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

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kwired said:
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.
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:thumbsup: agreed.
 
electrofelon said:
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.
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Let's say there is a 1 ohm path and a 10000 ohm path...and 100V. Path one is 100A. Path two is 10 milliamps.

Now let's say a 1000 ohm human finds himself/herself in series with each of those paths.

Path 1 is now 1001 ohms (1000 for the human and 1 for the original path). The current, within a decimal or two, is now 100 milliamps and the voltage is still 100. The voltage will be proportional to the resistances along the path. Within a decimal or two, the portion across the human is 100 milliamp at 100V.

Path 2 is now 11000 ohm (1000 for the human and 10000 for the original path). The current is now about 9 milliamps. Within a decimal or two the portion across the human is 9 milliamps at 9V.

A human in series with Path 1 is in trouble. A human in series with Path 2....probably OK.
 
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PetesGuide said:
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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One of the most common code violations I've seen throughout dozens of chemical plants/refineries is the absence of a GEC on Delta-Wye transformers.
The vast majority of ground fault will travel via the EGC within the circuit conductors - see NEC 250.134(B) handbook - however a small percentage will divide into the GEC at the XO. A lightning strike that hits the earth (nearby) will most likely also hit the GEC as well and travel into the XO.
 
JPinVA said:
Let's say there is a 1 ohm path and a 10000 ohm path...and 100V. Path one is 100A. Path two is 10 milliamps.

Now let's say a 1000 ohm human finds himself/herself in series with each of those paths.

Path 1 is now 1001 ohms (1000 for the human and 1 for the original path). The current, within a decimal or two, is now 100 milliamps and the voltage is still 100. The voltage will be proportional to the resistances along the path. Within a decimal or two, the portion across the human is 100 milliamp at 100V.

Path 2 is now 11000 ohm (1000 for the human and 10000 for the original path). The current is now about 9 milliamps. Within a decimal or two the portion across the human is 9 milliamps at 9V.

A human in series with Path 1 is in trouble. A human in series with Path 2....probably OK.
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Your example doesn't make sense for two reasons: first, the resistance values are skewed too far out of the realistic range. Second, it is very unlikely a person would find themselves in series with a fault.
 
PetesGuide said:
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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Here, it is stops the system from floating to some indeterminate voltage wrt ground.
 
JPinVA said:
...
...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.
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Not just to be picky, but to hopefully add to the understanding, to be correct the current varies in inverse proportion to the resistance.
Another way of saying the same thing is that the current varies proportionally to the conductance/admittance of the path. (Since conductance is the inverse of resistance and admittance is the inverse of impedance. They used to be measured in Mhos (Ohms backwards), but now have been systematized to the unit of Siemens.)
 
I suppose earthing does provide some reduction in potential difference for a small item, like a lamp post, where anytime you would be touching the lamp post, you would be very close to the electrode. There is a good diagram I cant find, maybe someone will post, that shows the voltage gradient as you move away from a typical electrode.
 
electrofelon said:
Your example doesn't make sense for two reasons: first, the resistance values are skewed too far out of the realistic range. Second, it is very unlikely a person would find themselves in series with a fault.
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Actual values are not pertinent to explaining the existence of the difference, nor is the liklihood of the event. I used the assumptions to make a quantitative analysis to show the difference.

I can also explain things in quantitative ways. In our charged world, the things around us become charged. Charged, as in containing different values of positive or negative charges with respect to ground. Without a low impedance path to earth, the charges can build up to the point of a very high potential difference (between ground and the objects so charged). And the bigger the object(s) so charged, the bigger the aggregate number of charges.

The only thing stopping the charges from getting to ground is the lack of a path. If the potential gets high enough, it will eventually exceed the voltage necessary to overcome the dielectric properties of the things between these charges and the earth. And when that moment comes....ZAP!

As a human has dielectric properties of less impedance than most high impedance "things", human interaction in the charged environment can often close the dielectric gap between the build up of charges and ground. The human becomes the pathway for the built up charges to reach ground.

Note this is similar to...but not the same as...those static discharges we get on cold winter mornings. Our morning zaps are due to charges that have built up on ourselves...so the flow is limited to that equalization....it's not through us...but from/to us. When equipment (or a building) builds up charges...the amount of charge gets ever bigger as the equipment or building gets bigger. These can be of sufficient quantity to be lethal when passing through a human that "closes the connection" to ground. Or, they can simply cause a fire or an explosion if the zap is sufficient to start something burning or initiate an explosive reaction.

Now...introduce the GEC and associated bonding. The existence of that ground connection allows those built-up charges to bleed away into ground in an orderly fashion. It prevents the build up of charges to dangerous levels. Directly dangerous, as when a human becomes part of the circuit (or, more precisely, simply the path required to equalize the charges)....or indirectly dangerous as when the zap causes a fire or an explosion.

