The aim of this article is to help you choose the correct Magnetic Contact for your needs.
Introduction
Magnetic contacts are a very useful, economic detection device and as such are widely used in alarm systems. They all work on the same principle - a small reed switched is opened/closed by an adjacent magnet. They are supplied as a pair comprising the magnet and switch and are available in Normally Closed (NC) and Normally Open (NO) versions. NC contacts are closed circuit when the magnet is in close proximity to the switch. NO contacts are open circuit when the magnet is in close proximity to the switch. NC contacts are almost exclusively used in intruder alarm systems where opening the switch (magnet on door moving away from the switch). NO contacts are used in applications where there is a requirement for the system to go closed circuit when the magnet is moved away from the switch, e.g some CCTV applications.
Magnetic contacts are available in two main physical subsets - Flush and Surface. Flush contacts are designed for the magnet to be concealed in the edge of the opening door or window and the switch in the frame such that when the door or window is closed the magnet and switch are in close proximity to each other. Surface contacts are fitted to the surface of the opening door or window and the corresponding frame, again such that when the door or window is closed the magnet and switch are in close proximity to each other.
NB; Surface magnetic contacts must always be fitted on the protected side of the door or window.
Flush and Surface magnetic contacts are available as either pre-wired (the switch is encapsulated and connection to the alarm system is made via a junction box) or with screw terminals on the rear of the switch.
Flush contacts result in a neater installation than surface contacts, but they take longer to install.
Flush or Surface?
Whether to fit flush or surface contacts depends on various factors.
- Flush contacts can only be fitted on wooden (or composite) doors and windows. Also note that it is not usual to fit magnetic contacts to windows in general and specifically flush contacts (easy to crack the glass when drilling).
- Flush contacts can be fitted very neatly - the cable can be brought out between the architrave and plaster. However if pre-wired types are used then a junction box may have to be used.
- Surface contacts can be fitted to wooden, PVC and Aluminium doors and windows, however if fitting to PVC or Alauminium doors Pre-wired types are better as they can be glued in position (sticky pads) rather than drilling the doors.
- Heavy duty contacts are available for fitting to roller shutter doors etc.
Designing A Hard-wired Intruder Alarm System
Designing A Wireless Intruder Alarm System
Designing A Hybrid Intruder Alarm System
How Alarms Work
About Alarm Systems
How to Design Your Alarm System
Alarm System Design Examples
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