# Weird Control Scheme Question



## HARRY304E (Sep 15, 2010)

*This is from the 2011 NEC but it is the same in 2008..*​ 
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404.2 Switch Connections.​

Click to expand...

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> *
> 
> (B) Grounded Conductors.
> Switches or circuit breakers​*
> ...





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> ​


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## IMM_Doctor (Mar 24, 2009)

*Wierd - depends on where you are from?*

Agreed, we U.S. electricians "switch" the ungrounded conductor, and the grounded conductor is allways unswitched. 

Well that all applies to wiring covered by NEC NFPA70.

I suspect that the engineer is European or Asian, where (NPN) or "Low Switching" is defacto standard.

The engineer may be just stuck in a rut.

Per your drawing, interestingly, the I/O voltage is going through "dry" (isolated / pontential free) contacts as shown. there is no need to do the "low true" switching.

BUT, I do NOT think there is any problem with this installation. I don't believe NEC applies to PLC input wiring.

I dont see any hazzards. The PLC input is a simple opto-coupler, and is polarity blind. The opto-copuler merely detects a voltage potential greater than x volts, and stimulates the isolated optical transistor to give the PLC a logical "1".

Being that the enginer drew it that way, he has a specific reason. Ask him.

Your drawing shows "MicroLogix" at the top of the 16-Pont Input Module.

"MicroLogix" is an Allen-Bradley trademarked name for 2 series of SLC Small Logic Controllers. It would either be MicroLogix 1500 (Bulletin 1769) or Micrologix 1100/1200/1400 (Bulletin 1762).

I checked the AB web-site the 1762 only has a 120vac 8-point input module.

The 1769 digital I/O has two 120vac input modules, but only one 16-point 120vac input module. (1769-IA16)

I read the instructions for the 1769-IA16 module and it merely shows L1 and L2 as source power for the I/O devices and does NOT specifically call out L1 as being switched and N being common. (N is not in the instructions) Just L1 and L2.

My opinion, the attached drawing is compiant with the manufacturers installation instructions.


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## Frank Mc (Nov 7, 2010)

IMM_Doctor said:


> Agreed, we U.S. electricians "switch" the ungrounded conductor, and the grounded conductor is allways unswitched.
> 
> Well that all applies to wiring covered by NEC NFPA70.
> 
> ...


I thought it was only the Japanese that used NPN logic...personally have,nt seen any European NPN stuff....My bet is the Engineer is an Electronics Engineer...;-)


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## BBQ (Nov 16, 2010)

HARRY304E said:


> This is from the 2011 NEC but it is the same in 2008..



I doubt the NEC applies to this machine, I believe it would be NFPA 79 that would be the standard to look at.


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## JRaef (Mar 23, 2009)

The problem with interpreting that NEC section to control circuits is that standard NEMA Overload Relay wiring flies right in the face of that. The NC aux. contact of the OLR is in fact on the neutral side of the coil on 120V systems.

Regardless of the NEC implications, the problem I have with doing that on a grounded neutral system is that if one of the wires between the Input and the relay contact were to go to ground, it would trigger the PLC input. The on-board diagnostics in the PLC would not be able to distinguish the difference between that condition and a true contact closure, whereas if you had a short on the hot side of the 120V input, the I/O card current would increase dramatically and/or the circuit fuse would blow and you would lose all of the inputs, either of which could be likely be flagged as an error in the PLC. That makes it a bad design right there if you ask me.


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## John Valdes (May 17, 2007)

Looks like the author likes to switch the common. Nothing a bottle of "White Out" can't fix.


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## oliquir (Jan 13, 2011)

NPN logic is used all over the world but just with DC system


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## mattsilkwood (Sep 21, 2008)

IMM_Doctor said:


> Agreed, we U.S. electricians "switch" the ungrounded conductor, and the grounded conductor is allways unswitched.
> 
> Well that all applies to wiring covered by NEC NFPA70.
> 
> ...


