"Normally Open" and "Normally Closed"
Normally Open and Normally Closed refer to the operation of
switches and detectors as well as the terminals of the system they are connected
to. The most simplistic example is a single Normally Open type of switch
that is just as it sounds.......Normally Open! Normal refers to the state
is in under normal circumstances. You can think of it as your front door
which is normally closed for most people. Not to confuse you. If you
attach a Normally Open (referred to as "n.o.") switch to the door, it should
remain open until the door is open. When the door is opened,
the switch closes. So in effect it is in the opposite state of the door!
In this case the alarm system would be setup to detect current flow in order to
"trip" or turn on the alarm.
In the simple circuit above, the siren sounds whenever the switch is closed.
This represents a n.o. switch that closes when the door is opened.
Switches and sensors can be purchased with normally opened or normally closed contacts. Some come with n.o. AND n.c. connections! In this case, you can use the same switch for either type of operation. So how do you choose? That can depend on many factors. Some alarm panels (the brains) only support one or the other type of operation. Others support either type of operation.
In general normally closed operation is more secure. In this mode the switch is closed and the panel monitors the electrical flow. If the flow stops, it triggers the alarm. So why is this better? It makes it slightly harder to bypass by intruders! In the normally open circuit, one simply cuts any wire and the panel will never "see it" because the flow is stopped by the cut wire. In a nc circuit the alarm will sound as soon as the wire is cut because the panel is waiting for the flow to stop.
On the alarm panel you will typically see "no", "nc" and "c" connection terminals. "c" stands for "common". One zone typically consists of 3 terminals (no, nc, c). The common is used with either a nc or a no terminal (connection) to create a "loop". Of course you can use both no AND nc loops. In some cases there is only one "c" for the entire panel. This can be a bit confusing, but just remember that you never make a loop from a no to a nc connection. The loops always use the common connection.
We will discuss more advanced protection in the next pages.