Inductance in adjacent wires

[SAF]

I'm a non-electrician involved in a project where inductance is creating control issues. I'm looking for informed thought or reaction to the situation, because it seems unexpected and not really understood by our supplier. The application is a 120 v residential sewage lift station, that involves a submersible pump; low level alarm float, a control float and a high level alarm float in a 1000 gal tank. All are 120 volt. The float and pump wires run together in conduit, underground, between a control panel with Logo plc and a junction box at the tank. Typical runs would be 15-30 feet. The low float circuit is normally energized. The middle float activates the pump when the circuit closes, shuts it off when it opens. The high alarm float is normally not energized. The PLC apparently looks for the control float circuit to drop below 40v to in turn shut the pump off. The apparent problem is that there is anywhere from 25 to 50 v induced in the pump control float wiring, presumed to be from the energized low float wire. It varies by installation, and also varies somewhat from one day to the next. There have been several situations where the PLC keeps the pump running to the point of tripping the low level alarm, apparently because it sees over the 40v threshold in the control float line. Suppliers are saying they have not had this issue at hundreds of other installed sites. Replacing the standard wires with shielded cable seems to lower the voltage readings a lot, and is the intended fix for all sites. Nobody can explain why we alone have the problem, or whether the size of the induced voltage is reasonable or not. I have no experience in this but I am really surprised that an insulated wire would show 40 volts just by running beside a 120v insulated wire. I would have a guessed a fraction of a volt would be seen.
What do more experienced people think about this? Have you seen this sort of thing before?

 

[ActionDave]

Approved as long as the discussion doesn't turn into DIY advice.

 

[ptonsparky]

We've seen the problem with controls but usually it was at several 100s of feet. This was prior to the advent of plc control, which would make it worse. The current required to hold a PLC input ON is much less than it would be for a motor starter coil.

The middle float would be augmented in the logic delaying the start of the pump at 'full' and/or the run time after it is 'empty. If not, the pump would cycle immediately at each open/close.

 

[ptonsparky]

We've seen the problem with controls but usually it was at several 100s of feet. This was prior to the advent of plc control, which would make it worse. The current required to hold a PLC input ON is much less than it would be for a motor starter coil.

The middle float would be augmented in the logic delaying the start of the pump at 'full' and/or the run time after it is 'empty. If not, the pump would cycle immediately at each open/close.

A neighboring village had the problem with a well control but only after it rained. Control wires were overhead. Again, older control.

 

[winnie]

Nobody can explain why we alone have the problem, or whether the size of the induced voltage is reasonable or not. I have no experience in this but I am really surprised that an insulated wire would show 40 volts just by running beside a 120v insulated wire. I would have a guessed a fraction of a volt would be seen.

What is your background, and is there any chance that the electrician involved in the project can join the discussion? We can't give you DIY advice, but I can answer the specific point above.

The issue of high voltage on an unconnected wire is known as 'phantom voltage' and it is seen is the instrument measuring the voltage has very high impedance. Even if the wire insulation were _perfect_ you get capacitive coupling between conductors, and this very slight capacitive coupling forms a voltage divider with the measuring instrument. If the measuring instrument is high impedance then the full voltage of adjacent wires can be seen on the unconnected wire.

I would expect that a proper PLC design would have input impedance selected to prevent this problem, but you should not be surprised by high voltages on unconnected wires. Electricians working with 480V systems and long conduit runs know that 'unconnected' wires can cause significant shocks because of capacitive coupling.

-Jon

 

[gar]

210401-1152 EDT

SAF:

You used the wrong word in your question. Inductance is not what you are concerned with.

Whoever designed the system does not know much about electrical circuits.

If you take two conductors and separate them by an insulator, then you have created a capacitor. The equivalent circuit for this capacitor may include some some inductance, and resistance, but both of these probably can be ignored for the problem you are describing.

I will not describe the troubleshooting procedure or possible solutions with you based on the forum rules. But you have a not uncommon problem that most electricians do not likely understand, or really provide a good solution. If the problem is what I think it is, then there are some very simple possible solutions.

.

 

[hbiss]

Nobody can explain why we alone have the problem, or whether the size of the induced voltage is reasonable or not. I have no experience in this but I am really surprised that an insulated wire would show 40 volts just by running beside a 120v insulated wire.

Actually you are correct. Someone isn't using the meter properly and isn't knowledgeable enough to know that the reading can't be right and why. I think you have some problem with the PLC or floats not working properly.

-Hal

 

[don_resqcapt19]

Actually you are correct. Someone isn't using the meter properly and isn't knowledgeable enough to know that the reading can't be right and why. I think you have some problem with the PLC or floats not working properly.

-Hal

For an open circuit conductor and a high impedance meter, 40 volts and more from that conductor to neutral or ground is very common. If the conductor is still connected to the PLC input, that is on the high side, but I have seen cases where the capacitively coupled voltage has prevented a PLC input from turning off where the field device was open and not sending voltage to the PLC.

I worked on projects where the engineering staff have added a load at the PLC input to the circuit neutral, to eliminate the effects capacitively coupled voltage. Those loads have been resistors, transformer type pilot lights and relay coils.

 

[ptonsparky]

For an open circuit conductor and a high impedance meter, 40 volts and more from that conductor to neutral or ground is very common. If the conductor is still connected to the PLC input, that is on the high side, but I have seen cases where the capacitively coupled voltage has prevented a PLC input from turning off where the field device was open and not sending voltage to the PLC.

