Remote building ground electrode - function?

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W@ttson

W@ttson

Senior Member
Location
USA
Hello,
So I know the drill regarding detached/remote building which get a feeder for a sub panel; feeder cable, EGC, create Ground electrode system, typically with two ground rods. EGC and new Ground electrode system bonded together.
My question is what is the function of the supplemental ground rods? I always explain it as assuring equipotential between the earth at the service entrance and the earth at the remote building. However, scouring this forum, I am seeing people talk about that the remote ground rods are to help the detached building deal with lightning strikes and static electricity. I find this tough to swallow. I think what makes more sense is the reasoning behind them is that the EGC may have a potential difference by the time it reaches that remote earth location and driving another rod there reduces the resistance of the overall ground electrode system and brings that potential of the egc back to “0v”. Any input greatly appreciated.
 
spraymax6 said:
Hello,
So I know the drill regarding detached/remote building which get a feeder for a sub panel; feeder cable, EGC, create Ground electrode system, typically with two ground rods. EGC and new Ground electrode system bonded together.
My question is what is the function of the supplemental ground rods? I always explain it as assuring equipotential between the earth at the service entrance and the earth at the remote building. However, scouring this forum, I am seeing people talk about that the remote ground rods are to help the detached building deal with lightning strikes and static electricity. I find this tough to swallow. I think what makes more sense is the reasoning behind them is that the EGC may have a potential difference by the time it reaches that remote earth location and driving another rod there reduces the resistance of the overall ground electrode system and brings that potential of the egc back to “0v”. Any input greatly appreciated.
Click to expand...
IMo it serves no purpose.
 
oldsparky52 said:
Do either of you know what the NEC gods used as their reasoning to have this requirement written into the code?
Click to expand...
I would guess that it's for the same reason why you need to have a GES where the service disconnect is located.
 
infinity said:
I would guess that it's for the same reason why you need to have a GES where the service disconnect is located.
Click to expand...
I think there is a difference though. A service may not be a grounded system, depending on the utility distribution system. the system serving a remote structure will definitely be grounded already, so the discussion is just about the proximity of electrodes to the structure
 
infinity said:
I would guess that it's for the same reason why you need to have a GES where the service disconnect is located.
Click to expand...
At the service, there is definitely something happening when you add to the ground electrode system. There is a whole IEEE book (don’t rememeber the number at the moment) which intensively goes into detail how different electrode configurations drop the resistance of the ground.
There is also the Fall of potential test to measure that ground resistance. Fluke goes into it here: https://www.fluke.com/en-us/learn/blog/grounding/earth-ground-fall-potential

That IEEE book basically goes into detail on how to arrange those ground electrodes and get them out of each others sphere of influence.
 
Greentagger said:
250.4(A)(1).
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As mentioned by Greentagger, NEC 250.4(A)(1) is the reason. Here is a bit more backup, although I do not downplay it as much as the others

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oldsparky52 said:
Do either of you know what the NEC gods used as their reasoning to have this requirement written into the code?
Click to expand...
My comments on this are limited to the NEC level of enforcement. There are valid reasons for electrical utility systems to be grounded or radio and cell phone towers.

There is a whole history of grounding/earthing that I have never taken the time to research but it goes back to the telegraph era. I think the reason mystique around earthing still exists today is it has been so ingrained in the thinking throughout the electrical world that no one questions it any more or no one wants to be seen among their peers as an oddball.
 
ActionDave said:
My comments on this are limited to the NEC level of enforcement. There are valid reasons for electrical utility systems to be grounded or radio and cell phone towers.

There is a whole history of grounding/earthing that I have never taken the time to research but it goes back to the telegraph era. I think the reason mystique around earthing still exists today is it has been so ingrained in the thinking throughout the electrical world that no one questions it any more or no one wants to be seen among their peers as an oddball.
Click to expand...


