AHJ's mandating two ground rods or testing - how STOP them?

[templdl]

They mandate greater than 6'

I got to thinking about this 25ohm issue and thought of it this way:
I thought of 2 parallel paths, one with 25 ohms is resistance and the other with less. The the same voltage is applied across which one would have more current flow through? The increase the voltage and even more current would flow. I realized that it is in our best interest to assure that the resistance of the ground rod should be as low as we are able to get because should another path to ground occur it would be that path where the current to ground be greater than that path to ground via the ground rod itself. As the potential to ground is increased as a result of a lightning strike which path to ground may be more favorable, the ground rod or another path that has less? Of course that second path to ground may not be present under normal conditions but but what would be the result if one unintentional ground should occur?
This is theoretical at best seems to be a reasonable to justify why it would be important to reduce the grounding resistance. Why was 25ohms selected as the standard? Good question though.

 

[kwired]

Can't view next page of the thread, this hasn't happened to me for a while now, but posting something usually fixes it.

edit: it worked.

 

[hurk27]

Can't view next page of the thread, this hasn't happened to me for a while now, but posting something usually fixes it.

edit: it worked.

Yep me too, I saw templdl posted but I wan't to wait till I could reply to it, you must have your post per page set to 40 like mine.

 

[hurk27]

I got to thinking about this 25ohm issue and thought of it this way:
I thought of 2 parallel paths, one with 25 ohms is resistance and the other with less. The the same voltage is applied across which one would have more current flow through? The increase the voltage and even more current would flow. I realized that it is in our best interest to assure that the resistance of the ground rod should be as low as we are able to get because should another path to ground occur it would be that path where the current to ground be greater than that path to ground via the ground rod itself. As the potential to ground is increased as a result of a lightning strike which path to ground may be more favorable, the ground rod or another path that has less? Of course that second path to ground may not be present under normal conditions but but what would be the result if one unintentional ground should occur?
This is theoretical at best seems to be a reasonable to justify why it would be important to reduce the grounding resistance. Why was 25ohms selected as the standard? Good question though.

The problem is that just because we have two rods, lets say both is 50 ohms, but they will not act like a single rod of 25 ohms and each will have separate shells and they do not reduce the resistance in the way we think when paralleling two resistors, this is because of the earth between the shells of the rod do not add up because each is its separate entity that is reference to the X/0 or center tap of the transformer and the MGN and miles of other reference points in the utilities supply system, when two rods are withing the sphere of influence or the other rod, it will reduce the resistance very little because you have not added any larger parallel paths for the current to flow,you are still within the area of soil of the first rod and have only added another connection to this same soil, this would be like taking a resistor and making another connection to it and expecting a lower reading from the same end of the resistor, with soil you might increase the surface of the connection to the resistor but you still have the same resistor.

The problem is that as I pointed out in the other post, a ground rod can only serve as a protection device if it was to be able to consistently open the OCPD, but we know it can not be depended upon this function as it is in many cases impossible to get a low enough resistance to consistently do this, as far a stopping step potential, if the ground rod can't not open the OCPD then it will sit there with the full applied voltage with the voltage dropping off drastically within the first few feet of the rod, and in most soils this would have a 75% or 90 volt drop that a person contacting these two points would receive this voltage across there body, now if the ground rod is remote from other metallic paths that are bonded to this ground rod then the difference of potential can be the full 120 volts because even at 12' from a rod you will start getting to the 98% of voltage drop, take a simple trailer such as the Coke a Cola concession stand in the thread that was linked to in the first post of the "Time to Eat Crow" thread, even if a ground rod was used and the EGC was not connected and there was a fault, if this ground rod was on the back side of the trailer away from the ordering window, the shell of this trailer could be as high or close to the full 120 volts to earth at this point, and a person who might be bare footed could be harmed, if the ground rod can not provide a fault path with a low enough impedance to open the breaker then it can not provide any protection from voltage to earth known as step potential.

It is simply because the earth that has more parallel paths the farther you get from the rod that will maintain the 0 volt reference to the transformer center tap or X/0 then the rod has the ability to bring the soil around the rod to the same potential of the rod because the rod only has a few parallel paths.

It doesn't matter the source of the voltage, a corner grounded 480 volt system will still have the same 75% voltage drop at the 3' shell from the rod it will just be a higher voltage or 380 volts, lightning striking something grounded to the rod while most of the energy will flow and other paths, the voltage left over will still drop across the earth around the rod at the same rate, so again the rod would not provide any protection from step potential even in a lightning strike, and because of the high frequency nature of lightning most of the current will flow on other paths.

