The final decade of the twentieth century saw many significant changes in the theory and practice of active fire detection utilising point-type fire detectors
Detectors fall into two primary types, conventional or intelligent. A conventional detector works as a simple on/off switch, with no communication other than an alarm signal between the detector and the panel. On the other hand, intelligent detectors have become increasingly complex, sophisticated and powerful as electronic technology has advanced, and the panel is constantly polling each detector in turn to interrogate its status.
The development of powerful, low-cost embedded processors has enabled system designers to incorporate numerous features and options into intelligent fire detection systems, both in the detector head and in the control panel.
However, not all installation will require the ultimate in facilities and features, and there is still a strong demand in smaller, less complex applications for the simpler and more cost-effective conventional system.
Conventional systems
A conventional fire alarm system normally consists of a control panel linked to a number of circuits of smoke detectors, heat detectors and manual call points with a similar number of sounder zones.
The control panel drives the detection lines and the sounder lines, provides LED indications of fire, fault or normal conditions and contains switches to allow sounders to be activated or silenced and the detectors to reset after an alarm.
Intelligent systems
These systems offer benefits in speed of detection, identification of the location of a fire and easier maintenance. Intelligent systems also offer tolerance to faults in the system wiring, which allows a single pair of wires to be used to connect up to 200 devices to the system, allowing cost savings in the wiring of large systems. In larger installations, the benefits of improved maintenance and reduced cabling cost are overwhelming, and, nowadays, the price difference between intelligent and conventional systems has narrowed to such an extent that even for small installations, the use of an intelligent fire alarm system should be considered.
Intelligent systems offer a number of advantages over conventional ones, including:
* The location of a fire can be precisely located from the control panel.
* The use of looped wiring allows the system to function normally even with an open circuit in the loop wiring.
* The wiring cost of a system can be reduced by the use of a single pair of wires for up to 200 devices.
* Detectors are constantly monitored for correct operation.
* The use of a 'pre-alarm' feature alerts staff to check whether a fire condition exists before the alarm is raised.
* The use of addressable loop-powered sounders allows the same wiring to be used for sensors, call points and sounders.
With intelligent systems the wiring is looped, and detectors, call points, sounders and interface modules can all be connected on the same pair of wires. Each device connected on the detection loop has its own 'address', usually set by means of a pair of switches on the back of the device.
The panel constantly communicates with each device in turn on the detection loop, and if any of the devices indicate an alarm condition or a fault, the panel can immediately identify which device generated the signal and trigger an alarm or fault condition. Most intelligent panels also include an alphanumeric display, which displays a user-configurable text message in case of a fire or a fault.
Multisensor detectors
One of the most interesting changes over the past decade was the growing acceptance of detectors that use more than one type of technology to detect fire. These detectors are generally known as multisensor detectors.
Multisensor detectors incorporate two or more sensors of different types - ionisation smoke or heat, and process the signals to produce one output on which a decision is based. The majority of multisensor detectors have microprocessors in the detector head to carry out this processing.
Multisensor detectors are invariably a combination of a heat and at least one type of smoke sensor. Detectors which combine more than one smoke sensor are treated by the regulating bodies as smoke detectors having more than one type of sensor. Multisensor detectors that incorporate CO sensing as well are planned.
The argument for fire detectors with two smoke sensors (ionisation, optical) and a heat sensing element is that they offer the best of both worlds, as far as smoke detection is concerned, ie the detection of large particle smoke with the optical chamber and the detection of small particle smoke with the ionisation chamber.
A multisensor detector has two or three signal inputs and these are processed in the detector using an algorithm written by the manufacturer for that particular type of detector and its associated equipment.
Different operating modes may be offered for particular environments. The range of modes and the method of selecting a mode will differ from one manufacturer to another.
Multicriteria detectors
A multicriteria detector is sometimes confused with a multisensor detector. If a detector measures smoke using the same sensing techniques but under two distinct operating conditions then it could be said to be multicriteria.
Laser detectors
Laser detection of smoke is a new technology that incorporates an extremely bright laser diode and integral lens that focuses the light beam to a very small point near the receiving photo sensor. The light then passes into a light trap and is absorbed. A scattering of smoke particles in this narrow light beam then activates the photo sensor.
In a typical optical or photoelectric detector the light beam given off by the LED is very wide and can reflect off chamber walls into the photo sensor. When dust accumulates in the detector the chamber walls change in colour from flat black to grey. With the laser detector the concentrated light beam does not touch the walls and is therefore much less susceptible to dust accumulation.
Smoke scatters light in all directions and in a typical optical detector, only a small portion of that scattered light reaches the photo sensor itself. In the laser detector a specially designed mirror reflects and concentrates most of the scattered light into the photo sensor. This is one of the reasons why the laser detector is much more sensitive than a standard detector.
The laser diode combined with the special lens and mirror optics allows the laser detector to achieve a signal to noise ratio that is much higher than traditional optical detectors. The features detailed above, coupled with the software algorithms allow differentiation between dust and smoke particles.
Because of these factors the laser detector can be set to extremely high sensitivity, yet can reject false signals caused by larger airborne particles such as dust, lint and small insects.
The future
Radio detectors, Filtrex detectors for harsh environments such as cement works, floor mills and sugar stores, and CO detectors, carbon monoxide sensors are becoming cost-effective enough for point-type sensors, bringing a new level of smoke sensing to the industry.
For details contact the FDIA on tel: (011) 307 1616, fax: (011) 397 1618, or e-mail: [email protected]
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