Yes, I am getting that Oddball treatment at the moment. :)
 
spraymax6 said:
wow I was not expecting this response.
My faith in the NEC is starting to shatter a bit. How can this requirement be for such frivolous reasons?
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The scales are falling off your eyes. Chunks of the code are frivolous and some of the worst examples are in 250. 250.122(B) makes zero sense, 250.148 is another stupid requirement. 250.64(C) is a horrible requirement because it lends validity to the notion that earthing is what makes a building electrical system safe when proper equipment grounding is where the focus should be.
 
ron said:
As mentioned by Greentagger, NEC 250.4(A)(1) is the reason. Here is a bit more backup, although I do not downplay it as much as the others

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Click to expand...
250.4(A)(1):
(1) Electrical System Grounding. Electrical systems that are
grounded shall be connected to earth in a manner that will
limit the voltage imposed by lightning, line surges, or uninten
tional contact with higher-voltage lines and that will stabilize
the voltage to earth during normal operation.
Informational Note No. I: An important consideration for limit
ing the imposed voltage is the routing of bonding and ground
ing electrode conductors so that they are not any longer than
necessary to complete the connection without disturbing the
permanent pans of the installation and so that unnecessary
bends and loops are avoided.

So here are the reasons:
1. limit the voltage imposed by lightning
2. limit line surges
3. limit the voltage imposed by unintentional contact with higher-voltage lines
4. stabilize the voltage to earth during normal operation

For 1 above, it is to reduce effects of lightning.
For 2 above, how is the ground electrode limiting line surges?
For 3 above, I guess since the higher voltage system and the lower voltage system both would be bonded to ground, a path can open up through the earth to trip the higher voltage system? I don't know just trying to explain this one. (From the helpful link that Ron posted: "Author’s Comment: For systems over 600 V, the earth helps the utility to clear a line-to-ground fault, but for systems operating at less than 600, the earth will not remove dangerous voltages on the metal parts of the electrical system from a line-to-ground fault. ")
For 4 above, I guess this is the "bring all voltage references to the same plane" explanation I was trying to give.
 
spraymax6 said:
Hello,
So I know the drill regarding detached/remote building which get a feeder for a sub panel; feeder cable, EGC, create Ground electrode system, typically with two ground rods. EGC and new Ground electrode system bonded together.
My question is what is the function of the supplemental ground rods? I always explain it as assuring equipotential between the earth at the service entrance and the earth at the remote building. However, scouring this forum, I am seeing people talk about that the remote ground rods are to help the detached building deal with lightning strikes and static electricity. I find this tough to swallow. I think what makes more sense is the reasoning behind them is that the EGC may have a potential difference by the time it reaches that remote earth location and driving another rod there reduces the resistance of the overall ground electrode system and brings that potential of the egc back to “0v”. Any input greatly appreciated.
Click to expand...
I like the way you explained it and agree with what you said about keeping the ground potential as close to earth as possible. If there was a voltage gradient across the land, you could have some voltage to ground potential at the remote building if additional ground rods were not installed. I don't know if it is really that big of a problem and I don't know if that is why the NEC gods wrote the requirement into the NEC.
 
spraymax6 said:
250.4(A)(1):
(1) Electrical System Grounding. Electrical systems that are
grounded shall be connected to earth in a manner that will
limit the voltage imposed by lightning, line surges, or uninten
tional contact with higher-voltage lines and that will stabilize
the voltage to earth during normal operation.
Informational Note No. I: An important consideration for limit
ing the imposed voltage is the routing of bonding and ground
ing electrode conductors so that they are not any longer than
necessary to complete the connection without disturbing the
permanent pans of the installation and so that unnecessary
bends and loops are avoided.

So here are the reasons:
1. limit the voltage imposed by lightning
2. limit line surges
3. limit the voltage imposed by unintentional contact with higher-voltage lines
4. stabilize the voltage to earth during normal operation

For 1 above, it is to reduce effects of lightning.
For 2 above, how is the ground electrode limiting line surges?
For 3 above, I guess since the higher voltage system and the lower voltage system both would be bonded to ground, a path can open up through the earth to trip the higher voltage system? I don't know just trying to explain this one. (From the helpful link that Ron posted: "Author’s Comment: For systems over 600 V, the earth helps the utility to clear a line-to-ground fault, but for systems operating at less than 600, the earth will not remove dangerous voltages on the metal parts of the electrical system from a line-to-ground fault. ")
For 4 above, I guess this is the "bring all voltage references to the same plane" explanation I was trying to give.
Click to expand...

