Section 23 - Fire Alarm and Detection Systems Flashcards
the purpose of a fire alarm
is to save lives by warning the occupants, early, of a fire condition.
A Manual pull station is comprised of….
a hinged or levered pull handle controlling an electrical contact or switch.
Location of manual pull stations
- located at every exit excluding elevators on every floor
- extra pull stations may be located between exits so that no location on a floor is more than 60m from a pull station.
Fixed Temperature Type heat detectors
Fusible (non resettable)
Resettable
Fusible (non resettable) - spring loaded plunder held under pressure - metal will melt at specific temperature - plunger is released - closes electrical contact - detector needs to be replaced - common temp settings are 57c, 93c, 140c
Resettable - Bi-metal disc is positioned in centre of detector housing - temperature increases to predetermined temp - 2 metals expand at different rates - causing electrical contact to close - as room temp decrease the system resets itself
Rate of rise type detector
- consists of a sealed air chamber with pressure relief vent
- vent automatically compensates for minor changes in temperature
- temp rises at a rate of 9c/min or quicker - increase in detector pressure cannot escape adequately
- increased pressure causes a diaphragm to distend thereby closing the normally open electrical contact
- as room cools detector will automatically reset.
Combination fixed temperature/rate of rise
and parts
- responds to temp increase of 9c/min and fixed temp.
- the most widely used detector today
Parts of combination fixed temp/rate of rise
-base, shell, heat collector fin, fusible link, plunger and spring, contact screw, bar, calibrated atmosphere, diaphragm, contact spring, calibrated orifice, contact buss
Rate of Compensation type
- detect a set temperature, but reacts to it quicker
- set of 2 metal strips - expand at a rate slower than outer shell
- shell at predetermined temp pulls the strips together closing the electrical contacts
- will automatically reset.
Smoke detectors
is an electrical device that is used to detect one or more products of combustion.
Ionization Detectors
- respond to both visible and invisible products of combustion
- best responds to particle size of 0.01 to 1.0 micrometer range(10 micrometers is invisible to the human eye)
- chamber consists of small radioactive source and 2 charged electrical plates of opposite polarity
- small current flow between plates
- larger molecules like products of combustion cause a decrease in the current flow
- Galvanometer measures this reduction in current flow and activates alarm
Photo-electric smoke detectors
Obscuration type
Reflected light type(light scattering type)
-sense the pressure of visible smoke
Obscuration type - certain types of smoke block or absorb light and consist of a light source and a light-sensing element.
Reflected light type(light scattering type) - light source and light sensitive cell housed within a light-absorbing chamber - grey smoke (not black) enters chamber the light is reflected to the photoelectric cell - activated the alarm
Special Application detectors
- Air duct mounted detectors
- Beam detectors
- Optical flame detectors
Air duct detectors - responds to a build up of smoke within the duct and then will shut down the air handling system
Beam detectors - transmitter and receiver parts - heat and/or smoke obstructs beam - has a delay mechanism to prevent accidental alarms.
Optical flame detectors - flash fire detector - Infrared detectors and ultra violet detectors are extremely fast and very reliable but subject to false alarms
Key Components of any sprinkler system that must be monitored
- Main water supply shut off valve
- Gate valve
- Pressure pumps
- Fire pump power
- air pressure supervisory devices on dry pipe systems
- post indicating valves
Automatic sprinkler supervisory service
- Waterflow
- Pressure
- Gate valve supervision
Waterflow device - a switch connected to a vane in the pipe and when there is a flow in the system the switch operates - is delay which allows for fluctuation common in such systems
Pressure switch - a diaphragm that moves when there is a pressure change in the system - in the retard chamber(wet system) - in the intermediate chamber(dry system)
Gate Valve Supervision - a switch that opens mechanically when the stem of the valve is moved to a partially or fully closed position
Alarm indicating devices
Devices to notify building occupants of a fire condition - Audible devices(horns, chimes, sirens, speakers, and bells)
Building where hearing impaired or noise levels are excessive visual alarms are used.(rotating reflective lights, strobe lights, spotlights)
Fire Alarm Control Panel
- Power
- Common Control
- Alarm initiating circuits
- Alarm signalling circuits