This isn't the actual drawing, it's just something I threw together to show what he was doing. I didn't want to post any actual drawings and get involved in any proprietary BS. 


JRaef said:


> The problem with interpreting that NEC section to control circuits is that standard NEMA Overload Relay wiring flies right in the face of that. The NC aux. contact of the OLR is in fact on the neutral side of the coil on 120V systems.
> 
> Regardless of the NEC implications, the problem I have with doing that on a grounded neutral system is that if one of the wires between the Input and the relay contact were to go to ground, it would trigger the PLC input. The on-board diagnostics in the PLC would not be able to distinguish the difference between that condition and a true contact closure, whereas if you had a short on the hot side of the 120V input, the I/O card current would increase dramatically and/or the circuit fuse would blow and you would lose all of the inputs, either of which could be likely be flagged as an error in the PLC. That makes it a bad design right there if you ask me.


Good point about a ground fault, I hadn't thought it through that far yet.:thumbsup:


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## mattsilkwood (Sep 21, 2008)

Finally got an answer. I'm told to wire everything as it should be, not like it is drawn. 
I haven't worked with this paticular engineer but he is with a pretty large national company so I'm assuming he has done this before. 
I found a few more mistakes today while I was going through it in detail. Do these guys ever proof read anything?


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## oldtimer (Jun 10, 2010)

electrolover said:


>




danger idiot at work !!


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## Jlarson (Jun 28, 2009)

Well that's quite a lame way to do inputs IMO too. 



mattsilkwood said:


> Do these guys ever proof read anything?


No. Ok a few do but most don't. 


Not like I'm perfect though, I know for a fact I've made my fair share of screw ups on drawings.


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## miller_elex (Jan 25, 2008)

I second-guessed an engineer's design once.... because I'd seen it done many times the other way. Well I did it my way.... and I ate humble pie afterwards.

RFI it.

EDIT: And I got layed off.


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## oliquir (Jan 13, 2011)

mattsilkwood said:


> Finally got an answer. I'm told to wire everything as it should be, not like it is drawn.
> I haven't worked with this paticular engineer but he is with a pretty large national company so I'm assuming he has done this before.
> I found a few more mistakes today while I was going through it in detail. Do these guys ever proof read anything?


this happen often when we make panels for engineer, there is a lot of errors on drawings that we correct when wiring and testing.


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## JRaef (Mar 23, 2009)

From his standpoint it may not be an "error". I had an Engineer working for me once who came from the Navy after spending 10 years on a Nuke Sub. They don't ground anything so for him, either side of a 120V circuit is the same as another. We had countless arguments over this. He was good and I didn't want to fire him over it, but he was stubborn on changing his ways in this regard because he felt it was more efficient for his engineering time to not worry about it. I finally had to give my shop guys lessons in how to spot his quirks and correct them. Pissed him off though...


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## John Valdes (May 17, 2007)

JRaef said:


> From his standpoint it may not be an "error". I had an Engineer working for me once who came from the Navy after spending 10 years on a Nuke Sub. They don't ground anything so for him, either side of a 120V circuit is the same as another. We had countless arguments over this. He was good and I didn't want to fire him over it, but he was stubborn on changing his ways in this regard because he felt it was more efficient for his engineering time to not worry about it. I finally had to give my shop guys lessons in how to spot his quirks and correct them. Pissed him off though...


Who said it was grounded. The engineer sees it as a 120 volt control loop. He calls it a common, not a neutral.
I agree with your Navy guy in control work. I never ever used the word neutral on any drawings I drew. I just gave it a number. Usually #2. And I always used the same color wire for everything in the panel except blue for DC.
The control xfmr may not be grounded in some control panels. So you have no neutral or grounded conductor. 

I would like to see the actual drawing. We are just taking this for granted.


