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Old 06-26-2010, 09:27 PM   #1
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Default Normally Open Normally Closed ?

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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Old 06-26-2010, 09:34 PM   #2
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Quote:
Originally Posted by Lone Crapshooter View Post
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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Old 06-26-2010, 09:37 PM   #3
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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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Old 06-26-2010, 09:45 PM   #4
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Originally Posted by Wingnut View Post
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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Old 06-26-2010, 09:51 PM   #5
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Altronix relay terminals are N/O - N/C sitting on the shelf, before applying power.
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Old 06-26-2010, 09:53 PM   #6
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Unless noted, it's referring to it's shelf state.
Plain, simple and correct
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Old 06-26-2010, 10:11 PM   #7
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Quote:
Originally Posted by mcclary's electrical View Post
Unless noted, it's referring to it's shelf state.
That's right.
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Old 06-26-2010, 10:38 PM   #8
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Originally Posted by mcclary's electrical View Post
Unless noted, it's referring to it's shelf state.
And all in favor said aye!!! Motion Passed.
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Old 06-26-2010, 11:04 PM   #9
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NO, or NC is the static state with no external influence on it.
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Old 06-27-2010, 08:38 AM   #10
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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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Old 06-27-2010, 08:46 AM   #11
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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.
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Old 06-27-2010, 08:57 AM   #12
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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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Old 06-27-2010, 09:01 AM   #13
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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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Old 06-27-2010, 09:03 AM   #14
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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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Old 06-27-2010, 09:08 AM   #15
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Originally Posted by Bob Badger View Post
I will go with that as well.

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


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


or vice versa
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Old 06-27-2010, 09:54 AM   #16
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Originally Posted by mcclary's electrical View Post
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. Multiply that by a couple dozen relays, and it was almost as bad as not having any prints at all.

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Old 06-27-2010, 10:40 AM   #17
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In that instance, the prints should show NC, mechanically held open


or vice versa
Well derr.

But they often do not.
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Old 06-28-2010, 10:55 AM   #18
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Quote:
Originally Posted by Fredman View Post
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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Old 06-28-2010, 08:06 PM   #19
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Now that we've solved contacts, lets move on to 3-way valves.
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Old 06-29-2010, 05:32 AM   #20
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Originally Posted by miller_elex View Post
Now that we've solved contacts, lets move on to 3-way valves.


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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