 

[templdl]

I agree with you hurk27,
What my thoughts are are not limited the number of growing rods used to reduce the resistance to ground but the unknown path to ground. The lower you can make the grounding electric resistance one would like to think that any current that may likely flow through another unknown parallel path to ground would be reduced. If there was a potential path to ground that would require a sufficient enough voltage to jump a gap or maybe a resistance, if you provide a low enough grounding resistance to ground my thoughts would be that it would be less likely that any voltage the would remain would be insufficient to flow to ground in another possible unintended EGC path to ground.
I agree that a lightning strike one would have no idea as to what the results would be as it would most likely be a crap shoot no matter how well you ground the system.

 

[Fulthrotl]

Exception: If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.

This wording requires to use a supplemental electrode unless you can prove the first electrode is 25 ohms or less.

Not really that much of a difference in requirements but a little rewording changed it to basically require 2 rods unless you want to measure the first.

http://www.fluke.com/fluke/usen/earth-ground-testing/fluke-1630.htm?PID=56021

clamp it on, read the reading, and act accordingly.

i've got a fair bit of work coming up, and grounding will be an issue,
as far as setting ground rods goes.

ground rods aren't a good choice for this work. driving ANYTHING 8' deep
into the ground in this area isn't a good choice. my plan is to put 25' of
4/0 bare into the ditch, and slurry the duct bank.

as for verifying that it's a good ground, this tool, in my humble opinion, is
worth it's weight in gold, to me anyway. it's $1,600 bucks. put it in the bid,
use it, and move on. sinking ground rods in this location will involve potholing
with a hydrolick, setting a chemical ground, and grouting the hole with conductive
slurry.

$1,600 is a walk in the park, by comparison.

 

[hurk27]

I agree with you hurk27,
What my thoughts are are not limited the number of growing rods used to reduce the resistance to ground but the unknown path to ground. The lower you can make the grounding electric resistance one would like to think that any current that may likely flow through another unknown parallel path to ground would be reduced. If there was a potential path to ground that would require a sufficient enough voltage to jump a gap or maybe a resistance, if you provide a low enough grounding resistance to ground my thoughts would be that it would be less likely that any voltage the would remain would be insufficient to flow to ground in another possible unintended EGC path to ground.
I agree that a lightning strike one would have no idea as to what the results would be as it would most likely be a crap shoot no matter how well you ground the system.

Your kind of loosing me with this unintended path theory, if a system has been referenced to earth anywhere after the transformer you will always have a potential between an ungrounded (hot) conductor and earth, this is why it is so important to make sure we have a low impedance fault path back to source which is usually the service neutral and main bonding jumper along with the EGCs, keep in mind that since the transformer has been bonded to the MGN it will not only have the weaker electrode at the service but an electrode every ? mile far out of the sphere of influence of the rod at the house, it almost makes it silly to even have an electrode system at the service because the utility's electrodes will be way more effective then the service electrode system.

Any other unintended path to earth will not have a low enough impedance to eliminate a voltage difference between earth and this voltage source, and because of this, current will flow, think about an EGC, it will be a far lower impedance to the transformer grounded conductor, just because you have a lower impedance if anything even more current will flow if a ungrounded conductor was to unintentionally make a connection.

Now if you are talking about a grounded conductor such as a neutral making an unintended connection to earth then if there is any voltage drop on the neutral the unintended connection to earth will still cause a current to flow and there will be a difference of potential but if the current is low the earth may lower the amount of difference of potential but it would be a very little effect, and it will not stop it, we find this problem in stray current around pools and is why we tell people grounding electrodes will not stop this problem, only if the current causing the stray voltage is very low can a electrode system lower the voltage difference of potential, but with all the grounding done on the utility side and at the service already, if you still have the difference of potential then just adding more rods will have very little to no effect, if this was possible then dairy farms would not be having such a problem, because ground rods are the first thing most try because they do not understand the science behind how current flows in earth.

Keep in mind that even if you are able to achieve the 25 ohm connection to earth, if there is a voltage drop on the grounded neutral system that is causing enough voltage to be a problem then the current on this neutral will most likely far exceed the 4.8 amps that the ground rod can possibly shunt to earth even if you achieve a 10 ohm impedance you are only talking about 12 amps of possible current flow, 5 ohms would be 24 amps, so as you can see grounding can be very ineffective as a safety system, about the one thing they did good at was removing static voltages on Aeriel lines that wind and solar events caused to build up on the old telegraph lines back when they switched from using wood to glass pin insulators as I posted about earlier in the thread.

