Fire detection systems

Fire and smoke detectors are always an integral part of a control loop which comprises sensors, alarm triggering equipment, transmission cabling, evaluation units, etc., and which together are generally referred to as a fire alarm system.

Fire and smoke alarm systems in road tunnels are designed to detect fires and smoke production as fast as possible so that safety equipment and procedures can be activated without delay. Their main objectives should be to:

• inform tunnel users at the earliest opportunity, so as to enable them to organize their self-evacuation and self-rescue;
• pass on all the possible fire parameters to the operational tunnel staff in order to enable them to change on going tunnel operations (traffic control and ventilation systems) according to the emergency procedures (so-called fire mode), and to call in the rescue services, medical staff, fire brigade, police, etc.
• to identify the locations of the fire or incident, in order to direct rescue service resources to the appropriate places to assist motorists, for example.

PRINCIPLES OF FIRE DETECTION

Fire detection principles are based on perceived parameters i.e. heat, smoke, radiation and production of typical chemical substances. Therefore, fire detection sensors include:

• Heat detectors: all materials for which the characteristics are sensitive to an increase in heat energy, whenever this implies a temperature rise. Examples are sensors which measure temperature differences with a reference temperature or rate of temperature rise, glass-fibre cables where light transmission characteristics are a function of temperature, linear sensor cables with built in electronic circuits, etc.;
• Flame detectors, based on their sensitivity to infrared and/or ultraviolet wavelength spectrums;
• Smoke detectors which measure the extinction of a infrared light beam through CO and CO2 ionisation areas;
• Detectors which combine different types of sensors.

Each of these detectors has their own specific application domain, related to its response time, robustness, reliability, etc.

Recently video AID systems have proven to be very efficient and fast in detecting fires. They detect incidents and any object or vehicle which does not conform to the normal expected traffic stream. The cameras can be automatically turned towards the incident scene, which enables the operator to discover the very early start of a fire.

Fire/smoke detection systems are described in Section 6.3 "Fire detection" of the report 2006 05.16.B.

REQUIREMENTS FOR FIRE DETECTION SYSTEMs

Generally speaking, fire detectors in road tunnels must be designed to withstand the following environmental conditions: air velocities up to 10 m/s, reduced visibility resulting from diesel exhaust fumes and abrasive wear stemming from tires and the road surface, increased and short-term fluctuating concentrations of pollutants (carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides and hydrocarbons), changing headlight intensities, engine heat and hot fumes, vehicle exhaust gases, electromagnetic interferences, mixed vehicular traffic (i.e., cars, small lorries, heavy load lorries, buses and tankers) that will result in varying degrees of tunnel cross section obstruction.

It cannot be stressed enough that they must have a high degree of fail-safe operation and be able to determine the location of the fire as precisely as possible. It is advisable that fire detection systems possess a certain level of intelligence in order to avoid false alarms, as to rectify them could entail significant expenses and even worse, may eventually discourage the operators from paying attention to the alarms.

Furthermore, it is imperative that the fire detection/alarm installation is reasonably priced, has low operating costs and is simple to maintain: refer Section 6.3 "Fire detection" of the report 2006 05.16.B.

PARAMETERS DICTATED BY CODES AND STANDARDS

The following parameters for automatic fire detectors are specified in national and international codes and standards : maximum time for a fire to be detected, determination of the fire location, minimum fire load to be detected, approved detection methods, assembly points for fire alarms, details pertaining to which tunnels should be provided with automatic fire alarm installations (e.g. length of tunnel, tunnels with mechanical ventilation, tunnels that are not permanently monitored by personnel, short tunnels with particularly high traffic densities).

A list of detailed reference material regarding fire detector parameters are described in codes and can be found in Section 10 "References" of the report 2006 05.16.B.

FIRE/SMOKE DETECTORS CURRENTLY IN USE

The efficiency of fire detection is not only based on the type of devices (temperature, light beam extinction, ionisation, etc.), but also on the detection strategy which has been developed, which includes the number of sensors and their level of surveillance in the tunnel.

Automatic incident detection, analysis of video images including AID systems, closed-circuit television (CCTV) observation, equipment such as fire extinguishers which activate alarms by the removal, as well as the emergency telephones are generally good means to raise an alarm.

Many detectors in use are based on heat and on the rate of temperature rise. When well calibrated, this type of system generates only few false alarms, but may have a slow reaction rate. Detectors based on smoke obscuration give early signals but suffer more false alarms because of smoke exhaust from diesel vehicles: refer to Section VI.3.1 "Fire detection" of report 05.05.B 1999.

The 2009 Routes/Roads article "Fire Detection Systems in Road Tunnels - Lessons Learnt from the International Research Project" deals with fire/smoke systems of road tunnels such as linear heat detection , optical detection of flames, detection by video imaging, localized heat detection and smoke detection by air sampling systems. It concludes that air sampling systems have good performance in terms of response time and their ability to accurately locate and monitor a fire and the effect on the road environment, when taking into account overall performance including false alarms, maintenance and fire detection. The information from this study can be used to determine the most appropriate technology for tunnel fire detection.