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System type and zoning

One of the key decision to make when installing a fire alarm system is whether to use an addressable or conventional system. The size/layout of the premises to be protected, user requirements, category of risk and budget will be the main factors that will drive the decision.

Conventional systems

Conventional detection systems offer a good basic system, ideal for small applications, or projects that are tight on funds.

The detectors in a zone of the building are all connected to the same pair of wires. If any of the detectors report an alarm, only a zone indication is shown on the control panel, meaning that the exact location of the alarm is not shown. There could be, say, 10 detectors in that zone and one cannot tell from the control panel which detector has triggered. Only the zone will be shown. (The visual audio devices are wired on a separate circuit.)

Advantages of a conventional fire alarm system over an addressable fire alarm system are:

• No sophisticated configuration is required and the setup is simple.

• Control panel and field devices are comparatively cheaper.

• There is compatibility of devices within a wide range of manufacturers.

Addressable systems

Addressable fire alarm systems are usually for larger, more complicated applications and will be generally more expensive than conventional systems. Detectors, sounders, manual call points and interfaces are all connected to the same loop/cable. Each device communicates directly with the control panel, therefore the control panel will be able to indicate the exact location of any alarm reported.

All the devices are individually addressed i.e. given a unique address and description of the area it is installed in to help identify the device. An example would be ‘Device number 1021, Optical Smoke Detector, Finance Manager’s Office’. The customised message will be displayed on the control panel in a text format or in a graphical display alongside the visual detection zone indicators.

The detector continuously sends data to the control panel. The control panel then uses this data to decide the condition of the device: fire, pre-alarm or fault. This feature allows some panels to increase or decrease the sensitivity of individual detectors.

Detectors are assigned to zones by a software program, which allows for easier zone changes if the building layout is altered at a future date. Addressable panels usually have an event log to recall past system events. They allow for cause and effect programming, where outputs are only triggered by certain detectors/inputs.

Whether one opts for a conventional or addressable system, there are some recommendations as stipulated in SANS 10139 that one needs to consider. One of these is zoning.

Fire detection zones

SANS 10139 Section 8.6.1 clearly states all buildings, other than very small buildings, need to be divided into detection zones. Detection zones need to be small enough for a fire to be located quickly. This applies even if the system’s addressable detection zone indication needs to be provided, as this often provides a quicker, albeit less specific, indication of the location of a fire than typical addressable text displays. Zone indicators also provide a simple ‘at a glance’ overview of the extent of fire or smoke spread.

If the total floor area of the building is greater than 300 m², each zone should be restricted to a single storey. If the total floor area of the building is less than 30 m², a zone may cover more than a single storey.

For voids above or below the floor area of a room, these may be included within the same zone of the room, provided that the voids and the room constitute a single fire compartment.

Additional recommendations applicable to detection zones that contain non-addressable automatic fire detectors. The floor area of a single zone should not exceed 2000 m². The search distance that has to be travelled by anyone responding to a fire alarm signal after entry to the zone in order for the location of the fire to be determined visually should not exceed 60 m.

Automatic fire detectors within any enclosed stairwell, lift well or other enclosed structure should be considered as a separate zone.

Manual call point zones

The floor area of a single zone should not exceed 2000 m², other than in the case of a zone comprising mainly a single, open plan area, e.g. an un-compartmentalised warehouse, which should not exceed 10 000 m² in area.

SANS 1019 section 8.6.2.1:

The manual call points on each level, other than a final exit level from the stairway, should be incorporated within the zone that serves the adjacent accommodation on that level. A manual call point located within the stairwell at a final exit to open air may be incorporated within the detection zone serving the stairwell.

Please note that it is a legal requirement in South Africa mandated by the Department of Labour for persons designing, installing, servicing or commissioning fire detection systems to be registered with SAQCC Fire in their respective competency levels.

Feel free to contact FDIA via email fdia@fdia.co.za for any questions, comments or concerns regarding fire detection systems.

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