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

  1. #1
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    How does a GEC limit overvoltage from lightning and grid surges?

    I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

    What is most confusing to me is that 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.?

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    I don't think it's effective for much at all other than giving a warm feeling to the dirt worshippers.
    If you don't think too good, don't think too much.

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    Quote Originally Posted by PetesGuide View Post
    I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

    What is most confusing to me is that 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.?
    The answer is that it doesn't, nor is it supposed to.

    What it is supposed to do is limit the voltage from earth to the grounded conductor of the electrical system when those events happen. Since there is a dead short between earth and the grounded conductor (the GEC) it probabl does indeed limit that voltage difference.
    Bob

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    Quote Originally Posted by PetesGuide View Post
    I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

    What is most confusing to me is that 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.?
    The contact resistance between the electrode and earth is too high to trip OCPD at household voltage, but it may still reduce the applied voltage when the source is distribution voltage. Or lightning, when compared to other, more destructive alternate paths.

    Sent from my XT1585 using Tapatalk

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    NFPA 780 covers lightning arrester systems. Only a whole house surge protector would protect against an overhead primary falling onto a secondary, or a power company surge.
    Electricians do it until it Hertz!

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    Quote Originally Posted by petersonra View Post
    The answer is that it doesn't, nor is it supposed to.

    What it is supposed to do is limit the voltage from earth to the grounded conductor of the electrical system when those events happen. Since there is a dead short between earth and the grounded conductor (the GEC) it probabl does indeed limit that voltage difference.
    Still not clear enough for me. Is this a question of semantics, where my use of lower means something different than limit? NEC 2011 250.4(A)(1) states: "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 in https://www.mikeholt.com/PopGraphic.php?id=4380 Mike Holt states, "Metal parts of the electrical installation are grounded to the earth to reduce voltage on the metal parts from lightning..."

    Is this not what actually happens? I'm trying to understand what the electrons are doing in these events and why.

    P.S. I'm interested in what happens in systems grounded as per NEC, but without NFPA 780 lightning protection systems installed.

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    Basically the idea is that if you have a sizable wire connection to the earth then if any potential difference that develops, for any reason, between the earth and the electrical system, current will take the wire route instead of a human or some other more easily damaged wire or other thing. I believe it's far more helpful in tripping ground detection ( if a high voltage source faults to a low voltage source ) than for helping with a lightning strike. Also protects somewhat against shock in certain fault situations. The lightning thing I think is less about what happens in a strike and more about limiting the static potential that can develop around thunderstorms.

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    Quote Originally Posted by PetesGuide View Post
    Still not clear enough for me. Is this a question of semantics, where my use of lower means something different than limit? NEC 2011 250.4(A)(1) states: "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 in https://www.mikeholt.com/PopGraphic.php?id=4380 Mike Holt states, "Metal parts of the electrical installation are grounded to the earth to reduce voltage on the metal parts from lightning..."

    Is this not what actually happens? I'm trying to understand what the electrons are doing in these events and why.

    P.S. I'm interested in what happens in systems grounded as per NEC, but without NFPA 780 lightning protection systems installed.
    Hello,


    I recommend you watch this video by Mike Holt:


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


    The EGC provides a low impedance conducting path to the power supply to eliminate the fault, if it has a grounding electrode, but it is not connected to the same ground in the system, there is a great possibility of having different gradients of potential during an event.


    The electrode and the EGC are not a protective mesh for the "step voltage".


    The grounding system, as stated in the section, limits the high voltage due to external events at the same reference potential. (Land)

    Regards
    Minor R.

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    Quote Originally Posted by PetesGuide View Post
    I'm trying to get a deeper and accurate understanding of how a Grounding Electrode Conductor works.

    What is most confusing to me is that 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.?
    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.
    Last edited by JPinVA; 11-07-18 at 03:04 PM.

  10. #10
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    Quote Originally Posted by jaggedben View Post
    Basically the idea is that if you have a sizable wire connection to the earth then if any potential difference that develops, for any reason, between the earth and the electrical system, current will take the wire route instead of a human or some other more easily damaged wire or other thing. .
    But current takes all paths, it doesn't matter if one is better or not. 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"
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

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