Various types of detectors are available operating on principles of thermal expansion, thermoelectric sensitivity, thermo conductivity or photosensitivity to detect presence of smoke, increase in temperature, and light intensity of total radiation. Their types are:
- Thermal expansion detectors
- Radiant energy detectors
- Light interference detectors
- Ionisation detectors
They should be properly located depending upon their range. They simply give alarm and cannot extinguish fire. They make us alert for fire fighting.
Through fire detection and fire alarm systems are separate systems but the latter has to operate just after the former operates. Therefore they are considered together. IS 2175 and IS 2189 also deal with together.
Two main functions of any fire detection system are:
- To give alarm to start up extinguishing procedure, and
- To give early warning to area occupants to escape.
It is wrong to speak ‘fire detectors’. Actually they detect sensible heat, smoke density or flame radiation to operate before actual fire follows. Their ‘sensor’ detects measurable quantity of these parameters. A decision making device coupled with the sensor compares the measured quantity with a predetermined value, and when it is different, an alarm is sounded.
A Fire detector both detects and signals.
Human being is good detectors as he can act in flexible way i.e. run away, put out the fire or call the fire department. No other detector can work in such selective manner.
Selection of the detector is important. For example, low risk areas thermal detectors, a ware house may have infrared and ionisation detectors and a computer area requires ionisation or combination detectors.
Location and spacing should be determined to obtain the earliest possible warning.
Sensitivity, reliability, maintainability and stability are important factors for selection.
Fire process has four stages-incipient stage, smouldering stage, flame stage and heat stage. Many types of fire detectors are available for various situations and useful at different stage of a fire.
Thermal detectors are of fixed temperature detectors, rate-compensated thermal detectors, rate of rise thermal detectors, line thermal detectors and the bulb detection system.
Smoke detectors are of photoelectric type and are of two classes – The beam photoelectric or reflected beam photoelectric detectors.
Flame detectors are of infrared (IR) or ultraviolet (UV) type.
Ionisation (combustion products) detectors are the single chamber or dual chamber ionisation detector and the low-voltage ionisation detector.
Fire Alarm System
Fire alarm system may be separate to run manually or connected with fire detectors and operable automatically. All workers must be made aware of the sound pattern and its meaning. Fire alarm sound should be distinguishable from other sound in that area. It should be clearly audible to all facility personnel. Sound for beginning of fire and end of fire should be kept different.