector consists of three main components; an air plenum chamber
with a centrifugal fan, a detection chamber with a focused 100 micron diameter laser
beam, and a single photon avalanche diode (SPAD) sensor. A powerful high-
efficiency centrifugal fan draws air continuously from the protected area into the
piping network and through the detection chamber without filtration or flow
restriction.A laser beam bounces light off small particles released by the combusting
materials in the protected area As a result, filter maintenance and loss of sensitivity
due to filter plugging are eliminated.
The SPAD photon sensor detects individual particles of smoke as they pass
through the laser beam. Smoke concentration is determined by counting the number
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of discrete particles detected, in a given time period. The system converts this digital
particle count to an analog signal directly proportional to the smoke concentration and
transmits this signal to the Display Control Panel.
The detector incorporates a particle size discriminator that provides outputs
only from particles in the size range of smoke while virtually eliminating outputs
from dust or other airborne contaminants. The laser used in the AnaLASER System is
designed to provide a minimum 10-year life and produce a long-term, stable output.
Changes in laser intensity resulting from temperature fluctuations, component aging,
or contaminant buildup are compensated for electronically. Supervisory circuits
monitor the operation of all critical components. The system also monitors airflow. in
the event of a component malfunction or variation in airflow, a FAULT signal is
automatically transmitted to the Display Control Panel.
This technology creates a high sensitivity smoke detection system up to 1000
times more sensitive than conventional ionization or photoelectric type smoke
detectors.
STRATOS –MICRA 25 AND 100
Stratos-Micras are air sampling smoke detectors.
Stratos-Micra uses the detector chamber from the Stratos-HSSD 2 to provide
the same sensitivity levels. Stratos-Micra 25 is the smallest high sensitivity aspirating
smoke detector available.
Stratos-Micra embodies innovative features which depart from accepted
techniques for detectors which operate at very high sensitivity. Perhaps the most
important feature of the system is the adoption of a patented Perceptive 'Artificial
Intelligence' known as ClassiFire-3D. This controls all aspects of the system
operation. ClassiFire-3D ensures that Stratos-Micra operates at maximum SAFE
sensitivity to give warning of problems earlier than previously considered possible.
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The picture below shows a Stratos-HSSD 2 with a Stratos-Micra 25 and a Stratos-
Micra 100 for size comparison
ClassiFire-3D is the most comprehensive intelligence found in a smoke
detection system to date. Not only does it determine the maximum reliable sensitivity
for the environment, but it also controls the dust separator monitoring for maximum
efficiency.
The detection principle used in Stratos-Micra is known as ' forward light
scattering' where the laser beam is diffracted by a small angle by smoke particles.
This principle not only offers high sensitivity, but sensitivity to a wide range of
Particle sizes.
A patented feature of the system is that compensation is made for any contamination
in electronic circuitry or hardware, ensuring a long and trouble free life.
The Stratos range of detectors are the only high sensitivity systems which are
routinely applied to the protection of very dirty and dusty environments. This is
achieved by using Laser Dust Discrimination (LDD) with a patented dust
management and separator system. These features have greatly extended separator life
service intervals. At the other extreme, Stratos-Micra 25 is capable of providing the
very highest levels of sensitivity in environments such as computer and clean rooms.
In these applications it is able to sense the very smallest amounts of smoke.
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Use of the latest semiconductor laser, electronic components and
manufacturing techniques enable the Stratos-Micra 25 system to be supplied and
installed with a significantly lower whole-life cost then alternative high sensitivity
systems.
Design Limitations:
Stratos-Micra 25 is intended to provide LOCALISED incipient fire detection only.
This means that it is suitable for the substantial range of applications typified by;
small non-compartmentalised rooms, warehouse racking, or pieces of electronic or
electromechanical equipment where it is desirable to achieve individual incipient fire
reporting. In compartmentalised rooms, each compartment would normally use
individual Stratos-Micra 25 detectors.