I worked on projects where the engineering staff have added a load at the PLC input to the circuit neutral, to eliminate the effects capacitively coupled voltage. Those loads have been resistors, transformer type pilot lights and relay coils.

Exactly what we did in the few instances we had prior to PLCs. Load it up.

 

[SAF]

For an open circuit conductor and a high impedance meter, 40 volts and more from that conductor to neutral or ground is very common. If the conductor is still connected to the PLC input, that is on the high side, but I have seen cases where the capacitively coupled voltage has prevented a PLC input from turning off where the field device was open and not sending voltage to the PLC.

I worked on projects where the engineering staff have added a load at the PLC input to the circuit neutral, to eliminate the effects capacitively coupled voltage. Those loads have been resistors, transformer type pilot lights and relay coils.

Thanks for this. I think that confirms what we are seeing / thinking. The voltages are measured from the PLC input to neutral, and it was suggested at one point that a resistor referred to as snubber be added to address the problem. I understand a proper one couldn't be identified so now the plan is to replace the wire with shielded wire in all the installations. The thing that was causing me some hesitancy was that our electrical people didn't seem too certain of it, and the fact there are thousands of similar Siemens LOGO PLC controlled systems in place, apparently without having this difficulty. But from the comments, it seems that it is not unheard of and we are probably on the right track.

 

[MTW]

A separate conduit run for the motor conductors would likely alleviate this problem, without resorting to special cables or other methods of shunting the capacitive coupling voltage.

You didn't mention how long the troubled run is, but I suspect it's longer than the typical runs, that have no issues.

 

[Dsg319]

For an open circuit conductor and a high impedance meter, 40 volts and more from that conductor to neutral or ground is very common. If the conductor is still connected to the PLC input, that is on the high side, but I have seen cases where the capacitively coupled voltage has prevented a PLC input from turning off where the field device was open and not sending voltage to the PLC.

I worked on projects where the engineering staff have added a load at the PLC input to the circuit neutral, to eliminate the effects capacitively coupled voltage. Those loads have been resistors, transformer type pilot lights and relay coils.

Do shielded cables help at out on digital inputs with capacitive coupling issues? I can’t see how they would but figure I would ask.

 

[SAF]

There are several dozen installations in the project, and the runs vary from about 15 feet to probably a maximum of 35 feet - so not that long. The voltage readings on the two open float circuits range from about 25v to about 50v. We have confirmed that we can drastically reduce the apparent voltages using shielded wire on the floats, but it is quite a lot of rework to pull new wire through all of the installs. There is nothing unique or unusual about our application, and specially shielded wire isn't spec'd by the supplier, which has just made me wonder if we are missing something.

 

[oldsparky52]

Seems you need an electrical engineer or a more knowledgeable electrician. I will say that I don't think the wire replacement is the direction I would be looking at, especially since the manufacturer says they don't have issues elsewhere.

 

[SAF]

Seems you need an electrical engineer or a more knowledgeable electrician. I will say that I don't think the wire replacement is the direction I would be looking at, especially since the manufacturer says they don't have issues elsewhere.

Well, that is difficult. I am not the one to fix this problem, just the one to represent the owners and pose questions of our engineering consultant and the contractor. So we have access to an electrical engineer through our engineering firm, and have had two electrical contractor's working on this. It was actually an electrical engineer friend not involved with the project who got us on to the high phantom voltage being an issue for the PLC, and suggested a capacitor; an electrical contractor suggested and tried the shielded cable. But no-one can explain why this very simple off the shelf set-up and widely installed PLC panel is needing this to be done. So while I am not looking for DIY advice, I was thinking there might be something else I could suggest they look at before we go ahead with this rewiring. Can you say what other direction you would explore?

 

[ptonsparky]

The snubbers are cheaper than pulling wire.

 

[MTW]

How much difference shielding makes is highly dependent on how the shields are terminated. If terminated incorrectly it will not help much.

 

[Barbqranch]

It may simply be something as simple as how close the wires are to each other in the conduit. Or how sensitive the particular plc is (input impedance and trigger voltage)

 

[jim dungar]

As others have mentioned, you just need some additional load, you don't need to get deep into the engineering, try several inexpensive options, such as standard RC snubbers used for motor starter coils. The problem is with the PLC output, but the solution is probably best done externally.
While not common, this certainly is not an unusual situation, I dealt with it several times particularly when LED pilot lights first came out.

If the PLC manufacturer changed an internal component or even simply a vendor, a problem that never existed now does, but when they go back to the old stuff the problems disappears as fast it it appeared.

 

[hbiss]

There are several dozen installations in the project, and the runs vary from about 15 feet to probably a maximum of 35 feet - so not that long. The voltage readings on the two open float circuits range from about 25v to about 50v.

That's because they are using a regular high impedance multimeter. The reading is termed as phantom because it isn't real. If there were any kind of load across the end of the wires instead of nothing connected to them that voltage would be zero. You can touch the ends of the meter probes with your fingers and cause it to display a voltage that is really just 60 Hz hum.

So that brings us to the question of why it's causing problems with the PLC input. Looking at the instruction manual gives information about the parameters of the inputs- less than 40VAC is off and more than 79VAC is on. What is telling is 40 VAC @ .003Ma and 79VAC @ .08Ma. So this would certainly indicate a high impedance input.

Your answer to this is the same as what I described above about how to eliminate phantom readings with a high impedance multimeter. Load it down. Certainly your EE friend should be able to figure that out.

-Hal