Complete information on Fire Detecting Systems


Manual fire alarm system – Hand bell

Automatic alarm systems based on heat, smoke.

(i) Fire alarm system provide an audible & or visual alarm. Signal as the result of manual operation of fire alarm station or automatic operation of sprinkler system. Manual station bells/horns.


Electric switches covered with a red colour housing located near exits or exit stair door.

Voice alarm speaker circuit with loud speaker amplifier & pre amplifier.

Four stages of fire is detected

(i) Incipient – An oxization chamber is used to detect aerosols.


(ii) Smouldering – Fire can be detected by smoke.

(iii) Flame – Radiation. Detecting infrared ultraviolet radiation emit­ted from fire source. Heat – Using the principle of expansion resulting from heat achieved by contacts assembled to make or break electrical circuit.

The type of detection system is based on smoke, thermal heat, flame, fire gas leakage, combustion, Water flow.

(I) Conduction/Convection Heat Detection:


Solid Point type:

(a) Backup fixed temperature type detector – thermal.

(b) Rate of rise temperature – thermal.

Electro preumatic detector combined rate of rise & fixed temperature type detector are used in kitchen.


(c) Rate compensation type. Solid line type –

Liquid expansion type;

(a) Back up fixed temperature type detector

Which respond when the detection element reaches a predetermined temperature.


Fire develops very slowly.

Housing of detector contains a fusible element that an exposure to a specific temperature melts rapidly causing electrical contact to activate a fire alarm. A faster acting variation of the fixed temperature detector is the rate compensation type (c) detector.

(b) Rate of rev temperature detector.

When respond to an increase in temperature at a rate greater than some predetermined value.

Very quick response to fire. Reach rapid changes in temperature.

Heat from fire causes air within the detector housing to expand more quickly then it can be vented. The increase in the air pressure then causes a diaphragm in the unit to operate compensation electric contact which initiate an alarm.

(c) Rate compensation on type detector-activates when the surrounding tem­perature reaches a predeter, mined value.

(II) Smoke Detector:

(a) Smoke:

Using Lyndell principle of light scater on the smoke particles operating on photo sensitive device.

Another classification of fire detectors based on the principle of operation:-

Smoke detectors are most likely to offer the earliest warning & so are gener­ally considered the most useful devices. They may be placed on ceilings for on walls near ceilings or in ventilation ducts Ceiling detectors should be placed where air activity is there. They are located inside air zones.

They will not detect fires as readily. One type of smoke detector project a beam across a space to receiver some distance away when smoke interrupts the beam the alarm sounds. This same principle has been used for devices that automatically open doors (in this use the beam is interrupted by the body of a person approaching the door).

Smoke detectors are used to detect fires in an incipient stage:

(a) Ionization type smoke detector.

(b) Photo electric detector, light construction, light scattering.

Chemical effect resistance bridge cloud chamber.

(a) Ionization type smoke detector:

This type of detector has two air sampling chambers one outside chamber & another inner chamber. When fire is caused it is invisible & results in the forma­tion of aerosols minute particles (may be visible or invisible).

Which travel in all directions out of which some are definitely to travel in the direction, penetrate the outer chamber called open chamber but inner chamber called enclosed refer­ence chamber) is closed to prevent easy entry.

Both chambers are kept ironed by a radioactive source of alpha particles, like. Americium 241 or Radium 226. Americium is chosen as a source because it has minimum radiation of particles other than alpha particles, the emission of alpha particles keep the air chamber ionized by the way of detecting splitting.

The air molecules are in ions. The ionized air particles will recombine (or neutralise each other) & become disasso­ciated again in a continuous process. If the chamber is not electrically charged.

However when a potential is applied to the chamber electrode mitigating charged air particles take place toward the electrode of the opposite sign, a minute but measurable electric current will flow across the chamber air gap.

Not all the particles will reach the electrodes. Some will collide with particles of opposite polarity & be neutralized. The current flow will therefore depend on the chamber & electrode configuration as well as & shape of the air particles. This latter parameter dictates the rate of drift of the particles across the chamber.

(b) Photo electric detector:

Use the principle of light obstruction or light scattering. As smoke particles enter the sensing chamber light reaching a photo sensitive device in the chamber is either reduced initiating an alarm or scattered on striking as smoke particles thus a photosensitive device designed to initiate an alarm on contact with light.

Photo electric detector is used:

Where a smouldering fire producing considerable quantity of smoke.

(III) Heat Detectors:

Respond to the high temperature caused by fire. They may be useful in small areas such as storage rooms where heat is built up could be rapid but they are used infrequently because by the time they sound the alarm, the fire is likely to be well under way.

Heat detector requires a definite increase in the temperature of the environ­ment, production of visible particles or aerosols.

Beam Detector:

The effect of hot air or gases passing through the beam causes it to break up in such a way as to produce a flickering effect which is superimposed on the brightness of the received high pulses. This flickering occurs at a frequency which normally lies between 2 to 20HZ & is of typical a fire condition.

By electronically demodulating the pulses & passing the separated modula­tions through an amplifier turned to typical frequency, a heat alarm signal is produced. There is nominal delay of 10 seconds during which time the modula­tion must sustain before an alarm is given.

Both heat & smoke detection signal remain latched on until manually reset. In the event of total obstruction of the state? a complete loss of incoming signal at the receiver & a fault warning is register.

Similar conditions are obtained in the event of transmitter failure. If a smoke alarm has been reset beforehand a fault signal will not cancel the smoke signal. Fault warning is self resetting when the beams are restored.

Such detector is being marketed by M/s steel industries age by name beam master in India.

Beam detectors have the disadvantage of having to be connected to as power failure or the plug being pulled if they are not wired directly to the function box.

Flame Detector:

They are most likely to be used in fuel storage area where fires would develop rapidly with no incipient stage where an explosive or rapid flame devel­opment occurs.

Electromagnetic Detectors:

Comprising of infrared & ultra­violet types, employs photocell sensitive to infrared & emitting from the fire. The prince of detection is that the radiation energy passes through air at the speed of light with raising its temperature is not dissipated by air currents & can be reflected.

When flames appear both ultra violet & or in feared region spectrum will show flicks frequencies. Photo cell current to these frequencies will give response with in a million of second subject to the flame being in the line of sight of the detector. Integrator time circuit of the detector will be so designed to give response only if the signal parish for 2 to 10 seconds.

Detection of the radiation from other sources such as sunlight heater is also possible.

Infrared Flame Detector:

In electromagnetic spectrum infrared radiation falls within the wave length 7600 Ao to 1000000 A°. Between 7500 A° to20000 A° the region is known as near infrared ft above 70000 A° is far infrared. It is infrared emitting from fire & responded by infrared detector.

An infrared detector essentially consists of a lens & optical filter, a photo electrically wave form filter, amplifier & an integrator or timer.

A photo electric cell basically converts light energy in to electrical energy, photo conductive Photo cells are usually made of materials such as halloos sul­phide, lead sulphide, lead, telluride, lead selenide, selenium & silicon It is neces­sary to project photo electrical & electrical components from dust & moist area.

Combustion Detector:

Depend on the current setup by the fire to carry the product of combustion in to the detector. It can operate when the fire develops is to such to produce sufficient energy to accelerate the products of combustion through the layer of warmer air in the vicinity of a detector, detector has to a time leg between the incidence of fire & detected response time 40-300 seconds.

Water Flow Detector:

It indicates because of the other detector types (smoke, flame or heat) acti­vated & starts activity of the sprinkler system.

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