How to configure detector zones and alarm zones within the premises?
A single, vertical detection zone should be provided for fire detectors within an enclosed stairwell, lift shaft or similarly enclosed flue like structure. However, any manual call point on the landing of a stairwell should be incorporated within the detection zone that serves the adjacent accommodation on the same level as the landing.
Detection Zone Safeguards
It is possible for addressable detectors to share one circuit all-round the building, thereby having several detection zones served by the same two-wire circuit. For conventional detectors, each individual detection zone is served by its own dedicated two-wire circuit. To ensure that an addressable system does not have a lower level of integrity than a conventional system, the code makes various recommendations that limit the effects of faults.
1. A single fault occurring on an automatic fire detector circuit should not disable protection within an area of more than 2,000m2, nor on more than one floor of the building. In conventional systems, this will normally be achieved as a matter of course since an open or short circuit condition will only affect the individual detection zone circuit concerned (See Figure 2). The detection zone will be no more than 2,000m2 in the area and, other than in very small buildings, will serve no more than one floor. In addressable systems where a number of zones share the same ring circuit or loop (See Figure 3), an open circuit is not too much of a problem (just so long as the fault is reported) since the loop can be driven in both directions. The case of a short circuit, however, is far more serious since this condition could prejudice every device (up to250) on the circuit. Short circuit protection is therefore required in all loop circuits. This is achieved by placing line isolator devices at appropriate locations in the loop circuit; so that the area protected by detectors between any two line isolators is no greater than 2,000m2 and these detectors are on the same floor level.
In addressable systems where detectors are connected on a ‘spur’ off a loop, (see Figure 4), to comply with the recommendation of the code then the spur should not serve more than one floor or an area of greater than 2,000m2.
2. Two faults should not remove protection from an area greater than 10,000m2. this recommendation imposes a maximum area of coverage for a single loop in an addressable loop system (see Figure 3). No loop in the system, therefore, should ever serve an area of coverage greater than 10,000m2. if the area to be protected exceeds this maximum limit, then an additional loop(s) should be used.
3. Open circuit and short circuit faults should be reported at the control panel within 100 seconds of occurrence this limitation will be satisfied if control equipment conforms to BS EN 54-2. When you have established the detection zone arrangement for the building, the next step in the design process is to decide which type of fire alarm system should be used, see section 4 (Which Type of Fire Detection and Alarm System?).
