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Discussion Starter · #1 ·
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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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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Discussion Starter · #5 ·
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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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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Discussion Starter · #7 · (Edited)
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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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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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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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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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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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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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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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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Discussion Starter · #18 ·
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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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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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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