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## oliquir (Jan 13, 2011)

It is written 120V(N) so it is the neutral and you cant have a floating neutral legally


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## mattsilkwood (Sep 21, 2008)

John Valdes said:


> Who said it was grounded. The engineer sees it as a 120 volt control loop. He calls it a common, not a neutral.
> I agree with your Navy guy in control work. I never ever used the word neutral on any drawings I drew. I just gave it a number. Usually #2. And I always used the same color wire for everything in the panel except blue for DC.
> The control xfmr may not be grounded in some control panels. So you have no neutral or grounded conductor.
> 
> I would like to see the actual drawing. We are just taking this for granted.


The transformer is bonded. In this case it actually is a mistake and one of many. I always find mistakes in drawings, wires misnumbered, something labeled wrong just simple stuff like that but these are pretty bad.


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## MDShunk (Jan 7, 2007)

oliquir said:


> It is written 120V(N) so it is the neutral and you cant have a floating neutral legally


I'm pretty sure you can if it's coming from a transformer less than 1000 va.


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## oliquir (Jan 13, 2011)

MDShunk said:


> I'm pretty sure you can if it's coming from a transformer less than 1000 va.


yes maybe but dont call it neutral, they should then call them L1 L2...


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## John Valdes (May 17, 2007)

oliquir said:


> yes maybe but dont call it neutral, they should then call them L1 L2...


This is the exact point I and the Navy guy are making. The common in the system is just the other wire that completes the circuit and nothing more.

IMO the word neutral should never be used as it is a CCC. It should always be called the grounded conductor.
In control systems the system itself must be bonded, but no conductor must be grounded. At least this is the way I see it?

Meaning the two 120 volt wires leaving the secondary of the control XFMR are the same potential if one is not grounded to the cabinet. And to take it one step further white or gray would not be required for this conductor.


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## Frank Mc (Nov 7, 2010)

John Valdes said:


> This is the exact point I and the Navy guy are making. The common in the system is just the other wire that completes the circuit and nothing more.
> 
> IMO the word neutral should never be used as it is a CCC. It should always be called the grounded conductor.
> In control systems the system itself must be bonded, but no conductor must be grounded. At least this is the way I see it?
> ...


John
As far as im aware the secondary of the transformer is grounded for safety reasons...If there was a short from primary to secondary ,the secondary voltage would still be present across the secondary terminals, but W.R.T earth would rise to the primary voltage....That,s my understanding of it...
HTH
Frank


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## John Valdes (May 17, 2007)

Frank Mc said:


> John
> As far as im aware the secondary of the transformer is grounded for safety reasons...If there was a short from primary to secondary ,the secondary voltage would still be present across the secondary terminals, but W.R.T earth would rise to the primary voltage....That,s my understanding of it...
> HTH
> Frank


I have seen multiple machines from ther US and overseas that have the secondary of the CONTROL XFMR not grounded. 
And the point being. What is the difference between the two wires that leave the secondary of a CONTROL transformer? Nothing. Unless you ground one of them.
There is no polarity and the potential is the same.

Now, I always ground one conductor leaving a CONTROL transformer, as that is how I was taught to do it. It makes troubleshooting easier as you can use use ground as the reference.


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## Frank Mc (Nov 7, 2010)

John Valdes said:


> I have seen multiple machines from ther US and overseas that have the secondary of the CONTROL XFMR not grounded.
> And the point being. What is the difference between the two wires that leave the secondary of a CONTROL transformer? Nothing. Unless you ground one of them.
> There is no polarity and the potential is the same.
> 
> Now, I always ground one conductor leaving a CONTROL transformer, as that is how I was taught to do it. It makes troubleshooting easier as you can use use ground as the reference.


I have also seen a few machines with the secondaries not grounded but you could count them on one hand......Grounding one leg sure helps when fault finding but as i mentioned in my last post i believe it is grounded for safety reasons........
Frank


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## Del (Feb 10, 2011)

*NPN versus PNP*

Its really a piece of poor design practice for 120v to be NPN switching now adays.