This product employs a very low-power aspirator and the aspirating capability
of the Stratos-Micra 25 detector is limited accordingly. Stratos-Micra 25 is NOT
intended to protect large areas, or to sample from areas where there may be any
difference in airflow rates or pressure differentials. Application of Stratos-Micra 25 in
these circumstances is not recommended. If detection in environments conforming to
these descriptions is required, alternative versions of Stratos products should be used.
It is recommended that a maximum sampling pipe length of 25 metres to be
used on the Stratos-Micra 25. Maximum single length of sampling pipe to be used on
the Stratos-Micra 100 detector is 100 metres in STILL AIR with 10 sampling holes
(or Capillary Remote Sampling Points). This will provide a transport time from the
end of the sampling pipe within 120 seconds. If the protected area has airflow present
the maximum permitted sampling pipe length will be reduced. In areas or applications
where the airflow rate exceeds 1 metre per second, maximum sampling pipe length is
reduced to 10 metres.
Stratos-Micra 100 has two pipe inlets and supports a maximum of 100 metres of
sampling pipe. Stratos-Micra is available with an optional ‘Piped Exhaust’ type
Docking Station. This is primarily intended to allow the Stratos-Micra detector to
sample from areas which may be at different air pressure to the detector location..
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Stratos-Micra provides four alarm level outputs and a fault output to the
fire alarm panel. Relays for Fire 1 and Fault are fitted as standard. When an
addressable panel is used the Addressable Protocol Interface Card (APIC) is used to
communicate using the addressable protocol. Using the APIC means that the alarm
and fault relays can be used for other purposes.
Specifications of Stratos-Micra 25:
Supply voltage: 21.6 - 26.4 Volts DC.
Current consumption: 250mA at 24 Volts DC.
Size: 135W x 175H x 80D.
Weight: 1.01 kg.
Operating temperature
range:
-10 to +60 deg.
Centigrade.
Operating humidity
range:
0-90% R.H. non
condensing.
Sensitivity
range(Obsc/m):
Min = 25% Max =
0.03%.
Particle sensitivity
range:0.003 to 10 microns.
Sampling pipe length:50 metres max. 25 metres
recommended
Cabinet rating: IP50.
“ PINNACLE ”
THE ULTRA HIGH SENSITIVE LASER SMOKE DETECTOR
37
Pinnacle, from System Sensor is an intelligent, addressable spot type smoke detector,
which is a laser-based and microprocessor controlled detector, achieving the highest
sensitivity and stability possible .
Laser Systems employing Pinnacle can be extremely flexible and cost
effective. Only critical areas that actually require ultra high sensitivity smoke
detection will use Pinnacle. Non-critical areas can simply use standard photoelectric
or ionization smoke detectors. But, regardless of type, all of the detectors install in the
same mounting bases. No special equipment is needed in order to install Pinnacle.
Pinnacle supercedes the performance of aspirated smoke detection systems:
In many ways, Pinnacle supercedes the performance of aspirated smoke detection
systems, traditionally the only way to achieve high sensitivity smoke detection.
Aspirated systems operate by drawing air and smoke through a network of pipe or
tubing that is routed throughout the protected space. Because of the nature of
detecting smoke in this way, aspirated systems are subject to the effects of dilution.
During an actual fire, smoke is drawn into the pipe through one of its sampling
ports. Unfortunately, the other sampling ports continue to draw clean air into the pipe
from areas that the smoke has not reached. This means that the smoke sensor in an
aspirated system must be set more sensitive to offset the effects of dilution.
Because Pinnacle is a spot-type smoke detector, it is not susceptible to
dilution. It is able to provide the exact location of the fire by identifying the address of
the detector sensing the smoke. This can greatly reduce response time in a real fire
situation since smoke at such low levels is not visible to the human eye. In addition,
each detector in a Pinnacle smoke detection system is fully supervised.
The main parts in a Pinnacle are:
Laser Diode
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Optical Amplifier
Photo Receiver
Laser Beam
The working of Pinnacle:
The principles of laser detection are similar to those of photoelectric technology. In a
photoelectric smoke detector, a Light Emitting Diode (LED) emits light into a sensing
chamber that is designed to completely restrict ambient light while allowing smoke to
enter. Any particles of smoke entering the chamber will scatter the light and trigger
the photodiode sensor.
