Perimeter alarm systems utilize technologies such as microwaves, active infrared, and electronic fences to form a visible or invisible "protective wall" at the boundary of the protected area. When a breach occurs, the detectors in the corresponding protected area will send an alarm signal to the alarm control host in the control center, which will issue an audible and visual warning while displaying the alarm location. It can also be linked to the perimeter simulated electronic screen, and even to video surveillance systems, access control systems, and high-voltage lighting systems.
In recent years, perimeter security systems have become a basic and indispensable subsystem of security systems. They are not only found in high-end fields such as military factories, barracks, airports, ports, and government agencies, but are also widely used in residential communities, maintaining a high growth rate in these areas. Cable theft prevention is a common application of this technology.
Several common cable anti-theft technologies
1. Anti-theft system for power line carrier communication monitoring
Power line carrier communication is a concept derived from the interdisciplinary field of electrical engineering and communications. In cable and wire theft prevention systems, power line carrier communication (as shown in Figure 1) utilizes signal transmission to load a detection carrier signal onto the cable within the detection range. The alarm at the monitoring center is connected to the carrier signal on the monitored cable. If a cable break causes an interruption in signal transmission, the alarm will automatically sound, serving as a warning. Power line carrier communication is currently one of the most widely used anti-theft methods because it transmits a carrier signal directly onto the monitored cable, requiring no additional wiring. Therefore, it has advantages such as easy installation, concealed detection, and a wide monitoring range (up to several kilometers). However, because it only triggers an alarm when cables are stolen and cannot deter criminals, and because video evidence cannot be obtained, single-mode power line carrier communication faces the challenge of integration and systematization.
2. Anti-theft Capacitance Detection Monitoring System
The principle of the capacitance detection method is to first short-circuit the ends of a pair of unused cables, and connect the other end to the multivibrator of the alarm. When the cable is working normally, the alarm's oscillator does not vibrate; when the cable is cut off, the unused cable becomes a simple capacitor, and the oscillator outputs a signal, triggering the alarm. The frequency of the oscillator's output signal changes depending on the location where the cable is cut. Figure 2 shows the basic model of the capacitance detection method.
3. Anti-theft system for current detection monitoring
This system determines the integrity of a cable system by detecting the presence of current in the cable loop online. In other words, the detection system does not react when the cable is functioning normally; it activates an alarm when no current is being transmitted through the cable. This current detection system cannot be used for cable anti-theft systems where there is voltage but no current, such as the power cables for blowers in subway or highway tunnels. Furthermore, the system only alarms when all branch cables in the circuit are disconnected (because the total current in the cable loop is zero only when all cable branches are broken). Therefore, current detection monitoring systems have the potential to "fail to report alarms."
Analysis of the application requirements of cable anti-theft technology in different fields
1. Buried cable anti-theft alarm for streetlights
With the rapid development of urban-rural integration in China, the pace of municipal construction has accelerated. Streetlights, as the main component of urban nighttime landscape lighting, play a vital role. Streetlight cables have largely shifted from overhead laying to underground laying, which includes direct burial, conduit laying, and cable trench laying. Regardless of the method, theft is a risk, leading to serious consequences such as economic losses and public security disruptions.
The most important requirement for an ideal underground cable anti-theft system for streetlights is that during the light-off period, the alarm and anti-theft system should operate without any power source at its endpoints; when the lights are on and the system is functioning normally, the alarm and anti-theft system should be independent of the circuit status (cable length, streetlight model, etc.). Ideally, multiple lines should be used, connected to the control room via a single main control device and communication equipment.
2. Highway power cable anti-theft alarm
Highways primarily use power cables, and one of the core challenges in developing highway cable anti-theft alarm systems is signal detection when cables malfunction. The main requirements for anti-theft systems in highway cable systems are: availability, reliability (low false alarm rate), sophistication (alarm differentiation), and scalability (networking with highway traffic police systems). Currently, power line communication is mainly used, but several issues remain to be addressed:
Transformers in power distribution systems block power line carrier communication information.
Three-phase power systems in circuits experience 10-20 dB of signal loss. When communication distances are short, carrier signals can generally only be transmitted on single-phase lines.
Power lines have their own pulse interference. The frequency of alternating current in my country is 50Hz. There is a second peak within a single cycle of the sine wave, which will generate second pulse interference. That is, power lines have 100Hz pulse interference, which is an inherent property of them.
Different signal coupling methods result in different power line carrier communication signal losses.
Power lines can cause data signals to deform.
The points mentioned above are directions worth researching and developing for power line carrier communication in cable anti-theft systems. This also indicates that my country's power line carrier cable anti-theft system technology is not yet fully mature. Currently, the best cable anti-theft system in China is produced by a company in Hangzhou that manufactures anti-theft cable systems.
