"cute little wire" on light poles (bleeding induced voltage)

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Markaaronky

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I've noticed several comments on Mike Holt's videos about the "cute little wire" on metal light poles that Mike points out isn't necessary because of the huge metal bolts that already ground the pole. Viewers point out that this wire is (unnecessarily, because of the bolts) there to bleed off induced voltage, rather than lightning protection.

My question relates to the concept of "bleeding": regardless of how the pole is grounded, as long as it's being subject to, say, EMF, isn't it true that there is a flow of current from the pole to ground? "Bleed" means current is flowing as long as a voltage is being induced, right?

I know this seems self-evident, as in current flowing in an antenna as long as it's being hit by a radio wave. But I've run into some people who stubbornly insist (forgive me, I know this sounds bizarre) that anything--in this case, humans in particular --that's grounded isn't subjected to the effects of induced voltage. They argue that if it's grounded, no current would flow. My understanding of the physics is different: a voltage is induced on the light pole or human or whatever, and current flows to ground, until you stop inducing the voltage. Do I have this wrong?

Thanks for entertaining what must seem like a very offbeat question.
 
Generally this forum is for those in the trade, however you seem to be asking questions for your own education and not to actually do any electrical work. If things change and it starts to sound like you are trying to do your own electrical work the thread will be closed.


So with that said, welcome to the forum. :)
 
iwire said:
Generally this forum is for those in the trade, however you seem to be asking questions for your own education and not to actually do any electrical work. If things change and it starts to sound like you are trying to do your own electrical work the thread will be closed.


So with that said, welcome to the forum. :)
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:thumbsup:
 
Somewhere I have pics of metal poles used for 138,000 volt lines. The poles are bolted into a concrete pad with over a dozen very large bolts.

There is also a #4 (or so) solid copper conductor on a lug on the side of the pole going to a single ground rod a few inches from the concrete pad.

I never could understand that.

To me that's like using a piece of twine for a safety loop on a logging chain.

:dunce:
 
clarification

clarification

iwire said:
Generally this forum is for those in the trade, however you seem to be asking questions for your own education and not to actually do any electrical work. If things change and it starts to sound like you are trying to do your own electrical work the thread will be closed.


So with that said, welcome to the forum. :)
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Thank you. This is purely an educational question, as you ascertain.:) To clarify, the claim that I've heard and think is incorrect is that a current cannot be induced in a grounded system. So when I read statements from experts in this forum, and comments on Mike's videos on YouTube, about a "bleed" wire, this seems to back up my understanding of the physics: that a current can be induced even if something is grounded, it's just that it flows to ground.
 
Outside of lightning, picture ground like this: just another type of resistor. Soil has no special electrical properties being simply another path electricity takes back to the source.
 
I have not seen the video to which you refer, but I do not understand what you mean by "bleeding off induced voltage." Induced by what? So let’s get down to some basic physics.


  • Any charged particle that is in motion (i.e., anything with current flowing) will create around itself a magnetic field.
  • If the current is constantly varying in magnitude and direction (i.e., it is an AC current), or if the wire (or other object) that is carrying the current is in physical motion, then the magnetic field that would be measured by a nearby observer (or felt by a nearby wire) will be constantly changing.
  • A charged particle (e.g., the electrons associated with a metal wire or a metal pole) that senses the presence of a changing magnetic field will feel a push. That push is the “induced voltage” to which I believe you were trying to refer.
  • If, and only if, there is a complete circuit available to the charged particle mentioned above (e.g., from the electron’s initial position internal to the secondary of a transformer’s windings, through the secondary conductors to a light bulb, through the light bulb, through another conductor back to the transformer’s secondary windings), then current will flow along that path.
  • The amount of induced voltage, and the amount (if any) of the resulting current, will depend on the strength of the original magnetic field, the distance between the source of that magnetic field and the charged particle, and a few other things.
  • I have a pencil sharpener on my desk, and it runs on AC power. When it is running, it generates a magnetic field that surrounds the pencil sharpener. During that time, there will be a voltage induced in the watch band that you are wearing right now. But the strength of that magnetic field is rather small, and there is a great distance between my desk and your wrist, so the induced voltage and resulting current are going to be too small to measure. Nevertheless, the phenomenon is real.
  • Similarly, the wires running up the light pole and allowing the light to shine will create around themselves a magnetic field. There are charged particles in the metal light pole. So you might conclude that there will be an induced voltage and a resulting current flow. That is true. But here again, the numbers are extremely small. That is, in part, because the magnetic field generated by the one wire that carries current to the light bulb is almost exactly the same in magnitude and opposite in direction as the magnetic field generated by the immediately adjacent wire that is carrying current down from the light bulb on its way back to the power panel. The two magnetic fields essentially cancel each other out, so that the metal light pole does not see any net magnetic field. Therefore, there will not be any voltage induced in the pole from this potential source.
  • So if the only wires that are carrying current anywhere near the pole are not inducing voltage in the pole, then what else would be doing that? This brings me back to me second sentence at the beginning of this post. “Induced by what?”

