Reprinted with permission by www.instrumentation.co.za
Fire safety in high-occupancy buildings is critical. Large numbers of people produce greater activity within these environments, which can lead to things going wrong. In particular, fire can escalate quickly into a catastrophe. This eventuality needs to be confronted in the design and protection of buildings and their occupants.
Smoke inhalation too can be fatal. Smoke visually obscures escape routes that can prevent fast and safe evacuation. It also inhibits the performance of fire rescue responders. These are just some of the dangers surrounding fire within high-occupancy buildings that place hard emphasis on early detection of fires and alerting occupants to the threat.
Fire sensing methods
The traditional method of sensing fire is to detect smoke by means of light obscuration within a sensing chamber. This is known as optical smoke sensing. However, there are challenges to sensing fires within rooms in high occupancy buildings using such methods. This is because of the human activity that occurs within these areas. Often bedroom applications result in unwanted fire alarms with many people believing that the smoke detector is too sensitive.
This is not the case. Spraying of aerosols such as deodorants, hair sprays or air fresheners mimic the obscuration caused by smoke. Similarly steam from showers can also do this. The key is to separate these unwanted phenomena from a genuine fire, while providing an early and stable alarm signal.
Alien Systems & Technologies utilises Protec 6000PLUS detectors that can discriminate between aerosols, steam and genuine smoke by utilising multi-sensor technology governed by an algorithm that makes the fire alarm decision. These detectors must have all three fire phenomena present – smoke, a rise in temperature and a rise in carbon monoxide.
Testing shows that even with hot steam or aerosols, the problem of unwanted alarms is solved. For example, in one such test steam was allowed to fill a Perspex test chamber with three detectors present: an optical detector, an optical/heat detector and an optical/heat/CO detector. The steam filled the test chamber and after approximately 35 seconds the optical detector produced a fire alarm. The test was allowed to run for a period of five minutes and the other two detectors ignored the steam and did not produce any fire alarms.
In a following test, steam was used to fill the Perspex chamber and after approximately 40 seconds the optical detector produced a fire alarm. Then after one minute, a small smouldering piece of towel is placed within a test chamber with the steam still present to simulate a small fabric fire that is common in a bedroom. After another minute, the optical/heat/CO detector produced a fire alarm because it sensed a genuine fire was present.
This is because it sensed the obscuration of light, a rise in temperature and a relatively high amount of carbon monoxide and the algorithm monitoring the sensor was able to make an accurate decision that a genuine fire was present. The optical/heat detector produced a fire alarm one minute afterwards.
Correct selection of detectors is vital
These tests show that the selection of fire alarm detectors in buildings is vital to prevent unnecessary evacuations from erroneous fire alarms. The disruption to occupants as well as the fire brigade is severe. Such unwanted alarms can lead to apathy, which then further increases the risk.
Detectors avoid this scenario as only genuine fires are sensed, and sensed early. There are also enhancements that can be added to the detector range. Programmable speech messages can also be added over and above standard audible tones. This allows for clear fire alarm evacuation commands to be broadcast without the need for extra annunciators.