Boundary-Penetration Sensors

Boundary-penetration sensors are designed to detect penetration or attempted penetration of perimeter barriers. These barriers include walls, ceilings, duct openings, doors, and windows.

Structural-vibration sensors detect low-frequency energy generated when someone attempts to penetrate a physical barrier (such as a wall or a ceiling) by hammering, drilling, cutting, detonating explosives, or other forcible methods of entry.

Glass-breakage sensors detect the sounds of breaking glass. The sounds of breaking glass include frequencies in both the audible and ultrasonic range. Glass-breakage sensors use microphone transducers to detect these sounds. The sensors are designed to respond just to specific frequencies of sound, minimizing false alarms that may be caused by someone banging on the glass. Glass-breakage sensors need not be mounted directly on the glass. They can also be standoff mounted and pointed at the glass. If glazing other than glass is being protected, ensure that the sensor works with that type of glazing material.

Passive ultrasonic sensors detect acoustical energy in the ultrasonic frequency range, typically between 20 and 30 kilohertz. They are used to detect an attempted penetration through rigid barriers (such as metal or masonry walls, ceilings, and floors). They also detect penetration through windows and vents covered by metal grilles, shutters, or bars—if these openings are properly sealed against outside sounds.

Magnetic switches typically are used to detect the opening of a door. These sensors also can be used on windows, hatches, gates, or other structural devices that can be opened to gain entry. The switch mechanism should be mounted on the doorframe and the actuating magnet should be mounted on the door. Magnetic switches usually are wired so that they are closed by the magnet when the protected opening is in its secure position.

Magnetic switches come in a variety of configurations that permit them to protect a wide range of openings. They can be visible or concealed.

For additional security, it may be appropriate to use balanced magnetic switches. A balanced magnetic switch has a three-position reed switch and an additional magnet (called the bias magnet) adjacent to the switch. When the door is closed, the reed switch is held in the balanced or center position by interacting magnetic fields. If the door is opened or an external magnet is brought near the sensor to defeat it, the switch becomes unbalanced and generates an alarm.

Grid-wire sensors consist of continuous electrical wire arranged in a grid. The wire maintains an electrical current. An alarm is generated when the wire is broken. The sensor detects forced entry through walls, floors, ceilings, doors, windows, and other barriers. An enamel-coated number 24 or 26 American wire gauge (AWG) solid-copper wire typically forms the grid.