I will conclude by asking you, as I have asked many others, including many members of the electrical trade and many electrical engineers, to once and forever discard the notion that current is seeking a path to ground. It is not. So in very long answer to your fairly short question (i.e., “isn’t it true that there is a flow of current from the pole to ground?”), no that is not true. Current can only flow if there is a complete path that begins at the source that set current into motion in the first place and that returns again to that same source. It is certainly possible for planet Earth to serve as a conductive medium for the flow of current. But that requires there to be a source of current and a path (that includes dirt) that will carry current back to that same source.
 
charlie b said:
I have not seen the video to which you refer, but I do not understand what you mean by "bleeding off induced voltage." Induced by what? So let’s get down to some basic physics.

I will conclude by asking you, as I have asked many others, including many members of the electrical trade and many electrical engineers, to once and forever discard the notion that current is seeking a path to ground. It is not. So in very long answer to your fairly short question (i.e., “isn’t it true that there is a flow of current from the pole to ground?”), no that is not true. Current can only flow if there is a complete path that begins at the source that set current into motion in the first place and that returns again to that same source. It is certainly possible for planet Earth to serve as a conductive medium for the flow of current. But that requires there to be a source of current and a path (that includes dirt) that will carry current back to that same source.
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Thank you for the thoughtful reply. I don't hold the notion that current necessarily seeks a path to ground, so that's not exactly where I'm coming from on this question.

In physics class we were taught that an electromagnetic field can induce a voltage-- a potential-- cause the buildup of electrons --on an object and that, given a conductive path to ground (defined as an object of sufficient size able to accept the electrons), those electrons will then flow to that ground. I came to the "bleed" wire on the light pole example only because it so well illustrates the problem I'm trying to describe.

I apologize if I'm missing the obvious in not seeing a complete path in this example, and/or perhaps I'm incorrectly referring to "current" as the flow of electrons, but if an electromagnetic field (e.g. radio waves) or static electricity in the air induces voltage on a pole with a "bleed" wire and electrons flow to the ground through that bleed wire, don't we have current flowing to ground (without a return path)? I understand from your example and accept that the charge may be small. Again, thanks in advance for the replies, patience with any misunderstanding I may have of the physics, etc.
 
Markaaronky said:
In physics class we were taught that an electromagnetic field can induce a voltage-- a potential-- cause the buildup of electrons --on an object and that,
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Hmm, build up a charge?

In most cases I believe the induced voltage goes away as soon as the induction stops.
 
K8MHZ said:
Wind and snow can build static charges that need to be 'bled off' to earth sometimes.

I doubt that would happen on a pole, but it does happen on wires, like those used for antennas.
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Antennas are the classic example I had in mind when I came up with this thought experiment Marky, thank you for bringing this up. I come from a ham radio background. I used the light pole example here mainly because Googling the topic led me to a video (on light poles) linked to this excellent forum and since I'm trying to wrap my head around the physics rather than wire anything, I went with the light pole example. :)

So I'm still trying to determine if this "bleeding off" constitutes the flow of current when it occurs. I've run into a claim that anything that's grounded can't have a current induced by EMF, and this just makes no sense to me. :happysad:

Thank you for the reply.
 
First of all, an electromagnetic field cannot induce a voltage. You need a “changing” field to do that. Here’s an experiment: Connect a coil of wire to a light bulb. Hold a bar magnet near the coil, and you will not illuminate the light bulb. Move the bar magnet towards the coil, and the light bulb will light up. It is the change in magnetic field that does the job. The change can be caused by a variation in the current that creates the field, or by physical motion (per the experiment described above).

Secondly, if the field does induce a voltage in a nearby piece of metal (be it the coil of wire I mention above or the metal light pole under discussion), it will tend to push electrons within that piece of metal to one side or another. In the case of the coil and light bulb experiment, there is a complete path available for current to flow along. In the case of the light pole, there is not. The metal atoms that make up the light pole do not have an infinite number of electrons, so you are not going to have electrons just flowing into the dirt. They have to come from somewhere, and they have to return to that same somewhere.

Third, it is not completely incorrect to refer to current as the flow of electrons. It is actually a good description for most of our purposes. A more nearly accurate definition of current would be “charge in motion.”

