Printer Friendly

Sensing your way to security.

Perimeter protection has been a concern since the beginning of time. While the methodology has changed drastically over the years, the essential components of detection and delay have remained unchanged.

In the Middle Ages, castles with thick walls surrounded by moats provided the necessary delay, while sentries spaced evenly around the castle walls provided the detection. Today, thick castle walls and moats have given way to perimeter fences, and sentries have been replaced by perimeter intrusion detection systems.

The need for an exterior perimeter protection system can arise from a number of conditions. Commercial, industrial, or defense-related sites require protection for various assets located within their perimeters or for the protection of outsiders from hazardous materials or conditions within the fence.

Correctional facilities have a dual concern: Inmates must be confined and unauthorized personnel must be denied entry.

Intrusion detection in the exterior environment presents a variety of challenges. If the sensor system is to function effectively-without excessive false alarms - it must be selected carefully.

At first glance, fences seem to be formidable barriers. However, tests have shown that fences, even those with arrays of barbed wire or razor ribbon, provide only minimal security and are easily penetrated by intruders.

Although fences provide only minimal security, they serve several useful purposes. Fences in an exterior perimeter protection system

* define the site perimeter,

* briefly delay an intruder,

* channel employees and visitors to authorized gates,

* keep honest people out, and

* serve as a sensor platform.

The first thought that typically comes to mind when discussing fences is the standard chain-link fence. While chain link is probably the most commonly used material, it is not the only choice available. Some additional options include the following:

* Wrought-iron fences, sometimes referred to as "embassy fencing," are commonly used in areas where appearance is important.

* Wooden-slat fences are often used when privacy is essential. Such a fence forms both a physical and a visual barrier.

* High-security fences incorporate special configurations of expanded metal that make unaided climbing difficult or impossible.

* "Living" fences are composed of dense shrubbery or hedges, which can be more difficult than chain-link fences to penetrate. One variation incorporates a shrub noted for its long, sharp thorns.

When a concealment or privacy fence is used, proceed cautiously. The fence can conceal an intruder while he or she observes your site or tries to defeat your perimeter protection system. In some instances, a traditional fence and a living fence are used together. This introduces the same vulnerability and must also be used cautiously.

While fences are a vital component of most physical protection systems, the combination of an appropriate fence with one or more carefully selected intrusion detection sensors forms a formidable combination of barrier and detection capability. *************** FENCES, OR FENCE-ASSOCIATED SENSORS, are divided into two broad categories: those that use the perimeter fence as a platforrwto support the detection device, and those in which the sensor also serves as a fence.

Fence-mounted sensors are attached directly to the fence fabric, posts, or both, and detect any attempt to cut,

climb, or jack up the fabric of the fence. The freestanding devices foffn a barrier in their own right and detect the same methods of attack against the structure. The following examples represent various fence sensor technologies.

Strain-sensitive cable sonsors. Strainsensitive cable sensors are constructed of a specially treated coaxial cable and an associated signal processor. The sensor cable is arrayed across the fabric of the fence and attached to the fence at intervals with cable ties. When an intruder attempts to cut, climb, or jack up the fabric of a protected fence, this sensor "hears" the mechanical movement of the fence. The sensor then uses sophisticated signal processing to compare the detected signals with defined intrusion signatures, differentiating between actual intrusions and nuisance alarms.

A strain-sensitive cable sensor is adjustable for sensitivity and often uses pulse-count technology. This technology requires a predetennined number of events to occur before declaring an alarm, further reducing the effect of nuisance alan-n sources.

Strain-sensitive cable sensors are typically available with detection cable lengths of up to 1,000 feet. Since the cable itself is the sensing media, detection sensitivity is generally un along the length of the sen

The strain-sensitive cable sensor is sensitive to the condition of the fence on which it is mounted. To ensure adequate detection, the fence should be tensioned to the manufacturer's recommendations and be free of noise inducing conditions, such as loose outriggers, loose tension or support bars, and signs mounted on the fence fabric.

