# Normally Open Normally Closed ?



## Lone Crapshooter (Nov 8, 2008)

How do you define Normally Open and Normally Closed? No brainier not really.
Had this debate with a coworker. This is the scenario you have a sensor switch in a vessel and this has a sensor that is driven be a electronics package. How the sensor detects level flow is not important what is important is the state of the contacts that operate when a condition is detected. 
Traditionally process control switches are hooked up to open a circuit when a condition is detected that way if a open in the wiring occurs the process is shutdown or a alarm is indicated (fail safe) .
The question is, is the normal state like it sits on the bench with no power applied or is it how it sits with power applied with a dry sensor or is there is no real standard and one should refer to the manufacturers instruction sheet on a switch by switch basis


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## McClary’s Electrical (Feb 21, 2009)

Lone Crapshooter said:


> How do you define Normally Open and Normally Closed? No brainier not really.
> Had this debate with a coworker. This is the scenario you have a sensor switch in a vessel and this has a sensor that is driven be a electronics package. How the sensor detects level flow is not important what is important is the state of the contacts that operate when a condition is detected.
> Traditionally process control switches are hooked up to open a circuit when a condition is detected that way if a open in the wiring occurs the process is shutdown or a alarm is indicated (fail safe) .
> The question is, is the normal state like it sits on the bench with no power applied or is it how it sits with power applied with a dry sensor or is there is no real standard and one should refer to the manufacturers instruction sheet on a switch by switch basis


 

Unless noted, it's referring to it's shelf state.


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## Wingnut (Jan 31, 2010)

unenergized-not pushed-just sitting there is its designation
if its normally closed its closed 
Intel you operate it then it opens.


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## McClary’s Electrical (Feb 21, 2009)

Wingnut said:


> unenergized-not pushed-just sitting there is its designation
> if its normally closed its closed
> Intel you operate it then it opens.


 
until not intel


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## B4T (Feb 10, 2009)

Altronix relay terminals are N/O - N/C sitting on the shelf, before applying power.


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## Ray Cyr (Nov 21, 2007)

mcclary's electrical said:


> Unless noted, it's referring to it's shelf state.


Plain, simple and correct :thumbup:


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## mattsilkwood (Sep 21, 2008)

mcclary's electrical said:


> Unless noted, it's referring to it's shelf state.


That's right.


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## yucan2 (Jun 9, 2009)

mcclary's electrical said:


> Unless noted, it's referring to it's shelf state.


And all in favor said aye!!! Motion Passed. :thumbup:


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## RIVETER (Sep 26, 2009)

NO, or NC is the static state with no external influence on it.


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## Fredman (Dec 2, 2008)

RIVETER said:


> NO, or NC is the static state with no external influence on it.


Unless you were an alarm/security guy in another life. 

The typical type of door/window contact used is a normally closed magnetic contact. The magnet holds the reed relay closed (i.e door shut) in what they consider its normal state.


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## Bob Badger (Apr 19, 2009)

mcclary's electrical said:


> Unless noted, it's referring to it's shelf state.


I will go with that as well.

That said sometimes when installed in equipment it's normal state has been reversed. :laughing:


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## sirius0 (Jun 13, 2010)

Yes agreed. But consider a switch with a mechanical actuator or similar situation. The contact block may be NO but the unactivated status might be closed. An old convention for this is NO, HC (normally open, held closed. Or NC HO, NO,HO, etc ).
You might have a vessel with a float switch. The contact block is one thing but if you have set up how it works mechanically then it is up to you to document it's operation properly.


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## Mike in Canada (Jun 27, 2010)

When push comes to shove, if it's a switch that I haven't worked with before then I check it with my meter when I install it.

Mike


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## drsparky (Nov 13, 2008)

I worked with someone who's mouth is NO instead of NC.







Had it put in ears plugs for the day.


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## McClary’s Electrical (Feb 21, 2009)

Bob Badger said:


> I will go with that as well.
> 
> That said sometimes when installed in equipment it's normal state has been reversed. :laughing:


 

In that instance, the prints should show NC, mechanically held open


or vice versa


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## Big John (May 23, 2010)

mcclary's electrical said:


> In that instance, the prints should show NC, mechanically held open or vice versa


God do I wish that was always the case. I've come across schematics where they had NC (normally de-energized) and NC (normally energized) on the same bloody print, but they didn't tell you which was which. :cursing: Multiply that by a couple dozen relays, and it was almost as bad as not having any prints at all.

-John


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## Bob Badger (Apr 19, 2009)

mcclary's electrical said:


> In that instance, the prints should show NC, mechanically held open
> 
> 
> or vice versa


Well derr. :laughing:

But they often do not.


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## RIVETER (Sep 26, 2009)

Fredman said:


> Unless you were an alarm/security guy in another life.
> 
> The typical type of door/window contact used is a normally closed magnetic contact. The magnet holds the reed relay closed (i.e door shut) in what they consider its normal state.


That would be it's "supervised " state. Its static state is as McClary said..."shelf" state.


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## miller_elex (Jan 25, 2008)

Now that we've solved contacts, lets move on to 3-way valves.:laughing:


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## McClary’s Electrical (Feb 21, 2009)

miller_elex said:


> Now that we've solved contacts, lets move on to 3-way valves.:laughing:


 

A single solenoid valve returns to it's detent position with the removal of control power.

A double solenoid valve needs power to shift either way


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## Fredman (Dec 2, 2008)

RIVETER said:


> That would be it's "supervised " state. Its static state is as McClary said..."shelf" state.


 Supervised ? :blink::no: 

That is an alarm term but essentially has nothing to do with N/O or N/C. Either one of those circuits can be a supervised circuit.
Go out to Ademco and buy some contacts. Ask them. Look at the package. Google them. They are called N/C contacts. They use a magnet to hold the contact in it's normal state.


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## RIVETER (Sep 26, 2009)

Fredman said:


> Supervised ? :blink::no:
> 
> That is an alarm term but essentially has nothing to do with N/O or N/C. Either one of those circuits can be a supervised circuit.
> Go out to Ademco and buy some contacts. Ask them. Look at the package. Google them. They are called N/C contacts. They use a magnet to hold the contact in it's normal state.


When the system is armed and monitored by the system it IS supervised . If it is laying on the desk, unaffected by anything...that is it's static state.:thumbsup:


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## Bob Badger (Apr 19, 2009)

Fredman said:


> Go out to Ademco and buy some contacts. Ask them. Look at the package. Google them. They are called N/C contacts. They use a magnet to hold the contact in it's normal state.


Yeah all that proves is alarm installers and suppliers are too stupid to know what is normal. :laughing:











(FWIW, we service security systems:jester


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## Fredman (Dec 2, 2008)

Sure,, thanks Bob. 

I knew I should have resisted the urge to respond to this thread.


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