Basics

For thousands of years man has been fascinated by fire, and dependent on it. He has used its energy to cook, heat and process materials. And yet fire can be as dangerous as it is useful. Even if nowadays electricity has by and large replaced open fires, unintentional fires caused by sparking, electrical short-circuits and explosions are still an all too frequent occurrence. When it first breaks out, a destructive fire can easily be extinguished with a glass of water; however, after just a few minutes, a whole bucket of water is required and, later still, tons of water will not be enough to put out the fire. That is why automatic fire detection systems play such an important, not to say critical role in the race between the spread of a fire and the arrival of the fire-fighting teams or fire brigade.

There are essentially three detection principles involved in fire detection. A fire alarm is triggered whenever certain stipulations or limit values are exceeded:

• Smoke detection: Measures the number of smoke particles in the air. Photoelectric smoke detectors are used here.
• Heat detection: Measures the rise of temperature within a given interval and the maximum temperature. This group includes rate-of-rise temperature detectors and fixed-temperature heat detectors.
• Flame detection: Measures the radiation stemming from the fire (infrared, visible, ultraviolet). Infrared and ultraviolet flame detectors are mainly used in this instance.

When selecting fire detectors we need to consider the following questions to ensure the best possible, most reliable detection: What type of fire can we expect or is most likely? What are the fire phenomena during the outbreak phase? What possible deceptive factors are they? Room height? What are the ambient conditions like (paper, plastic, timber, paints and varnishes, fuels, electrical equipment, computer installations, etc.)?

An automatic fire alarm system is a reliable means of detecting the outbreak of a fire without the need for human detection; it warns any persons at risk, and mobilises the fire-fighting forces. The system consists of fire detectors, the fire alarm control panel, the power supply, the alarm unit and the connecting lines. The function of the individual components is as follows:

• Within the monitored area, automatic fire detectors permanently detect the respective fire phenomena and convert them into an electrical signal. This signal is recorded by the fire alarm control panel, evaluated and, where appropriate, displayed both visually and acoustically as a fire alarm.
• Manual call points (emergency buttons) are used for triggering manual alarms.
• Local alarms are triggered to give early warning to all people present.
• The fire alarm is forwarded to a remote operating and control centre using a transmission unit. The fire-fighting measures are then initiated by the control centre (fire brigade).
• Two independent power sources are required for the electricity supply. One of the two sources is always the mains while the other consists of a backup battery.
• Faults are signalled visually and acoustically in the fire alarm control panel and forwarded to a remote monitoring control centre using a transmission unit. Troubleshooting is then initiated by the control centre.
• The fire alarm control panel is designed so that different automatic fire control systems can be triggered as required (e.g. stationary extinguishing systems, air conditioning and ventilation systems, fire rated doors, lift installations).

In cases where the use of conventional point detectors becomes difficult for practical, technical or aesthetic reasons, special fire alarm applications are the optimum solution. The two main product groups are aspiration smoke detectors and linear heat detectors. The precise mode of operation of these systems can be found under the product descriptions.