Define "Normally Open" vs. "Normally Closed"

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wrote:

Actually, even with fire alarms you can have series string (normally closed) or parallel string(normally open) circuits.
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On Thu, 29 Apr 2010 22:12:35 -0400, snipped-for-privacy@snyder.on.ca wrote:

Fire alarm relays are "normal" in the energized state so they fail safe.
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Or you can look at the switch you pulled out and wire the new one the same way.
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As an electrician, I've been through this numerous times over the years. It clearly depend on who you are talking to, and what trade they're in. IMO, the "normal" position is the state the switch is in when nothing affects it. Alarm people have the opposite take on it. My solution is to always buy devices that come in the "form C" which is both open and closed circuits, this way you can't go wrong
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Yeah, then you can put one wire on the "common" and flip a coin. :)
Wire it NO and if the light goes off when it should come on then wire it NC.
My best guess is that you should wire it NO and the magnet will close the switch.
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wrote:

That's fine if he wants to know when the door is CLOSED - Generally people set it up that they know when it is OPEN. (or NOT closed)
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On Thu, 29 Apr 2010 22:19:30 -0400, snipped-for-privacy@snyder.on.ca wrote:

No. I think you and I disagree whether the switch is "normal" with the magnet near or far.
I don't know. I am guessing.
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wrote:

Well, I'm not guessing at what "normally open" or "Normally closed" mean on rees switches or momentary contact switches of any type., so it's possible we do dissagree. A "normally closed" switch conducts electricity in it'e un-activated state. A reed switch proximity switch is unactivated when there is no magnet close.
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On Thu, 29 Apr 2010 22:19:30 -0400, snipped-for-privacy@snyder.on.ca wrote:

Oh yeah...........never mind :)
Reminds me of when we installed some lights in an underpass. The emergency lights burned all the time and the main lights came on in the daytime. :)
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RBM wrote:

I agree.
In all the following the contacts are closed with the magnet next to the reed switch.
From an alarm perspective *NC* ("normal" is when the door is closed):
http://en.wikipedia.org/wiki/Burglar_alarm_control_panel "Most switching devices are N.C. (normally closed) circuits, so when the device is not in an alarm condition, the circuit is closed."
http://www.alibaba.com/product-gs/218836027/Magnetic_Door_Alarm_Switch_5C_31A.html "Output signal: normally closed (switch contact is closed when the door closed)"
From a component manufacturer *NO*
C&K: "ELECTRICAL CIRCUIT: SPST NO (Contact Form A). Reed switch opens when magnet is removed from proximity. Contacts are held closed when magnet is within actuation range."
To know what the switch does you need to have a description like all of the above.

For the magnetic switches I looked at from alarm companies, none said NO or NC. A few said NC loop. They were probably all "NC" from an alarm perspective. Except a few were Form C.
--
bud--

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wrote:

Not quite right. An alarm SYSTEM being normally closed requires normally open SWITCHES. This is a series string system, where when one - any one - switch opens, the "normally closed" loop is open and the alarm sounds. The SWITCHES are normally open, and closed when the magnet activates the switch.
A Normally OPEN system is a parallel string circuit - where the normally CLOSED switches are activated when the doores are closed and the magnet is close - and ANY ONE switch being CLOSED activates the system.
The techs (if they know what they are doing and talking about) KNOW this.

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On Apr 30, 9:02 pm, snipped-for-privacy@snyder.on.ca wrote:

