SYSTEM ZONING AND LAYOUT
It is recommended that one of the first stages in fire alarm system design is to divide the property into precise areas or detection zones. This is a fairly straight forward process so long as the following criteria is adhered to.
- A zone should not extend beyond a single fire compartment.
- The floor area of a zone should not exceed 2000m2 or (10000m2 for annalogue/addressable systems using short circuit loop isolators on a single closed loop).
- A zone should not extend beyond a single floor area, therefore each floor level in a building will constitute at least one zone.
- As an exception to the above, stairwells, lift shafts and other vertical shafts should be designated as an individual zone.
- Small premises with multipul floor levels not exceeding an area of 300m2 in total may be considered as a single zone.
- The search distance in a azone should not exceed 30m from point of entry, i.e. that distance which needs to be travelled for a searcher to visually establish the position of the fire. This can be made less complex with the use of remote indicators removong the necessity to open every door and inspect each room in a zoned area.
AUTOMATIC FIRE DETECTION
The various types of heat and smoke detectors are summerised as follows.
- OPTICAL (photoelectric) type smoke detectors normally provide the best method of smoke detection in stairwells, corridors and sleeping areas. This is because they are designed to detect large smoke particles that can be expected in such areas from smouldering and developing fires.
- HEAT detectors are best suited to areas where clean burning or flaming fires might be expected. In addition they are also suitable for storerooms, plant rooms and where the presense of smoke or steam is usually encountered under normal conditions. (e.g. kitchens and hallways next to shower units, garages etc). Heat detectors are also used if there is a high level of airborne contamination from dust or pollen.
- RATE-OF-RISE heat detectors will detect an abnormally rapid heat build up that might be expected to occur under fire conditions in factories and shops etc.
- FIXED TEMPERATURE heat detectors will operate only when their rated temperature is exceeded so kitchen areas, boiler rooms and hot industrial environments will utilise this type of detection.
It is important to take all of the points into account so that the optimum detector choice is made to ensure that adequate detection is provided with the minimum risk of false alarms.
What is important to understand is where heat detectors are sometimes prefered to prevent a false alarm, they are not as sensitive as smoke detectors (becasue you need a developed fire to activate it) and the risk to property value and human safety should be considered very carefully. In addition to these notes smoke detectors should not be sited near air conditioning outlets, bathrooms and dead air areas and areas prone to insect infestation and chemical storage areas where abnormally high airflow rates and gas discharge may accur.