driven ground rods 6' apart

[G._S._Ohm]

@Petersonra, it sounds like fraud.
Maybe nobody cares about that in the US except for the Madoff business. In MD only 0.2% of inmates are there because they defrauded somebody.

Thanks, Hurk27. I"ll add this to my bookmarks because this question will certainly come up again.

 

[petersonra]

Until they changed the wording in 2008 or 2011 (I forget which year it was changed) you did not have to drive two rods unless you determined the first was more than 25 ohms. Now you must verify the first is less than 25 ohms or drive the second rod. Before you found most installations only had one rod, and nobody even knew what the resistance was because they had no way or knowledge or just did not want to check the first one. Now you have to use two rods unless you actually do check the first one and it ends up being less than 25 ohms.

and what makes 25 Ohms the magic number? if it is the magic number why isn't that the requirement rather than a second rod?

 

[kwired]

and what makes 25 Ohms the magic number? if it is the magic number why isn't that the requirement rather than a second rod?

I kind of remember someone on this forum bringing up the fact that 25 ohms has been the magic number since the telegraph days.

Otherwise I have always heard if you still can't get 25 ohms with two properly installed rods, a third or fourth rod are not going to improve things much.

That doesn't mean you won't get specifications from a designer at times where they want a specific resistance or lower, but that is specification not NEC.

 

[hurk27]

and what makes 25 Ohms the magic number? if it is the magic number why isn't that the requirement rather than a second rod?

IMHO the 25 ohms is a left over from the days of old, it really serves no purpose other then a starting point that has no significant value in our electrical world, 25 ohms @ 120 volts will only pass 4.8 amps, so it provides no protection from any shock hazard,

You wont believe it but it was a number from the days of telegraph when they switch to glass insulators on the poles, with the wood insulators the static build up would drain through the wood and pole to earth, but the glass was such a good insulator static build up became a problem and they started the requirement of 1 ground rod for every mile, which turned into 4 rods per mile when power transmission came along, I had a book with this in it at one time but I haven't found it on the net yet and most info about how telegraph systems work jumble old info with new, at one point on long runs they actually started using a SWER system that would also drain the static charge but during solar flares enough energy would burn out the coils of the receivers and shock the operators so ground rods became the norm.

 

[Dennis Alwon]

Hey Dennis, what is CEE ? What is the American Electricians Handbook? Don't recall hearing about it.Thanks all. A very merry & Blessed Christmas to one & all !!!

CEE is a Concrete Encased Electrode- I preferred this to ground rods as they generally have a much lower resistance.

 

[hillbilly1]

CEE is a Concrete Encased Electrode- I preferred this to ground rods as they generally have a much lower resistance.

Yeah, that's what I used on my house when I built it 20 years ago, long before uffer grounds were used in residential, it's been used in commercial a long time.

 

[G._S._Ohm]

Some more grist for the mill
http://ecmweb.com/site-files/ecmweb.com/files/archive/ecmweb.com/images/archive/0900groundrods.pdf

 

[hurk27]

Yeah, that's what I used on my house when I built it 20 years ago, long before uffer grounds were used in residential, it's been used in commercial a long time.

Actually when I was in Florida back in the 70's it was a requirement in many of the areas there, being that central Florida is the lightning capital of the United States it was what seemed like a good idea at the time, but farther study into lightning has reveled that even using a CEE very little lightning current will take the path to the CEE.

Here was an old thread that talked of the usefulness of ground rods and CEE's for lightning protection and the report, but the links to the report have long been disconnected.

http://forums.mikeholt.com/showthread.php?t=77454

 

[petersonra]

Some more grist for the mill
http://ecmweb.com/site-files/ecmweb.com/files/archive/ecmweb.com/images/archive/0900groundrods.pdf

However, when served from overhead lines (where the fault
current return path(s) could break and become an open-circuit), grounding system designs should reduce the potential gradients in the vicinity of
the ground rods. This helps you achieve safe step and touch potentials
under ground fault conditions.

Does anyone here actually believe that ground rods provide much in the way of step and touch potential safety?

 

[hurk27]

Does anyone here actually believe that ground rods provide much in the way of step and touch potential safety?

I hope not.

I was surprised in seeing that in the article, it looks like it was published around 2008,
the correct info that debunks this myth was out there at this time, the graphic I posted was published around 1999-2000 so its been out there for some time, but these myths still continue and are taught by some of the most respected instructors because they are still using older books.

 

[gumbey]

paralleling of earth rods

paralleling of earth rods

I have heard the further the better and also twice the length of the rods. 8' rod then drive the second one 16' away. Persomally I use the CEE whenever possible.

i would agree with you ,twice the lenght of the rod to prevent over lapping of the area. the books that i have read say minimum of six feet apart without mention the rods lenght,what if its two ten feet rods?

 

[kwired]

If you have a rod that is 5/8 diameter, why is making it 2 feet deeper going to change the horizontal dimensions of the "sphere of influence"? It will change the vertical dimension but horizontally why wouldn't it remain the same?

 

[K8MHZ]

If you have a rod that is 5/8 diameter, why is making it 2 feet deeper going to change the horizontal dimensions of the "sphere of influence"? It will change the vertical dimension but horizontally why wouldn't it remain the same?

In order to remain a sphere, as any one dimension changes, so do all the others.

