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48 Tunnel Fire Detection and Ventilation Impact The detection of a fire is of paramount importance since not detecting an event could mean the loss of precious time to save lives and property. Timely fire detection allows for timely evac- uation, rescue, control of tenable environment and firefighting. Fire detection is conducted based on exceeding threshold values for a prescribed duration. Fire detection should provide a means of detecting a fire in all parts of a tunnel and ancillary areas, provide a means of activating an alarm for evacuation and activating other fire safety measures such as the tunnel ventilation system, and send an alarm to the fire services. NFPA 502 requires to detect, identify and to locate fires in all tunnels over 800 ft (240 m) long and in shorter tun- nels with a high volume of traffic or high level of risk [1]. The NFPA 502 Standard allows for manual means to identify and locate fires within tunnels with no fixed firefighting systems that are equipped with CCTV cameras with incident manage- ment software and a warning notification system. At least two means of identifying and locating fires are required. Automatic fire detection is required for tunnels equipped with a fixed firefighting system. With no operator supervi- sion (unmanned tunnels), an automatic fire detection system is needed for tunnels over 1000 ft. The key objective of the automatic tunnel fire detection and warning systems is to provide prompt, accurate, and reli- able fire detection while preventing nuisance alarms. Prompt and accurate fire detection will result in timely activation of a tunnel ventilation system in the predetermined mode of operation to maintain a tenable environment for evacuees. The goal should be to detect a fire of 5 MW (17 MBtu/hr) or less within 60â90 seconds in a testing environment with 3 m/s (600 fpm) of air velocity. It should accurately detect the fire within a fire zone which defines the activation limits of the ventilation and FFSS for a particular length of the tun- nel. Quicker detection of a smaller tunnel fire in a shorter time has been proven possible. Automatic activation of the ventilation and firefighting systems could be considered but is not currently required by the NFPA 502 Standard due to concerns related to the reliability of the tunnel fire detection systems [1]. Depending on the nature of the fire, any of the following can start developing first: smoke, flame, or heat. Consequently, multi-sensor alarm systems are better suited for automatic control. Induced airflow in road tunnels may reduce air tempera- ture, surface temperature, dilute smoke concentration and deflect flames, which impacts the performance of the tunnel fire detection devices. This may result in delayed fire detec- tion, detection of the wrong fire zone, or inability to detect fire. In relatively short tunnels, just before the fire event occurs, the mechanical ventilation seldom operates at high speed, and induced airflow is primarily driven by the pis- ton effect and other natural ventilation sources. Hence, the impact of mechanical tunnel ventilation on the fire detec- tion systemsâ performance could be of interest for very long tunnels only. However, the impact of natural ventilation has to be analyzed when selecting the fire detection device. In addition, the tunnel slope may also have some impact on the accuracy of detecting the fire zone. Additional information on tunnel fire detection and warn- ing systems design can be found in Appendix A. C h a p t e r 4