Pinnacle works on the same light-scattering principle, but with 100 times
greater sensitivity. This ultra-sensitivity is due to the laser itself, which is literally
amplified light (the word “laser” is an acronym for Light Amplification by Stimulated
Emission of Radiation).
Using an extremely bright, controlled laser diode, the laser beam is
transmitted through the chamber to a light trap that eliminates any reflection.If a
particle of smoke (or dust) enters the chamber, light from the laser is scattered and the
detector, using patented algorithms, verifies the nature of the scattered light to
determine whether the source is dust or smoke. If a determination of smoke is made,
the alarm is signaled.
Smoke particles, especially those by-products of an early fire, are
extremely small, hence the need for the high sensitivity of the laser.
Pinnacle Specifications
Voltage Range : 15 – 32 volts DC peak
Standby Current (max. avg.) : 230 µA @ 24 VDC (without communication)
330 µA @ 24 VDC (one communication every 5 sec)
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LED Current (max.) : 6.5 mA @ 24 VDC (on)
Operating Temperature Range : 32° to 100°F (0° to 38°C)
Velocity Range : 0 – 4000 fpm (0 to 20.3 m/s)
Relative Humidity : 10% – 93% noncondensing
Smoke Sensitivity (9 levels) :0.02, 0.03, 0.05, 0.10, 0.20, 0.50, 1.00, 1.50,
2.00%/ feet obscuration.
(0.06, 0.10, 0.16, 0.33, 0.66, 1.65, 3.24, 4.85,
6.41 %/m obscuration.
MULTICRITERIA SMOKE DETECTOR ESM12251TEM
ESM1225TEM is a true multicriteria detector with microprocessor at its heart .
There is a combination of optical smoke sensing chamber and thermal sensing
element in the detector . Detector issues an addressable fire alarm , prewarning , fault
warning and maintainence warning.
Special algorithms ‘Drift compensation ‘ and smoothing element nuisance
alarms provide a consistent progressive alarm sensitivity threshold .The first feature
compensates automatically for the build up of contaminants in the sensing chamber
keeping the sensitivity constant up to a defined maximum level. Smoothing takes into
account short term environmental noise effects. When detector needs cleaning , it will
give an addressable service alarm.
Two LEDs provide information of alarms and are visible to all
directions.Detector has 2 rotary switches for addressing of the detector.Detector is
easy to install, it is placed into its base and turned into position.
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Technical specifications:
Dimensions 102x43mm
Operating temperature -30C-+60C
Humidity 10-93+/-2%
Operating voltage 15-32V DC
Standby current 250-300microAmp
Infra-Red BeamMaster-H Smoke Detector
The Chubb BeamMaster-H Smoke Beam Detector consists of a wall mounted emitter
and receiver pair incorporating a near infra-red beam to detect smoke using the light
obscuration method.
The Chubb BeamMaster-H Smoke Detector is designed for detection of
smoke in large spaces such as halls, warehouses, museums, theatres etc., where point
detection is impractical or more costly. The unit detects smoke linearly over the
protected range enabling early detection before the fire spreads.
Operation :
The emitter projects a near Infra-red beam which is detected by the receiver . The
beam is pulsed to reduce the overall current consumption and improve the noise
rejection characteristics. Powered directly from the Zonemaster conventional zone
without the need for an external supply , when the zone is reset , the detector is
automatically reset.
The emitter and receiver are synchronised via a direct 2 wire link which
also supplies power to the emitter. All other field wiring is connected to the interface
on the receiver. If smoke obscures the beam the receiver detects this and indicates a
41
fire. Any gradual reduction in received light through environmental contamination is
automatically compensated for within the detector.
In normal operation two status LEDs indicate a fire or fault condition, these are
viewed through a unique lens that allows good visibility from any viewing angle
particularly from beneath the unit. An output is also provided from the receiver for a
remote fire indication.
