Securing the Great Outdoors

Everything technical we have learned in the past via “Tech Talk” will be pushed to the max when applied to outside perimeter security. Perimeter security can apply to everything from the perimeter of a vault, a room, a building, a building complex, a community, or even the border of a country.

That means we may be dealing with alarm sensing circuits that can go from feet to miles in distance, placing a large demand on electrical characteristics and principles such as Ohm’s Law, Kirchoff’s Law, voltage drops, ground loops, signal attenuation, EMF/RFI, temperature, UV, just to name a few. 

The Trouble With Long Wire Runs

When powered sensors, such as microwave and photoelectric beams or camera equipment, are placed at long distances, the internal resistance of the power cabling can become an intimidating factor and influence the reliable performance of the sensors. Remember the relationship in Ohm’s law (E = IR) between voltage, current, and resistance?

If the powered device draws too much current, and the internal resistance of the cable is higher than normal due to the long distance of the cable (remember resistance in cable is measured in ohms per foot; also you must take into account the return cable path distance as well), then, again using Ohm’s Law, the cable voltage drop would be too large for the device to operate properly.

This again calls into play another rule called Kirchoff’s Law, which states that all of the voltage drops of a circuit equal the voltage source of that circuit (see “Tech Talk,” September 2006). This voltage-drop problem is actually re-emerging today with the newer applications of PoE (power over Ethernet) on devices such as IP cameras. A good guideline is that the voltage available at the powered device out on the perimeter should not be less than 95 percent of the source voltage, which equates to an allowable 5-percent voltage drop by the cable resistance.

Some ways to avoid this problem include using heavier gauge wire or power the perimeter devices locally from a remote AC power source or via remote solar power sources from vendors such as Kingston, N.Y.-based SunWize (

(Perimeter Sensor Tip: You do not have to have cable traveling the whole distance of a perimeter sensor system. Use sensor technology such as microwave or infrared beams to traverse much of the distance. Have the beams remotely powered at strategic transmit and receiver points around the perimeter. Then any breaking of the beams will activate an alarm. The receiver circuit only needs to be monitored at the building receiver end point.)

When low-voltage cable needs to travel externally at very long distances, every effort should be made to have the cable buried in protective conduit or shielding.  When cable is run a long distance above ground, such as on a fence, in a perimeter application, it can act like a radio antenna and pick up a variety of EMI/RFI (electromagnetic and radio frequency interference), such as radio and cell phone transmissions. Depending on how well the security sensor’s filtering circuits are designed, this interference can cause false alarm triggering of alarm circuits.

In some situations, the added use of toroidal-shaped RF chokes may need to be applied to perimeter cabling that is picking up unwanted energy. This is often a simple matter of wrapping the cable a few times through the doughnut-shaped rings close to the sensor’s circuit input connectors. This will help to detune the input circuits.

Dealing With Other Electrical Issues

Another potential electrical problem with long sensor equipment cable runs is ground loops.

The Alliance for Telecommunications Industry Solutions (ATIS) defines ground loop thusly: “In an electrical system, an unwanted current that flows in a conductor connecting two points that are nominally at the same potential, i.e., ground, but are actually at different potentials. Note: For example, the electrical potential at different points on the surface of the Earth can vary by hundreds of volts, primarily from the influence of the solar wind. Such an occurrence can be hazardous, e.g., to personnel working on long grounded conductors such as metallic telecommunications cable pairs.”

The unwanted voltage is not only a safety hazard but can cause sensor performance problems as well. Some possible solutions are to only ground cabling at the system’s source points or the use of isolation transformers.

Electrical problems, especially with outside perimeters, can be considerably reduced with the application of fiber-optic cable type sensors. Technology such as Optical Time Domain Reflectometers (OTDR) can detect a slight change in the flexing of a fiber-optics cable from movement along a perimeter fence line.  Additionally, video information can also be sent through fiber cables and is a popular method of transmission for exterior CCTV as it avoids other dangers such as lightning surges due to its nonconductivity.

Technology Brings New Methods

I have recently noticed some major shifts in perimeter security technology. The use of video motion detection is now becoming more common. While earlier systems used other sensor technology such as beams and motion sensing of fences that, in turn, would activate a particular CCTV FOV (field of view), better and better video algorithms are allowing video motion recognition to directly signal a perimeter alarm.

With new video motion detection, alarms are triggered by a configuration of changes in pixels in a digital video image. Viewing areas are programmed in which a perimeter boundary can be identified (see diagram). Video detection should be tested to make sure areas, such as tree branches swaying in the wind or birds and small animals, can be adjusted so as not to cause false alarms. Also, make sure to use cameras that go into the infrared range and/or use IR illumination for motion detection and identification at night.

Another new form of perimeter detection is the use of a radio technology that was being explored as far back as the 1960s. This technology is called UWB, or ultra-wide band, which typically covers a large range of the frequency spectrum at greater than 500MHz. Much of the bandwidth is used instantaneously by low power, ultra-short pulses.

One company taking the lead in the security industry in this area is Salem, N.H.-based UltraVision Security Systems with its buried UltraSensor™ devices. The sensors, which can be networked, detect movement of objects and can also relay back data such as rate and direction of movement to software display programs. It is like having a below-ground radar system on your secured perimeter. 

Challenges Many, Look for Answers

As we have seen, the mixing of security technologies to secure a large perimeter can be challenging and warrants much more research than can be expressed here. One good comprehensive reference source on perimeter security, although a little outdated, is the free “Perimeter Security Sensor Technologies Handbook” available from the National Institute of Justice’s (NIJ) “Justice Technology Information Network” Web site.

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