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Thread: How does a GEC limit overvoltage from lightning and grid surges?

  1. #11
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    you ever see a picture of lightning, it travels through the sky and to the earth, the earth is obviously a good enough conductor for it.

    https://www.youtube.com/watch?v=hG2p9N1MHlk&t=108s

    skip to 5:36,
    to see that distribution voltage will also move pretty good through earth. also consider the fact that in substations we use a ground grid to reduce step potential. earth is usually considered a high resistance relative to 120V yes most people would agree with that, at a higher voltage many people might say it's conductive. it will definitely conduct electricity.

    or go ask a dairy farmer (cattle are very sensitive and are good ground fault indicators(actual ground(earth and currents from electrical system)))
    Last edited by Wire-Smith; 11-07-18 at 08:07 PM.

  2. #12
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    Quote Originally Posted by PetesGuide View Post
    ... I know a GEC doesn't help with ground faults from hot to the safety ground path--so if it doesn't help in that case, how does it lower the voltage from lightning strikes, unintentional contact with high-voltage lines, grid surges from switching feeders, etc.?
    250.4 General Requirements for Grounding and Bonding.
    The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.

    (A) Grounded Systems.
    (1) Electrical System Grounding.
    Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.


    So, 250.4 says it identifies what grounding and bonding are required to accomplish. And it is "prescriptive". It is going to tell us what to do and how to do it. No selection or choices on our part.

    250.4.A.1 tells you what the grounding and bonding is going to do.
    "will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation"

    So, follow the rules and all good things will happen - or not.
    1. "and that will stabilize the voltage to earth during normal operation" So, is a lightning strike "normal operation. No. Wait a minute, it's normal operation, what is to stabilize.


    2. So, is there any expectation that grounding/bonding "will limit the voltage imposed by lightning"? No. Lightning strike hits close and raises the distribution MGN to 50KV. And the house? The house neutral/ground is tied right to the MGN. It goes right up with it.

    3. "unintentional contact with higher-voltage" Ok consider dropping a substation 69KV line across a 7200V feeder to suburban housing transformers. 69KV is 40KV to ground, the 7200V line jumps to 40KV, the service to the house jumps to 1300V and it is going to stay there until the substation protective relays open the feeder CBs - Or until the house CB opens or the meter disintegrates. And the ground rod is .................
    ......................................
    .......................................
    ......................................
    doing nothing until - Ahhh something flashed over.

    4. "line surges" So, what is that? It is not a switching transient, nor contact with a higher voltage line, nor lightning strike induce. "Surge" is not defined in the NEC. It is not defined in IEEE 100.

    I have this picture in my mind of a Gary Larsen cartoon. House covered is arcing slime. Guy standing next to the house looking at the distribution line. There is a bulge traveling down the distribution line looking like a snake that swallowed a pig. "Oh, oh, here comes another surge."

    Not to be confused with Art 280, 285 Surge Arrestors, Surge Protective Devices.

    So 250.4, 250.4.A.1, No clue what they are talking about.

    ActionDave in post 2 nailed it.


    the worm (is being driven out of the ground by the "surges")
    Without data you’re just another person with an opinion – Edwards Deming

  3. #13
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    Quote Originally Posted by electrofelon View Post
    But current takes all paths, it doesn't matter if one is better or not.
    Current takes paths proportionally to their resistance/impedance, so it certainly may matter which path is 'better.'

    Additionally, there's the effect on potential difference. If some touch potential develops between earth and some metal part, that potential is there until a path -maybe you - comes along to carry current. Of there is already a low resistance path, then that potential is somewhere between significantly reduced or effectively gone.


    The only ways I can see bonding to dirt helping is:

    1. It causes enough fault current to trip an OCPD (unlikely at less than 600v of course)

    2. There is some sort of ground detector which opens the circuit.

    3. Ita a low energy source such as Capacitive coupling or a high impedance fault, in which case the voltage difference can be "shunted"
    I think you're overlooking the mitigation of a variety of possible touch and step potentials from situations other than a ungrounded conductor faulted to ground.

