# PLC Wiring



## backstay (Feb 3, 2011)

Got a call out to a sewer treatment plant. Some trouble with a bad float. But in trouble shooting I find the hots and neutrals reversed in the control cabinet. They have a cord plug landed on the terminal strip and plugged into a power conditioner unit. I'm told the unit was replaced a year ago, but the way it's setup, you just plug in the cord and that is hard wired to the strip. So it been this way for a long time. So my question, how reliable is a backwards wired PLC? It's a little Micro Logics unit.


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## erics37 (May 7, 2009)

Backwards wired as in reverse polarity? I don't think it would care. If it references itself to ground it'd probably be through the chassis and not the neutral.


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## backstay (Feb 3, 2011)

erics37 said:


> Backwards wired as in reverse polarity? I don't think it would care. If it references itself to ground it'd probably be through the chassis and not the neutral.


 All the controls are hot neutral reversed. Makes troubleshooting a pain. Everything is an open neutral.


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## macmikeman (Jan 23, 2007)

It seems pretty odd to me that somebody with enough sophistication to program a plc would not be the kind of electrician to reverse the hots and neutrals on the inputs and outputs. Some hack must have been messing around in there as an afterwards ''gee I bet I can fix this'' fugundo. 

Usually around here the handyman trucks list all the types of work they claim to be able to do well such as window screens, toilet repairs, gutter cleaning and the like. So far I haven't seen '' program plc'' on the side of their trucks , but I guess it was only a matter of time...........


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## backstay (Feb 3, 2011)

macmikeman said:


> It seems pretty odd to me that somebody with enough sophistication to program a plc would not be the kind of electrician to reverse the hots and neutrals on the inputs and outputs. Some hack must have been messing around in there as an afterwards ''gee I bet I can fix this'' fugundo. Usually around here the handyman trucks list all the types of work they claim to be able to do well such as window screens, toilet repairs, gutter cleaning and the like. So far I haven't seen '' program plc'' on the side of their trucks , but I guess it was only a matter of time...........


It almost looks like it was original. As this wiring ages in the waste water plant, the broken corroded wiring will make troubleshooting impossible. I'm going to call the company that built the controls and see if they have a reason to reverse wire it. All these hots looking for a return is going to give someone a hell of a shock.


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## John Valdes (May 17, 2007)

If you mean the power for the power supply and its 120 volts it does not matter.
If you mean the inputs, there are no neutrals.
If you mean the outputs, you are only providing the circuit so it does not matter. What voltage are outputs? 

Control people do not recognize/treat neutrals like an electrician does. This is why many control panels are built with one color wire.
They don't care and really it does not matter.
Is the neutral grounded? At the transformer?

If you fixed the issue and it works, collect your check and forget about it.


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## backstay (Feb 3, 2011)

John Valdes said:


> If you mean the power for the power supply and its 120 volts it does not matter. If you mean the inputs, there are no neutrals. If you mean the outputs, you are only providing the circuit so it does not matter. What voltage are outputs? Control people do not recognize/treat neutrals like an electrician does. This is why many control panels are built with one color wire. They don't care and really it does not matter. Is the neutral grounded? At the transformer? If you fixed the issue and it works, collect your check and forget about it.


 This is 120 volt controls, the power for the controls is separate(isolated) from the incoming power and the neutral is grounded. I get voltage between hot and the cabinet, but not between neutral and the cabinet. Output and input cards are switching neutrals, not hots.


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## tates1882 (Sep 3, 2010)

the plc i just finished come from the equipment manufacture with all the switching/signaling done on the -24vdc (negative).


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## erics37 (May 7, 2009)

backstay said:


> This is 120 volt controls, the power for the controls is separate(isolated) from the incoming power and the neutral is grounded. I get voltage between hot and the cabinet, but not between neutral and the cabinet. Output and input cards are switching neutrals, not hots.


This is like a sink vs source kind of thing but with AC. If it works, it works - you just have to reverse polarity your brain to figure it out.


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## ScooterMcGavin (Jan 24, 2011)

backstay said:


> This is 120 volt controls, the power for the controls is separate(isolated) from the incoming power and the neutral is grounded. I get voltage between hot and the cabinet, but not between neutral and the cabinet. Output and input cards are switching neutrals, not hots.



AC conducts in both directions regardless of polarity. The PLC IO doesn't care how it's hooked up. Although if you hooked up DC to an AC output it would never shut off because the output needs the zero cross to stop conducting.


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## macmikeman (Jan 23, 2007)

scameron81 said:


> AC conducts in both directions regardless of polarity. The PLC IO doesn't care how it's hooked up. Although if you hooked up DC to an AC output it would never shut off because the output needs the zero cross to stop conducting.


Yes we know that ac works both directions, you could wire a buildings lights with the neutrals switched instead of the hot side, but that would not meet code either......... 

I am not about to go grabbing my code book to find the place where it says controls need to have the neutral hooked up to the return side of the circuit cause I did that code book reporting stuff for a full decade on the web, some of it here and got burned out by it. Let Dennis do it. Better yet, go to holt and get 55 guys on it. Hell, I can go do that for you now.


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## macmikeman (Jan 23, 2007)

So I went over there and somebody who looks just like BBQ whom I believe to be just about as expert on code as you are going to find anywhere pretty much agreed with me- and the part I wasn't thinking about- if you open the hot at the same time as the neutral then it is ok, otherwise no you cannot switch the neutral side of controls and meet National Electrical Code code.....


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## EBFD6 (Aug 17, 2008)

macmikeman said:


> So I went over there and somebody who looks just like BBQ whom I believe to be just about as expert on code as you are going to find anywhere pretty much agreed with me- and the part I wasn't thinking about- if you open the hot at the same time as the neutral then it is ok, otherwise no you cannot switch the neutral side of controls and meet National Electrical Code code.....


Every motor starter I've ever worked on breaks (switches) the neutral thru the overload relay.


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## backstay (Feb 3, 2011)

erics37 said:


> This is like a sink vs source kind of thing but with AC. If it works, it works - you just have to reverse polarity your brain to figure it out.


Exactly, my old brain doesn't like it. One issue I have, all the white wires are hot.


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## ScooterMcGavin (Jan 24, 2011)

macmikeman said:


> Yes we know that ac works both directions, you could wire a buildings lights with the neutrals switched instead of the hot side, but that would not meet code either.........
> 
> I am not about to go grabbing my code book to find the place where it says controls need to have the neutral hooked up to the return side of the circuit cause I did that code book reporting stuff for a full decade on the web, some of it here and got burned out by it. Let Dennis do it. Better yet, go to holt and get 55 guys on it. Hell, I can go do that for you now.


I wasn't arguing that it's right or ok, only that functionally the Plc doesn't care how you do it. That's the same reason I don't like sourcing inputs and sinking outputs, it's backwards from everything your taught as an Electrician and confuses the hell out of people who aren't expecting it.


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## Jlarson (Jun 28, 2009)

We see that a lot when someone used to doing sinking DC stuff wires up AC I/O, usually electronics techs.


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## KennyW (Aug 31, 2013)

Jlarson said:


> We see that a lot when someone used to doing sinking DC stuff wires up AC I/O, usually electronics techs.


Yes it seems weird in an industrial environment but overall for electronics people (people used to logic level dc inputs and outputs ) sourcing inputs and sinking outputs make more sense and are more common (admittedly they are DC but then again the United states is one of the only last holdouts where AC controls are common. Canada too but it's changing) Inputs are pulled up internally to positive voltage and "float" high, a "true" or "1" state happens when the input is switched to ground.

Have a look at an electrical schematic for a car for example. You'll see lots of sensors that switch to ground. 
Not common for AC or industrial environment s though for sure. You could argue it's against code but the only "load" You're switching is the PLC input itself so I think even most inspectors would tell it's weird but I doubt you be mandated to change it in most cases.


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## backstay (Feb 3, 2011)

KennyW said:


> Yes it seems weird in an industrial environment but overall for electronics people (people used to logic level dc inputs and outputs ) sourcing inputs and sinking outputs make more sense and are more common (admittedly they are DC but then again the United states is one of the only last holdouts where AC controls are common. Canada too but it's changing) Inputs are pulled up internally to positive voltage and "float" high, a "true" or "1" state happens when the input is switched to ground. Have a look at an electrical schematic for a car for example. You'll see lots of sensors that switch to ground. Not common for AC or industrial environment s though for sure. You could argue it's against code but the only "load" You're switching is the PLC input itself so I think even most inspectors would tell it's weird but I doubt you be mandated to change it in most cases.


Not code though.


