Perimeter Security Devices
 

GLASS  BREAKAGE  DETECTORS

One of the most innovative devices and probably the best physical intrusion sensor on the market today. This device responds to specific audio frequencies associated with breaking glass. It consists of a sensitive microphone mounted inside a plastic case and quite often is "teamed" with either a seismic or air density sensor. The two technologies working in tandem provide a virtually false alarm free detector. The physical act of dropping a glass on a tile floor directly beneath one of these types of sensors was usually sufficient to trigger a false alarm. The industry has addressed this problem by providing "dual-technology" sensors.

When an exterior window pane is broken the glass itself actually "flexes" or bulges in the few microseconds after an impact and just before it physically breaks or shatters. This action, coupled with the sound made by the breaking glass is how the sensor is able to distinguish between an actual window being broken, and the accidental dropping of a drinking glass. "Flex" is by no means the only other part of the "dual-technology" equation. Other sensors incorporate what resembles a seismic sensor as part of their alarm discrimination circuitry. These types of sensors have to be mounted fairly close to the windows they are protecting, and their installtion requires other "additional" considerations. They have to be attached to a portion of the building structure that is capable of transmitting the "shock" part of the equation. This may not always be possible.

Your alarm consultant will also caution you with respect to curtains, and other window "treatments" that may interfere with the sensor and preclude it from actually detecting an intrusion. It's also very important for the installer to verify the sensor's capacity to detect an alarm condition prior to it's actual physical installation. What this means is that the installer should manually test each detector BEFORE determining the unit's final installation location.

Here are some pictures of different dual technology glass break detectors:
 

Ademco® 2500 Series

Paradox® GlassTrek

C&K® FG-730 

CONTACTS  (DOORS  AND  WINDOWS)

Of the two main groups of devices that comprise a modern alarm system, contact type switches are the simplest by far, and the most powerful.  When properly utilized, they can detect an intrusion into the protected space even before physical entry is gained.  Contacts on certain entry doors also act as "triggers" that enable the system to detect an entry by an actual user and give them the time required to access the control unit and disarm or turn it off.  The most common types of contacts employ a micro switch made of very thin metal reeds that are sealed into a plastic or aluminum housing.  They are either surface mounted or flush mounted into the door or window frame.  Surface mounted contacts are usually secured with double sided tape or screwed onto the door frame.  Wires connected to the switch terminals are in turn terminated into one of the zones of the control unit.  A simple magnet is mounted immediately adjacent to the switch on the door or window itself.  When the two units (magnet and switch) are in close proximity, the control unit recognizes this as a "normal" condition.  When you open the door/window, the reed switch is activated and the alarm system interprets this as a "break" in the wire which in turn triggers the zone to which the switch is terminated.

Most alarm systems are capable of recognizing when a switch is "tampered" with, and it is up to you as the concerned customer to ensure that this feature is activated.  In most instances, most alarm companies will install "low security" normally closed switches and in the case of a flush mounted installation, this provides excellent protection.  Surface mounted switches, however, are far more accessible to a knowledgeable burglar or thief, and in some applications a higher security switch may be the more prudent choice.  Your professional security consultant should present these options to you, the customer, during the course of his normal walk around inspection of your premise.  In most instances the difference in cost between the lower and the higher security switches is not even worth mentioning.

Some alarm systems require even higher security contacts.  These employ a reed switch with three terminals.  When an "alarm" condition is achieved (by the opening of a door or window), one "side" of the terminals will "open", and the other will short.  The combination or a single action (an opening or closing terminal) will result in a faulted or tripped zone.  Still other types of contacts employ a biased magnet/switch combination.  When an intruder attempts to bypass the magnet on the door or window with another magnet, the switch will immediately open and display an alarm condition. The simplest contact switch device is one that is rarely utilized today, although there are still legitimate applications that do crop up from time to time. This device employs a spring loaded micro-switch, which looks very similar to the switches used in your car to turn on the interior lights when you open your door.