In a dangerous situation, emergency alarm systems (fire alarm systems or burglar alarm systems) should signal ‘actively', and remain ‘passive' in safe situations according to DEHN UK. Malfunctions of these systems (no response in case of danger, or alarm signal in case of no danger) are undesirable and expensive. False alarms sent by emergency alarm systems result in expenses, which, in the industrial countries, amount to several hundred million Euros per year. Another aspect of malfunctions is the possible direct or indirect danger to human lives. In this context, we may remember the malfunction of the fire alarm system in the tower of the Frankfurt Rhein-Main airport in 1992, where a false activation of the fire extinguishing system occurred because of a lightning strike. Within a few minutes, the air traffic controllers had to leave the control room. In this critical situation, approaching airplanes had to be redirected to other airports. Considerable delays occurred in the air traffic. False alarms of emergency alarm systems are also disturbing in another respect:
- When false alarms accumulate, the operator can no longer rely on the system and questions the significance of the system (investment) as such.
- The guard starts ignoring alarm messages.
- Neighbours will be disturbed by acoustic alarms.
- Fire-fighting forces (e. g. fire brigade) will be bound unnecessarily.
- The activation of the fire extinguishing system causes interruptions of operations.
- Damage is caused by not signaling existing dangers.
All these factors cause unnecessary expenses. They can be avoided, when possible causes for false alarms are already recognised in the design stage and are eliminated by suitable preventive measures. For this purpose, the German Insurance Association (Gesamtverband der Deutschen Versicherungswirtschaft e. V. - GDV) published VdS guidelines (VdS 2095; VdS 2311; VdS 2833). One of the measures also requested in the VdS guidelines is lightning and surge protection.
A coordinated lightning and surge protection prevents a false alarm caused by atmospheric discharges and improves the availability of the early detection of dangers and alarms. When installing comparable alarm transmission systems, for which, out of financial reasons, a VdS approval is not used (in residential building for example), the guidelines may also be used for project design and for the construction as well as for agreeing individual measures between constructors and operators. Indeed, fire alarm systems installed nowadays have an increased surge immunity in accordance with IEC 61000-4-5 for primary and secondary wires as well as for the mains inputs. However, a comprehensive protection against damage by lightning discharge and surges can only be achieved by external and internal lightning protection measures.
Different monitoring principles are applied for emergency alarm systems:
Impulse line technology
The information from the triggering alarm device is transferred in digital form. This allows recognition of the alarm device and the exact localisation of the trouble spot (Fig. 9.9.1).
DC line technology
Each alarm line is permanently monitored according to the closedcircuit principle. If an alarm device is activated in the line, this line is interrupted and an alarm is triggered in the control and indication equipment. Hereby, however, only the alarm line can be identified but not the individual detector.
Regardless of the used monitoring principle, the lines of the emergency alarm system must be integrated into the lightning and surge protection of the complete system.
For protection of alarm lines with dc line technology, Blitzductor CT BCT MOD BE. is recommended. It is chosen according to the voltage of the alarm lines, which is normally 12 or 24 V. Blitzductor CT BCT MOD BE is recommended to avoid having to change the loop resistance of the alarm lines too much.
Regardless of the line topology, the outputs of the control and indication equipment, for acoustic and visual signalisation for example, should be protected by Blitzductor CT. Care should be taken to ensure the nominal current of the protective devices is not exceeded. In case of nominal currents > 1A, the company suggests a DEHNrail DR 24 FML protective device be used. (see Table 9.9.1). The control and indication unit is normally connected to an exchange line of a fixed-network operator by means of a telephone dial unit. For this application, the SPD type Blitzductor CT, BCT MOD BP 110 would be suitable. The surge protection of the power supply is important, too. For alarm systems, which are certified by the German Insurance Association, (systems recognised by VdS), the manufacturer of the alarm system should be contacted. The installations as well as the lightning and surge protection equipment have to be set up in accordance with VdS 2095, VdS 2311 or VdS 2833.
A distinct increase in the operational reliability of these systems can be reached with specific lightning and surge protection of alarm systems, including the prevention of false alarms when no danger exists, and the prevention of costs arising from this. This allows effective damage limitation by informing the auxiliary personnel reliably., counteracting potentially catastrophic conditions including danger to human lives and pollution of the environment.
In the event of injuries to persons or environmental damage, the operator of a plant is liable first. This comprehensive responsibility for security can normally be expected from managers or executives of a company. However, in the legal sense, an operator of a plant is a technical layman, who is not able to assess the potential risk involved in a technical solution. Therefore, skilled persons as suppliers of technical solutions must ensure in each individual case, the solutions offered correspond to the actual requirements.
Regardless of the fact, whether fire alarm systems are VdS-approved systems or not, they should be furnished with a surge protection.