Note that there are indeed instances when even the GEC isn't sufficient to protect someone. A lightning event, even an indirect event, can deposit charges more rapidly over a short duration than the GEC and bonding can handle. This raises the possibility of the human once again closing the air gap during such an event. But all said, a GEC, even though it can't prevent this from happening in all instances, will still provide a measure of protection (better than nothing) during the event and a larger measure of protection as it equalizes the charges after the event.
 
electrofelon said:
I suppose earthing does provide some reduction in potential difference for a small item, like a lamp post, where anytime you would be touching the lamp post, you would be very close to the electrode. There is a good diagram I cant find, maybe someone will post, that shows the voltage gradient as you move away from a typical electrode.
Click to expand...
Is this the one?

touch potential.jpg
 
Keep in mind we're dealing with different (and fundamentally separate) situations where the GEC and grounding are at play.

The street lamp example shows grounding's effect on a shorted wire from a continuous source. That is not the same effect as what can happen on a line surge, or a lightning or static event. We are also dealing with the difference around the rod versus within the structure.

In the case of a source effect, we will get a ground potential rise as shown in the figure....and if the current is not sufficient to trip the OCPD, that rise and potential will CONTINUE TO EXIST as long as the source continues to provide power and the short to ground still exists. It's a constant step and touch danger close to the rod.

With a line surge...or a lightning event, a ground potential rise will indeed occur, but it will dissipate quickly as everything equalizes. Without a constant source to feed the ground potential rise, things equalize. So if you're standing next to the rod as it dumps the current...there is a possible danger in the same manner as shown in the figure. But this danger rapidly subsides. As for being within the structure, my previous post explains the danger and how the connection helps makes things safer.
 
JPinVA said:
Actual values are not pertinent to explaining the existence of the difference, nor is the liklihood of the event. I used the assumptions to make a quantitative analysis to show the difference.

I can also explain things in quantitative ways. In our charged world, the things around us become charged. Charged, as in containing different values of positive or negative charges with respect to ground. Without a low impedance path to earth, the charges can build up to the point of a very high potential difference (between ground and the objects so charged). And the bigger the object(s) so charged, the bigger the aggregate number of charges.

The only thing stopping the charges from getting to ground is the lack of a path. If the potential gets high enough, it will eventually exceed the voltage necessary to overcome the dielectric properties of the things between these charges and the earth. And when that moment comes....ZAP!

As a human has dielectric properties of less impedance than most high impedance "things", human interaction in the charged environment can often close the dielectric gap between the build up of charges and ground. The human becomes the pathway for the built up charges to reach ground.

Note this is similar to...but not the same as...those static discharges we get on cold winter mornings. Our morning zaps are due to charges that have built up on ourselves...so the flow is limited to that equalization....it's not through us...but from/to us. When equipment (or a building) builds up charges...the amount of charge gets ever bigger as the equipment or building gets bigger. These can be of sufficient quantity to be lethal when passing through a human that "closes the connection" to ground. Or, they can simply cause a fire or an explosion if the zap is sufficient to start something burning or initiate an explosive reaction.

Now...introduce the GEC and associated bonding. The existence of that ground connection allows those built-up charges to bleed away into ground in an orderly fashion. It prevents the build up of charges to dangerous levels. Directly dangerous, as when a human becomes part of the circuit (or, more precisely, simply the path required to equalize the charges)....or indirectly dangerous as when the zap causes a fire or an explosion.

Note that there are indeed instances when even the GEC isn't sufficient to protect someone. A lightning event, even an indirect event, can deposit charges more rapidly over a short duration than the GEC and bonding can handle. This raises the possibility of the human once again closing the air gap during such an event. But all said, a GEC, even though it can't prevent this from happening in all instances, will still provide a measure of protection (better than nothing) during the event and a larger measure of protection as it equalizes the charges after the event.
Click to expand...

JPinVA said:
Keep in mind we're dealing with different (and fundamentally separate) situations where the GEC and grounding are at play.

The street lamp example shows grounding's effect on a shorted wire from a continuous source. That is not the same effect as what can happen on a line surge, or a lightning or static event. We are also dealing with the difference around the rod versus within the structure.

In the case of a source effect, we will get a ground potential rise as shown in the figure....and if the current is not sufficient to trip the OCPD, that rise and potential will CONTINUE TO EXIST as long as the source continues to provide power and the short to ground still exists. It's a constant step and touch danger close to the rod.

