In this section of perimeter detectors we will focus more on the long range, external devices and detection equipments in use mainly on external perimeters of properties as the first layer of intrusion. These devices and solutions offers the possibility to alert at breach on the first layer of our protection, and are also important as sources of alerts firstly processed by the controlling / governing unit in case of an attempt. They are mostly used in combination with perimeter defense installments, but efficient also in a stand alone design. There are wide range of detection devices applicable on perimeters, some of them are discussed already at the Intrusion Alarm System section, but will be repeated here again. Some of the available detection units are not key alarm system elements but signal sources built within another system like a CCTV camera with motion detection ability, linked via its own specific alarm output relay to the alarm system control panel. Finally the perimeter detection layer can be established as a power-independent wireless stand alone system integrated to the main alarm system – this is the solution Kerberos set up with the most success in the past.
PERIMETER DETECTION DEVICES:
Wired Photoelectronic Detectors
Wired PE detectors are coming in pairs, establishing one or multiple invisible beams over the fence or within the perimeter to create an additional perimeter barrier. The length of a beam between detector pairs are depending of the capacity of the device but a single pair can cover over 100 meters distances in a straight line or even more. Once any of the beams is broken or obscured the device sends its signal about the fact of a breach to the control panel. Wired sort refers to power source link, as these detectors require external power source for their operation, that must be properly set up and installed prior of installation of the units, possibly with a tampers proof design. The units are highly reliable due to the ability to detect infinite events thanks to the continuous power supply, while the event signals themselves can be transmitted on wi-fi or radio frequency besides the hard wired topology.
Wireless Photoelectronic Detectors
With the development of solar technology external power independent detectors appeared on the security market as well. As the applications got closer to expectation and passed standards, Kerberos started to implement them as stand alone installations, but more often as integrated subsystems. Theses detectors require no power wiring as they gain the current required for operation via their onboard solar panels, also they communicate with other systems via protected and encrypted radio transmission. The distance coverable is at the moment 100 meters, but the development is continuous. In other features they provide the same methodology for alarming as the wired beams, but they have a limit for night operation: after a certain events transmitted, their power backup exhausts and need to be disposed again to sunshine to get back into transmission. This limitation however was never met under proper operation and the units are working with no complaint ever since their installation. The manufacturers in the same time offered proper connections for backup hard wirings for communication and powering, those can also be used in case of any doubt.
Vibration (shaker) or inertia sensors
These devices are mounted on barriers and are used primarily to detect an attack on the structure itself. The technology relies on an unstable mechanical configuration that forms part of the electrical circuit. When movement or vibration occurs, the unstable portion of the circuit moves and breaks the current flow, which produces an alarm. The technology of the devices varies and can be sensitive to different levels of vibration. The medium transmitting the vibration must be correctly selected for the specific sensor as they are best suited to different types of structures and configurations. A rather new and unproven type of sensors use piezo-electric components rather than mechanical circuits, which can be tuned to be extremely sensitive to vibration.
Passive magnetic field detection
This buried security system is based on the Magnetic Anomaly Detection principle of operation. The system uses anelectromagnetic field generator powered by two wires running in parallel. Both wires run along the perimeter and are usually installed about 5″/12cm apart on top of a wall or about 12″/30cm below ground. The wires are connected to a signal processor which analyzes any change in the magnetic field. This kind of buried security system sensor cable could be embedded in the top of almost any kind of wall to provide a regular wall detection ability, or can be buried in the ground. They provide a very low false alarm rate, and have a very high chance of detecting real burglars. However, they cannot be installed near high voltage lines, or radar transmitters.
This proximity system can be installed on building perimeters, fences, and walls. It also has the ability to be installed free standing on dedicated poles. The system uses an electromagnetic field generator powering one wire, with another sensing wire running parallel to it. Both wires run along the perimeter and are usually installed about 800 millimeters apart. The sensing wire is connected to a signal processor that analyses:
- amplitude change (mass of intruder),
- rate change (movement of intruder),
- preset disturbance time (time the intruder is in the pattern).
These items define the characteristics of an intruder and when all three are detected simultaneously, an alarm signal is generated.
The barrier can provide protection from the ground to about 4 meters of altitude. It is usually configured in zones of about 200 meters lengths depending on the number of sensor wires installed.
The operation of a microwave barrier is very simple. This type of device produces an electromagnetic beam using high frequency waves that pass from the transmitter to the receiver, creating an invisible but sensitive wall of protection. When the receiver detects a difference of condition within the beam (and hence a possible intrusion), the system begins a detailed analysis of the situation. If the system considers the signal a real intrusion, it provides an alarm signal that can be treated in analog or digital form.
Taut wire fence systems
A taut wire perimeter security system is basically an independent screen of tensioned tripwires usually mounted on a fence or wall. Alternatively, the screen can be made so thick that there is no need for a supporting chainwire fence. These systems are designed to detect any physical attempt to penetrate the barrier. Taut wire systems can operate with a variety of switches or detectors that sense movement at each end of the tensioned wires. These switches or detectors can be a simple mechanical contact, static force transducer or an electronic strain gauge. Unwanted alarms caused by animals and birds can be avoided by adjusting the sensors to ignore objects that exert small amounts of pressure on the wires. This type of system is vulnerable to intruders digging under the fence. A concrete footing directly below the fence is installed to prevent this type of attack.
Fibre optic cable
A fibre-optic cable can be used to detect intruders by measuring the difference in the amount of light sent through the fibre core. If the cable is disturbed, light will ‘leak’ out and the receiver unit will detect a difference in the amount of light received. The cable can be attached directly to a chainwire fence or bonded into a barbed steel tape that is used to protect the tops of walls and fences. This type of barbed tape provides a good physical deterrent as well as giving an immediate alarm if the tape is cut or severely distorted. Other types work on the detection of change in polarization which is caused by fiber position change.
This system employs an electro-magnetic field disturbance principle based on two unshielded (or ‘leaky’) coaxial cables buried about 10–15 cm deep and located at about 1 meter apart. The transmitter emits continuous Radio Frequency (RF) energy along one cable and the energy is received by the other cable. When the change in field strength weakens due to the presence of an object and reaches a pre-set lower threshold, an alarm condition is generated. The system is unobtrusive when it has been installed correctly, however care must be taken to ensure the surrounding soil offers good drainage in order to reduce nuisance alarms.