 

[Fliz]

I think the OP should consider that the geotechnical conditions probably contribute to those two jurisdictions requiring this. I know San Francisco is one of them, and I cannot count how many times I've been able to drive a rod at least halfway into the ground there with my bare hands. Half that city was sand dunes before they built on it. Sinking a rod into sand lessens its ability to conduct to ground compared to other soils. In fact, Ufer grounds were invented in WWII out in the sandy southwest for this very reason, their rods weren't working. Concrete encased electrodes make a superior electric ground than any type of soil.

Not sure what code cycle is being enforced there but the 2011 NEC section 250.53(A)(2) does require a single rod, pipe or plate electrode to be supplemented by an additional electrode unless you meet the exception which allows for the resistance testing.

Pete

In 2008, its 250.56: Asingle electrode consisting of a rod, pipe, or plate that does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of any of the types
specified by 250.52(A)(4) through (A)(8). Where multiple
rod, pipe, or plate electrodes are installed to meet the requirements
of this section, they shall not be less than 1.8 m
(6 ft) apart.

 

[templdl]

This issue is just getting to be too theoretical. Should there be a lightning strike there is no way for any of us to determine what the result wound be anyway.

 

[mdpolan]

Ground rods

Ground rods

Hey Really? You guys don't know why we install ground rods? Ever heard of lightning? Yea it's a pain I actually purchased a ground rod resistance meter ( 3 point). But after nearly all of the single rods exceeding 25 ohms I gave up and just double rod-ed. More importantly I have never found an actual spec that documents how that 25 ohms was to be tested. At what soil moisture content? at what depth? Can I pea on it to increase the soil conductivity? What if I happen to have a bucket of horse pea? I once installed a rod and the first question the inspector asked was "so how far down did you get?" There is shallow bed rock here and nobody has ever gotten 8' in this town! Keep rodding!

 

[kwired]

Hey Really? You guys don't know why we install ground rods? Ever heard of lightning? Yea it's a pain I actually purchased a ground rod resistance meter ( 3 point). But after nearly all of the single rods exceeding 25 ohms I gave up and just double rod-ed. More importantly I have never found an actual spec that documents how that 25 ohms was to be tested. At what soil moisture content? at what depth? Can I pea on it to increase the soil conductivity? What if I happen to have a bucket of horse pea? I once installed a rod and the first question the inspector asked was "so how far down did you get?" There is shallow bed rock here and nobody has ever gotten 8' in this town! Keep rodding!

You should have started reading this forum sooner and maybe you wouldn't have wasted money on a ground rod meter


Many participating in the conversation are well aware that one of primary purposes of grounding electrodes is lightning. That doesn't mean because you have a grounding electrode you will never have any damage from a lightning event. Even with a pretty low resistance electrode, you still have currents that find alternate paths. The more direct the strike the more you will find alternate paths seems to be pretty common.

 

[Smart $]

... Even with a pretty low resistance electrode, you still have currents that find alternate paths. The more direct the strike the more you will find alternate paths seems to be pretty common.

Must be feeling pitnicky at the moment , but I believe a better word would be "additional".

 

[kwired]

Must be feeling pitnicky at the moment , but I believe a better word would be "additional".

I agree that would be a better word to use.

 

[templdl]

When I upgraded my person load center some time ago there was only one rod directly below the load center in my basement as well as being bonded to a copper water pipe nearest to the load center. I drove (2) rods at least 6' apart outside my home and then ran a ground all the may across my basement bonding it directly to my water service where it enter my basement.
Yes, it is a great improvement over what I had. Will it provide better protection against a lightning strike? I wound like to think so but there are still no guarantees. If a lightning strike is intense enough the (2) ground rods and even bonding to an additional grounding electrode such as the water service may not be sufficient enough to dissipate the lightning discharge to ground and the charge may be intense enough to seek other paths to ground. I would like the lightning strike to play by my rules but I know that it isn't the case. One ends up playing the odds that the installed grounding system would be sufficient enough to protect the distribution system but there as still no guarantees.
Lightning protection aside I believe that when the neural is grounded in and of by itself I believe helps stabilized the electrical distribution system sort of like acting as if the earth was a big capacitor. Some while back I may have read about that providing a benefit.

