two ground rods negating the requirement to check for impedance

[infinity]

I really don't see any difference in the function of the code rules between 2008 and now. Just a different way of saying the same thing.
250.56 in the 2008 said either test and prove that your single rod meets the 25 ohm rule or install two rods. To me that is the same as what the exception to 250.53(A)(2) says in the 2014 code.

Yes, the ultimate meaning is the same but now when using a rod a second electrode required unless you can meet the exception. As Kwired pointed out prior to the 2008 it never really stated who would be providing the burden of proof.

 

[david luchini]

I really don't see any difference in the function of the code rules between 2008 and now. Just a different way of saying the same thing.
250.56 in the 2008 said either test and prove that your single rod meets the 25 ohm rule or install two rods. To me that is the same as what the exception to 250.53(A)(2) says in the 2014 code.

I agree. I don't see any change in the "burden of proof" between current Code and earlier versions on this issue.

 

[kwired]

I agree. I don't see any change in the "burden of proof" between current Code and earlier versions on this issue.

Doesn't outright say anything about burden of proof, but as worded now it puts the burden of proof on the installer where before one could drive one rod and ask the inspector to prove resistance before making you install the second one. Maybe didn't work that way everywhere but here they didn't make you drive a second rod because they were not going to test the first one, now they want two rods unless you can prove resistance of the first one - all because of the wording change.

 

[John120/240]

How many out there have actually confirmed a single ground rod complying to 25 ohms?

NEVER

 

[david luchini]

Doesn't outright say anything about burden of proof, but as worded now it puts the burden of proof on the installer where before one could drive one rod and ask the inspector to prove resistance before making you install the second one. Maybe didn't work that way everywhere but here they didn't make you drive a second rod because they were not going to test the first one, now they want two rods unless you can prove resistance of the first one - all because of the wording change.

I don't see that at all. The burden is on the installer to install the grounding electrodes per Code requirements under both wordings.

 

[kwired]

I don't see that at all. The burden is on the installer to install the grounding electrodes per Code requirements under both wordings.

Sorry you walk through this world with blinders on.

Don't mean to sound mean but it was happening, the new wording makes it easier for the inspector. He can more easily now ask for proof of resistance, before as worded and with no local amendments to clarify things, he potentially had to prove it himself or let it go.

 

[david luchini]

Sorry you walk through this world with blinders on.

Don't mean to sound mean but it was happening, the new wording makes it easier for the inspector. He can more easily now ask for proof of resistance, before as worded and with no local amendments to clarify things, he potentially had to prove it himself or let it go.

I don't walk through the world with blinders on. The change to the wording of the Code didn't change the requirements for the installation of a single rod, pipe or plate electrode. They are the same in the 2014 Code as they were in the 1999 Code.

As for "it was happening," it wasn't happening here, or in any area I've worked in.

I've never seen anything in the Code that makes an inspector "prove it himself." I've got to wonder about the inspectors in your area if the installer can tell the inspector "prove my installation doesn't comply."

 

[augie47]

In this area even if you went to the expense of buying the meter the soil conditions would still require the 2nd rod. The number of instances where you could omit the 2nd rod would not offset the cost of the meter.
( a majority of my inspections have a UFER or building steel so it's a mute point)

 

[kwired]

I don't walk through the world with blinders on. The change to the wording of the Code didn't change the requirements for the installation of a single rod, pipe or plate electrode. They are the same in the 2014 Code as they were in the 1999 Code.

As for "it was happening," it wasn't happening here, or in any area I've worked in.

I've never seen anything in the Code that makes an inspector "prove it himself." I've got to wonder about the inspectors in your area if the installer can tell the inspector "prove my installation doesn't comply."

I agree the requirements never really changed. How they may be interpreted is what changed.

Maybe we need to go back to ROP's and look at why the change was made? I seem to recall it being to better clarify the intentions of the rule.

 

[augie47]

I agree the requirements never really changed. How they may be interpreted is what changed.

Maybe we need to go back to ROP's and look at why the change was made? I seem to recall it being to better clarify the intentions of the rule.

That's certainly true in our State. Prior to the change in wording somewhere in our hierarchy someone deemed that the inspector needed to show a resistance of greater than 25 ohms in order to require a supplemental electrode.

 

[kwired]

I never drove a second ground rod until this change unless otherwise specified by a designer.

I was aware of the rules that said a second rod may be necessary, but if no one is making you or your competition do it, you can about imagine most are not doing it, once they were required to do it I would assume others were also charging customers more because of it - still keeping the competition on a even playing surface from that aspect.

 

[gadfly56]

How many out there have actually confirmed a single ground rod complying to 25 ohms?

