Why 2-Ground rods???

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tom baker said:
Its based on the law of diminishing returns. Adding a second ground rod reduces the resistance by 60%, 3 gets a down to 40%, 3 is 30%.
Some one a long time ago determined that 2 was enough. This was all worked out probably by 1920. 6 feet apart is because closer than that the concentric shells of the earth overlap and it becomes one ground rod.
But 25 ohms? No one knows. Mike Holt emailed me once he thought he had the reason. But I don't think he ever found out for sure.
So....
Drive two and go home
Or
Use a concrete encased electrode.
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Or....test it.
 
If you want to spend the money (and lots of it, mind you) to properly measure a rod's resistance, it's far quicker and cheaper to just drive a second rod.
 
tom baker said:
Its based on the law of diminishing returns. Adding a second ground rod reduces the resistance by 60%, 3 gets a down to 40%, 3 is 30%.
Some one a long time ago determined that 2 was enough. This was all worked out probably by 1920. 6 feet apart is because closer than that the concentric shells of the earth overlap and it becomes one ground rod.
But 25 ohms? No one knows. Mike Holt emailed me once he thought he had the reason. But I don't think he ever found out for sure.
So....
Drive two and go home
Or
Use a concrete encased electrode.
Click to expand...

Where would the "Sphere of Influence" come into play here?

I lay one down and drive the second greater than the length apart. Don't care about 6', just the way I was taught...
Second rod is way cheaper than a test set.

Here is one reason from ECMweb: Why do I need to reach 25 ohms? The most credible answer to this question is: 25 ohms is a reasonable value to strive for, given the average soil resistivity for most regions of the United States.http://ecmweb.com/content/who-cares-about-25-ohms-or-less

Of course, he could have been guessing...
 
jap said:
Don't get me wrong. I agree with installing grounding electrodes. I just think there should be 2 required and the clause about the "Unless 1 will do it" should be removed.
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I agree. The mythical 25 ohms resistance should go away completely. How silly is it to say that one rod must be 25 ohms or less but two rods can be more than 25 ohms and still be OK?
 
Hv&Lv said:
Where would the "Sphere of Influence" come into play here?

I lay one down and drive the second greater than the length apart. Don't care about 6', just the way I was taught...
Second rod is way cheaper than a test set.

Here is one reason from ECMweb: Why do I need to reach 25 ohms? The most credible answer to this question is: 25 ohms is a reasonable value to strive for, given the average soil resistivity for most regions of the United States.http://ecmweb.com/content/who-cares-about-25-ohms-or-less

Of course, he could have been guessing...
Click to expand...

25 ohms is a holdover from the land line telegraph era. That value was based upon the average expected distances from telegraph repeaters, whose structures housed the batteries for the systems. They used a SWER scheme for the system in order to conserve on wire. The sounders for the system could work on a wide range of voltages, but did have a minimum. Once distance between sounders, also commonly found at the repeater sites as few were not manned, caused enough voltage drop, voltage from the batteries were used to bump it up for the next trip. 25 ohms was a value determined to be a compromise between electrode installation costs and repeater construction costs.

"A reasonable value to strive for" is hardly a scientific answer. The real answer is simply that it was adopted from a technology no longer used and is archaic and meaningless for the electrical systems we work on.
 
roger said:
Zog, you could but, it takes longer and is more expensive to test one rod than it is to drive a second rod and go home.



Roger
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Yeah I know, just saying it is an option. For me personally I can test it faster than I could drive a rod in our NC clay. But that's just me.
 
From a 1940's handbook:

6footground2.gif



6footground.gif
 
K8MHZ said:
25 ohms is a holdover from the land line telegraph era. That value was based upon the average expected distances from telegraph repeaters, whose structures housed the batteries for the systems. They used a SWER scheme for the system in order to conserve on wire. The sounders for the system could work on a wide range of voltages, but did have a minimum. Once distance between sounders, also commonly found at the repeater sites as few were not manned, caused enough voltage drop, voltage from the batteries were used to bump it up for the next trip. 25 ohms was a value determined to be a compromise between electrode installation costs and repeater construction costs.

"A reasonable value to strive for" is hardly a scientific answer. The real answer is simply that it was adopted from a technology no longer used and is archaic and meaningless for the electrical systems we work on.
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I'm not doubting your reason, it sounds like the most logical. I don't know the exact reason, and only have these type publications to go on. I have wondered the reason for the 25 ohms for a couple of years. (along with so many others that guess)

Can you cite a source, so if I use that I don't have to say "cause K8MHZ said so"...
 
Hv&Lv said:
I'm not doubting your reason, it sounds like the most logical. I don't know the exact reason, and only have these type publications to go on. I have wondered the reason for the 25 ohms for a couple of years. (along with so many others that guess)

Can you cite a source, so if I use that I don't have to say "cause K8MHZ said so"...
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So, "cuz K8MHZ says so isn't good enough?"

