Detection & Activation Devices
CONTACTS (SWITCHES)
Contacts are switches fitted to doors and windows to detect unauthorised intrusion. In any reputable alarm system the switches are in a make (closed circuit) position when the door or window is fully closed.
Most modern switches are magnetic reed types. They consist of two metal strips, mounted within a sealed glass tube. One fixed with one contact on, the other a spring which is fitted with a contact point which are positioned parallel to each other, but not touching. The assembly is fitted into a glass tube to seal it from damp etc. The reed assembly is then fitted into a plastic case.
When a magnet is brought close to the pair of contacts, the spring contact is magnetically pulled up against the fixed contact, and completes the circuit.
Magnetic switches are of two basic types, flush or concealed, and surface fitting.
FLUSH CONTACTS
The flush fitting switch is countersunk flush with the surrounding woodwork The operating magnet is countersunk in the top or bottom leading edge of the opening part of a door or window.
The contact is countersunk opposite the magnet in the surrounding framework. It is fitted to ensure that when the door or window is fully closed, the magnet and contact are in alignment, and the contacts are pulled together to make a closed circuit. The two components do not have to be touching to allow them to operate. They are also concealed from view and protected from interference.
SURFACE CONTACT
A surface reed contact has a similar internal construction as a concealed or flush contact. As the name suggests, the housing is usually rectangular and is screwed to the surface of a door or window frame, with the matching magnet assembly being fitted to the adjacent leading edge or top of the door or window.
SHUTTER CONTACTS
A variation on surface contacts is the Shutter Contact.
This consists of a reed contact in a heavy cast housing that is relatively flat, and is designed to be screwed down on the floor next to a roller shutter, usually in an industrial premises,or garage, where it can be safely driven over.
The magnet is also in a heavy-duty housing, and is screwed onto the shutter lower rail.
This type of contact is also suitable for domestic up-and-over garage doors etc. Shutter contacts are most tolerant to movement and misalignment.
CONTACTS -- Advantages and Disadvantages
The advantages of concealed contacts are: neater appearance, hardly visible when the door or window is closed and a greater degree of protection. The door and frame protect the contact, although the relevant wiring is still accessible, but only from the inside of the protected premises.
The disadvantage is that when fitted to external doors and windows, they have a tendency to fill with water by capillary action etc. and the wiring rapidly corrodes, causing false alarms.
Flush contacts take a little more skill and time to fit neatly.
The advantage of surface contacts are that they are simple to fit and align and as they are fitted to the surface of the protected opening, and are often of a sealed design. They are usually not affected by damp and corrosion.
The disadvantage of surface fitted contacts is the cosmetic appearance and the easy access to the components, which in theory would allow tampering of the circuit. The incidence of this being done though is very rare, especially in domestic intruder alarm systems.
Surface fitting contacts should not be fitted to steel or iron framed windows, as the metal frame tends to affect the magnet, and cause sticking, setting and false alarm problems.
They all are subject to accidental damage because of their exposed fitting.
It could be possible to use magnets to try to bypass the protected opening, but this is practically unheard of. For high security installations, a reed contact, protected against the use of an external magnet by the fitting of an additional reed switch and magnet, is produced-but its use is unnecessary on all but the most high-risk application.
The physical operation of a reed type contact can be checked by listening very carefully as the magnet approaches the reed switch—a click will usually be heard, In flush fitting reed contacts always unscrew the plastic housing and check that the magnet is not loose or turned away from its correct position, before looking for electrical faults. The reed switch itself is usually very reliable.
With thanks to Tony Render, Managing Director and Dave Almond, Service Director of RENDER ALARMS Middlesbrough 01642 230696, For their help with the equipment illustrations and support.