In theory, if a source of high voltage (lightning or downed high voltage line) comes in from the service, electrodes give the current somewhere that it is likely to want to go and that would reduce the amount of current that flows through the building wiring. It would generally be the case that the electrodes will make some difference, no matter how small. In practice, when you do the math, it seems that in the typical case that difference will not be meaningful for an individual rod electrode. Now if all the buildings in a nieghborhood are grounded to a metal water distribution system, as was the case in many cities decades ago, then you have a very effective widespread grounding-grid that really helps if any voltage is imposed on the grounded conductors that run through the utility distribution system. Even lacking that, the idea would be that if all the neighborhood properties services are grounded, then those nearer the source of the problem mitigate the voltage for those that are farther away, essentially spreading out and minimizing the damage to any particular property. It is a collective insurance program, only really works if everybody does it.

In this context, the likelihood that additional electrodes at feeder-fed outbuildings do anything particularly worthwhile is quite questionable. First, if the high voltage source is imposed between the service and the outbuilding (direct hit on the property), then my guess is most likely that property suffers serious damage regardless. Whereas if the high-voltage source comes from outside the property, and the service grounding isn't enough to mitigate damage in the particular circumstances, it would be unlikely that the additional outbuilding electrode makes a meaningful difference on the local property. Either the voltage coming in has been mitigated by all the neighborhood grounding in between the source and the property or it hasn't. Finally, with regard to reducing shock potential for improperly-bonded-and-accidentally-energized parts, the effect of an electrode is extremely localized; pretty much doesn't do anything unless your standing right on top of it.

Given all these factors, I think I agree with electrofelon. :cool:
 
spraymax6 said:
250.4(A)(1):
(1) Electrical System Grounding. Electrical systems that are
grounded shall be connected to earth in a manner that will
limit the voltage imposed by lightning, line surges, or uninten
tional contact with higher-voltage lines and that will stabilize
the voltage to earth during normal operation.
Informational Note No. I: An important consideration for limit
ing the imposed voltage is the routing of bonding and ground
ing electrode conductors so that they are not any longer than
necessary to complete the connection without disturbing the
permanent pans of the installation and so that unnecessary
bends and loops are avoided.

So here are the reasons:
1. limit the voltage imposed by lightning
2. limit line surges
3. limit the voltage imposed by unintentional contact with higher-voltage lines
4. stabilize the voltage to earth during normal operation

For 1 above, it is to reduce effects of lightning.
For 2 above, how is the ground electrode limiting line surges?
For 3 above, I guess since the higher voltage system and the lower voltage system both would be bonded to ground, a path can open up through the earth to trip the higher voltage system? I don't know just trying to explain this one. (From the helpful link that Ron posted: "Author’s Comment: For systems over 600 V, the earth helps the utility to clear a line-to-ground fault, but for systems operating at less than 600, the earth will not remove dangerous voltages on the metal parts of the electrical system from a line-to-ground fault. ")
For 4 above, I guess this is the "bring all voltage references to the same plane" explanation I was trying to give.
Click to expand...
My thoughts:

#1. Just not real confident a few (more) rods will do anything meaningful to mitigate damage from a lightning event. I suppose I could be convinced if there were some facts or detailed scientific analysis on the subject.

#2. This should be deleted. This statement has been questioned by many members of this forum over the years.

#3. This is valid

#4. I think this should be deleted or reworded. It is true but I fear many read too much into it. You just create a fixed voltage reference by grounding a system. It gets rid of capacitive coupling that would give unpredictable and varying voltage readings to ground. The key is the "during normal operation" part - it all goes to hell during a fault. Also you don't need a low resistance to ground to get your stable ground reference. In the context of the specific situation of an outbuilding, the system is already grounded (ignoring ungrounded systems). If you have some difference of potential between dirt and the egc system at a remote structure, adding some more ground rods isn't really going to do anything, I mean maybe you could close it up some fraction of a volt, but that's about it.
 
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