I think Bud and RBM who are both electricians have this one right. There is no consistency on this. At the very least, I can show you an alarm switch I quickly found that clearly calls switches that are CLOSED when the magnet is next to them NORMALLY CLOSED SWITCHES. You can pull up the PDF "manual" as well and it is consistent with that terminology. It didn't take long to find this, it was the first place I looked after googling alarm switch.
I think Buds statement that you have to carefully read the full description to figure it out is spot on. And even then, in some descriptions you may not be able to tell because they really don't say. In this one, it is very clear:
http://www.smarthome.com/7113-10/Surface-Mount-Magnetic-Contact-Switch-Normally-Closed-10-Pack/p.aspx
"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack
Essential Info The most basic sensors of any alarm system are the door and window Surface Mount Magnetic Contact Switches. There are 10 pairs of switches included in this kit allowing you to monitor multiple areas around your home. This type of contact switch is for normally closed circuits; this means that the when the magnets are lined up, the switch is closed."
Note that they call the circuit normally closed and also call the switch for that circuit normally closed.
It all gets back to what your definition of "normal" is. If it means the condition with no outside signal or magnet on it, which is reasonable and how we approach relays then I agree with your interpretation. But I also see how what RBM said is true. That in the alarm industry, which BTW is an obvious big user of these devices, they call a switch "normal" if the window is shut and the magnet is next to it. It becomes not-normal when the window is moved away and it trips the alarm.
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On Sat, 1 May 2010 06:31:46 -0700 (PDT), snipped-for-privacy@optonline.net wrote:

No, they do not call the switch a "normally closed" switch. They say when the magnets are lined up they are normally closed. The definition of a "normally open" switch meets that spec.
You DO have to read carefully what they are saying - and you have to understand what is really going on.

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On May 1, 10:53 am, snipped-for-privacy@snyder.on.ca wrote:

Yeah YOU sure do need to read carefully what they are saying. And you obviously didn't. Because if your read the description in the link I provided, they CLEARLY call that switch normally closed. It's right in the freaking one line TITLE:
"Surface Mount Magnetic Contact Switch, Normally Closed (10-Pack)"
Here's another link to a switch supplier that discusses the issue and says this:
http://www.iheartswitch.com/blog/choosing-right-magnetic-switch-your-project
"For unknown reasons, there are some companies classifying their magnetic switches against the grain. Their NC switches turn things OFF when the two halves are apart, while NO does the opposite. This is why even some salespersons are confused as well. Therefore, before purchasing any magnetic switches, read the product descriptions carefully."
The OP can make up his own mind. Listen to two electricians and a switch company that says confusion does exist and the only way to know is to carefully read the description, if it's even contained in the description. Or you, who says the terms are only used one way. Clearly in the case I just showed you, if you went by the one line title of the switch, you'd be getting the opposite of what you expect. And in most cases, that one line is about all they say about the switch, ie there is no further explanation.

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On Sun, 2 May 2010 06:34:49 -0700 (PDT), snipped-for-privacy@optonline.net wrote:

A doorbell button is a clear and easy to understand example of a normally open switch.
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wrote:

Indeed, it must be.... Assuming you didn't get some behind-the-scenes tutoring.
--
EA

>



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On Sun, 02 May 2010 09:56:37 -0400, snipped-for-privacy@dog.com wrote:

From the site referenced :
Surface Mounted Normally Open Magnetic Reed Switch Philmore # 30-17050
Use this switch if you'd like to turn something OFF when the two halves of your switch are apart. Commonly used with security systems.
Specifications
Action: Closed Loop / Normally Open (when magnet is beyond the actuations range) Maximum Gap (actuation range): 1-1/4" Max Mounting Hole Centers: 2-3/32" Terminal Type: Screw Type: SPST Current Rating: 0.5A @ 100VDC
There is NOTHING contrary in this description. With the magnet beyond actuation range, the NO switch is turned off - exactly as it should be.
Radio Shack's is normally closed when contacts are apart (poor description - but basically when magnet is beyond actuation range)
Digi-Key gives no description other than "normally closed"

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On Sun, 02 May 2010 14:20:13 -0400, snipped-for-privacy@snyder.on.ca wrote:

Here is my post that you just responded to.
I think you meant to respond to someone else, as your response made no sense as a response to what I said.
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You use a reed switch, with TWO magnets. When the door is down, place the two magnets so they oppose each other, the reed switch will stay open if you position the magnets correctly. one magnet on the door, one on the frame next to the reed relay/switch. When the door goes up, the one magnet will move away from the reed relay/switch and the remaining magnet will close the switch and turn on your alarm/light bulb/whatever. This is high-school 9th grade stuff, not rocket science!!!!
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hr(bob) snipped-for-privacy@att.net wrote:

If I recall, the first thing they taught back in 9th grade was to answer the question that was asked, not tell the inquirer how to build a convoluted Rube Goldberg device ;-)
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