  4. #14
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    Quote Originally Posted by JPinVA View Post
    As you noted, it doesn't help with a "source" fault as source faults, by design (and code) take the low impedance neutral back to source. Well, technically, they also take the ground path...but for OCPD purposes, the neutral path is required.

    We live in an electrically charged world. The electricity we create in our generating plants and distribute on wires to our homes is but our taming of the charges. There are many "untamed" charges around us...and the positive charges like to hook up with negative charges. Charges being imposed on things in and around our house may be different than the charges on the ground. They want to get together.

    If we make it easy for them to get together, then it becomes an orderly everyday hook up. We make it easy by bonding all the electrical things together in the house, to include the dedicated EGC. This completes the easy path to the panel, and the GEC gets it to the electrode and to the earth. This orderly every day hookup ensures charges don't build up to the point where the potential difference gets to an unsafe level...and violently comes together.

    As for lightning, the GEC really doesn't help much on a direct strike...you need LPS for that. But lightning doesn't just affect what the bolt touches. There are all sorts of charges associated with lightning...some get imposed on the structure...some ride the wires...both inside the house as well as on utility poles. As these charges flow through the house and the wires, they are trying to equalize...headed to (our from) the earth. A house "grounded" to ground (the GEC playing an important role in the connection) makes it easy to complete the connection.

    As for man made sources along the distribution lines....surges...and power line faults...these desire to go back to source. If these faults are imposed on the house in some way, the low impedance path within the house to ground helps ensure dangerous buildup does not occur in the house itself.
    I like a lot of what you said here. Lightning can be many thousands of volts, and even high frequency involved, that means impedance is a factor and not just resistance. Also keep in mind it is seeking path to earth and not to the source like we have with a fault from an ungrounded system conductor. A reason we must use many items if present as grounding electrodes is because those items generally have a low impedance to earth. Now you may still have some stray currents finding other paths because current does take all available paths, but the lower impedance paths will carry more current than the higher impedance paths.

    Quote Originally Posted by electrofelon View Post
    But current takes all paths, it doesn't matter if one is better or not.
    Correct it takes all paths, but divides in different levels dependent on impedance of each path.

    A ground rod at a building service with resistance to earth of 25 ohms, may just be low enough to let the thousands of volts imposed on it through the rod instead of arcing across an air gap someplace on the EGC elsewhere in the building.

    Somewhat of a crapshoot what may happen, especially the more direct the lightning hit is.
    I live for today, I'm just a day behind.

  5. #15
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    Quote Originally Posted by jaggedben View Post
    Current takes paths proportionally to their resistance/impedance, so it certainly may matter which path is 'better.'
    How? Please explain.

    Additionally, there's the effect on potential difference. If some touch potential develops between earth and some metal part, that potential is there until a path -maybe you - comes along to carry current. if there is already a low resistance path, then that potential is somewhere between significantly reduced or effectively gone.
    Yes but you have to pretty much be standing right on the electrode for it to make any difference.




    I think you're overlooking the mitigation of a variety of possible touch and step potentials from situations other than a ungrounded conductor faulted to ground.
    Im open, please explain!
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #16
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    Quote Originally Posted by electrofelon View Post
    Originally Posted by jaggedbenCurrent takes paths proportionally to their resistance/impedance, so it certainly may matter which path is 'better.'



    How? Please explain.


    Remember Ohm's law?

    If you have a 1000 volt source and place both a 10 ohm and a 10,000 ohm resistor in parallel across the source doesn't both carry current? One happens to carry much more current than the other but both are a part of a closed circuit with a voltage across them.

    Same reason a 1500 watt heater and a 5 watt lamp both operate when connected in parallel to the same voltage source.
    I live for today, I'm just a day behind.

  7. #17
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    Quote Originally Posted by Wire-Smith View Post
    you ever see a picture of lightning, it travels through the sky and to the earth, the earth is obviously a good enough conductor for it.

    https://www.youtube.com/watch?v=hG2p9N1MHlk&t=108s

    skip to 5:36,
    to see that distribution voltage will also move pretty good through earth. also consider the fact that in substations we use a ground grid to reduce step potential. earth is usually considered a high resistance relative to 120V yes most people would agree with that, at a higher voltage many people might say it's conductive. it will definitely conduct electricity.

    or go ask a dairy farmer (cattle are very sensitive and are good ground fault indicators(actual ground(earth and currents from electrical system)))
    Oh, wow! I had no idea there were those kinds of interactions. That video is very insightful to understanding lightning's interaction with the ground.

