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

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    #46
    Originally posted by JPinVA View Post
    There is a reason code now requires all (with maybe some special exceptions) electrical components to be grounded at the same point. There is at least one case study I'm aware of where a cable TV system was grounded on one side of a residence, separate from the common ground. The EGC to the TV outlet was bonded normally. A potential gradient across the house resulted in a high flow of current through the TV. Think of the GEC connection as a positive terminal and the cable ground as a negative terminal (e.g., the essence of voltage gradient). The positive terminal is connected to the TV via the EGC. The negative terminal is connected to the TV via the cable. The "short" occurs across/through the TV. Granted, there is a parallel path through the earth, but it is higher impedance than the path through the low impedance wires.

    If the cable is bonded at the COMMON ground point (not just ground...but the common ground....at the same point) then there is no potential between the GEC ground and the cable ground...as they are the same.

    Oh, and if memory serves me, I think Mike has a picture with comments about something similar happening when grounding a photoelectric array via a ground rod. A current caused by a gradient goes up the rod...and fries the array electronics on it's way to/down the GEC.
    what your essentially saying equates to what my example to be is that electricity only flowed because i connected the circuit (see drawing)
    but the truth is, there was a difference of potential without me in the circuit and it would have flowed through the light bulb in my scenario and the house in yours.

    now replace me in the drawing with a copper wire outside the house, you can save the house, what would happen in the light bulb scenario? it wouldn't turn on would it? even if i had huge fuses on the primary of my transformer it would run dead shorted for a while and that bulb might not even flicker.

    i'm not saying a ground rod will save you from surges every time, but A) its cheap B) its easy C) it seams to do something

    now i will say, something related to your scenario, you can definitely direct surges into your house, but thats why we do grounding before anything enters the house, and some/or a lot can still run through the house but that doesn't mean less would run through it without grounding(that's before the line enters the house). remember lightning doesn't care as much about the conductivity levels we commonly consider, much higher voltage, less care about that (it goes through miles of air)
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    Last edited by Wire-Smith; 11-10-18, 05:34 PM.

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      #47
      The ground current does not flow through the metallic path because the combination of wire resistance and the two electrode resistances is lower than the earth resistance between the two points. But if the earth current is large enough to develop a substantial voltage there will be objectionable current in the metallic + rods path even if the total resistance is higher than the earth resistance.

      Sent from my XT1585 using Tapatalk

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        #48
        Originally posted by electrofelon View Post
        I just want to emphasize that this is BONDING. We have gotten a bit off topic from the OP who, I believe, was question system earthing and equipment earthing.
        OP here. I was indeed asking about system earthing (e.g. at the meter/service entrance/etc. wherever the GEC is connected), but not bonding or other equipment earthing.

        All of the answers are helping me get closer to understanding this; thanks!

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          #49
          Originally posted by JPinVA View Post
          I agree that the number of times the earthing actually saves a life from electrocution is very small. I was just trying to give examples of "possible" situation where it makes a difference. The big issue is secondary safety. Grounding bleeds off charges. These charges can cause machines to fail, create sparks that can set off combustible material or fires, or cause reflex reactions from personnel that could result in harm (falling off a ladder....recoil of an arm into moving machinery parts).

          As for electricity "going to ground", we really need to step back to the ROOT PHYSICS. Opposite charges attract...Equal charges repel. That is the ROOT physics. Everything we do in power creation and distribution does not change the root physics. The reason we can say electrons return to source is not because they are "salmon" and desire to return to their place of birth. It's because we CREATE the potential difference under which the root physics plays out. We can say the current from the transformer on our pole outside want to return to the source because by DESIGN we create the potential difference under which the current (obeying the root physical laws) will find its way back to the source. Absent outside interference from mother nature (or even some man-made events), if a path exists, current returns to source. As long as our DESIGNED circuit is operating as designed in the expected environment, we can say current using a ground path back to source is not "going to ground"...it is returning to source via the ground path.

          But if an external event occurs that drives the charges on the line, or the earth underneath, to some other potential outside of the expectation under which the circuits was designed....the electrons that leave the source are going to obey the laws of physics within whatever environment is so created. If a lightning leader of positive charges strikes close enough to affect the electrons leaving the transformer, those electrons (acting under the laws of physics) might find themselves in the cloud...far away from the source from which they came! The lightning has effectively thrown the proverbial wrench into our CREATED circuit (where electrons return to source), upending the "return to source" mantra for a brief time. And it is quite possible that as the lightning event subsides, there might be electrons that were pulled from the ground that find themselves sitting on the GEC the moment everything gets back to normal. These electrons might find their way through ground and even the neutral back to a transformer from which they never came...again following the laws of physics. Once this cycle is complete, the circuit returns to normal operation...current leaving...and returning to source.

          I guess in a qualitative way, we can say the GEC and grounding is a way to help mitigate the problems that occur when things throw a wrench into our carefully designed systems. There are lots of events that can happen...and grounding makes those effects less pronounced on humans and our equipment, and assists in getting things back to normal when a bad event occurs. Although I didn't get into it in the above, the GEC and grounding is not always good. There are some events where the connection to earth can cause more harm than good. We live in a crap shoot world. The powers that be (engineers with far more experience than I) have determined over the years that the good things from our grounding exceed the bad things. It's not all good. But here we are...and it seems to do pretty good.
          absolutely. In a lightning event or even when say a higher voltage distribution line falls on a lower distribution line you have two sources connected in some manner to the same circuit conductors. Grounding at the service entrance does help to shunt some of that undesirable voltage/current to earth, which in the case of lightning is one terminal of the source or in the case of higher voltage distribution, is quite often a pretty fair return path to the source. Earthing does little or nothing to help clear faults on under 1000 volts systems.
          I live for today, I'm just a day behind.

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