Fire alarm system:

A fire alarm system consists of one F. A., panel which detects the entire state of the system glass enclosed switches called manual call point (M. C. P. S), and audio alarm here in for pounding in alarm, public addressing speakers, a set of battery and the carding which connects all these components to the panel. The amplifier for the P. A. system is housed in the panel/the system described below is a manual fire alarm system. In an automatic Fire Alarm System the will be fire detectors in addition to the aforementioned components. These components are of great importance in fire alarm system.

Power supply to the system is 24 VDC. This is derived from single phases, 2SOV, 50 Hz AC by means of a rectifier. A backup battery of 24 V within automatic charge over switch is provided to ensure the uninterrupted operation of the system, in the event of a power failure during fire. In a manual system, the glass enclosure of the MCP has to be broken by means of the hammer provided along with it, thereby closing the circuit, which activates heaters of the corresponding zone through the panel. An alternating sound indicating fire can be heard now.

This warns the fire condition automatically and transforms it into electrical signals. This is signal activated activate by hooters of a particular zone where the fire has occurred. The MCP’s can also be used the system manually, if so desired. The P. A system can be used by the fire force personnel to guide the inmates during rescue operation.

Fire Detectors:

Fire detectors of three types are normally used in automatic fire alarm installations.

They are:

  • Ionization type smoke detectors.
  • Optical type smoke detectors.
  • Heat detectors.
  • Cards used in control and Indicating Equipment

Power Card:

  • It gives all the details about the healthiness of the power supply used in this system. it gives the following indications .
  • D. C. on: It indicated that the DC supply is on.

Blown Fuse:

It indicates that the fuse is blown.

D. C. Fuse:

It is a capsuletypefuserated3 Amps. When the fuse blown the BLOWN FUSE indicator will light us In this condition the panel will not work.

Blown Fuse:

It is red LED indicator for BATTERY CHARGINGFUSE blow. When the red lamp glows check the CHG Fuse and replace it.

CHG. Fuse:

It is a capsule typefuserated3 Amps. When the fuse is blown the BLOWN FUSE INDICATOR will light up.

Batt. Low:

It is a battery low indicator. When the battery is below the average charge the indicator will light up.

Then check the battery meter and check the CHG. Fuse then keep the Trickle/Boos switch to the Boost position for emergency charging. When battery gets enough charge keep the switch in Trickle position.

A. C. Fall:

It is a red LED indicator. When the system is on the Indicator will light up.

Function Card:

It has the following functions- Zone cards.

Fire:

It is a push switch. This switch is for testing the Five Circuits and Fire LED indicators in the zone cards. In each card, there is a ‘Test’ switch in the particular zone card” the t wore Fire LED indicator in the zone card will light up and also a buzzer will sound, the test shows the healthiness of Fire condition circuits in the particular zone card.

Shot:

It is a push switch. This switch is for testing the short LED Indicator and its circuits in the zone cards. for testing the ‘short, push the short push switch in the function card simultaneously short LED indicator will light up (in the zone card) and also a Buzzer will sound. This test shows the healthiness of the short detection circuits-in the particular zone cards.

Open:

It is a push switch. This switch is for testing the open LED indicator and its circuits in the zone cards for testing the ‘open’, push the open push switch in the function card simultaneously the open LED indicator will light up (in the zone card) and also a Buzzer will sound. This test shows the healthiness of the ‘Open’, detection circuits in the particular zone cards.

Accept:

It is a push switch. In Fire Short and open condition the panel will give buzzed sound. Tn step this, push the accept switch. In the fire condition the Hooter will sound continuously. To stop this use Hooter Off switch.

Rest:

It is a push switch. In the fire on other fault condition after rectifying the complaint if any, then push the ‘Reset switch Zone Cards: It is plug in type P. C. B. module for easy maintenance, It’s Fire Detectors circuit number will very according to the number of zones. It has got the following functions:

Fire:

This is two parallel red LED indicators. In fire condition these two LED indicators will light up also the buzzer and hooter will sound.

Short:

This is the LED indicator for short circuit in detector cabling if there is any short in detector wiring. Then the LED will light up and the buzzer will sound.

Open:

This is LED indicator for open circuit in detector cabling. If there is any open circuit in detector wiring, then the LED will light up and the buzzer will sound. Isolate: This is a push on and pushes off switch. In off position a red LED indicator will High up. We can switch off a particular zone for maintenance purpose without disturbing other zones.

Test:

This is push switch for testing the Fire, short and open circuits, Push fire, short and open push switches in the function card, keeping anyone of the above pressed and then push the test switch, It is for the system self checking.

Ionization Type Smoke Detector:

If the atom is subjected to radiation from a radioactive source ns electrons become detached. As a result the atom becomes Ionization Type Smoke Detector positively charged (t. e, it has more electrons than protons). These ‘new atoms are called ‘ions‘ and the process that created them is called ‘ionization’. If the atoms of air in a container are subjected to radiation, ionization will take place in this way, and the ions will move about haphazardly.

