Overhead line Distance

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joe k.

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This there a simplified method for determining the EMF field/voltage from an overhead line. Example. If you hold a volt meter under a transmission line and ground one leg it will read a voltage.

I understand the NESC has a safe distance defined but not a method for calculating approximate voltage at a given distance outside the safe disctance.
 
Re: Overhead line Distance

Yes. It's very simple. There is none. :eek:

Current flowing through the overhead line creates a magnetic field that surrounds the line. That is true of any current and any type of line. The physical property that you can measure under (or near) an overhead line is the strength of the magnetic field. You don't get "voltage" from the overhead line unless you have an electrical circuit that is "linked by" the lines of magnetic flux. The voltage will be equal to the number of turns in the coil times the rate of change in the magnetic flux.

Put a coil of wire under the overhead line, and there will be a voltage impressed on that coil. The voltage, in turn, will cause current to flow in the coil. The current in the coil will itself, in turn, create a magnetic field. The direction (orientation) of this new magnetic field will be such that it is in opposition to the field from the overhead line. This is essentially what happens in a transformer.

But if there is no coil of wire, just a person standing under the overhead line, there is no circuit for the line's magnetic field to link with. Therefore, there is no voltage.

May I ask what you are trying to accomplish with this line of questioning?
 
Re: Overhead line Distance

Is there a milligauss to voltage potential conversion? If so, you can use a Gaussmeter to measure the EMF.
 
Re: Overhead line Distance

Originally posted by bphgravity: Is there a milligauss to voltage potential conversion?
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No. Not in the sense that you can convert "feet" to "inches" by multiplying by 12.

"Gauss" is a unit of "magnetic flux density." The word "density" gives it a notion of "amount per unit area." You go from "magnetic flux density" to "magnetic flux" by multiplying by area. But first, you have to have an "area" under consideration. The "area" is the space occupied by a coil of wire (for our practical purposes). Without that coil, you have no "area," and you can't go from "Gauss" to a unit of "magnetic flux."

Why does that matter? Who cares about "magnetic flux"? If you are looking to find "voltage," you get there by taking the rate at which "magnetic flux" is changing, as seen within the area surrounded by the coil of wire, and multiplying by the number of turns of wire in the coil.

Here's the "short and simple answer": Voltage does not just exist in the air; voltage is not called into existence by having a wire nearby. You have to have some type of electrical circuit, such as a coil of wire, in order for the magnetic field to induce a voltage.
 
Re: Overhead line Distance

May I add that the magnetic field is proportional to current, not to voltage.
 
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By useing a radio tuned to a blank AM station, you can detect the magnetic flux field from the overhead high voltage lines. you will hear a loud hum or static, but it will not give you a voltage measurement. The volume will change based on the flux field intensity and distance from the lines.
 
Re: Overhead line Distance

Originally posted by charlie b:
Current flowing through the overhead line creates a magnetic field that surrounds the line. That is true of any current and any type of line. The physical property that you can measure under (or near) an overhead line is the strength of the magnetic field.
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But when you hold a non-contact voltage detector next to a circuit under no load (hence no current) it still detects a field.

When I stand 200' from the transmission lines on my property, my tick tracer detects the voltage running through the lines. Since the voltage is in the neighborhood of 230,000 volts, it can be detected at a considerable distance. The wire is not very large, so there it stands to reason that not many amps are flowing through the wire.

I hold the same detector to the 4/0 wires feeding a home, and I need to be within 2" for the detector to light.

I am by no means saying ya'll are wrong, I know you're right. But I don't understand.
 
Re: Overhead line Distance

George, there is also an electric field which is a function of voltage. Your tracer senses this field which of course is stronger with higher voltage.

As you walk around in your insulated boots, your bod forms one plate of a capacitor, the other plate is the earth. When you walk near a power line, another cap comes into play between your bod and the line. So you have a capacitive divider which will allow your bod to experience an AC voltage. Hold the tip of one lead of a hi-Z AC VM with the other lead grounded, and you will measure a voltage, but it won't mean much.
 
Re: Overhead line Distance

You got it Geo.,

Magnetic fields are produced by moving charges, for example, electric currents in a relay coil or solenoid or electric motor.

Electric fields are produced by a difference in voltage as in a cathode ray tube. Place the back of your hand near a TV screen, and the electric field will pull the hair toward the screen. Same thing supposedly happens just before you are struck by lightning. But, you might not live to tell about it.
 
Re: Overhead line Distance

It is not the presence of voltage that creates an electric field. It is more the other way around. Here are the four fundamental ?rules? of the science of electromagnetism:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">A charge will create around itself an electric field. If it is a point charge, then the electric field will go out in all directions.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">If a charge is located within an electric field (i.e., a field created by some other charge in some other location), it will feel a force. The force will be along a straight line from the source of the field to itself. The force will be either a push or a pull, depending on the nature of the second charge and the first charge (i.e., that created the field). Opposite charges attract; like charges repel. This is the phenomenon that calls ?voltage? into being.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">A charge in motion will create around itself a magnetic field. That is why we say that it is current, and not voltage, that creates the field.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">A charge in motion within a magnetic field will feel a force. By the same token, a stationary charge in the presence of a changing (or moving) magnetic field will feel a force. Imagine two lines: The line that is the direction in which the charge (or the field) was moving, and the line that is the direction of the field. The direction of the force will be perpendicular to the plane that contains both of these two lines. This is the phenomenon that creates a current in the secondary of a transformer.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">Does this help, George?
 
Re: Overhead line Distance

Quote from Charlie B:

"It is not the presence of voltage that creates an electric field. It is more the other way around. Here are the four fundamental ?rules? of the science of electromagnetism:"

Charlie, you are nit picking. This is a chicken and egg problem. The presence of an electric field is accompanied by a change in voltage. Field strength is often expressed in volts/meter instead of newtons/coulomb. One could argue that it is the difference in potential energy of the electrons, i.e, voltage, that creates the field.

Whatever though, the point is that an isolated body, human or otherwise, functions as a common plate of two capacitors. that is a capacitive divider.
 
Re: Overhead line Distance

Originally posted by rattus:Charlie, you are nit picking.
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A ?nit? is the larvae stage of a flea. If you don?t pick the nits, then someday the fleas will come back to bite you. :eek:

By definition, ?voltage,? or ?potential difference,? is the amount of energy required to move a unit of charge from one point in space to another point, in the presence of an electric field.
 
Re: Overhead line Distance

Charlie, I contend that voltage and field strength are so interrelated that one cannot say one causes the other.

Furthermore, "The potential at a point is considered to have a value even though there may be no charge at the point." (Frances Weston Sears, Electricity & Magnetism, Addison-Wesley, 1951)

[ November 26, 2005, 11:07 AM: Message edited by: rattus ]
 
Re: Overhead line Distance

I need to get a book on electricity. I think the last time I studied this stuff was in 7th grade, and while some of it makes sense, some of it is flying right over my head. :)
 
Re: Overhead line Distance

Originally posted by rattus: Charlie, I contend that voltage and field strength are so interrelated that one cannot say one causes the other.
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I contend otherwise. This really isn?t a chicken and egg thing.
Originally posted by rattus: "The potential at a point is considered to have a value even though there may be no charge at the point."
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Certainly. I can only agree. You may note that I have not said anything to contradict that observation.

The ?Potential? at a point is the amount of energy it would take to move a unit charge from that point to an infinite distance away. There is a ?difference in potential,? or ?voltage,? if the amount of energy required to move a unit charge from one point to infinity is different than the amount of energy required to move a unit charge from the other point to infinity.

If there is a charge at a point within an electric field, that charge will feel a force. But the potential exists even if there is no charge at that point. The potential exists at that point specifically because there is an electric field that would, if a charge were present at the point, exert a force upon that charge.
 
Re: Overhead line Distance

Originally posted by georgestolz:
I need to get a book on electricity. I think the last time I studied this stuff was in 7th grade, and while some of it makes sense, some of it is flying right over my head. :)
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George....maybe we can be study partners, from a distance of course. :(

[ November 26, 2005, 12:45 PM: Message edited by: iwire ]
 
Re: Overhead line Distance

You guys don't actually believe that we understand this stuff, and that we are not just making it up as we go, do you? :D
 
Re: Overhead line Distance

I believe Charlie is saying that this high flying discussion is not germane to the question.

I have performed a little experiment. I measured the AC voltage on my bod as I walked near a HV distribution line. Connecting the meter common to a metal fence post causes my bod to measure some 300 millivolts. If I touch the fence post, the voltage goes to zero.

If I connected the common to the EGC of an outside switch, I measure some 3 volts. Now this could be inductive pickup on the EGC. Must check that out.

Now let Cbg be the capacitance between body and ground, and let Clb be the capacitance between line and body. Then, the voltage on my bod is:

Vbg = Vline*Clb/(Clb + Cbg)
 
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