What Do Normally Closed and Normally Open Mean?
Normally opened or closed refers to the state of a contact attached to an opening. When talking about circuits or zones on an alarm panel, "normal" indicates the normal state of the opening being protected. For doors and windows, this is usually closed. Fire devices are normally open.
For the most part, normally closed contacts are used on normally closed protection points. If for no other reason, because it's much less confusing! What this means is, if you have a magnetic contact, usually consisting of a reed switch inside a plastic housing, and a separate magnet in a matching plastic housing, connected to a door, when the door is closed, the reed switch within the contact is closed, and when the door is opened, or the magnet and the contact are separated, the reed switch within the contact is also open. Put another way, if you have a meter capable of displaying when it sees a closed circuit (short), or an open circuit, and you attach the meter to the wires or screw terminals for the contact, with the magnet next to the contact (the "normal" position) if you read an open, then the device is normally open, and if you read a short, then the device is normally closed.
Why do we care? The alarm panel does what we tell it to do, and different panels have different capabilities when it comes to wired zones. Older panels usually required an End of Line Resistor be installed on each wired zone. The resistor orientation (series or parallel) depended on whether the contact(s) on the circuit were Normally Open, or Normally Closed. So, in order to wire the zone correctly, you had to know the contact(s) normal state. Newer panels offer the option of using End of Line Resistors, or specifying Normally Open, or Normally Closed. There may also be other options, but we won't discuss those here. In order to properly program and wire each zone, we must know what the normal state of each contact is. For example, if we tell the panel to treat a zone as Normally Closed, we're telling the panel that when it sees a short on the zone, it is in the normal, non-faulted condition, and when it sees an open on the zone, it should show that zone faulted. If we program for Normally Closed, but wire Normally Open, the system will tell us the door is faulted when it's closed, and that it's in the normal condition when it's open.
When replacing an existing contact, you also need to know if the contact being replaced was normally open or closed, especially if more than one contact is wired into a zone. It is difficult, and in some cases impossible, to mix Normally Open and Closed contacts on a single zone. When buying contacts, look for Form C, SPDT (Single Pole, Double Throw) contacts if you require a normally open device. Honeywell contacts usually indicate Form C with a -2 after the part number. Form A, SPST (Single Pole, Single Throw) contacts are always normally closed. There are hundreds of different contacts to choose from in various sizes and configurations, from surface mount, to recessed mount. Many of these are offered by Alarm Grid.
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- Answered By
- Julia Ross