If we then introduce a positively charged plate and a negatively charged plate to the container, the positive and negative ions are attracted towards the Opposite charged plate. This will produce small current in the external circuit.

Optical Type Smoke Detector:

While the ionization detector responds to the invisible products of combustion the optical detector, as its name implies, react to the visible products of combustion, the particles of Carbon an? Other chemicals which give smoke its characteristic appearance. Art amount of light falling on the photo-electric cell, which is the critical factor in the operation of the optical detector, Some optical detectors are designed so that, in a fire situation MORE light is thrown on to the photo- electric cell. These are called the ‘light-scatter type’. Others are designed so that LESS light is thrown on to the photo-electric cell. These are called ‘obscuration‘ type.

Heat Detector:

Heat ‘detectors are designed to detect fire in the more advanced stage when the temperature in the protected area stage when the temperature in the protected area starts to rise.

Given that the effects of here are easy to observe it is not surprising that the at detectors were the earliest form of detector to be developed. The effects of heat which provide the basic operating principles for the heat detectors are:

  • Melting (or fusion) in metals or plastics.
  • Expansion in solids, liquids and gases.
  • The electrical effect.

Smoke & Flame Detectors:

Success in controlling fire lies in its early detection. With the passage of each moment the fire spreads progressively and within a few minutes it turns violent. Fire detection systems are designed to detect fire at the earliest practicable moment so that appropriate actions can be taken for its control.

Main Objectives:

  • Quick detection.
  • Reliable transmission of detection signal.
  • Translation of signal into dears alarm and indicates location of fire.

General Requirements:

 

  • Fire Detectors should be reliable and should not be liable to disturbance by any other Svel whether associated with it or not.
  • The system should not be rendered in operative partially or totally by the fire.
  • Operation of an installed system should be tested for satisfactory performance periodically.
  • The system should not create direct or indirect hazards.
  • The power supply to the system shall be from two sources with an automatic change over switch.
  • If the signals are not received or acknowledged within two minutes, an audible alarm- shall be automatically sounded.
  • Fire Detectors shall be operated by heat, smoke or other products of combustion like flame or combination of these factors. Flame detectors shall only be use in addition to smoke or heat detectors.

Categories of Fire Detection System:

These fall into two important categories:

  • Heat Detectors.
  • Smoke Detectors.

Heat Detectors:

Mainly heat detectors are of two types. Which operate at fixed temperature.

Which operate when the rate of rise in temperature of the surrounding air reaches a set limit.

Fixed Temperature Type:

A bimetallic strip is used to make-or break an electric circuit at a preset temperature. When the temperature near the bimetallic strip-rises to a pre set temperature the electric circuit is made which makes the bell to ring.

Rate of Temperature Rise Type:

A sealed chamber when subjected to heat expands and pushes a flexible diaphragm to make an electrical contact complete for the alarm.

Smoke Detectors:

There are three types of smoke detectors

  • Light Obscuration Type.
  • Light Scatter Type.
  • Ionization Type.

Light Obscuration Type:

A beam of light is arranged to fall on a photoelectric cell and if the atmosphere between the light source and the cell is clear, the balance of the electric circuit is not disturbed.

When smoke passes between the source of light and the photocell, the intensity of light falling the photocell is reduced. This change in the electric output is used to operate an alarm at a level.

Light Scatter Type:

When a beam of light extend across a transparent medium e. g. , air, its intensity is reduced by absorption and partially by scattering. Scattering takes place due to suspended particles such as dust, liquid particles or smoke.

Ionization Type:

If the atom is subjected to radiation from a radioactive source some electrons become detached. As a result the atom becomes positively charged. Such charged atoms are called “Ions” and the process that created them is called “Ionization”.

If the atoms of air in a container are subjected to radiation, ionization will take place and the ions will move haphazardly. If we then introduce a positively charged plate and a negatively charged plate to the container, the positive and the negative ions are attached to the oppositely charged plate. This will produce a small current in the external circuit.

Flame Detectors (Infrared Type):

In addition to light, fires generate infrared and ultra violet radiations which ate less affected by Sensor, the presence of flames can be detected in spite of the smoke generated. The signal from the photoelectric cell with infrared filters or an ultraviolet sensor can be arranged to trigger the smoke than normal white light. By employing a photo electric cell with inferred filters or an ultraviolet alarm.

These devices suffer from the disadvantage that they will react to infrared or ultraviolet radiations from other sources and hence are prone to spurious alarms. This disadvantage is overcome in a combined smoke and heat detecting system in which a modulated infrared beam is transmitted across the area to be protected. Smoke and heat haze interfere with the, beam and this is detected in the receiving unit which initiates the alarm.