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Old January 31st, 2018, 04:36 AM   #1
joshi14
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A little help regarding a pressure switch!!!

i am a beginner plc programmer and have some confusion, a little help would be appreciated . why do we use a normally close contact when we are dealing with low pressure switch and use normally open contact when dealing with high pressure switch? i saw a programmer doing this while writing his program in ladder diagram and i got confused. should not it be that both the switches are normally close in both low and high and opens if the pressure drops down a certain level or increases above certain level?

thanks in advance
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Old January 31st, 2018, 10:24 PM   #2
OkiePC
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Don't think of the contacts in the program as normally open and normally closed. Use the terms "examine if 0 (open)" and "examine if 1 (closed)" The contents of the addresses being a 1 or 0 is what the ladder logic instructions are checking. Using the terms normally open and normally closed for ladder logic instructions can lead to confusion even though Automation Direct actually names their instructions this way...

Now with that out of the way, the switch itself could be normally open or normally closed. If you make the decision about which contacts to select for use in your system, think about what you want to happen if the wire from the sensor breaks. Do you want the PLC to "think" that a pressure problem has occurred or do you want it to "think" that everything is okay?

In many cases both sensors act exactly the same way and are wired the same way but the low pressure switch closes under normal operating conditions and the high pressure switch closes under a high pressure condition. In most cases, in ladder logic, you are looking for a "0" (off) input from the low pressure switch to indicate a problem and you are looking for a "1" from a high pressure sensor to indicate a problem.

Clear as mud?

All newbies should watch these videos in order:

https://www.youtube.com/watch?v=T3tn...PxGunBTtPcS3Ez

It's based on Allen Bradley PLCs but the concepts are pretty much universal.
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Old January 31st, 2018, 10:40 PM   #3
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Quote:
Originally Posted by joshi14 View Post
i am a beginner plc programmer and have some confusion, a little help would be appreciated . why do we use a normally close contact when we are dealing with low pressure switch and use normally open contact when dealing with high pressure switch? i saw a programmer doing this while writing his program in ladder diagram and i got confused. should not it be that both the switches are normally close in both low and high and opens if the pressure drops down a certain level or increases above certain level?

thanks in advance
Ideally, all switches should be NC, and open in a fault condition, but we don't live in a perfect world. Sometimes, you just have to deal with what you have.
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Old February 1st, 2018, 10:47 AM   #4
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Quote:
Originally Posted by Bit_Bucket_07 View Post
Ideally, all switches should be NC, and open in a fault condition, but we don't live in a perfect world. Sometimes, you just have to deal with what you have.
Not always. For example in cases where you have to know position of certain part before other parts can move. Sorry my English but I believe you know what I mean. Of course in most of cases nc is safest but like I said, not always
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Old February 1st, 2018, 12:33 PM   #5
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I would say the correct way for high and low pressure will be low wired to NO this will send power to the input in a normal condition. If a wire is broken, you have a fault. In the case of the high pressure side it should be wired NC. This will give you a high input in normal operation. If you have a high pressure issue or a bad wire, you get a fault. If High was wired NO and you have a wiring problem, the PLC will never see the High presser Issue. Wired NO & NC, Safe on both counts. We need to think about not only the normal state but also the possibility of a wiring problem. You want to fail in a safe state.

Last edited by Racer5039; February 1st, 2018 at 12:36 PM.
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Old February 1st, 2018, 05:28 PM   #6
Geoff White
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Normal is a confusing term.

A pushbutton is normal when unpushed
A relay is normal when de-energised
A safety device is normal when un-interrupted

Pressure limit switches both have the same normal state but the pressure is normally high in one and low in the other.
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Old February 1st, 2018, 06:46 PM   #7
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Quote:
Originally Posted by joshi14 View Post
i am a beginner plc programmer and have some confusion, a little help would be appreciated . why do we use a normally close contact when we are dealing with low pressure switch and use normally open contact when dealing with high pressure switch? i saw a programmer doing this while writing his program in ladder diagram and i got confused. should not it be that both the switches are normally close in both low and high and opens if the pressure drops down a certain level or increases above certain level?

thanks in advance
Okay...it appears we are using NO contacts on both PRESSURE SWITCHES...

I think interpreting this may depend on if the "contact" you refer to is in plc or switch itself.

For example.... if you have 50 PSIG operating pressure and the low is adjusted at 40 PSIG and the high is adjusted at 60 PSIG.

In reference to 0 PSIG (Atmospheric pressure) the low would be energized because you are above 40 PSIG. And the high is not energized till you get to 60 PSIG.
So...In the PLC LOGIC....
The Low PS would be seen as closed (NC) for okay, because you are above Low Press limit. And switch will open below setpoint for alarm.
Thee high PS is seen as NO for okay, because you are below the high pressure limit. And the switch will close above setpoint for alarm.
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Last edited by NetNathan; February 1st, 2018 at 06:48 PM.
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Old February 1st, 2018, 07:57 PM   #8
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Quote:
Originally Posted by Geoff White View Post
Normal is a confusing term.

A pushbutton is normal when unpushed
A relay is normal when de-energised
A safety device is normal when un-interrupted

Pressure limit switches both have the same normal state but the pressure is normally high in one and low in the other.
I agree that "normal" can be a confusing bit of terminology sometimes.

For instance, I've seen normally open micro-switches operated by cams in conveyor e-stop enclosures that provide a closed contact when the switch operator is not tripped, or is in the "normal" condition.
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Old February 1st, 2018, 08:10 PM   #9
jaden
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With devices such as pressure or temperature switches, I prefer the terms such as Open on Rise, Close on Fall, etc. Takes out some of the confusion.
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Old February 1st, 2018, 08:12 PM   #10
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with devices such as pressure or temperature switches, i prefer the terms such as open on rise, close on fall, etc. Takes out some of the confusion.
+1
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Old February 2nd, 2018, 12:27 AM   #11
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With devices such as pressure or temperature switches, I prefer the terms such as Open on Rise, Close on Fall, etc. Takes out some of the confusion.
It seems even simpler to use...Open Above or Closed Below, etc.
My equipment is vacuum/pressure, so that is why I use these terms.
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Old February 2nd, 2018, 06:41 AM   #12
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Thanks a lot guys, i think i got it now.
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