There is a video where Mike Holt drives I think about four ten-foot sections before he gets close to 25 ohms. Also has a discussion of step potential.

Aha, found it! http://forums.mikeholt.com/showthread.php?t=159914&highlight=ground+rod

 

[Smart $]

...

I've never seen anything in the Code that makes an inspector "prove it himself." I've got to wonder about the inspectors in your area if the installer can tell the inspector "prove my installation doesn't comply."

Depends on the area, but in most areas, the onus is on the inspector to provide a reason for a failed inspection.

Back under 1999 NEC edition, if the inspector failed for ...

250-56. Resistance of Made Electrodes. A single 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 in
Sections 250-50 or 250-52. Where multiple rod, pipe, or
plate electrodes are installed to meet the requirements of
this section, they shall not be less than 6 ft (1.83 m) apart.

...some AHJ's felt the language obligated them to prove the resistance was greater than 25 ohms.

 

[augie47]

Depends on the area, but in most areas, the onus is on the inspector to provide a reason for a failed inspection.

Back under 1999 NEC edition, if the inspector failed for ...

...some AHJ's felt the language obligated them to prove the resistance was greater than 25 ohms.

:thumbsup:

 

[kingpb]

My understanding has been that the 25ohm or less is the threshold for a single connection to earth that would allow a single rod to perform its function when either a fault or lightning incident occurs. Above 25ohms, if there is only one path it may not be capable of operating properly. So, a second rod is added, thus giving a better connection/multiple paths and at the point the fact that it is still possibly above 25ohms is not a factor. Hence no measurement is required.

A lower value is sometimes specified by an engineer or designer to account for electronic equipment but typically that's not even a real issue; only perceived.

Proper testing is time consuming, confusing, and costly. A cheaper alternative is to use a clamp on earth ground resistance tester, but they can be inaccurate and not considered reliable.

In the end, It's probably much cheaper to just add the second rod, and skip the rest of the nonsense. Especially, in most cases you would probably end up driving a second rod anyway to get below 25ohms, even after you performed the tests.

 

[mwm1752]

There is a video where Mike Holt drives I think about four ten-foot sections before he gets close to 25 ohms. Also has a discussion of step potential.

Aha, found it! http://forums.mikeholt.com/showthread.php?t=159914&highlight=ground+rod

Saw that but I never had anyone drive a 40' grd rod:happyno:

 

[GoldDigger]

kingpb,
The existence of multiple paths has no particular relevance to either AC fault or lightning related performance.
And 25 ohms is not sufficient to trip even a 15A breaker on a 120V to ground circuit. On a US 240V (120-0-120) circuit a ground fault will not produce a greater current than on a 120V circuit. It requires a ground electrode resistance of less than 6 ohms to have a good chance of tripping a 15A breaker, even if the ground electrode resistance on the POCO side is assumed to be zero.
There is, however, nothing wrong with your conclusion that it is best to drive two rods without testing. Assuming you need a ground rod in the first place.

 

[big john]

My understanding has been that the 25ohm or less is the threshold for a single connection to earth that would allow a single rod to perform its function when either a fault or lightning incident occurs. Above 25ohms, if there is only one path it may not be capable of operating properly. So, a second rod is added, thus giving a better connection/multiple paths and at the point the fact that it is still possibly above

But even a 25 ohm rod wouldn't clear a LV fault. You wouldn't trip 1 phase faults until you had total circuit impedances in the single digits.

And given there is a far more effective MGN connection at the pole, I don't even see the service electrode being useful for clearing high voltage.

Like you said, drive two rods and be done. I think the NEC overcomplicates the issue by dragging ohmic values into it.

 

[mivey]

kingpb,
The existence of multiple paths has no particular relevance to either AC fault or lightning related performance.
And 25 ohms is not sufficient to trip even a 15A breaker on a 120V to ground circuit. On a US 240V (120-0-120) circuit a ground fault will not produce a greater current than on a 120V circuit. It requires a ground electrode resistance of less than 6 ohms to have a good chance of tripping a 15A breaker, even if the ground electrode resistance on the POCO side is assumed to be zero.
There is, however, nothing wrong with your conclusion that it is best to drive two rods without testing. Assuming you need a ground rod in the first place.

The origins of the 25 ohm requirement (and the dual rods for that matter) had nothing to do with LV breakers tripping but was for lightning shunting and activation of MV & HV protection.

There were some telecom requirements for low ohm grounds, but that is a different issue; the best I recall it was 20 ohms for one of the telecom specs.

 

[iwire]

Here is a lot of info from Mike Holt on the origin of 25 ohms. http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/That-25-Ohm-Rule-Revisited~20050505.php