An inherent audio listen-in assessment capability assists the system operator in evaluating alann signals. Strainsensitive cable sensors are extremely effective, offering a high probability of detection and a low nuisance alarm rate in a cost-effective package.

Other than periodically examining the cable to ensure that it has not been damaged and is still securely attached to the fence, this sensor requires no maintenance.

A variation of this sensor uses a magnetic sensor cable, containing a combination of continuous permanent magnets and conductors deployed along the fence fabric. Movement of the fence associated with an intrusion attempt causes movement of the sensor cable, which translates into movement of the

conductors within the magnetic field generating an electrical current. The generated electrical current is evaluated by the signal processor to deterrnine if the alarm criteria have been met.

Mochanical fence disturbance sensors. Mechanical fence disturbance sensors use a seismic mass, weight, or other inertia-based device, which moves in response to fence movement, to detect fence motion associated with an intrusion attempt. Typically, the seismic mass, or weight, is positioned so that at rest it forms part of a closed electrical circuit. Movement of the sensor

causes the seismic mass or weight to jump off its contacts, opening the circuit and causing an alarm. A variation of this device uses a normally open circuit that is closed by movement of the seismic mass.

This sensor is sensitive to the condition of the fence on which it is mounted. Loose or noisy fences will cause excessive movement of the sensor and excessive nuisance alarms. Most variations of this sensor incorporate some form of pulse count circuitry to reduce nuisance alartns. Additionall , variations of this sensor are ex-

tremely sensitive to their orientation (vertical versus horizontal) and whether they are mounted on the fence fabric or poles.

Careful attention must be given to spacing of the individual sensors along the fence. While the strain-sensitive cable sensor is linear, with a relatively even detection sensitivity along its entire length, die mechanical fence disturbance sensor consists of a series of individual detectors, spaced at 10- to 30-ft. intervals. Sensitivity is greatest closest to the sensor, and it tapers off towards the midpoint between individ-

ensors must be placed close enough together to ensure full detection capability from fence top to bottom at the sensor midpoint.

Electromechanical fence disturbance sensors. Electromechanical fence disturbance sensors use piezoelectric crystal devices, or geophones, mounted directly to either the fence fabric or fence

posts to detect intrusions. Unlike the mechanical version of this sensor, which simply opens or closes an electrical circuit, electromechanical devices translate fence motion into electrical signals that are proportional to the amount of fence motion. The electrical signals are processed against established intrusion signatures to detennine if an alarm exists.

Although the electromechanical device processes fence movement indications in a different manner, it is subject to all of the conditions identified for the mechanical version of this sensor. Care must be given to the sensor orientation, location on the fabric or poles, spacing between devices, and fence condition.

Generally, the electromechanical version of this sensor is more resistant to nuisance alarms than the mechanical version, due to a combination of pulsecount technology and enhanced signal processing capability.

Electrical field and capacitance sensors. While electrical field and capacitance sensors are somewhat different in their methods of operation, they accomplish essentially the same result. Each consists of a number of wires arrayed horizontally along the perimeter fence. Electrical field systems usually incorhD>aporate e to five wires while capaci-

tance devices may use as many as 18 to 20 wires.

In either case, the wires, which are insulated from the fence and ground, create an invisible electrical field extending from the ground to a height of up to 10 feet. An intruder attempting to cut, climb, or jack up the fabric of the fence will disturb the electrical field, causing an alarin.

Electrical field and capacitance array sensors are typically installed as fence associated sensors, protecting the fence fabric, the top guard, or both. They may also be configured as freestanding sen-

sors on their own supporting hardware.

Strain-sensitive cable and fence disturbance sensors are sensitive to the condition of the fence on which they are mounted, but neither the electrical field nor the capacitance array sensors are affected by fence condition as long as the fence posts are holding the sensors in position.

Both electrical field and capacitance array sensors present a formidable appearance and should have a substantial deteffent effect on a potential intruder. Both sensors are somewhat maintenance sensitive, requiring periodic retensioning of the sensor wires and establishment of a foliage-free zone directly beneath the sensors.

