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

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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.?

2. I don't think it's effective for much at all other than giving a warm feeling to the dirt worshippers.

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Originally Posted by PetesGuide
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.

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Originally Posted by petersonra
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.

6. Originally Posted by jaggedben
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"

7. 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.

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.

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Originally Posted by electrofelon
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.

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Originally Posted by PetesGuide
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:

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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Originally Posted by INGMRS
Hello,
I recommend you watch this video by Mike Holt:

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!