Contacts and Switches
Contacts typically refer to the different types of switches connected to an alarm system that detect opening of doors and windows. Contacts probably originated from the old term "contactor" that refers to the action of a switch when it is "turned on". In essence, two pieces of metal make contact and "complete the circuit". Contacts and switches act like valves in plumbing. When turned on, they allow electrical current to flow. Alarm systems are designed to detect this flow in or to take some action like sound the siren or notifying the police. Alarm systems are also designed to detect a lack of flow and take action based on the interruption of flow. This is discussed in more detail in the N.O./N.C. section.
Switches come in many flavors. The most basic of which is
the mechanical switch which opens or closes based on physical contact.
These are rarely used in modern systems due to their higher failure rate.
You may still see door jamb switches used in some systems. It consists of
a small plunger or ball that opens or closes when pressure is applied.
The jamb switch above would typically be mounted in a door frame.
The more common reed switch is used in most "hard wired"
systems. Hard wired refers to the use of wire to connect to the switches
and sensors in a system. The other option is wireless discussed in that
section.
The reed switch is based the reaction of certain metals when placed within a
magnetic field. In essence the metals connect when in the magnetic field
and the conneting action is used to close a contact! So a reed switch has
two parts, the "switch" with the magnetically sensitive metal contacts, and a
magnet.
The 3 reed switches above are designed for different applications. All 3
have a switch and magnet component. The magnet is mounted to the moving
component (door, window) and the switch is connected to the frame. In many
cases the switch can be completely hidden from view because these switches can
be small and painted over after installation. The critical consideration
for the installation of a reed switch is the magnetic gap. You must be
sure that the two components are installed in very close proximity AND in the
correct orientation! Some are designed to be installed "side by side" as
in the 3rd above and others are designed to be installed "on end" as in the 2nd
example. A little experimentation with a continuity tester will usually
suffice. Also make sure that you allow for movement of the components due
to moisture, humidity, etc. Always be sure that they are installed WELL
within the range of operation of the magnet. I have seen installers who
purposely install them at "the edge" of their operational gap only to find
intermittent false alarms down the road. The though was that the edge of
operation would be more sensitive to and attempted violation.
Wiring a single zone typically requires 2 wires (conductors) and their are some rolls of cable that are designed specifically for alarm systems. Many installers are now using category 5 cable for alarm wiring. "Cat 5" cable is used for many other purposes and allows some standardization. Since cat 5 has 8 conductors, it will more than suffice for alarm installation and may be used for other purposes in the future.