When properly installed, both sensors provide a high probability of detection, however, the nuisance alarm rate is higher than for the strain-sensitive cable sensor.

Taut wire sonsors. Taut wire sensors consist of a series of barbed wires arrayed across the fabric or top guard of a fence. Each of the wires is under considerable tension and is connected to one or more switches at vertical sensor posts along the perimeter. Deflection or cutting of one or more of the wires causes the associated switches to move, setting off an alartn. A variation of this device uses piezoelectric crystal switches in place of mechanical switches.

As with the electrical field and capacitance sensors, this sensor may be installed in conjunction with a fence or as a freestanding device. In either configuration it presents a forrnidable appearance and has a high deteffent value.

The primary maintenance requirement of this sensor is the need to periodically retension the sensor wires. Foliage growth does not effect the operation of the taut wire sensor. When properly installed and maintained, the taut wire sensor incorporates a high probability of detection and a low nuisance alarm rate. However, it is expensive to install and maintain.

FIBER-OPTIC BASED SENSORS ARE RELAtively new and becoming more popular. All fiber-optic sensors operate by transmitting a light signal down a length of fiber-optic cable and monitoring the received signal at the end of that cable.

Two different concepts are used in the fiber-optic monitoring process. Some fiber-optic sensors simply look for the presence of the light signal as an indication of secure status, and the absence of the light signal as an alarin indication. Rese are continuity-effl sensors and require that the light-canying fiber actually be broken or cut to cause an alarm. Simple disturbance of the cable will not be detected. Because these sensors respond only to an actual cable break, they are relatively free of nuisance alanns.

However, they are easily defeated by a knowledgeable intruder.

The second monitoring concept looks for changes in the light signal that occur when the cable is slightly disturbed or moved. Although several different processes may be at work here, these sensors are generally refeffed to as microbending devices.

Microbending fiber-optic fence sensors. The microbending based fiber-optic fence sensor appears as a single, insulated cable approximately /4-inch in diameter, with a related signal processor. This sensor cable is affayed along and coupled to the fence in a manner virtually identical to that of a strain-sensitive cable sensor. Movement of the fence associated with an intrusion causes movement of the cable, and movement of the cable disturbs the light signal, causing an alarm.

This sensor is similar to the strainsensitive cable sensor in that it is sensitive to excessive movement of the fence. While testing of this sensor is not complete, it appears to have a high probability of detection. It also appears to have an excessive nuisance alarm rate.

Breakwire fiber-optic fence sonsors. The breakwire fiber-optic fence sensor

is composed of an array of fiber-optic strands, each embedded in a piece of barbed tape and arrayed across the fabric of the fence. This configuration forces the intruder to break the barbed tape and embedded fiber-optic cable, causing an alarm. This sensor may be deployed as a full multiwire fence array or as a top guard protection device.

A variety of fence sensors are available to accommodate virtually every fence sensor requirement. While each of the technologies was discussed here generically, a variety of manufacturers and products are available, each offering different features and capabilities.

Careful evaluation of the generic technologies as well as the individual features and capabilities of each sensor, coupled with the consideration of all data developed during a comprehensive site survey, will enable the selection of an appropriate sensor or combination of sensors to effectively protect your assets. N

Martin L. Vitch, CPP, is a senior secu- rity consultant with C. H. Guernsey and Company Inc. in Atlanta. He is a member of ASIS.
COPYRIGHT 1992 American Society for Industrial Security
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:perimeter intrusion detector systems
Author:Vitch, Martin L.
Publication:Security Management
Date:Jul 1, 1992
Previous Article:Detecting intrusion with fiber optics.
Next Article:Out of the showroom and into the manufacturing plant.

Related Articles
Perimeter protection - define, detect, and defend.
Intrusion detection: inside and out.
A cause for alarms.
The principle of timely detection.
Prehistoric protection.
Taking the nuisance out of intrusion detection.
Getting a sense for danger.
Motion detector.
Alarm design that rings true: in designing alarm systems, consultants and end users often forget to consider the basics--such as how the system can...

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters