Induced voltage (Phantom Voltage) vs. cable length

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gar

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151102-0727 EST

HSB_007:

Post #8 from you.
I collected some more information to see if one could look further and provide some help into figuring out if there will be a problem of inductive coupling at a distance of 3000 meters.
I believe we are now concerned with a 3000 meter cable, about 10,000 ft. Also #18 wire in this post, 6.39 ohms / 1000 ft at 20 C. Post #13 has #16 wire and 0.33 A. #16 copper is 4.02 ohms / 1000 ft at 20 C.


Post #1 from you.
We have an instrument that requires 120 V ac to work and we mandate that the conductors length from control room (safe area) to the instrument do not exceed 1000 ft (305 meters).
I would not call a horn or strobe an instrument. At what voltage does it fail to pull-in (start to work)? At what voltage does it drop-out (quit working if already started)?


Post #5 from you.
The situation I have is a multi-pair cable (12 pairs) each one of them is of the same signal type, 120 Vac digital output serving horns and beacons.
Apparently a very low data rate. Is a single pair uaed for both power and control, or just a signal to activate the device?


Post #6 by GoldDigger.
With a signal level of 120VAC I think that you are really talking about powering the horns and beacons rather than just supplying a control signal to turn them on and off with the power provided by another circuit.
But in any case, there is no specific distance at which a problem will suddenly appear.
If the devices will work just as well on 100VAC as 120VAC and will not operate at an input of 20VAC, then it is very unlikely that there would be any crosstalk problems even at 1000 feet.
You are much more likely to get a problematic voltage drop in that length of wire from the wire resistance than a problematic stray voltage from coupling.


Post #8 by you.
1- Multi-pair (12 pairs)
2- All conductors are in the same cable and will serve the same purpose (power and control Beacons and Horns through 120 V ac).
3-We are complying with IEEE 518 when it comes to spacing with other adjacent signals. In this case, there is enough spacing on the same cable tray between this multi-pair cable and adjacent ones (about 6 inches from signal level 2, and 8 inches from signal level 4).
4- Twist: 60 mm
5- Conductor: Circular copper,7 strands minimum; tinned per ASTM B33
6- Conductor size: 18 AWG

7- Primary Insulation material: thermoset cross-linked polyolefin, temperature rating 90 C.
8-
Primary insulation voltage: 300 Vac.
9- Primary Insulation thickness: 0.38 mm
The loop DC resistance of #18 copper for a 10,000 ft one way distance at 20 C is about 2*64 ohms or 130 ohms. If a horn is 10 W at 120 V, then current is greater than 0.08 A. Assume a current of 0.1 A, then loop voltage drop is about 13 V neglecting inductance. What is the threshold current to operate a horn?

With my experimental setup of two #14 Romex cables lashed together over a 12 ft distance and running 10 A 60 Hz thru one cable (this obviously means shorting one end and feeding the other from an adjustable voltage transformer with a current meter in series) , and shorting one end of the second Romex cable and placing a 75 ohm load at the opposxite end, results in a voltage of 2.45 mV across the 75 ohm resistor from magnetically induced voltage in the second Romex cable. For your 10,000 ft application this might mean a maximum ballpark estimate of 2.5 V.

I believe the parallel arrangement of this experiment provides the maximum magnetic coupling between the two cables. You have twisted pairs in your cable and thus coupling could be less, and there are differences in wire spacing. You have a statistical problem and probably should try some experiments on possibly 1000 ft of your cable. You also have to determine the thresholds of operation of your different loads.

Your individual currents are much lower than 10 A, but there is some composite flux vector at the wire pair of interest that you must estimate from the sum of all the magnetic fields of the various pairs.

As an alternative use DC instead of AC.

.
 

FionaZuppa

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Part Time Electrician (semi retired, old) - EE retired.
Why do you ask? Is there something you suspect? Just curious.


That should be an RMS rating if it is normal AC cable. Is it something else?
1) just making sure there's enough headroom for the dielectric
2) its an indicator to insulation thickness

i would just call Teldor and ask them. from the data given thus far, i would choose a FFTP w/ drain wires.
 
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Barbqranch

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I don't believe a precise answer can be given for a long run of cable, but it is possible to make a very good estimate if the factors are known.

I would be inclined to, if possible, either short or have a low impedance at the source when the signal is not present. For instance, a relay closing to sent the signal, but NC shorts the wires.
 

gar

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Location
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EE
151102-1544 EST

Barbqranch:

I don't believe a precise answer can be given for a long run of cable, but it is possible to make a very good estimate if the factors are known.
That is a pretty safe statement.