Finally, looking at the photo in post #7, tell me what that “bleed wire” is going to accomplish that would not also have been accomplished with far greater effectiveness by the huge metal pole to which it is attached?
 
charlie b said:
First of all, an electromagnetic field cannot induce a voltage. You need a “changing” field to do that. Here’s an experiment: Connect a coil of wire to a light bulb. Hold a bar magnet near the coil, and you will not illuminate the light bulb. Move the bar magnet towards the coil, and the light bulb will light up. It is the change in magnetic field that does the job. The change can be caused by a variation in the current that creates the field, or by physical motion (per the experiment described above).

Secondly, if the field does induce a voltage in a nearby piece of metal (be it the coil of wire I mention above or the metal light pole under discussion), it will tend to push electrons within that piece of metal to one side or another. In the case of the coil and light bulb experiment, there is a complete path available for current to flow along. In the case of the light pole, there is not. The metal atoms that make up the light pole do not have an infinite number of electrons, so you are not going to have electrons just flowing into the dirt. They have to come from somewhere, and they have to return to that same somewhere.

Third, it is not completely incorrect to refer to current as the flow of electrons. It is actually a good description for most of our purposes. A more nearly accurate definition of current would be “charge in motion.”

Finally, looking at the photo in post #7, tell me what that “bleed wire” is going to accomplish that would not also have been accomplished with far greater effectiveness by the huge metal pole to which it is attached?
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Thank you again for the reply and patience Charles. When I said EMF, I was referring to electromagnetic radiation, such as radio waves, which I know (or have been taught in physics classes and ham radio courses) can induce voltage and current. I don't believe radio communication could work otherwise. So my apologies if I used incorrect terminology. I agree that a stationary magnet won't do the job. In my areas of study, EMF doesn't refer to a stationary field. For example, a microwave oven or your pencil sharpener example would be considered EMF sources. I know an A/C power source can induce a voltage, for example: I've measured it. This is what I was referring to as EMF and that term is (possibly poorly!) commonly used.

I like your "charge in motion" definition better for what I'm trying to describe; thanks for clarifying.

Certainly no argument from me that the wire in post #7 would accomplish anything. I'm completely deferring to the experts here and Mike Holt's criticism/expertise regarding the practice. He very clearly points out what you are saying--how useful would a wire be compared to the pole? :) And to reiterate, I'm not trying to wire anything, especially a pole! This is purely a physics exercise for me--I've heard a very strong claim that if an object is grounded, you cannot induce a flow of electrons through that object. This seems to fly in the face of physics to me, having seen charges induced and then discharged on objects in physics class, knowing how the antennas in my ham radio shack work, etc.
 
Markaaronky said:
I've heard a very strong claim that if an object is grounded, you cannot induce a flow of electrons through that object. This seems to fly in the face of physics to me, having seen charges induced and then discharged on objects in physics class, knowing how the antennas in my ham radio shack work, etc.
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I don't know much of anything about radio but maybe you know this.

If we were to firmly ground the radiating part of an antenna would it still be able to send a transmission via that antenna?
 
iwire said:
I don't know much of anything about radio but maybe you know this.

If we were to firmly ground the radiating part of an antenna would it still be able to send a transmission via that antenna?
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I will answer this if it's OK.

Yes.

In fact, there are several antennas that are DC grounded by design. A 'J-Pole' is one.

At radio frequencies, impedance is dependent on several things. What DC 'sees' as a 'short' to ground, RF may see as just the opposite.

Now, if you take an antenna, like a dipole or a Yagi, which was not designed to be DC grounded, grounding the radiating elements will have a detrimental effect. The antenna will still radiate some, but with the extra wire and earth connection, more of the energy sent to the antenna will be reflected back into the transmitter and eventually dissipate in the form of heat in the feed lines to the antenna and the radio itself.
 
Three comments:

1. You can have a current which is not an actual flow of charge as a result of what is called "displacement current' in a capacitor. Among other things this means that you can have current flowing through a pole to ground just from unbalanced capacitive coupling from the hot wires.
2. When dealing with antennas capacitance and other transmission line phenomena are critical.
3. When a physicist talks about an electromagnetic field it is not assumed to be a static field unless specifically stated, so in that context an electromagnetic field can induce voltage and drive current.
 
K8MHZ said:
Iwire,

I can also 'shunt feed' a solidly grounded metal pole and that grounded pole will work as a vertical antenna if the shunt is designed properly.

People have shunt fed fences and railroad tracks and got them to radiate just fine as antennas.
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I do appreciate your responses but I am not really interested in antennas that are designed to be grounded or ones that take a special design to work while grounded.

I was really wondering if an antenna that is not intended to be grounded would stop working if grounded. You say yes at a reduced output.
 
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