# Normally open or closed



## The_Modifier (Oct 24, 2009)

zen said:


> If I have a door jam switch it, I think its normally open. When the door is open it becomes closed turning the light on. Correct? So if I want the dj switch to open the circuit I would ask for a normally closed dj switch. I often confuse myself with the state of something when its refered to. Any help would help.
> 
> Sent from my SPH-D710BST using electriciantalk.com mobile app


When you pull it out of the box that you purchased it in. That is the state that you consider it to be.

N.O.= doorbell switch (you have to push it closed)
N.C.= Door jamb switch (you have to push it open).


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## micromind (Aug 11, 2007)

It depends on the definition of 'normal'. 

In this case, it's very likely that normal would be door open, because the door closed would depress the spring in the switch.


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## zen (Jun 15, 2009)

So I need to ask for a normally open dj switch. Is this a common item

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## dronai (Apr 11, 2011)

You have the kind that goes behind the jam or a magnetic that looks like an alarm contact above the door ?


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## The_Modifier (Oct 24, 2009)

zen said:


> So I need to ask for a normally open dj switch. Is this a common item
> 
> Sent from my SPH-D710BST using electriciantalk.com mobile app


A Normally closed- it will open (break the circuit) if the door is closed (and the switch is mounted in the frame work.


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## zen (Jun 15, 2009)

Yes this is a x ray room and they are asking for a dj switch that opens the circuit if someone opens the door . It comes from their generator cabinet and will tell the x ray to stop shooting the x ray

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## sseivard (Apr 25, 2012)

Like the other guys said contacts are labeled for their shelf state. It might help for you to think of the limit switch as being normally closed held open.


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## sseivard (Apr 25, 2012)

Float switches are like this too. I wish i Had a dollar for every one I had seen on drawings backwards.


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## EBFD6 (Aug 17, 2008)

zen said:


> Yes this is a x ray room and they are asking for a dj switch that opens the circuit if someone opens the door . It comes from their generator cabinet and will tell the x ray to stop shooting the x ray
> 
> Sent from my SPH-D710BST using electriciantalk.com mobile app


You want N.O.


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## zen (Jun 15, 2009)

Thanks

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## bill39 (Sep 4, 2009)

sseivard said:


> Like the other guys said contacts are labeled for their shelf state. It might help for you to think of the limit switch as being normally closed held open.


The above is the traditional accepted definition but the contacts on most safety gate switches are shown in the state with the keys inserted into the switch. 

I guess they think the normal state is with the contacts actuated by the key. 

Seems like this just confuses the issue.


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## oliquir (Jan 13, 2011)

a lot of swith have both NO and NC contact


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## zen (Jun 15, 2009)

oliquir said:


> a lot of swith have both NO and NC contact


When I had used them for houses it seems they were only one way. Open the door circuit is closed light comes on. When we did a hotel it seems they could be either depending on which connector it was hooked to.

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## RIVETER (Sep 26, 2009)

zen said:


> If I have a door jam switch it, I think its normally open. When the door is open it becomes closed turning the light on. Correct? So if I want the dj switch to open the circuit I would ask for a normally closed dj switch. I often confuse myself with the state of something when its refered to. Any help would help.
> 
> Sent from my SPH-D710BST using electriciantalk.com mobile app


It sometimes is confusing. Normally closed would be, in my opinion, the state it would be in when the conditions in which you are using it are...NORMAL. You can have a switch which when in your hand has open contacts. When on the door jamb and the door is closed, it is a Normally Open, Held Closed application. There are a handful of ways to do it and some will depend on if you want "supervisory" capabilities.


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## A Little Short (Nov 11, 2010)

RIVETER said:


> It sometimes is confusing. Normally closed would be, in my opinion, the state it would be in when the conditions in which you are using it are...NORMAL. You can have a switch which when in your hand has open contacts. When on the door jamb and the door is closed, it is a Normally Open, Held Closed application. There are a handful of ways to do it and some will depend on if you want "supervisory" capabilities.