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## robojoe (Mar 23, 2010)

It may not be code but how many industries care about code in control wiring. We see a lot of that in foreign machinery. You just have to look at the wiring and how it's wired before you start troubleshooting. An electrician and a controls electrician think totally different. For example 24v control wiring is color coded blue and 240v 3 phase is black red blue. They could easily be confused in a control panel if you don't look. Just my two cents.


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## macmikeman (Jan 23, 2007)

Let me rephrase what I posted earlier to '' Chapter three wiring'' . I don't think the nec cares or covers what way the low voltage dc is wired and like some here say it is done to change state from +1 to zero. When you are controlling pumps and warning lights and horns and the like however.......... not allowed to switch the neutral side.


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## backstay (Feb 3, 2011)

robojoe said:


> It may not be code but how many industries care about code in control wiring. We see a lot of that in foreign machinery. You just have to look at the wiring and how it's wired before you start troubleshooting. An electrician and a controls electrician think totally different. For example 24v control wiring is color coded blue and 240v 3 phase is black red blue. They could easily be confused in a control panel if you don't look. Just my two cents.


Just to bring this back to reality, the prints had the wiring to code, the panel was wired in Minneapolis Minnesota, not a foreign country, we're not talking about DC and someone sometime swapped the feed to the terminal strip so now it's hot neutral reverse. I troubleshot the thing just fine, it had a bad float. My question was "how reliable is the PLC going to be with the reversed wiring".


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## macmikeman (Jan 23, 2007)

backstay said:


> Just to bring this back to reality, the prints had the wiring to code, the panel was wired in Minneapolis Minnesota, not a foreign country, we're not talking about DC and someone sometime swapped the feed to the terminal strip so now it's hot neutral reverse. I troubleshot the thing just fine, it had a bad float. My question was "how reliable is the PLC going to be with the reversed wiring".


I think I can answer that- More reliable. Reason- you are not bringing a foreign voltage to their board if the neutral is connected to the bus. That step can avoid some damages that may occur to the board. Current will flow thru the contacts, but the voltage is going to be on the other side of the resistance until you bare hand touch it on the neutral side of the resistance and complete the circuit thru yourself and get electrocuted, which is why it is prohibited.


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## KennyW (Aug 31, 2013)

macmikeman said:


> Let me rephrase what I posted earlier to '' Chapter three wiring'' . I don't think the nec cares or covers what way the low voltage dc is wired and like some here say it is done to change state from +1 to zero. When you are controlling pumps and warning lights and horns and the like however.......... not allowed to switch the neutral side.


 But those inputs ARE just turning a 0 into a 1. The "load" they are switching is a plc input inside the panel, not a light fixture or a hot water heater. 

As was said, by your definition, basically every starter bucket ever produced is "not code" - the OL contact switches the neutral side of the contactor coil.

I will agree it's unconventional and if it were me I'd be looking into fixing it unless there was a reason it was do that way, but I wouldn't typically expect an industrial electrician to use the term "it's not code". Things like this are nonetheless done and there are bigger fish to fry.


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## Big John (May 23, 2010)

In both the NEC and NFPA 79 for industrial controls there are exceptions that allow putting OLs in the neutral.

The setup described in this thread doesn't sound technically correct but I can't prove it.


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## dronai (Apr 11, 2011)

Just finished a job, where all the controls we installed are 24V DC with negative switched. Confused me when wiring the icecube relay coils, and the unswitched side was 24V + Negative was labeled, and went to the I/O. Thought about this thread. The company I work for, always wires this way in DC.


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## RIVETER (Sep 26, 2009)

erics37 said:


> backwards wired as in reverse polarity? I don't think it would care. If it references itself to ground it'd probably be through the chassis and not the neutral.


Really !!!


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## JRaef (Mar 23, 2009)

dronai said:


> Just finished a job, where all the controls we installed are 24V DC with negative switched. Confused me when wiring the icecube relay coils, and the unswitched side was 24V + Negative was labeled, and went to the I/O. Thought about this thread. The company I work for, always wires this way in DC.


Probably learned PLC connections using or copying from Japanese systems where they use sourcing and sinking I/O in ways that appear opposite to the way it is generally done in the US and Europe (in the EU they call it "positive logic and negative logic" which in my opinion is even more confusing). There are lots of debates as the merits and detractors of both methods, and those debates will never end. But I always like to keep the end user in mind because they are the ones who will have to live with it in the long run.


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## DriveGuru (Jul 29, 2012)

I absolutely hate controls wired like that, if I have a wire to a switch or prox ground out, I want it to blow a fuse, not give the PLC a false input. Just sayin


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