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    Fitting Magnetic Contacts

    Fitting Magnetic Contacts

     This article describes firstly how to select the correct magnetic contact for your needs and secondly how to fit the contact correctly.

    About Magnetic Contacts

    All magnetic contacts comprise two parts - a reed switch and a magnet.  The reed switch changes its state depending on the position of the magnet.  If you look at the lists of magnetic contacts on our website you will notice that there are quite a lot of different types

    • Flush types and Surface types
    • Normally Closed (NC), Normally Open (NO) and Change-Over (C/O)
    • Grade 1, Grade 2, and Grade 3
    • Simple Contacts, contacts pre-fitted with specific resistors, and contacts with selectable resistors

    Magnetic contacts fitted in alarm systems should be tamper protected.  On simple contacts the alarm pair from the control panel is connected across the alarm terminals or to the relevant cores from a pre-wired contact, and the tamper pair twisted together and secured under a spare teminal, or connected to the relevant cores from a pre-wired contact.

    Contacts fitted with resistors are designed to be connected as a  Fully Supervised Loop (FSL) where the alarm and tamper circuits are carried on a single pair - the resistance values differentiate the type of alarm.

    The different types are described below

    Flush Magnetic Contacts

    flush magnetic contact

    Flush Magnetic Contacts are designed to be fitted with the magnet in the edge of the door and the reed switch in the door frame such that the face of the magnet and switch are aligned when the door is closed.  Flush contacts are available as terminal (left) or pre-wired types.  They are also available with selected or selectable resistors.  Pre-wired types need to be connected to the cable from the control panel (or to another detector) using a junction box.  The contact shown left has 5 terminals; the two chrome terminals are the reed switch terminals, the other three brass terminals are not connected to anything in the detector and are used to loop the detector to other detectors, or in the case of a single contact to simply secure the twisted-together tamper pair.

    Surface magnetic Contacts

    surface contact

    Surface magnetic contacts are designed to be fitted with the magnet fitted to the face of the door and the reed switch on the door frame such that the face of the magnet and switch are aligned when the door is closed.  Surface contacts are available as terminal (left) or pre-wired types.  They are also available with selected or selectable resistors.  Pre-wired types need to be connected to the cable from the control panel (or to another detector) using a junction box.  The contact shown left has 5 terminals; the two chrome terminals are the reed switch terminals, the other three brass terminals are not connected to anything in the detector and are used to loop the detector to other detectors, or in the case of a single contact to simply secure the twisted-together tamper pair.

    Normally Closed Contacts (NC)

    Magnetic contacts used in alarm systems are usually NC contacts where the reed switch opens when the magnet is moved away from it.  Alarm control panels are designed to detect a circuit that goes open circuit, i.e. there is no circuit through the alarm terminals on the control panel.  Contacts used in FSL installations are also NC contacts but in this case the control panel detects a different resistance value when the switch circuit is broken (and another resistance value when the tamper circuit is broken).

    Normally Open Contacts (NO)

    These contacts work in the opposite way to the NC contacts above, i.e. the circuit is made when the magnet is moved away from the switch.  They are not normally used in conventional alarm systems, but are typically used to switch a low voltage/current appliance on in conjunction with a relay to switch higher voltage/current appliances.

    Change-over Contacts

    These contacts are basically both NC and NO contacts in the same unit.  They have three terminals - Common, NC and NO.

    Graded Contacts

    Please read the article on alarm grades for information about alarm grades.

    Grade 2 and grade 3 contacts are available.  They operate in the same way as ungraded contacts but have a tamper switch and can also have pre-selected or selectable resistors for use in FSL (Fully Supervised Loop) installations.

    Wiring Types

    There are two main ways of connecting magnetic contacts (and other detectors) to alarm control panels - Closed Circuit Loop (CCL) and Fully Supervised Loop (FSL).  All control panels can have detectors connected as CCL.  Higher specification panels can also utilise FSL wiring.  Among the control panels that can utilise FSL wiring are listed below.

    • Texecom Veritas R8+, Excel, and all the Premier range of panels
    • Scantronic 9x5x series
    • Honeywell Galaxy G2
    • Menvier 'M' series

    The advantages of FSL wiring are that it is more secure in that it is difficult to simply short-out the detector, and the amount of cabling is potentially reduced - FSL needs only 2 cores for a non-powered detector and 4 cores for a powered detector.

    4-wire CCL (Closed Circuit Loop) Wiring - single contact

    ccl wiring

     The diagram above shows the wiring for a single simple contact.  The alarm terminals (shaded grey) may be in a different position to those shown, but they are easily identifiable by the chrome finish or the presence of the wires to the reed switch.  If using a single simple pre-wired contact there will be four cores from the contact - 2 for alarm and 2 for tamper.  The alarm pair are usually end-stripped of their insulation.  If you are in doubt it is easy to find the pairs using a continuity tester.

    4-wire CCL (Closed Circuit Loop) Wiring - multiple contacts

    ccl wiring

     The diagram above shows the wiring for two simple contacts connected to the same zone.  Note 1: the connections between the two contacts can be made from one contact to another or, if the cable from each detector is brought back to the panel, the connections can be made at the panel.  The important thing is that the alarm loop must be one continuous loop such that if either contact goes open circuit then the alarm will be triggered. Similarly the tamper loop must form one continuous loop. if the tamper loop is being connected to other tamper loops then these must form one larger single continous loop.

    2-wire FSL (Fully supervised Loop) Wiring

    ccl wiring

    The diagram above shows the FSL wiring for a single detector and for two detectors connected to the same zone.  No more than 3 detectors should be connected to the same zone when using FSL wiring.  The resistor values shown are for a Texecom Premier control panel and other panels may have different resistor values. Control panels which support FSL wiring are supplied with resistors of the correct values and those resistors can be physically installed at the detector.  However, magnetic contacts are available with the resistors already installed in the contact and there are also magnetic contacts with selectable resistors to suit any control panel. There is a third type of wiring, commonly known as Triple End of Line (TEOL) which is used with Passive Infra-Red (PIR) detectors.

    Physical Installation

    Magnetic contacts must always be fitted within the protected area i.e. on the inside of an external door and opposite the hinge edge.  When fitting magnetic contacts to external doors the contact should be fitted as near the top of the door as is practical in order to minimise corrosion caused by damp.  The magnet and switch parts should be fitted as close together as possible, but there is some leeway and they will usually work correctly with a gap of around 10mm.  If you are in doubt it is quite easy to test the operation using a continuity tester before finally fitting the contact.

    Surface Contacts

    Surface contacts are simply screwed in position such that the magnet and switch are in close proximity to each other. It is usually best to have the magnet and switch both in the same plane but one or both can usually be rotated if required. If in doubt check the contacts operation using a continuity tester before finally fitting the contact pair. With terminal contacts connect the cable to the terminals before fixing the cable. The cable can then be neatly clipped working back towards the panel or junction box.

    Flush Contacts

    Flush contacts need to have holes drilled into the door edge and frame. For terminal type contacts the hole in the frame needs to be deep enough to accomodate the terminal connections. When deciding where to fit the flush contact avoid nails and end-grain and also avoid positions where there is a large gap between the door and frame or where there is a lot of movement in the door.

     


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    2013-06-18 15:15:26