I had a project where ever piece of machinery in the factory came from the US(sorry guys), I spent six months there bringing the kit up to eu safety standards, also doing some project work there.

During that time we had to move a conveyor, where the control cct was 120v AC NPN through the proximity switches, some lucky devil trapped a neutral under a raceway cover shorting out the signal,and giving an input to the PLC, which held off from running the line.Because it was the neutral that was grounded we had a hell of time (on a friday PM) trying to track it down. Found it eventually, got the plant back up, and enjoyed the row from the missus for being home late(not).

Since that day, I have generally only seen NPN in rare circumstances. Normally involving electronics, like on Servo drives. Yes its still out there, but its reducing in quantity every year(being ripped out in upgrades)

It generally seems to be used where a electronics designer has moved across to the big bang juice, until someone has taken them aside and beaten some sense into them with a 2" flexible conduit.


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## Frank Mc (Nov 7, 2010)

Del said:


> Its really a piece of poor design practice for 120v to be NPN switching now adays.
> 
> I had a project where ever piece of machinery in the factory came from the US(sorry guys), I spent six months there bringing the kit up to eu safety standards, also doing some project work there.
> 
> ...


Hi Del

I would have thought 24vdc i/o would be the norm on your side of the pond...??...

Do you guys ground the 0v when using 24vdc i/o...??

I stated previously that i also believe an electronic,s designer does this npn switching stuff...All the old Microprocessors use active low signals ,so i believe they carry that method thru to plc stuff...
Frank


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## Mike in Canada (Jun 27, 2010)

Frank Mc said:


> Hi Del
> 
> I would have thought 24vdc i/o would be the norm on your side of the pond...??...


 24V is certainly getting more common, but you still see a LOT of 120V.


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## mattsilkwood (Sep 21, 2008)

Frank Mc said:


> Hi Del
> 
> I would have thought 24vdc i/o would be the norm on your side of the pond...??...
> 
> ...


One place I work for does ground all their 24v on the 0v side. They use it for a wire break signal on their stranding machines.


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## Del (Feb 10, 2011)

*24v*

I crossed over the pond from east to west, and then some.

I always used to ground out the Ov of 24Vdc, one time I didn't do it on a small machine panel I'd built, one of the relays was chattering for a few minutes, until I realised what I'd done, the link went in straight after that.

A floating common always seems to cause errors I've noticed.


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## oliquir (Jan 13, 2011)

sometimes i ground the 0v but some devices tells not to ground 0v to avoid some ground noises


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## Frank Mc (Nov 7, 2010)

mattsilkwood said:


> One place I work for does ground all their 24v on the 0v side. They use it for a wire break signal on their stranding machines.


It wouldnt by any chance be a wire braiding m/c for rubber hose??

Frank


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## Frank Mc (Nov 7, 2010)

oliquir said:


> sometimes i ground the 0v but some devices tells not to ground 0v to avoid some ground noises


Yeah i believe some dont ground when using with analog signals...

Frank


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## Frank Mc (Nov 7, 2010)

Del said:


> I crossed over the pond from east to west, and then some.
> 
> I always used to ground out the Ov of 24Vdc, one time I didn't do it on a small machine panel I'd built, one of the relays was chattering for a few minutes, until I realised what I'd done, the link went in straight after that.
> 
> A floating common always seems to cause errors I've noticed.


Wouldnt have thought a floating common would cause relays to chatter....

Frank


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## cmdr_suds (Jul 29, 2016)

Del said:


> *NPN versus PNP*
> 
> Its really a piece of poor design practice for 120v to be NPN switching now adays.
> 
> ...


NPN and PNP are transistor types are only used in DC circuits. If it is AC, the output will be an SCR or triac or maybe a FET. As far as why NPN sinking is common, in the early days of instrumentation, NPN output circuits(current sinking) were used because they were much simpler, uses fewer components and cost less. But they fly in the face of current source switching used in relay logic and the rest of the the industrial world. Now a days, most manufacturers can provide either one.


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