As we discussed last month, intrusion detection systems (IDSs) should prevent burglars and trespassers from entering your facility in the first place. Using sensors, these systems are sensitive to sound, vibration, motion, or a change in capacitance due to penetration of an electrostatic field. Some also sense when someone breaks an electrical circuit or interrupts a light beam.

Understanding how the different devices of IDSs really work can help you develop the smartest plan for your facility. So let's look at some of the basic principles behind their operation.

An IDS consists of three basic components: sensors, control units, and audible alarms. Sensors are devices that sense or respond to certain conditions in and around a protected area. One control unit is typically required in each secure area. Its function is to receive signals from the sensors and to transmit these signals to the monitoring unit and local audible alarm. Each control unit must report to a separate monitoring unit status indicator module.

Each monitoring unit typically contains one signal and power status indicator module. The monitoring units connect to a local audible alarm (most typically a siren or buzzer). This alarm serves two purposes. Initially, it may scare the intruder away. Secondly, it alerts local security personnel and police of a breech in security.

The local audible alarm has limited value for areas where there are no response personnel. You should not use a local audible alarm without a remote monitor unit.

Let's take a closer look at the different types of sensors employed by these IDSs.

Interior sensors. Facility directors use interior sensors to detect an intruder inside a building. The most effective and useful interior detectors use either ultrasound or infrared light. Although both are effective, each has its own advantages.

Ultrasonic detectors operate at very high frequencies, at a much higher pitch than the human ear can detect. When these sound waves bounce off stationary objects, they return to the detector at the same frequency at which they were emitted. However, if these sound waves bounce off a moving object, they will return to the detector at a different frequency than the one at which they were emitted. When this happens, the detector sends an alarm signal to the controller.

Infrared detectors sense heat. They sense the level of infrared energy (heat) given off by every object in its field of view. If the system detects a change of temperature, it sends an alarm signal to the controller. For example, if a human with a body temperature of about 99 DegrF walks into a room with 70 DegrF walls, the sensor detects this and sends an alarm signal to the controller.

When deciding which type of interior detectors to install, it is important to understand the characteristics of each. In general, you should use passive infrared detectors when an intruder's movement will be within the detector's field of view. Choose ultrasonic detectors when the intruder's likely movement is toward the detector.

You must also be aware that heating systems can trigger some types of infrared sensors. Most new designs eliminate this problem by using a segmented lens. Each segment detects heat independently, so the system will not send an alarm signal to the controller unless it detects heat moving from one segment to another. This means a heat vent will not trigger the sensor.

Pay close attention when aiming the sensors. Specifically, you should keep the lowest point of the sensor's field a couple of feet off the ground in areas where guard dogs may be present. In these areas, you should use a detector that does not have a low-angle field of view.

Ultrasonic detectors can also activate falsely. The forced air from a heating/cooling system can move the drapery or a plant's leaves and set off a false alarm.

Hybrid sensors. Another type of interior detector, these devices use a combination of passive infrared and ultrasonic technology in the same unit. If the infrared sensor activates, but not the ultrasonic sensor, no alarm will sound. Only when both sensors trigger will an alarm signal travel to the controller.

As far as cost goes, interior sensors are typically more expensive than perimeter sensors. However, you only need them in areas where an intruder might make it past the perimeter detectors.

Perimeter sensors. Perimeter sensors protect the exterior boundaries of a facility. Although these sensors are usually inexpensive, you must install them at every possible point of entry, including all doors and windows. The big expense comes in running a cable to every detector in the system.

Magnetic contact switches are the most common perimeter sensors. These devices consist of two components. One part of the detector attaches to the movable portion of the door or window; the other part attaches to the stationary frame. You must install these two components so they almost touch each other (1/16 in. spacing between). One part of the detector contains a magnet, while the other incorporates a specially constructed switch. When both parts are in close proximity, the magnet keeps the switch mechanism closed. But when the two parts are separated (as they would be if the door on window were open), the switch will open, and the controller picks up an alarm condition.

Glass-break sensors. This sensor contains a tiny tuning fork, which vibrates when it detects a high-pitched sound. For example, when someone breaks through a window, the sound produced by the breaking glass forces the tuning fork to vibrate. This sets off the sensor to send an alarm signal to the controller. But remember: You must install this detector on the window's glass to detect breakage. It does not work when placed on the window frame.

Seismic detectors or shock detectors. These devices pick up vibrations produced when a stationary object is set in motion by external forces. They're sometimes used at windows, which will vibrate when they are opened, or on fences, which will shake when climbed.

Microwave motion detectors operate on the same principle as ultrasonic detectors. However, unlike ultrasonic detectors, the microwave sensors operate in the Gigahertz band of the radio frequency spectrum. This means wind currents or changes in temperature do not affect the device. The limitation of microwave sensors is its detection pattern can actually penetrate walls. Thus, they also detect movement in areas you may not want to monitor. Here's another tip: Remember metal objects easily reflect microwave signals.

Stress sensors. These devices detect a person walking, crawling, or jumping into a protected area. You typically install stress-detecting sensors under floors, stairs, or roofing systems and connect them to an adjustable electronic processor. An intruder's body weight is enough to cause the protected area to flex or stress when he or she walks across it. The sensors detect this, and alert an alarm condition. You can easily conceal these sensors, adjust them for sensitivity, and even install some types under carpets.

Pressure mat sensors are sometimes used at an entrance. These mats send a signal to the controller when someone puts weight on them.

Depending on which IDS you choose, more than one type of sensor is typically necessary to provide adequate protection for an area. If possible, check with persons already using such systems or devices before purchasing your own system.

In next month's lesson, we'll discuss voice evacuation systems, commonly called voice evac. We will explore why these systems have tremendous potential for use in facility safety.


Sidebar: Selecting an IDS

When selecting an intrusion detection system (IDS), consider:

• Location and response time of security personnel;

• Value of facility, material, or the sensitivity of material you want to protect;

• Area environment (including building construction, sound levels inside/outside, climate, etc.);

• Radio and electrical interference; and

• Operational hours of installation or facility.