Fire Detection and Warning Systems Flashcards
Describe the operation of an Ionisation detector and where they are generally located.
radioactive, ionisation, electrical current
(i) Single open chamber within detector containing small radioactive source, emitting alpha and gamma particles.
(ii) radiation causes the air in the chamber to become ionised, promoting the flow of electricity between two electrodes.
(iii) smoke enters the chamber interfering with flow of electricity = reduction in current flow.
(iv) when current drops below predetermined level, the detector activates.
- Primarily used for detecting fast burning fires which give rise to small particles
- Commonly found in environments with highly flammable materials or those prone to combustible gas leaks.
Describe the operation of an Optical detector and where they are generally located.
IR LED, photodiode, smoke interference
- Reacts to visible products of combustion.
(i) Contains an InfraRed LED light source and a light receptor (Photodiode receptor).
(ii) The amount of light falling onto the receptor is the critical factor.
(iii) Smoke particles enter the housing, reflecting light onto the diode receptor, creating a small electrical current.
(iv) detector preset so when smoke density reaches a predetermined level alarm is sounded.
- Best suited to areas with slow burning fires.
- Recommend for spaces containing materials that might not ignite quickly but can produce dense smoke eg bedrooms, spaces with furnishings of synthetic materials.
Describe the operation of a heat detector and where they are generally located.
thermister, rate of change
(i) Hot air from the fire enters the sensor chamber and the thermistor.
(ii) once temp reaches a fixed level (58c) signal sent to integrated circuit which sounds alarm.
- Rate of Heat Detector:
Some heat detectors use a matched pair of thermistors to sense heat, one ambient and one enclosed.
(i) under normal conditions both register similar temps,
(ii) in event of fire the rapid increase in temp causes exposed censor to register an imbalance causing alarm to sound.
Describe the operation of an Aspirating detector and where they are generally located.
nephelometer
(i) Air samples drawn in through pipes running through a protected area.
(ii) each air entry hole in the system considered a separate smoke detector.
(iii) air exhausted in central unit containing a special smoke detector, known as a Nephelometer.
(iv) Nephelometer measures size and concentration of particles suspended in the air/smoke via the means of light they scatter and responds accordingly.
Describe the operation of a Flame detector and where they are generally located.
IR, UV, Viible light, optical sensor, algorithms
- Responds to radiation emitted by flames.
- Can be in the form of:
Infrared radiation (IR)
Visible light
Ultraviolet radiation (UV)
(i) energy travels in waves radiating from point of origin to the detector, some designed to respond to infrared others respond to IR and UV.
(ii) optical sensor receives IR or UV waves from flame, analyses their frequency, intensity and duration to determine if they match internal algorithms, if so alarm triggered.
(iii) If signals do not match algorithms, considered false sources and are rejected.
- Found at oil and gas installations, chemical plants, aircraft hangers.
- Useful where smoke detection may be slow to respond or unreliable.
Describe the operation of an Optical Beam detector and where they are generally located.
thermal turbulance, smoke obscuration
Consists of two units:
Transmitter - producing a pulsed beam of IR light.
Receiver - tuned to accept the IR frequency.
- usually mounted at opposite ends of the space being protected.
- IR beam analysed for fluctuations in thermal turbulence and for loss of strength by smoke obscuration.
if either exceeds a preset level the smoke or heat LED is illuminated and fire signal triggered.
- Used in large open space like warehouse and atriums.
- Good when multiple point detectors are impractical.
Describe the operation of a
Multi-sensor detector and where they are generally located.
- Combine smoke and heat sensors into a single unit (may include CO detection).
- Designed so that both slow smouldering fires and fast flaming fires with rapid temp increase can be detected.
Describe the functions of the control unit of a modern fire detection and alarm system.
• Automatically monitor and control the equipment system ( such as fire detection, fire alarm circuits and power supply).
• Indicate fire and fault signals and their location.
• Operate the alarm either throughout the building or in a particular sequence related to the evacuation plan.
• transmit the signal to an alarm receiving Centre (ARC) for onwards transmission to the FRS or key holders.
• Operate equipment such as deactivation of door-holding devices, open smoke vents to.
• Operate fixed fire fighting equipment such as sprinkler, foam or gas systems.
• Indicate which zone is giving the signal not only at the main panel but also at repeater panels/indicators of installed.
State the ways fire warning/detection systems can notify occupants of an emergency.
(i) audible
(ii) visual
(iii) sensual
How are fire alarms designed in large buildings
Maybe divided into Detection and Alarm zones
Detection - detector and manual call points based on zones enabling indicator boards to show the origin of the signal, directing Ff’s.
Alarm - MoE arranged around a more sophisticated evacuation process
Describe the three Evacuation Processes which may be Implemented by Fire Alarm Systems
Staged
* evacuation signal given to zones where occupants are at the greatest risk, eg fire zone and the zones immediately adjacent or above.
- in all other zones an alert signal is given that maybe be converted to an evac signal if required.
Phased
* occupants evacuated in a pre-determined sequence according to the degree of risk they may be in from fire.
- instructions normally given by voice annoucement system, trigger in the first instance.
- subsequent phasing of evacuation normally controlled manually.
Progressive Horizontal
* used in hospitals and rsidential care premises.
- patients/occupants move horizontally from a zone where at risk to an adjacent safe zone on he same floor.
- building divided into individual alrm zones.
- zoning determined by evac strategy from the building.
- fire alarm sounds or voice system grouped so all sounders in each zone transmit the appropriate signal.
