Problem with induced low voltage across 120V line
Problem with induced low voltage across 120V line
(OP)
Do to a combination of problems in the field I have as much as 45 volts induced across a 120VAC control circuit. The power wiring (480VAC) is run in the same conduit along with several control loops and even water has been found in part of the underground conduit, but rather than correcting the field conduit problems at this time I need to suppress the induced voltage at the actuator. The induced voltage across the control circuit picks up relays intermittently and is pulsing an actuator motor.
Could someone help me with the best way to lower the induced voltage and where would it be most effective, at the field device connection or within the control building? Thanks for any help you can offer.
Could someone help me with the best way to lower the induced voltage and where would it be most effective, at the field device connection or within the control building? Thanks for any help you can offer.





RE: Problem with induced low voltage across 120V line
If this is a common-mode voltage to ground, I'd suspect that the main problem is not induced voltage, but rather a missing neutral-ground bond at the upstream separately-derived system. I'd definitely check that out first. If this is truely an induced voltage problem, a small 120 volt transformer at your load could be used to eliminate common-mode voltage, BUT YOU SHOULD REALIZE THAT THIS WOULD ONLY BE MASKING OTHER POTENTIALLY SERIOUS PROBLEMS.
If instead you're reading 165 volts, line-to-neutral, the only way that induced voltage could cause that situation is if you have not run all circuit conductors together in a common raceway system, which would be a violation of code and for which the easiest, cheapest, and fastest correction would be to properly rewire it.
Provide more installation details if you'd like a more specific answer.
You should keep in mind that "correcting the field conduit problems at this time" could likely be the easiest, cheapest, and fastest way to correct your problems, as well as the only legal and safe way.
RE: Problem with induced low voltage across 120V line
Again thank you for your help.
RE: Problem with induced low voltage across 120V line
RE: Problem with induced low voltage across 120V line
I still recommend you get the thing fixed right, though.
How long a wiring run are you talking about here?
RE: Problem with induced low voltage across 120V line
First, are the "control loops" installed complying to NEC Article 725? That could be the biggest problem.
There are three voltmeter readings necessary to troubleshoot the problem. Besides across the pair of wires, what is the voltage from each conductor to ground? How much do those readings change when paralleled with the 10KΩ resistor?
As for water in raceways—is the wiring suitable for the application at hand? There is almost no way to prevent moisture ingress in an outdoor underground raceway over the life of the installation. In most cases, NEC Chapter 3 requires wiring suitable for wet locations for jacketed or non-jacketed cables.
RE: Problem with induced low voltage across 120V line
Thanks again for your help.
actuator
RE: Problem with induced low voltage across 120V line
RE: Problem with induced low voltage across 120V line
Used this fix to solve a problem on Dumbo at Disneyland in Calif.... about 20 years ago. Dumbo's still running...
HTH
RE: Problem with induced low voltage across 120V line
RE: Problem with induced low voltage across 120V line
as an F Y I:
In actual test run at Reliance Electric circa 1970, twisted pair was compared with shielded cable, and with twisted shielded cable.
..... and the winner was: tight twisted pair....
The shield serves as an antenna.... and reduces the effectivity of the twisted pair (do you ground one end?.....if so which end?...do you ground both ends?....What is the consequence?...)
Certainly, reducing the strength of the magnetic flux field to which the wires are subjected, improves attenuation. However, the addition of the shield can exacerbate the situation by acting as an antennae and providing a closer coupling.
The straight twisted pair, when tightly twisted, usually provides sufficient cancellation of the induced fields.
RE: Problem with induced low voltage across 120V line
RE: Problem with induced low voltage across 120V line
While the wiring may meet code, the code does not address the issue of noise immunity. Mixing power wiring with 120v (and below) control wiring has always been an invitation to cause the type of interaction you stated in your original post. Usually, such issues are addressed by equipment OEM's on their installation drawings. Responsible OEM's will provide wiring do's and don'ts... in the form of notes on the installation drawings or wiring diagrams, etc.
Electricians that are good at bending conduit and pulling wires, and hooking them up are, for the most part, oblivious to issues such as EMI, EMC, RFI, etc.... and noise in general. If a straight line is defined by two points, and you give them the two points, they will go in a straight line. But don't expect them to be able to handle parabolas and hyperbolas. Their education, training, and experience, generally doesn't equip them for such tasks.
But they are exceptional within their limitations.
(Conversely, most graduate engineer are ill prepared to function as electricians).
When the drawings that are given to the electricians to use for the installation contain such notes, they can plan accordingly. i.e., if the drawings say that 120v control wiring should not be run with power wiring or should be run in separate ferrous metal conduits, electricians will heed these notes and plan the installation so as to comply.
These are my opinions, which have been formulated in over 40 years of experience in both domains.
For what its worth...........
RE: Problem with induced low voltage across 120V line
Jbartos: as an F Y I:
In actual test run at Reliance Electric circa 1970, twisted pair was compared with shielded cable, and with twisted shielded cable.
..... and the winner was: tight twisted pair....
///Very impressive.\\\
The shield serves as an antenna.... and reduces the effectivity of the twisted pair (do you ground one end?
///Recommended up to 1MHz.\\\
.....if so which end?
///Sending end, not receiving end.\\\
...do you ground both ends?
///Recommended above 1MHz.\\\
....What is the consequence?...)
///A reduction of noise in very low power signals.\\\
RE: Problem with induced low voltage across 120V line
Appreciate your comments.
We used to ground shields at the drive end. I think it had something to do with the impedance to ground of the shield. IF grounded at the source... then you had the cumulative impedance over the length of the shield that would (as the impedance became significant) support noise. Never did get to evaluate the hows and whys.... but on more than one occassion, a noise problem was solved by moving the ground end from the source to the terminating end (drive)...
Most of the electrical noise encountered over the years wasn't in the HF domain... Grounding at both ends of the shield introduced more noise than it cured. Something about the difference of ground potentials and the impedance in-between.
Have you observed that some Belden Shielded cables also have a drain wire....? Usually see that in cables with a foil shield as opposed to the braided wire shield.
Well, sir, I think we've beaten this one into the "ground"
(pun intended.... no apologies offerd)...
Cheers
RE: Problem with induced low voltage across 120V line
1. IEEE Std 142-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, (Green Book),
Page 213 Par. 5.7 Grounding of Shields. (Excerpts or Eng-Tips)
1. The overall shield is grounded whenever and wherever possible for low frequency application. (My Remark: This is an obvious intent since the shield replaces Conducive Metallic Conduits, e.g. RGS, EMT, IMC, etc., where there are no major shielding questions.) The individual shields are grounded according to the applied frequency: low frequency at one end and high frequency at multiple points. ......
Shields may be:
1. Braided copper wire
2. Metalized foil with a copper drain wire
3. Metallic conduit (if steel conduit it also serves as a magnetic shield)
4. Other shielding methods
...
For signal frequencies of up to about 1MHz, it is a good practice to ground a shield at only one end, preferably at the source of the signal end leaving the load end insulated from ground. This is to prevent the shield from acting as a conductor for voltage differences at the two ends.
The IEEE Standards are often interested in volunteers; especially, if the volunteers are experts as you are. Visit
http://www.ieee.org
for more info.
RE: Problem with induced low voltage across 120V line
You have earned a star.
I am impressed and appreciative of the information from IEEE Std 142-1991. I was not aware of this reference and will order it come Monday.
Kind regards,