The unit indicates a fault on the zone under the following conditions:
Compensation limit exceeded
Total Obscuration of Beam (Under this condition a fire signal will also be
generated following a pre-defined time delay.)
Receiver cover left open
Receiver unit removed from zone
Line smoke detector Fireray 1401 based on infrared beam:
Line smoke detectors are used mainly in premises with high ceilings such
as shopping centres, atriums in hotels, churches, hangars, etc., and in rooms in which
detectors cannot or may not be mounted on the ceiling , for example in historical
buildings and museums.
The line smoke detector consists of a transmitter that transmits a modulated infra-red
light beam to a receiver , along with a control unit for power supply and signal
conversion. The received light beam is analysed , and if smoke is present for more
than five seconds, the fire alarm is activated.
Transmitter and receiver should be positioned so that the beam of light runs parallel
with the ceiling at a distance of 0.3 to 0.6 metres. The maximum range is 100m and
coverage is 7m on either side of the beam.
.
Fireay 1401 consists of one transmitter and one receiver in aluminium enclosures of
the dimensions, 128x90x85mm and a 250x200x148mm control unit.
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Video Smoke Detection:
43
Due to the size and environmental conditions in some locations , detecting
small amounts of smoke using standard methods of detection may prove
inadequate.A video based fire and smoke detection system will detect fire in the
very early stages in such environments thereby providing the ideal detection
solution.
Features And Benefits:
Upto 8 cameras per system and it can utilise existing CCTV surveillance
cameras. 10 independent alarm zones per camera .
Automatic checking for video signal loss,obscuration, low level light and low
contrast level 5000 event log .
Relays for interfacing with other equipment.
Flexible configuration between camera zones and alarm outputs.
In cavernous environments such as a turbine hall or exhibition halls , detecting
smoke quickly using standard methods of detection can be inadequate.‘Point’ type
or beam’ type detectors may prove to be too slow. Similarly in such environments a
fire may be well under way before being detected by a heat detector. In such
conditions video based smoke detection offers an ideal detection solution.
The Chubb video smoke detection system consists of a standard closed
circuit television (CCTV) cameras linked to a self contained processing system
which is capable of recognizing small amounts of smoke , within the video
image. The system uses highly complex algorithms to process video information
for up to eight cameras simultaneously. Following detection the system operator is
alerted both at the processor and by a variety of remote outputs.
The system can be divided into ten independent alarm zones per camera, giving
80 detection zones per system. The system also has sixteen opto - isolated
alarm outputs that may be individually assigned to any combination of zones and
sectors. These system outputs can be easily interfaced on to our
Controlmaster range of analogue addressable control panels.
Video smoke detection can be installed in a variety of applications, such
electrical power generating stations , toxic waste plants , cement works , paper
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mills , aircraft hangers , road and rail tunnels and historic buildings.
Alarm output devices:
Upon receiving an alarm notification, the fire alarm control panel must now tell
someone that an emergency is underway. This is the primary function of the alarm
output aspect of a system. Occupant signaling components include various audible
and visual alerting components, and are the primary alarm output devices. Bells are
the most common and familiar alarm sounding device, and are appropriate for most
building applications. Horns are another option, and are especially well suited to areas
where a loud signal is needed such as library stacks, and architecturally sensitive
buildings where devices need partial concealment. Chimes may be used where a soft
alarm tone is preferred, such as health care facilities and theaters. Speakers are the
fourth alarm sounding option, which sound a reproducible signal such as a recorded
voice message. They are often ideally suited for large, multistory or other similar
buildings where phased evacuation is preferred. Speakers also offer the added
flexibility of emergency public address announcements. With respect to visual alert,
there are a number of strobe and flashing light devices. Visual alerting is required in
spaces where ambient noise levels are high enough to preclude hearing sounding
equipment, and where hearing impaired occupants may be found..
