A Peek Inside Perimeter Security

Technically, the installation and servicing of perimeter security systems is the most challenging and demanding of all security applications. A good electronic perimeter security system must be able to adapt to dramatic weather conditions with a minimal amount of false alarm activity. On the other hand, it must be electronically smart enough to counter attempts to compromise the system in remote locations.

The increased demand of meeting homeland security requirements has put additional emphasis on perimeter security installations, as well as heightened expectations of its effectiveness and capabilities. This month, we will look at some of the different technologies and how to correctly apply them in a reliable perimeter security system.

Many Types of Perimeter Sensors
It seems that every time I turn around, another company has designed a new perimeter security device. Don’t get me wrong; new technologies are what keep us all very busy. But keeping track of it all can sometimes be a bit overwhelming. Most new applications are built around a handful of core perimeter technologies. They are:

Taught-wire fence — a reliable design detects climbing or cutting, and provides good reliability from bad weather environments (see photo on page 29 of February issue). Discriminatory technology, such as those made by Safeguards Technology of Hackensack, N.J., make the system more than just a cut/pull detection system.

Photoelectric beam towers — Provide a transmitter/receiver configuration of photoelectric beams in a pedestal tower array. Can be overlapped and the alarm circuit repeated around a complete perimeter. Requires no fence support. Typically installed within a physically secured area. Problems sometimes arise with equipment blocking security paths. Extra lane markings may be needed.

Microwave beams — Overlapping microwave beams provide a long-distance perimeter system. Typically installed within a secured fenced area.

Passive infrared curtains — Similar to interior PIR sensors but with additional discrimination electronics for outdoor applications. Companies, such as ASIM Technologies of Billerica, Mass., provide “gap-free” curtain coverage for distances of up to 500 feet and at a height of 13 feet. These more advanced systems come with special telescope-like precision mirror glass optics.

On-fence cabling — Cabling and sensors that are strapped to the fence to detect climbing or cutting of fence. Cable may detect suspicious sounds, or flexing of fence. Companies such as Tempe, Ariz.-based Southwest Microwave (I know you thought they just did microwave) provide intelligent cabling systems (see diagram on page 28 of February issue). Digital signal processing (DSP) allows for detection within three meters of the intrusion. I also like Southwest’s “Cut simulation tool,” which is used to test for fence cut detection of the system. (Installation tip: Make sure to use UV [ultraviolet]-resistant tie-wraps when securing sensor cabling to fence. You may also need to use a calibrated tie-wrap gun to avoid damage to the sensor cable.)

Below-ground sensors — This can either be a coax cable (sometimes called a “leaky coax”) that acts like an antenna or seismic-type detectors sensing above- and below-ground digging activity. Another type of system actually detects if intruders have metal weapons as they cross the perimeter. Seismic systems need to set up discrimination areas to counter false alarm signaling if perimeter is near a roadway. Many prefer below-ground installations since they provide considerable protection from bad weather. However, care needs to be taken when installing cable and equipment in a trench for reliable performance (see sidebar on page 29 of February issue).

Fiber-optic cabling — I have put this in a category of its own since it is a popular technology that can be used on fencing or below ground. Fiber-optic cable that has laser light going through it can detect the slightest movement or pressure using a combination of DSP and time domain reflection measurement technologies. Fiber is very popular since it is immune from EMI and lightning.

Video motion detection (VMD) — This is probably the hottest perimeter technology going today. VMD systems can be programmed to look at multiple perimeter areas while ignoring other noncritical false alarm areas.

This was just a sampling of the many variations and combinations of perimeter sensor technologies. There are also some very specialized military-type technologies such as air turbulence detection for helicopter attack.

Design to Offset Interference Issues
When planning a perimeter system, one should learn to mix combinations of physical and electronic security. One of the best is a double-fence combination in which sensors between or on either side of one of the fences must be activated simultaneously to cause an alarm. The outer fence needs to only be high and strong enough to protect wildlife from causing false alarms by sensors protecting the main inner fence.

From a technology installation viewpoint, remember you will typically be placing large amounts of sensitive cable at very long distances and often in large open areas. This cabling can act like a big radio antenna, and there may be times when radio frequency interference (RFI) and EMI filtering will need to be added per the manufacturer’s instructions.

This is especially true if you install near areas such as a radio tower. In many cases, you will use shielded cable so remember the basic rule of grounding the shield only at the head end of the system so as not to cause any “ground loop” problems. Additionally, lightning can be a problem and proper surge suppression should be implemented. Even buried cable can have a lightning ground surge induced on it.


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