Fire Alarm Systems
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Conventional 4 Wire System

 Fig. 4.1.1

A conventional fire alarm system normally consists of a control panel linked to a number of lines of fire detectors and manual call points, normally called detection zones, and a number of sounder or alarm circuits. A simple system is shown in Figure 4.1.1

The control panel drives the detection zones and sounder circuits, provides LED indications of fire, fault or normal conditions and contains switches to allow the sounders to be activated or silenced and the detectors to reset following an alarm. The control panel is powered from the mains (230VAC) and will contain back-up batteries to allow the system to function for a minimum of 24 hours, dependant on the application, in case of a mains failure.

Most conventional fire alarm panels have several detection zones comprising a mixture of automatic fire detectors and manual call points. In order to limit the effect of faults, and to limit the search area in the case of a fire, the size of a fire detection zone is limited to 2000m2, with a maximum travel distance within the zone to locate a fire of 60m. In addition, zones should not cover more than one storey, unless the total floor area of the building is less than 300m2. As a result unless the site is very small, the system will comprise several detection zones.

A fire alarm (or sounder) circuit may cover more than one detection zone, but it must follow the boundaries of the relevant detection zones, and the boundaries should be of fire resisting construction.

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Two Wire Conventional System

 Fig. 4.1.2

The Twinflex conventional fire alarm system incorporating the Multipoint combined smoke and heat detector with built in sounder means the whole system can be installed using only one pair of wires.

Using the Multipoint detector as part of the Twinflex 2-wire fire alarm system means that when a detector is wired in, a sounder is wired in too -- with no extra wiring, therefore greatly reducing the number of points that need to be installed and the time it takes to install them.

As the Multipoint offers 7 different modes of detection, the installation is made even simpler as this one device suits all applications. Whatever type of detection is required, for any part of the installation it can be selected by the flick of a switch at the time of commissioning. Any one of 3 different smoke modes, 2 fixed temperature heat modes, a rate of rise mode and a combination smoke or heat mode can be selected.

As the Twinflex Multipoint detector is available with or without a full specification 90 dB(A) sounder no extra devices need to be purchased when audible warning is required. (Meets audibility levels recommended in BS5839 Part 1:1988). Twinflex Manual Call Points can be run on the same pair of wires as detectors. Both detectors and Manual Call Points have a built in EOL switch so that any device may be set as the end of line monitor. The panel can also differentiate between a Manual Call Point and a Detector alarm. If extra sound is required, or for areas that do not require detection, then the Twinflex Hatari sounder may be used on the same two wires, offering sound output of 100dB(A). It also has a built in EOL monitoring switch.

The panel can accommodate up to 32 devices per zone, has separate fault monitoring displays for each zone, and a one man walk test facility. Zones are configured without the need to use resistors or capacitors on unused zones.

The system is installed on 2 core and earth cable for each zone which supports automatic detectors, combined detectors/sounders, manual callpoints and stand alone electronic sounders. Each of the devices in the system has the facility of a built in end of line fault monitoring device operated by means of a DIL switch.

Multi criteria detector includes 7 detector modes by means of a DIL switch within the device. These modes include 3 'optical smoke detection' of varying sensitivity with thermal enhancement to comply with EN54 part 7. 3 heat detection modes include rate of rise (EN54 part 5 grade 1), fixed temperature (EN54 part 5 grade 2) and high fixed temperature (EN54 part 8 temperature range 1). A combined smoke or heat detection mode which can alarm on smoke or heat.

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Analogue Addressable System

 Fig. 4.1.3

Conventional fire alarm systems provide an adequate and cost effective fire alarm system for many small buildings. In larger, more complex buildings however, more sophisticated 'intelligent' fire alarm systems tend to be used. 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 198 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. Currently, the point at which an intelligent system becomes economical is around 6 zones in the UK.

Figure 4.1.3 demonstrates an example of a single loop intelligent fire system layout. The wiring is looped, and connects to the control panel at each end. All detectors, call points, sounders and interface modules are wired directly to the loop, each having its own address. The control panel communicates with each device on the loop, and if an alarm or fault condition is signalled, or if communications are lost with one or more detectors, the appropriate response is triggered. The loop can be powered from each end so that if the loop is broken at any point, no devices are lost.

In order to give tolerance against short circuits on the loop, short circuit isolators are placed at intervals on the loop. Should a short circuit occur on the loop the isolators directly on either side of the fault will isolate that section. The panel will detect the loss of the devices, signal a fault and drive the loop from both ends, thereby enabling the remainder of the loop to operate correctly and ensuring minimum loss of coverage.

Short circuit isolators are available as separate modules or incorporated into a detector base.

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