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Home Alarm Switches – Part One Magnetic Contacts

Because it’s relatively difficult to break into a building through its walls, most intruders gain access through doors and windows.  For this obvious reason, how well you install sensors on windows and doors play a major role in the effectiveness of the alarm system you install.

Magnetic Surface Switches

Magnetic surface switches are the most fundamental contact switches used today.  A typical contact consists of a switch and a magnet that mount opposite one another on an opening.  The switch mounts on the stationary part and the magnet mounts on the movable part of a door or window (as a general rule but for wireless switches this can be untrue).

The switch itself consists of a set of mechanical contacts, a spring mechanism that holds the contacts apart, two or three connecting screw terminals, and a plastic case.  Early surface switches used spring-loaded mechanical contacts to make or break alarm circuits.  Today, miniature reed switches are used, because they are more reliable.  Reed switches also respond faster in the absence of magnetism, such as when doors and windows are opened.

Reed switch contacts are enclosed in small gas-filled glass vials.  This reduces the switch damage that can be caused by humidity, dirt and corrosive materials in the environment.  Also, early models incorporated contacts that often stuck together on seldom used doors.  When these doors finally were opened, the switch contacts would not open, preventing an alarm condition.  Manufacturers began plating contacts with various metals, until they found that rhodium did the best job of preventing contacts from sticking.

The use of reed contacts has enabled magnetic surface switches to become very small.  The small size enables alarm installers to place switches in very tight quarters.  It also makes it more difficult for intruders to see a switch and possible tamper with it.  Magnetic surface switches are equipped with either external terminal screws (two-terminal models support normally closed-circuit alarm systems: three terminal switches support either normally closed or normally open circuits) or attached and soldered lead-in wires.  Some manufacturers offer customized lead-in lengths.  This benefits companies that maintain a standard of placing all first-floor sensor splices in a basement.

Another reason to order longer lead-in wires is for easy switch replacement.  Standard lead-in lengths usually are no more than 6 to 12 inches.  With shorter lead-in lengths, installers must place splice connections inside wall cavities.  When a switch must be replaced, it can be difficult to pull these splices out of the wall.  Magnetic surface switches are suitable for both residential and commercial applications, but they are not always aesthetically pleasing.  They also can become a target for enterprising intruders.

Press-fit Switches

Press-fit switches mount inside door and window frames and jambs.  They are virtually hidden from view, making them more difficult to compromise and more aesthetically pleasing than surface mounted switches.  Like a surface mounted contact, a press-fit contact consists of a switch and a magnet.  The magnet is installed in the movable part of a door or window, such as the frame.  The switch is installed in the door or window jamb.  These switches are popular because they are small and easy to install.  They usually are available in 1/4 and 3/8 inch diameter and in 1/2 to 1 inch lengths.  They also are available in 3/4 and 1 inch diameters.

On doors, press-fit switches can be mounted either vertically in the top of the frame, or horizontally in the side of the frame.  They also can be mounted vertically at the bottom of a door.  Recessed and surface switch should almost always be installed on the side opposite the hinges.  In this position, even the slightest door opening will trigger an alarm.  When installed on the hinge side, a door can be almost fulled opened before an alarm occurs.

To install a press-fit switch in a door, begin by drilling a hole in the door jamb.  If connecting wires are to be pulled up from a basement, then the hole must be drilled approximately 1 inch from the bottom of the threshold.  If the wires are to be pulled from an attic, the hole can be drilled either vertically into the attic or horizontally toward the top of the door frame.  In the latter case, a second hole will need to be drilled from the outside of the switch hole, on an angle into the attic or basement.

Connect the switch with the circuit wires using a standard Western-Union splice, solder and tape.  The two splices (one on each wire) should be staggered so one does not interfere with the other.  Once the connections are soldered and taped, insert the switch into the hole until it is flush with the frame.  Install the magnet in the same manner but opposite the switch.  One type of recessed switch is tapered on one end and has small “ribs” that help keep it in place in a hole.  To install, simply press the switch and magnet into their respective holes.  If a hole is too tight do not hammer the switch into place.  YOU WILL DAMAGE IT!  Simple remove the switch and drill a slight larger hole.

Other press-fit switches are installed with an epoxy sealant.  Put a small amount of sealant around the switch or magnet before inserting it in the hole.  The epoxy will harden and hold the switch or magnet in position.  Magnets and switches must be positioned properly for optimum operation over a long period of time.  Magnets and switches should be installed end to end, never in a “T” configurations.  An improperly installed switch may work now, but eventually it can cause false alarms as the magnet begins to lose holding power.  Test your magnet, it will have a larger working gap one way compared to the other.

The normal gap between any magnetic contact switch (surface or press-fit) can be extended by using wide-gap technology.  Wider gaps are used to prevent false alarms from loose-fitting doors.  They also offer installers room for error in their installation of switches and magnets on poor-fitting doors and windows.  Wide-gap switches and magnets are constructed differently than conventional models.  Once or more reed switches, along with an equal number of “helper” magnets are installed inside common plastic housings.  The North and South magnetic poles of the helper magnets are arranged in a particular way.  In a wide-gap version, magnets of equal magnetic polarity are mounted opposite from a multiple reed switch.

Stacked reed contacts are magnetically polarity conscious.  If the wrong magnet comes in proximity with the switch, an alarm occurs.  The result is the same if wide-gap magnets are moved too far or close to a switch.  This makes it difficult for a thief to compromise them.  Heavy steel doors are excellent applications for wide-gap switches.  Normal gap magnetic switches are susceptible to the steel pulling magnetic flux away from the switches.  This can lead to false alarms when magnets lose their holding power.  False alarms also can occur when vibrations in the structure shake the already weakly held reed contacts.

Surface switches are more secure when the are polarized.  Polarization refers to the manner in which a magnet is applied to its switch.  By reversing the polarity of a magnet in relation to a corresponding switch, false alarms will occur.  Wide-gap switches also are called polarized, biased and high-security switches.  Manufacturers usually provide ways for installers to readily identify the polarity of wide-gap switches and magnets.  For example, the polarity of a recessed switch and magnet is sometimes discernible by a painted band on one end.  A switch and magnet are magnetically polarized when painted ends are installed in a particular way, according to the manufacturer’s instructions.

Magnetic surface switches and magnets also are manufactured with explosion-proof housings, which provide a very high degree of security for applications such as military facilities and nuclear plants.  The basic premise of these devices is to surround the polarity-conscious reed switches, helper magnets and corresponding magnet with a ferrous shield.  The switch mechanism remains in a closed position with the shield in place.  However if someone attempts to remove the ferrous shield to substitute the stacked magnets, the switch mechanism will detect this and trigger an alarm.

Stay tuned for part two cover mechanical switches