:)

I am going to have to dig for links, and I don't fault you for requesting them, I would to the same.

Until I find one, FWIW, us hams kind of have an unusual interest in the history of telegraphy, using Morse Code, leading to the advent of radio telegraphy using Morse Code. It's from that genre, not from my profession as an electrician, that the info was purveyed to me.
 
This might be a good topic for a poll, along these lines:

Having two grounds became necessary when:

(A) The ground rod mafia conned the code panel into helping them double their sales;

(B) Inspectors suddenly had attacks of 'the vapors' over the uncertainty of code language, even though 70 years of experience failed to identify any problems; or,

(C) Everyone suddenly decided electricians were but apes with tool belts, and needed every detail of their work determined by chairborne instant experts.
 
renosteinke said:
This might be a good topic for a poll, along these lines:

Having two grounds became necessary when:

(A) The ground rod mafia conned the code panel into helping them double their sales;

(B) Inspectors suddenly had attacks of 'the vapors' over the uncertainty of code language, even though 70 years of experience failed to identify any problems; or,

(C) Everyone suddenly decided electricians were but apes with tool belts, and needed every detail of their work determined by chairborne instant experts.
Click to expand...

Renosteinke, check out the qualifications of the "chairborne instant experts". Their names and affiliations are in the front of the book. The guys I know on the code committees have worked decades in the trade and have probably forgotten more than you or I will ever know.
 
fmtjfw said:
Renosteinke, check out the qualifications of the "chairborne instant experts". Their names and affiliations are in the front of the book. The guys I know on the code committees have worked decades in the trade and have probably forgotten more than you or I will ever know.
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If he would have used the same type satire on the 25 ohm requirement I would have seen humor in it.

So here goes

The 25 ohm requirement became necessary when:

A) 1899 when the IBEW went on a national strike. The telegraph companies had figured out that they could use less wire (and less labor) by using the earth as a ground. Knowing that 25 ohms would not be easy and take additional materials and labor, they settled for that compromise in order to go back to work.

B) 1987 when cows started getting nervous and cross eyed.

C) 1906 when Henry Biddle was making a device that could measure electrode resistance and owned the NEC.

D) 1913 when several different values were tried, they were 16, 19 and 25. 25 won because it wouldn't be confused with the others if written upside down.
 
Very good, K8, very good! :D

Yes, I'll agree the 25 ohm requirement is a wee bit arbitrary.

Code committe qualifications? How about them being bamboozled by a couple Holt members - but, more importantly, a handful of home inspectors, who made the proposals for the code change.

A subtle shift in the burden, with the effect of ALWAYS requiring two rods be driven. After all, who can guarantee that the 25 ohms you measure today will be the same all day, every day, forever? What's next- requiring homeowners whose homes have but one rod to subscribe to a 'certified' testing service and have their electride tested every six months?

Yup, those committee members are smart fellas. I'm quite confident that if they really wanted to say "Two rods will be driven," they would have said so. Nope - toss in a few waffle words, and they got hoodwinked into just making a 'change in the grammar.' I'm sure they were as surprised as anyone when the seminars took to the road, telling everyone you now had to drive two rods.

FMT, while I was being facetious, you're absolutely correct if you infer that I consider the code change to be silly.
 
K8MHZ said:
If he would have used the same type satire on the 25 ohm requirement I would have seen humor in it.

So here goes

The 25 ohm requirement became necessary when:

A) 1899 when the IBEW went on a national strike. The telegraph companies had figured out that they could use less wire (and less labor) by using the earth as a ground. Knowing that 25 ohms would not be easy and take additional materials and labor, they settled for that compromise in order to go back to work.

B) 1987 when cows started getting nervous and cross eyed.

C) 1906 when Henry Biddle was making a device that could measure electrode resistance and owned the NEC.

D) 1913 when several different values were tried, they were 16, 19 and 25. 25 won because it wouldn't be confused with the others if written upside down.
Click to expand...

I pick D, but really think 88 is an even better answer as it would be even less possible to tell if it were upside down in many cases, and easier to achieve with one rod:)
 
Just Askin'...

Just Askin'...

So, exactly how would you measure the resistance to earth? Not being afraid to display my ignorance and trying to fulfill my goal of learning at least one new thing a day (it's been a slow day in that regard), I ask in all seriousness. 480sparky cut things off just when it looked to get interesting. :weeping:
 
In our locality we do not require a second rod. I won't talk you out of a second rod but one will pass. Our reasoning isn't due to any electrical theory but our position on what we can legally enforce. We cannot prove a single rod doesn't meet the requirement and do not expect the contractor to purchase necessary testing equipment. That being said, I'm glad the 2011 language was changed. We are currently under the 2008. May not be the most sound reasoning but the contractors like us....until the next Code change!;)
 
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