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

[Wire-Smith]

One I can think of occurs whenever there are two groundings, either intentional (e.g., when a ground rod is provided for a generator) or when some other ground node exists that is part of the bonding in the house, with a higher ground impedance to the electrode underneath. These situations create a lower impedance path THROUGH the bonding between the nodes than through the earth.

When a lightning even occurs, a potential gradient is produced around the lightning strike. A house that is in this gradient, will experience the gradient...one side to the other....one ground node to the other. This voltage between ground nodes will result in current flowing up one node (GEC or the other ground point)...though the bonding EGC...and down the other node (other ground point or GEC). Equipment in the bonding path will be subject to whatever current/voltage exists due to the lightning strike and related impedance and couplings.

i believe you are looking at that wrong, please let me try to explain why i think that.

consider a copper wire, call it a #12AWG not that's its too relevant but to make sure we are on same page. say this #12 is feeding a light (100w incandescent if you want, but again not really relevant) , and the circuit is energized.120V

now say for what ever reason say we are in an electrician training lab and this wiring is all exposed and not buried in a wall or anything and its just single conductor thhn run on plywood

now say the "hot"(many call it that)(black wire)(neutral is white), now say the "hot" is stripped in two areas about 5' apart. now say you have rubber boots on whatever, say we are running this all off an isolation transformer with an ungrounded secondary. now i grab with one bare sweaty hand one of the bare spots in the black wire and the other bare spot with my other hand. i don't feel anything.

now if i would have a bare spot in the neutral wire and grab neutral with one hand and hot with the other i would get shocked.

now, how does this apply to what you were talking about

when i grabbed the black in two spots i was connected to the circuit in two spots. the thing is there was very little resistance between those two points in that black wire.

then when i switched it up i got shocked, not because white wires hate me, but because there was more resistance(the light) between the points i was grabbing.

so now i ask you, how is it that solidly bonding things creates more voltage gradients. you yourself just said a lightning stroke alone produces voltage gradients.

why do you think we use equipotential grounding grid in high voltage substations? the lower the resistance between everything the less the voltage will be between them.

if theres no resistance/impedance (me grabbing two parts in the black) there is a lot/almost no voltage/difference of potential, when there is resistance/impedance between the two parts (grabbing black and white) then there is a larger voltage/ difference of potential

now before you say i am short circuiting the circuit, i am, and thats what your bonding would do instead of something else(a person or flammable material) in the situation you referred to.

 

[electrofelon]

why do you think we use equipotential grounding grid in high voltage substations? the lower the resistance between everything the less the voltage will be between them.

if theres no resistance/impedance (me grabbing two parts in the black) there is a lot/almost no voltage/difference of potential, when there is resistance/impedance between the two parts (grabbing black and white) then there is a larger voltage/ difference of potential

now before you say i am short circuiting the circuit, i am, and thats what your bonding would do instead of something else(a person or flammable material) in the situation you referred to.

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. You cant bond dirt. An equipotential grid at a substation isnt about trying to bond the dirt. You could have an extraordinary single point ground of .1 ohms and that still doesnt get you there. You are making an extensive conductor network or "mat" so you are like a bird on a wire when a MV fault happens. I just want to emphasize that because people often take the takeaway from your example of less resistance=less shock and falsely extend it to grounding electrode resistance.

 

[Wire-Smith]

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. You cant bond dirt. An equipotential grid at a substation isnt about trying to bond the dirt. You could have an extraordinary single point ground of .1 ohms and that still doesnt get you there. You are making an extensive conductor network or "mat" so you are like a bird on a wire when a MV fault happens. I just want to emphasize that because people often take the takeaway from your example of less resistance=less shock and falsely extend it to grounding electrode resistance.

thanks, yeah good point on the ground rod resistance. a ground rod isn't going to do much for step and touch potential.

but i will say i actually do try to use the right terminology "bonding" and "grounding", and think it's very necessary don't get me wrong i really do.

but i actually see grounding as a type of bonding, bonding the earth to the non current carrying metal electrical equipment and also usually the electrical system itself as well(current carrying conductors). i don't completely disagree with you that they are completely different topics. but i see grounding as a type of bonding. its just special because the earth isn't usually as conductive as metallic normally non current carrying parts and even more disproportionate with low voltages.

 

[JPinVA]

i believe you are looking at that wrong, please let me try to explain why i think that.

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.

 

[Wire-Smith]

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.

your scenario details don't all sound right to me, but i'll play along.

a gradient would have been across the house if the cable didn't have a ground rod. i do agree the cable should be brought in at electrical system ground though.

a gradient would have been across the house without either ground rod, lightning doesn't discriminate against things because they're not metal, go ask a tree or lightning victims(the guy that was holding the meter in your scenario).

how did they measure this current?
how did they know this lightning was coming to set up these meters of which there are none large enough ever made to measure this current?

 

[Wire-Smith]

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)

 

[GoldDigger]

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.

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[PetesGuide]

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!

 

[kwired]

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.