With a line surge...or a lightning event, a ground potential rise will indeed occur, but it will dissipate quickly as everything equalizes. Without a constant source to feed the ground potential rise, things equalize. So if you're standing next to the rod as it dumps the current...there is a possible danger in the same manner as shown in the figure. But this danger rapidly subsides. As for being within the structure, my previous post explains the danger and how the connection helps makes things safer.
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I think you are giving too much credit to equipment earthing, and most of what you are discussing is mitigated by bonding not earthing. What are all these charges that keep building up? Are people leaving their Van Degraaff generators on all day ;) I dont see how equipment earthing will mitigate a "line surge" (whatever that is exactly). I think much of this plays on the theory that "electricity goes to ground". Yes charges can develop relative to ground in some cases, but in other they could care less about the ground. I am not saying it does absolutely nothing, just that it is very minor, and exceedingly rare that it results in a life or property saving event.
 
Do you think lightning protection systems are a joke as well? the whole lightning protection code book and engineering designs.

and another one, they used to use the ground as a conductor for the old telegraph systems
 
Wire-Smith said:
Do you think lightning protection systems are a joke as well? the whole lightning protection code book and engineering designs.

and another one, they used to use the ground as a conductor for the old telegraph systems
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Using the ground as a return conductor for wired telegraphy only worked because the signal was a very simple binary signal, wherein the dashes were three times longer than the dots, so any noise or other problems created by the lossy and high-resistance Earth were basically moot. Thus, using one wire instead of two was a cost-saving measure, not related to noise or safety.

As soon as people started trying to use telegraph wires for telephone circuits, the noise caused by this basically forced telephone circuits to use two-wire circuits.

I don't know if once the telephone circuits switched to two-wire distribution, if the remaining telegraph circuits (which lasted until the 1970s) were converted to two-wire circuits or remained as ground-return one-wire circuits.

It would be interesting to research when telegraph/telephone circuits began to be grounded for lightning protection.

--
DE K6WEB
General-class Ham, licensed since 2003.
 
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PetesGuide said:
Using the ground as a return conductor for wired telegraphy only worked because the signal was a very simple binary signal, wherein the dashes were three times longer than the dots, so any noise or other problems created by the lossy and high-resistance Earth were basically moot. Thus, using one wire instead of two was a cost-saving measure, not related to noise or safety.

As soon as people started trying to use telegraph wires for telephone circuits, the noise caused by this basically forced telephone circuits to use two-wire circuits.

I don't know if once the telephone circuits switched to two-wire distribution, if the remaining telegraph circuits (which lasted until the 1970s) were converted to two-wire circuits or remained as ground-return one-wire circuits.

It would be interesting to research when telegraph/telephone circuits began to be grounded for lightning protection.

--
DE K6WEB
General-class Ham, licensed since 2003.
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i just meant it will obviously conduct

also i guess i should add for clarity, i in know way think the grounding electrode is for the opening of OCPD. poorly installed conduit being used as EGC or long wire runs with no do regard for increasing the size of circuit conductors for OCPD operation and ran close to ampacity will keep a breaker from opening if you hold the hot and neutral directly together at the far end. earths a poor conductor at 120/240, so is a long #12 copper wire when its heated up.
 
electrofelon said:
I think you are giving too much credit to equipment earthing, and most of what you are discussing is mitigated by bonding not earthing. What are all these charges that keep building up? Are people leaving their Van Degraaff generators on all day ;) I dont see how equipment earthing will mitigate a "line surge" (whatever that is exactly). I think much of this plays on the theory that "electricity goes to ground". Yes charges can develop relative to ground in some cases, but in other they could care less about the ground. I am not saying it does absolutely nothing, just that it is very minor, and exceedingly rare that it results in a life or property saving event.
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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.
 
i agree grounding electrode systems(except equipotential style) do little for shock prevention, but they help with fire prevention. one of the if not the main reason we started grounding residential services is back in the day transformer primaries would fail and energize secondaries, when the distribution(primary) was using a multi grounded neutral your interior home wires could become energized at the primary voltage to ground (the primary system was grounded) and if you have ever seen or could imagine medium or high voltage on wood in contact with ground/earth(with a grounded system) without proper insulators it makes for a good bon fire.

another thing. overhead lines are a huge grid of conductors, if any area is near a lightning stroke the entire line can see a surge voltage to ground, this is why the GEC is at the service disconnect, to give any residual of that surge a path to earth that is more favorable to the home insurer and heavy sleepers.

another item to think about, every utility pole i know of (except where copper thieves hit) has a little number 6 run all the way down and usually uses a butt ground on bottom of pole. if there's anybody willing to experiment with cost saving measures, many utilities are in that group. they usually run a static line at the top of the poles (it serves as a lightning protection system, not perfectly but it does) and connect it to that little number 6 so the poles don't burn. they use lightning arrestors connected to that little number 6 and butt ground and poles don't burn and my power often cuts out during lightning strikes, how about yours? but i often see many trees hit by lightning burn.

i do admit there are some grounding codes that are antiquated with the way things are done nowadays which add to confusion and should be cleaned up just for that. but tying to earth does have good reason.
 
Wire-Smith said:
i'm interested if your willing to share a scenario where it will do more harm
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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.
 
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