 

[kwired]

When I upgraded my person load center some time ago there was only one rod directly below the load center in my basement as well as being bonded to a copper water pipe nearest to the load center. I drove (2) rods at least 6' apart outside my home and then ran a ground all the may across my basement bonding it directly to my water service where it enter my basement.
Yes, it is a great improvement over what I had. Will it provide better protection against a lightning strike? I wound like to think so but there are still no guarantees. If a lightning strike is intense enough the (2) ground rods and even bonding to an additional grounding electrode such as the water service may not be sufficient enough to dissipate the lightning discharge to ground and the charge may be intense enough to seek other paths to ground. I would like the lightning strike to play by my rules but I know that it isn't the case. One ends up playing the odds that the installed grounding system would be sufficient enough to protect the distribution system but there as still no guarantees.
Lightning protection aside I believe that when the neural is grounded in and of by itself I believe helps stabilized the electrical distribution system sort of like acting as if the earth was a big capacitor. Some while back I may have read about that providing a benefit.

Grounding electrodes do help stabilize voltage to ground. But when current is imposed on the electrode you can kind of forget that as there will be a voltage drop on most electrodes because they will all have some impedance.

And as Hurk has mentioned the high voltage/high frequency currents in a lightning strike may find some unintended paths to be of less impedance than they would be to 60 Hz. low voltage current.

Any time lightning has struck a building or nearby a building when I have been called to repair damages, seems like the telephone lines always take a good beating, yet they would have a high impedance to a 60 Hz 600 volt or less system.

 

[jaggedben]

There are now two jurisdictions in No Calif. mandating we install two ground rods in lieu of having a testing agency prove less than 25 ohms resistance to actual earth.... So, hats off to San Francisco and Palo Alto Calif. forcing homeowners to spend more money on a pointless installation. And a bow to your hard-headedness in sticking to it.

... So, yielding the point that this is a safer installation, for now in particular with respect to lightning, why do not all areas mandate the two rods or testing?

In the case of San Francisco, it's because very few houses are new enough to have a UFER. Many still don't have any grounding at all.

In the case of Palo Alto, it's because it's Rhonda.

 

[templdl]

Grounding electrodes do help stabilize voltage to ground. But when current is imposed on the electrode you can kind of forget that as there will be a voltage drop on most electrodes because they will all have some impedance.

And as Hurk has mentioned the high voltage/high frequency currents in a lightning strike may find some unintended paths to be of less impedance than they would be to 60 Hz. low voltage current.

Any time lightning has struck a building or nearby a building when I have been called to repair damages, seems like the telephone lines always take a good beating, yet they would have a high impedance to a 60 Hz 600 volt or less system.

Those are my thoughts to a tee.

 

[grasfulls]

Rhonda

Rhonda

In the case of San Francisco, it's because very few houses are new enough to have a UFER. Many still don't have any grounding at all.

In the case of Palo Alto, it's because it's Rhonda.

Only those of us who have worked there get the levity involved here. Too funny. But, I must now say, after all has been said in here, it is because of her making us comply with the NEC, so I can no longer fault her. Glad you mentioned names, I sorta left that open. That said, again after all of this thread, I am embarrassed by my statements implying their mandate by stupidity. Though they may do this with ignorance as to why, they have just cause. (I have learned to enjoy CROW in these forums, BTW).

 

[jmellc]

And if you have one little hole in the concrete walkway where the underground electric and gas come up, there may not be anywhere to go 6' away without drilling or crossing the walkway.

I had 1 install where I had a few feet of earth under the meter, then concrete walkway/patio for 20-30 feet. I drove 1 rod near the meter, looped from it to the wall. Strapped to the wall along 1st mortar joint, turned a corner & went a few more feet til I had earth again & drove my 2nd rod there. I just priced it into the job.

 

[jmellc]

You should have started reading this forum sooner and maybe you wouldn't have wasted money on a ground rod meter


Many participating in the conversation are well aware that one of primary purposes of grounding electrodes is lightning. That doesn't mean because you have a grounding electrode you will never have any damage from a lightning event. Even with a pretty low resistance electrode, you still have currents that find alternate paths. The more direct the strike the more you will find alternate paths seems to be pretty common.

Speaking of lightning, does anyone know a good book or manual on it? I have had customers ask why lightning did this and didn't do that when a path seemed obvious but was not followed. I always told them "I don't know. Lightning follows its own rules, if any".