An alarm glass is a pane of glass with built-in monitoring devices that trigger a burglar alarm system if the pane is damaged or destroyed . For this purpose, an alarm wire is embedded over a large area in the glass or a small alarm loop, a so-called "alarm spider", is applied in a window corner. If the glass breaks, electrical conductors are interrupted in both versions and an alarm is triggered. Alarm glasses are often used in the shop windows of jewelers .
Alarm glass consisting of laminated safety glass
The alarm glass consists of a laminated safety glass with a meandering inserted wire , the alarm wire . This type of alarm glass is therefore also called "alarm wire glass". The wire is part of a detection line of an intrusion alarm system. If the glass pane breaks, the alarm wire will break and the burglar alarm system will trigger an alarm.
Alarm glass consisting of toughened safety glass
The alarm glass consists of single-pane safety glass , often in combination with another pane in an attack-resistant design as insulating glazing . In the event of destruction, the toughened safety glass shatters into small cubes at every point on the pane. An alarm spider (small-area electrical conductor loop) that is usually screen-printed and burned in is destroyed as a result and the intrusion alarm system triggers an alarm. The alarm glass is installed in the window on the side from which a break-in attempt is to be expected. The alarm spider is attached by the manufacturer in such a way that it lies in the space between the panes of the insulating glass pane. The other pane of the insulating glass is then the attack-resistant pane.
The window equipped with it is monitored with alarm glass for breakthroughs or access. When monitoring access, the maximum permissible distance between the wires in the glass is 10 cm. When monitoring for penetration, the distance is 4 cm. The type of monitoring that must be used depends on the type of object to be protected.
To connect the alarm loop (alarm wire or alarm spider) there is a flexible cable on the edge of the alarm glass pane or a so-called surface connection to which a cable can be connected near the edge. The cable is connected to the network of the intrusion alarm system. The alarm loop is looped into a conventional alarm line of the intruder alarm system. If the alarm glass is intact, a constant quiescent current flows. If the alarm glass is destroyed, the alarm loop is also destroyed and the alarm line is interrupted. The intrusion alarm system detects the interrupted flow of electricity and triggers an alarm.
Alarm wire glass
The connection resistance of alarm wire glass depends on the size of the glass surface and the required type of monitoring (access or penetration). The larger the disc and the smaller the distance between the wires, the longer the wire becomes and the resistance of the alarm wire increases.
The connection resistance of alarm spiders does not depend on the size of the glass surface and the type of monitoring. The loop resistance is a few ohms, depending on the manufacturer .
According to the usual standards, every alarm glass pane is subjected to a functional check before it leaves the manufacturer's works. Manufacturers with increased quality requirements carry out a control measurement of the alarm glass before processing and after processing into insulating glass. A suitably trained assembly company will carry out a simple passage check immediately after each transport of the panes.
Typical installation errors of alarm glasses are, for example:
- Failure to use the assembly materials supplied.
- Damage to connections and cabling.
- Installation with the "attack side" inwards.
- Deformation of the glass during installation and subsequent interruption of the alarm wire.
The relevant manufacturing and installation principles are:
- Requirements for alarm glasses VdS 2270 (2002-03)
- Test methods for alarm glasses VdS 2317 (2002-07)
- Guidelines for installation VdS 2311 (2017-04)
- Leaflet "Information on alarm glasses" VdS 5038 (2005-12)
- DIN VDE 0833-3 (2009-09)
- VdS guideline 2311: VdS guidelines for burglar alarm systems - planning and installation. 2017-04 Chapter 3
- VdS guideline 2311: VdS guidelines for burglar alarm systems - planning and installation. 2017-04 Chapter 9.3.5
- VdS guideline 2311: VdS guidelines for burglar alarm systems - planning and installation. 2017-04 Chapter 188.8.131.52
- Bodo Wollny, Alarmanlagen , 4th edition, page 23f, Pflaum, ISBN 3-7905-0903-5