Types of devices and equipment for fire alarm systems
All of the devices listed below can be standard conventional or non-standard conventional or addressable in type.
Manual call points:
Generally either have a rigid glass or flexible resettable plastic element that when pressed, the glass snaps or cracks or the plastic element bends allowing the micro switch to drop, which then sends a positive signal to the control panel.
Optical smoke detectors:
Have an internal pulsing LED and a photo-diode at an obtuse angle. In clear air conditions the photo-diode receives no light from the LED and produces a corresponding negative signal.
When smoke enters the chamber it scatters light onto the photo-diode, activating a positive signal to the control panel.
Optical smoke detectors respond best to slow smouldering type fires.
Ionisation smoke detectors:
Have a radio-active source between two electrical charged plates with current flow across the air filled space producing a negative signal. When smoke enters the chamber the current flow is reduced or interrupted completely, activating a positive signal to the control panel.
Ionisation smoke detectors respond best to fast burning type fires.
There are two types, rate of rise or fixed temperature, both work upon similar principles to smoke detectors, but by sensing changes in temperature or heat instead of particles of smoke.
Under normal circumstances the thermistor elements sense the surrounding air temperature and produce a negative signal. When the air temperature rises quickly by approximately 20 degrees C (rate of rise) or if a set temperature in degrees C (fixed) is reached a positive signal is activated to the control panel.
Rate of rise heat detectors tend to respond best to fast burning type fires.
Fixed temperature heat detectors respond to all types (slower or faster) burning type fires.
Have both an optical smoke chamber and heat thermistor, the smoke chamber has an internal pulsing LED and a photo-diode at an obtuse angle, when smoke enters the chamber it scatters light onto the photo-diode, activating a positive signal, while the thermistor elements senses the surrounding air temperature and when the air temperature rises quickly by approximately 20 degrees C sends a positive signal to the control panel.
Multisensor detectors respond to both smouldering and fast burning fires.
Have coated electrodes that under normal conditions return a negative signal, upon the presence of carbon monoxide a reaction occurs that causes current to flow between the electrodes sending a positive signal to the control panel.
It should be noted carbon monoxide detectors are only able to provide an early fire warning in certain well defined circumstances and so should only be used to supplement fire detection systems not as a replacement for smoke or heat detection.
Optical beam detectors:
Can either be transmitter / receiver or reflective type units which send out infra-red light to the receivers directly or are reflected back giving a negative signal, when smoke obscures the beam of light to a set pre-determined level the alarm is triggered giving a positive signal to the control equipment.
Optical beam detectors are ideal for protecting large open spaces such as sports halls, atria and warehouses.
Aspirating smoke detectors:
Consist of a central detection fan unit which draws air through pipework from sampling holes to the protected area to detect smoke.
Air samples are filtered for contaminants or dust to avoid false alarms and passed across a laser detection unit, clean air gives a negative output signal, when smoke particles are detected they obscure the laser scattering the light across the unit providing a positive alarm output.
Several levels of alarm can be configured ranging from several times more sensitive than a normal conventional smoke detector to several times less sensitive.
Aspirating detectors are ideal for clean rooms, computer server rooms, telecommunication centres large storage warehouses, or buildings of significant historical value or importance.
Linear heat detection:
Consists of a two core cable terminated by an end of line resistance which returns a negative signal directly or via a controller unit to alarm control equipment, the conductors are separated by polymer plastic designed to melt at a specific designated temperature which causes the cores to short circuit or alarm sending a positive signal to the control equipment.
Linear heat detection is ideal for power stations including cable ducts and trays, coal conveyors, large industrial freezers and cold stores, waste recycling centres, marine or shipping applications or chemical works hazardous areas.
Units can be conventional or addressable, wall or base (ceiling) mounted that produce standard matching tones that operate as an output to provide the primary warning from an alarm control panel.
Electronic sounder indicator:
Units can be conventional or addressable, wall or base (ceiling) mounted that produce standard matching tones with red or white flashing LEDs that operate as an output to provide primary and secondary warning from an alarm control panel.
Units can be conventional or addressable, red or white flashing LEDs or xenon filament in type that are wall mountable that operate as outputs to provide secondary warning from an alarm control panel.
Visual alarm devices (VADs):
From January 2014 driven by disability legislation, units should be fitted where required or identified by risk assessment in areas of high ambient noise or to alert deaf or hard of hearing persons to provide a specific illumination of 0.4 lumens/m2 on a perpendicular surface with a frequency between 0.5 and 2.0 Hz as primary warning of a fire alarm activation. VADs can be conventional or addressable, wall (W) or base (ceiling C) or open (O) mounted, either as stand-alone high intensity LED or combined with a sounder that operate as an output from an alarm control panel.
Addressable loop isolators:
Are fitted at strategic intervals along the loop in case of a short circuit fault on the cable. Isolators can be stand alone or contained with other items such as call points, sounders, sounder indicators or interface units.
In the event of a short circuit fault the isolators either side of the fault will activate, because the loop is powered both ways the minimum amount of devices become non-active. If any isolators are operated the amber LED on the isolator illuminates constantly when the short is cleared the isolator is automatically reset.
Input / output interfaces:
Are devices used on addressable systems and relays are used on conventional systems to link external third party items to and from the fire alarm.
The input part of the unit or relay contacts allow items such as sprinkler systems, building management systems or other fire systems such as shop to precincts or shopping centres, or suppression systems for computer or telecommunication rooms to signal into the alarm panel to activate the fire alarm system appropriately ( fire, alert, warnings).
The output part of the unit or relay contacts allows items such as gas or boiler shutdowns, lifts and door releases / holders to be operated upon a fire alarm activation appropriately (shut off gas or boilers, bring lifts to ground and open or close doors).