Figure 8-23. - Conductive foil on picture window.

Figure 8-24. - Complete glass-door protection.

ADDITIONAL COMMERCIAL/INDUSTRIAL COMPONENTS OF SECURITY/FIRE-ALARM SYSTEMS

Industrial security/fire-alarm systems are essentially the same as those used for residential applications. There are, however. a few additional components that are used mostly in industry.

Vibration detectors are often used on industrial buildings to detect vibrations caused by forced entry. Such detectors have been used on a variety of construction materials. such as hollow tile, plaster and lath, brick, concrete. metal ceilings, and wood. Once mounted in place, they may be adjusted with a setscrew for the desired sensitivity.

Some factories maintain a security fence euipped with fence-guard detectors. This type of dectector will detect climbing, cutting, or any other penetration of the fenced area. Most of these detectors operate on standard closed-circuit controls as described previously.

Fence-guard detectors use a vertical-motion detector that is sensitive to movement created by climbing or cutting the fence. Normal side motions, such as wind or accidental bumping, do not affect the detector and cause false alarms. The detectors are normally mounted about midway up the fence and every 10 feet (3 m) of fence length. Most of these devices set off the alarm if they are tampered with or if the wire is cut. They may be connected to a control panel and the alarm will sound in the form of a bell or horn, or it will silently dial the local law-enforcement agency.

Another type of detector that is used is the outdoor microwave detector. This detector is used for pro- tecting large outdoor areas like car lots, construction sites, and factory perimeters. In operation, a solid, circular beam of microwave energy extends from a transmitter to the receiver over a range of up to 1,500 feet (457 m) for some brands. Any movement inside of this beam (fig. 8-30) will activate the alarm.

Thermistor Sensors

The continuous linear thermal sensor is a small- diameter coaxial wire that is capable of sensing temperature changes along its entire length. The sensor is made up of a center conductor and an outer stainless steel sheath. The center conductor is electrically insulated from the outer sheath by a ceramic thermistor material, as shown in figure 8-31.

Since the thermistor has a negative coefficient of resistance, the electrical resistance between the center wire and the outer sheath decreases exponentially, as the surrounding temperature increases (fig. 8-32).

The changing resistance is monitored by one of several control panels that then can actuate extinguishing systems or any other electrically controlled devices.

Such sensors have a diameter of approximately 0.080 inch (0.2 cm) and, therefore, have a small mass that permits them to sense changes in temperature rapidly. They can sense temperatures from 70°F (21°C) up to 1200°F (649°C) if the thermistor material is properly selected.