The deeper the rod, the larger the diameter of the sphere of influence. Just as the shallower, the smaller.

It's more geometry than electrical theory

 

[hurk27]

If you have a rod that is 5/8 diameter, why is making it 2 feet deeper going to change the horizontal dimensions of the "sphere of influence"? It will change the vertical dimension but horizontally why wouldn't it remain the same?

In order to remain a sphere, as any one dimension changes, so do all the others.

The deeper the rod, the larger the diameter of the sphere of influence. Just as the shallower, the smaller.

It's more geometry than electrical theory

I think there has been a few misconceptions brought on by the guides of twice the length of the rod apart.

The term of twice the length of the rod was just a guideline but really has very little to do as to how big the sphere of influence is, using this, one could say that the SOI of a ten foot rod would be 20' when it would be much closer to 26'-28', a longer rod will for the most part extend the amount of soil in contact with the rod to 2' deeper, as well as the SOI would also extend 2' deeper, so the SOI would just look like a longer tube with a round bottom under the bottom of the rod, it does not increase the diameter of the SOI but maybe by a fraction of the 2' increase in depth.

If we go back and look at Gary's experiment we can see that even though he only drove these rods 5 feet into the ground the diameters of the SOI's he got are very close to what one would expect from a 8'-10' rod, as a matter fact because of his high conductive soil his SOI were smaller as a result of the conductivity of the soil, not the shallow depth of his rods as one might be lead to believe, if he had driven the rods deeper it might have increased the diameter a few inches but that would have been it.

In the afore posted PDF on the second page in the left paragraph he came close to the real life diameter of a common SOI:

Earth electrode resistance is the number of ohms of
resistance measured between the ground rod and a
distant point on the earth called remote earth. At this
point, the earth electrode resistance
no longer increases
appreciably when the distance
measured from the grounding
electrode increases. This distance
is typically about 25 ft
for a 10-ft ground rod.

In real life test it is more like 26' to 28' for about 100Ω per cm of soil, I have never seen much of a change in this distance per the length of the rod as he stated, the bigger difference is more dependent upon the resistance of the soil, as this plays a much larger role in what the size of the SOI will be.

But I can see where being that it is common to install an 8'-10 rod, it would be easy as a directive to just use the 2 times the length of the rod as a good reference?

Now since I know Mark is into radio as I was on the commercial side, in doing many radio towers grounding grids under the tower it is easy to get crossed up on what is required for a ground plane (AM broadcast) then as for lightning protection, although the two work together and function as both, we used to install as common 40 to 50 foot sectional rods that were cad welded together as they were driven, these were driven (in most specs) every 20 foot in a circular pattern around the base of the tower in rings every 20 foot out to a point that depended upon the frequency of the transmitter (325' was common for stations around 1250kz), these rods would be cad welded together with copper conductors in a circle pattern and in a radial pattern to form a grid ground plane that would mirror the tower in hight, as at these frequencies the whole tower was the transmitting antenna, I have also seen where 40' spacing was spec as since the rods were out of the SOI it really didn't change anything.

so we can see two very different requirements but with the one for radio towers can have a very much longer distance between the rods or closer as because of the long wave length it really doesn't effect the function of the ground plane because once you reached the outside distance of the SOI it doesn't change the resistance between the rods.

Oh Mark I have also helped a couple hams design a grounding grid for a 160m ground mounted ground plane using this same system of rods and grid. oh are they fun to do, or should I say allot of work if you don't have a trencher and rod driver.

 

[kwired]

In order to remain a sphere, as any one dimension changes, so do all the others.

The deeper the rod, the larger the diameter of the sphere of influence. Just as the shallower, the smaller.

It's more geometry than electrical theory

Is the "sphere of influence" actually a true sphere or is it more of a cylinder with semi-sphere at the ends?

 

[hurk27]

Is the "sphere of influence" actually a true sphere or is it more of a cylinder with semi-sphere at the ends?

For the most part if the soil is the same within the area of the SOI it will be, but different types of soil in this area can affect its shape, there is a good graphic in that PDF link posted of what it would look like, it is more of a reference that puts this sphere equal distance from the rod including below where it would form a round bottom kind of like the aluminum cigar tubes cigars came in , if you were to take a string with a length of this SOI and moved down the rod and under it the outer end of the string would form the shape of this tube.

Now other conductive paths that trespass into the SOI can alter the shape of the SOI but depending upon how close to the rod this conductive path is will determine by how much.

Most of the experiments I did back in the late "70's" in Florida that were very similar to Gary's "Time to Eat Crow" thread were done out on my property that was away from any other possible influences, the first set of experiments I did was from a totally isolated power supply an old Onan 5kw generator) at the time I didn't understand how the current flowed in the earth so I didn't want any influences of the service electrode system or the electrodes at the POCO's poles, later I repeated the experiments using power from my house and about fell over when I got the same results and discovered that the current didn't try to flow more in the direction of my service electrodes, as in Gary's experiment I found that the drop off in voltage was the same in all directions from my injection rod, and not like in Gary's my soil was not as conductive at about 200Ω per cm which resulted in about 35Ω rod resistance I think Gary's was much lower at about 8Ω for his 5' rods which where he was doing his experiments was an area where coal was shipped out and the carbon rich soil was a great conductor.