I would be inclined to, if possible, either short or have a low impedance at the source when the signal is not present. For instance, a relay closing to sent the signal, but NC shorts the wires.
Actually an open circuit at the sending end would be better. The twisted pairs are individually shielded, and therefore, there is nil capacitive coupling of any useful energy into the twisted pair.

If you short the sending end, then there is a one turn loop with a load ( the destination ). Thus, current can flow in the loop. If the sending end is open, then there is no closed loop and no current flow. There is distributed capacitance berween the wires in the twisted pair of about 20 to 100 pfd per foot. Assume 100 pfd per foot and 10,000 ft, then total shunt capacitance is 1 mfd, this can be considered a partial short, but it is distributed. At 60 Hz 1 mfd has a reactance of about 2700 ohms. If you could induce 120 V, then maximum current flow would be 44 mA. But all this is unreasonable, and any real current would be much much lower.

At 10,000 ft and 60 Hz you do not need to consider transmission line factors. One wavelength = 3100 miles.

Don't short the source for this application. But, there might be applications where a short might be useful.

These off the cuff calculations are just to define some limits.

.
 
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FionaZuppa

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151102-1544 EST

Actually an open circuit at the sending end would be better. The twisted pairs are individually shielded, and therefore, there is nil capacitive coupling of any useful energy into the twisted pair.

how do you know if the pairs have foil? did i miss that?

I don't believe a precise answer can be given for a long run of cable, but it is possible to make a very good estimate if the factors are known.

I would be inclined to, if possible, either short or have a low impedance at the source when the signal is not present. For instance, a relay closing to sent the signal, but NC shorts the wires.
i can hypothesize. an FFTP w/ drain wires is the "best" cable to get, and should not be budget busting. this also allows for re-use of the wire is the application changes in the future, hence you are building flexibility into the solution (bonus).
 

gar

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Location
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EE
151102-2012 EST

FionaZuppa:

Posts 1 and 14 reference electrostatic shielding of individual wire pairs. Almost certainly these are foil shields.

.
 

FionaZuppa

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Location
AZ
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Part Time Electrician (semi retired, old) - EE retired.
151102-2012 EST

FionaZuppa:

Posts 1 and 14 reference electrostatic shielding of individual wire pairs. Almost certainly these are foil shields.

.

ok, my bad.
have a gander. not sure if this is too technical.
http://www.uio.no/studier/emner/matnat/ifi/INF5460/v09/undervisningsmateriale/F2-3-en.pdf

aluminum foil FFTP wont help much with magnetic shielding. the foil shielding needs to be of magnetic type. an FFTP or FFTP w/ drains that has mag foil shielding will be best, and, in this application you can likely go beyond the 1000ft w/o having issue with induced voltage from adjacent mag fields. always a caveat, as in you cant rid the mag field to zero, but it should be so small that it becomes insignificant for the application at hand.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151103-0651 EST

HSB_007:

You seem to have been asking for an equation that will tell you that your cable will work at X-1 feet, but won't work at X+1 feet. To this my comment is that there is no such equation.

You might develop some approximate equation that would provide a probability distribution curve of the likelyhood of a certain error rate vs distance. But this will require that you run experiments, make assumptions, and judgements on your entire system.

A single equation is not available.

.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
151103-0651 EST

HSB_007:

You seem to have been asking for an equation that will tell you that your cable will work at X-1 feet, but won't work at X+1 feet. To this my comment is that there is no such equation.

You might develop some approximate equation that would provide a probability distribution curve of the likelyhood of a certain error rate vs distance. But this will require that you run experiments, make assumptions, and judgements on your entire system.

A single equation is not available.

.
well to be fair, if you can build an approximation equation with "X" in it and you know the threshold level you cant exceed "Y", then you can simply solve for "X". building the equation requires some non-trivial work.
 

mivey

Senior Member
151103-0651 EST

HSB_007:

You seem to have been asking for an equation that will tell you that your cable will work at X-1 feet, but won't work at X+1 feet. To this my comment is that there is no such equation.

You might develop some approximate equation that would provide a probability distribution curve of the likelyhood of a certain error rate vs distance. But this will require that you run experiments, make assumptions, and judgements on your entire system.

A single equation is not available.

.
A pretty good assessment. While a calculation might be fun, and probably not exacting, experimental data, tables, and practical experience will rule the day.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
151103-2122 EST

HSB_007:

It is a day and a half since you last posted. If you come back with more information and are willing to run some experiments, then we might be able to help you determine how to solve your problem.

The characteristics of the loads at the end of your cable are a starting point.

.
 
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