The switch, NO/NC is the state it is in when at rest. That is what they are referring to as "normally". It changes to "not normally" (not what it is officially called, just my definition) either electrically or manually changing it's state.
For instance a N/O contact on a relay is only open when it's at rest or not powered up. It closes (changes state) when power is applied.
Also, the switch in the OP is whatever state it is in with no action on the button, plunger, etc. When the button...etc is activated it changes state.


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## Big John (May 23, 2010)

oliquir said:


> a lot of swith have both NO and NC contact


 For ease of description, that's a "form C" contact. 

There are a bunch of other types, but NO, NC, and "Form C" are the most common switch configurations we seem to use.


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## zen (Jun 15, 2009)

Thanks Big John. I saved that to my phone

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## ScooterMcGavin (Jan 24, 2011)

A Little Short said:


> The switch, NO/NC is the state it is in when at rest. That is what they are referring to as "normally". It changes to "not normally" (not what it is officially called, just my definition) either electrically or manually changing it's state.
> For instance a N/O contact on a relay is only open when it's at rest or not powered up. It closes (changes state) when power is applied.
> Also, the switch in the OP is whatever state it is in with no action on the button, plunger, etc. When the button...etc is activated it changes state.


There are a couple of exceptions to the shelf state rule.

1) temperature switches: normal is considered below the setpoint so if its set for 50 deg and its 80 deg in the room the switch will be in the non normal state. 

2) vacuum switches: normal is considered a perfect vacuum or 0 PSIA. So on the shelf at atmospheric pressure it would be in the non normal state.


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## drsparky (Nov 13, 2008)

Schrodinger's cat says that it is both open and closed until you measure it.:nerd:


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## Tsmil (Jul 17, 2011)

Just out of curiosity, in your jurisdiction, who makes the call about what level of safety is required for the application?

The door jam switch in this application is not just turning on a closet light or a bathroom fan, it is actually being used for a safety interlock.


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## Big John (May 23, 2010)

Tsmil said:


> Just out of curiosity, in your jurisdiction, who makes the call about what level of safety is required for the application?
> 
> The door jam switch in this application is not just turning on a closet light or a bathroom fan, it is actually being used for a safety interlock.


 Good question. The couple of times I've done personnel safety interlock systems they were pretty complicated and often had a good bit of redundancy.

For example, in this situation, what's to prevent the system from automatically starting back up as soon as the door closes with the person in the room? The way I've done this in the past was to install a lockout that had to be reset from _outside _the room once the door had been opened.


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## zen (Jun 15, 2009)

When we met with the tech. He gave us some drawing he said were only a guide for different ways to install. I asked . Yes big john there was a redundant double door jam that may have had other connections. . He said often a dj that opens the ray circuit in the generator is all that is needed and that our inspector would have the verdict on to what extent . The way he explained it to me , they have simplified these x ray for anyone installs and that there is a interlock in the generator .

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## Tsmil (Jul 17, 2011)

So, are you going to install then hope the inspector agrees?


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## zen (Jun 15, 2009)

We have a good relationship with the inspector so we will call him and ask him what he is going to expect

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## derekwalls (Dec 18, 2012)

drsparky said:


> Schrodinger's cat says that it is both open and closed until you measure it.:nerd:


(not to interrupt this thread)
I got a kick out of the Schrodinger's cat reference from working on my Physics degree in college. I would drive myself MAD going in circles with trying to think out the outcomes of this "experiment".


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## LARMGUY (Aug 22, 2010)

zen said:


> When I had used them for houses it seems they were only one way. Open the door circuit is closed light comes on. When we did a hotel it seems they could be either depending on which connector it was hooked to.
> 
> Sent from my SPH-D710BST using electriciantalk.com mobile app


Just wait till you get into door strikes. 

FS = Fail Safe
NFS = Non Fail Safe

Then you have the manufacturers that use

FS = Fail Secure
NFS = Non Fail Secure


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## Tsmil (Jul 17, 2011)

derekwalls said:


> (not to interrupt this thread) I got a kick out of the Schrodinger's cat reference from working on my Physics degree in college. I would drive myself MAD going in circles with trying to think out the outcomes of this "experiment".


If it wasn't for "the Big Bang theory" I wouldn't know what Schrödinger's cat reference was all about. Thank you Sheldon.


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