  8. #18
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    Quote Originally Posted by INGMRS View Post
    Hello,
    I recommend you watch this video by Mike Holt:


    https://www.youtube.com/watch?v=qNZC782SzAQ


    Regards
    Minor R.
    This video is giving me the best understanding of earth resistance, impedance, and earth-to-EGC connections I've yet come across. This should be required viewing for all electricians and EE majors (and their professors).

    Thanks!

  9. #19
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    Quote Originally Posted by electrofelon View Post
    How? Please explain.
    Example: Let's say we have two paths that current can flow across a potential difference. Let's say the potential difference is 100V, and the current source is sufficient to deliver whatever the resistance will allow. One path is 100 ohms. The other path is 1 ohm. One amp will flow across path one. 100 amps will flow across path two.

    Note that current did not take ONLY the path of least resistance (the 1 ohm path). If it ONLY took the path of least resistance, then we would have 100A in path two and zero amps in path one. Current takes all paths. Hence, while the lion's share of the current takes the path(s) of least resistance, 100A in this instance. There is still 1A flowing through that 100 ohm path.

    So there are two sides of the coin we must careful address here. On the one hand, we don't want to say that current takes ONLY the path of least resistance. It takes all paths. On the other hand, although all paths are taken, we don't want to infer that the current on each path is equal. The current varies proportional to each path's resistance/impedance of the path. In our example, both paths are taken, but 100A flows on one path and 1A flows on the other...a significant difference.

    Lastly, if the second path is of significant enough resistance...say 1M ohm...we're talking current in that path that is measured in micro amps. In the limit, as the alternate paths approach ever higher impedance (with respect to a low impedance path), to the point that the measurable/detectable current in the high impedance path is effectively zero, then for all practical purposes (in that unique situation), current is indeed taking the path of least resistance. But that is an edge case that can cause confusion, so forget what I just said. Stick with current takes all paths.

  10. #20
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    Quote Originally Posted by kwired View Post
    Remember Ohm's law?

    If you have a 1000 volt source and place both a 10 ohm and a 10,000 ohm resistor in parallel across the source doesn't both carry current? One happens to carry much more current than the other but both are a part of a closed circuit with a voltage across them.

    Same reason a 1500 watt heater and a 5 watt lamp both operate when connected in parallel to the same voltage source.
    Quote Originally Posted by JPinVA View Post
    Example: Let's say we have two paths that current can flow across a potential difference. Let's say the potential difference is 100V, and the current source is sufficient to deliver whatever the resistance will allow. One path is 100 ohms. The other path is 1 ohm. One amp will flow across path one. 100 amps will flow across path two.

    Note that current did not take ONLY the path of least resistance (the 1 ohm path). If it ONLY took the path of least resistance, then we would have 100A in path two and zero amps in path one. Current takes all paths. Hence, while the lion's share of the current takes the path(s) of least resistance, 100A in this instance. There is still 1A flowing through that 100 ohm path.

    So there are two sides of the coin we must careful address here. On the one hand, we don't want to say that current takes ONLY the path of least resistance. It takes all paths. On the other hand, although all paths are taken, we don't want to infer that the current on each path is equal. The current varies proportional to each path's resistance/impedance of the path. In our example, both paths are taken, but 100A flows on one path and 1A flows on the other...a significant difference.

    Lastly, if the second path is of significant enough resistance...say 1M ohm...we're talking current in that path that is measured in micro amps. In the limit, as the alternate paths approach ever higher impedance (with respect to a low impedance path), to the point that the measurable/detectable current in the high impedance path is effectively zero, then for all practical purposes (in that unique situation), current is indeed taking the path of least resistance. But that is an edge case that can cause confusion, so forget what I just said. Stick with current takes all paths.
    No disagreement. But how does this reduce shock potential? Unless its a real high impedance source (such as capacitance, or you are standing pretty much right on the electrode, the other path doesnt change anything about the path shocking the person.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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