Another key function of the output function is emergency response notification. The
most common arrangement is an automatic telephone or radio signal that is
communicated to a constantly staffed monitoring center. Upon receiving the alert, the
center will then contact the appropriate fire department, providing information about
the location of alarm. In some instances, the monitoring station may be the police or
fire departments, or a 911 center. In other instances it will be a private monitoring
company that is under contract to the organization. In many cultural properties, the
building's in-house security service may serve as the monitoring center.
Other output functions include shutting down electrical equipment such as computers,
shutting off air handling fans to prevent smoke migration, and shutting down
45
operations such as chemical movement through piping in the alarmed area. They may
also activate fans to extract smoke, which is a common function in large atria spaces.
These systems can also activate discharge of gaseous fire or preaction sprinkler
systems.
Smoke Alarm Response:
Smoke alarm response was
measured by direct recording of the
voltage signal from both ionization and
photoelectric smoke alarms arranged for
analog output, instead of the more common
alarm threshold. By recording analog
output, the performance of smoke alarms at
any desired threshold setting as well as the
potential use of algorithms that reduce
nuisance alarms can be evaluated. The
analog signal was calibrated against
unmodified alarms purchased in local, retail
outlets, in the laboratory to verify that the
modifications did not affect the alarm performance. Alarms were located in typical,
code-required locations, as well as in the room of origin, in order to determine the
effectiveness of alternative siting rules. In the room of fire origin, three unmodified
alarms were used to avoid destruction of the limited supply of analog-modified alarms
Interconnecting Smoke Detectors:
Battery-powered smoke detectors are stand-alone units.
Home smoke detectors should be interconnected. This means that an alarm in one
smoke detector will cause all others in the home to go into alarm. Typically the
detectors are connected by a pair of wires to transfer an alarm signal from one
detector to all the others in the chain
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This sort of wiring guarantees that if one alarm in the house goes off, they all
go off. Even if the fire starts and is detected in the basement, people asleep upstairs
will hear the alarm because of this safety feature -- every alarm in the house goes
off.
If we buy an AC-powered smoke detector today, it will have three wires --
black, white and red. Black accepts 120 volts AC, white is neutral, and red is the
intercommunication wire. All of the alarms operate off the same circuit from the fuse
box and are normally connected using normal wire for three-way switches this wiring
contains black, white and red wires in a Romex casing.The red wire is run from alarm
to alarm to interconnect them.
When any alarm detects a fire, it sends a 9-volt signal on the red wire. Any
alarm that detects a 9-volt signal on the red wire will begin sounding its alarm
immediately. Most alarms can handle about a dozen units intercommunicating on the
same red wire. It's a very simple and a very effective system.
Working of three way switches:
This is explained by looking at how a normal light is wired for residential wiring of a
light switch. The figure below shows the simplest possible configuration
In this diagram, the black wire is "hot." That is, it carries the 120-volt AC current. The
white wire is neutral. In the figure the current runs through the switch. The switch
simply opens (off) or closes (on) the connection between the two terminals on the
switch. When the switch is on, current flows along the black wire through the switch
to the light, and then returns to ground through the white wire to complete the circuit.
To run power from the fuse box to the switches and outlets in the house ,a romex wire
is normally used. A piece of Romex is shown here:
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Romex consists of an outer plastic sheath (white in this picture) with
three wires inside. The black and white wires are insulated, while a bare, third wire
acts as the grounding wire for the circuit. Most normal household applications use 12-
or 14-gauge Romex.
Installation of smoke detectors:
Any smoke detector that should be installed should have a test button. When the
button is depressed, the audible alarm sounds the warning signal. If there is a hearing
impaired person in the house, consider the installation of a hearing impaired smoke
detector. These are special units that feature a powerful strobe light to alert the
hearing impaired in the event of a fire.
Smoke detectors should be wired directly to the 120VAC electrical circuits.
Units that depend on batteries as their sole source of power should be avoided.
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References:
1. Mechanical and Industrial Measurements By R.K.Jain.
2. www.msnsearch.com\ smoke detection systems.
3. www.google.com\ laser beam smoke detectors
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