Where to measure 25 OHMS

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jxofaltrds

jxofaltrds

Inspector Mike®
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Mike P. Columbus Ohio
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ESI, PI, RBO
(B) Electrode Spacing. Where more than one of the electrodes
of the type specified in 250.52(A)(5) or (A)(7) are
used, each electrode of one grounding system (including
that used for strike termination devices) shall not be less
than 1.83 m (6 ft) from any other electrode of another
grounding system. Two or more grounding electrodes that
are bonded together shall be considered a single grounding
electrode system.

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.

If I have a metal water line (pipe) can I measure for 25 OHMS there?
 
Dennis Alwon said:
Also even if the metal water pipe has 25 ohms you still need the rod(s)
Click to expand...

Yes. But how many rods, if you choose rods, is my question.

(2) Supplemental Electrode Required. A metal underground
water pipe shall be supplemented by an additional
electrode of a type specified in 250.52(A)(2) through
(A)(8). If the supplemental electrode is of the rod, pipe, or
plate type, it shall comply with 250.53(A). The supplemental
electrode shall be bonded to one of the following:
(1) Grounding electrode conductor
(2) Grounded service-entrance conductor
(3) Nonflexible grounded service raceway
(4) Any grounded service enclosure
(5) As provided by 250.32(B)
 
Dennis Alwon said:
What are you using to measure? Not an ohmmeter I hope
Click to expand...

This brings up another point. Where does the NEC require the EC to prove to the AHJ that the 25 ohms was meet? Does the EC also have to prove voltage drop? True 120 or any other voltage?

I say if the AHJ wants to verify 25 ohms then they must perform the testing.

Does the EC have to stand there with a tape and show proper wall or counter top spacing?
 
jxofaltrds said:
Yes. But how many rods, if you choose rods, is my question.

(2) Supplemental Electrode Required. A metal underground
water pipe shall be supplemented by an additional
electrode of a type specified in 250.52(A)(2) through
(A)(8). If the supplemental electrode is of the rod, pipe, or
plate type, it shall comply with 250.53(A). The supplemental
electrode shall be bonded to one of the following:
(1) Grounding electrode conductor
(2) Grounded service-entrance conductor
(3) Nonflexible grounded service raceway
(4) Any grounded service enclosure
(5) As provided by 250.32(B)
Click to expand...

You need a supplement for the rod either another rod or an electrode from that list. The water pipe is not on the list.
 
jxofaltrds said:
This brings up another point. Where does the NEC require the EC to prove to the AHJ that the 25 ohms was meet?
Click to expand...
In the laws governing the licensing of an electrical contractor the laws govern that the electrical contractor installs a compliant electrical system here in NC. See 250.53(A)(2) of the 2011 cycle for the rule governing the installation of rod, pipe or plate electrodes

jxofaltrds said:
Does the EC also have to prove voltage drop? True 120 or any other voltage?
Click to expand...
This is a design issue and it is not enforceable by any code official.

jxofaltrds said:
I say if the AHJ wants to verify 25 ohms then they must perform the testing.
Click to expand...
I can see nothing wrong with a code official testing but unless they have some sort of handle at the end of their name I wouldn?t pay a lot of attention to their findings. As a code official I wouldn?t accept any testing done by an electrical contractor without the same handle at the end of their name.

jxofaltrds said:
Does the EC have to stand there with a tape and show proper wall or counter top spacing?
Click to expand...
See the answer to the first quote of this post.

In order to circumvent any testing I would simply drive two rods and be done.
 
jxofaltrds said:
Understood.

What I am asking is, if I have the metal waterline and add a rod, first is it one system?

Second must I take my reading 'at' the rod?
Click to expand...

Mr. Jxofaltrds:

It is my understanding that yes, if you have a metal underground water pipe which is supplemented by a ground rod you have single system with two components.

If you have such a system described above, no ohm measurements are necessary.

In my opinion, the only time an EC would need to ?prove? that the grounding electrode system had a resistance of less than 25 ohms is if he or she had only one ground rod per 250.53(A)(2).
And the situation of having only 1 ground rod would be the only circumstance where an Inspector should ask for ohm measurements.

At least this is what I think.
 
jxofaltrds said:
Understood.

What I am asking is, if I have the metal waterline and add a rod, first is it one system?

Second must I take my reading 'at' the rod?
Click to expand...
First yes it is one system and second there must be a second rod or proof that the contractor has 25 ohms of resistance.

The new change requires two rods every time a rod is used unless there is documentation that there is 25 or less ohms of resistance
 
jwelectric said:
First yes it is one system and second there must be a second rod or proof that the contractor has 25 ohms of resistance.

The new change requires two rods every time a rod is used unless there is documentation that there is 25 or less ohms of resistance
Click to expand...


The way I see it is:

First, in Jxofaltrds illustration, the metal water pipe is the primary grounding electrode. This requires a supplemental grounding electrode per 250.53(D)(2).

Second, according to 250.53(A), if a ground rod is the primary or only grounding electrode, it requires a supplemental grounding electrode unless it is measured at 25 ohms or less.

At least this is the way I read it.

As others will say, this section can be interpreted different ways.
 
Just another example of how archaic the NEC can be at times.

The 25 ohm thing should be removed. That is a measurement that must be taken with specialized equipment and also requires a bit of specialized training. Most people, even electricians and EEs don't know how earth resistance is measured and what the 25 ohms is a resistance between.

Some companies are selling what they claim to be as clamp on ground resistance testers. They are not. Read the fine print. They check for ground loops and can't be used with the electrode system connected to the power system for anything but that.

The actual value, 25 ohms, is a remnant from the telegraph era where they used earth return systems to save wire. Those were DC, battery powered systems. Nothing at all like today's power distribution system.

Who is to say that the 25 ohms will stay 25 ohms? Electrode to earth resistance changes fairly regularly.

If I were an inspector I would need to see a certification from a third party testing outfit to accept the 25 ohm value in lieu of the electrode requirements it negates, and would reluctantly do so at that. Cheaper to put in two rods.
 
realolman said:
I don't know if this will do you any good or not, but it was interesting to me. Made me wanna buy one of those fluke jiggers.

http://support.fluke.com/find-sales/Download/Asset/2633834_6115_ENG_A_W.PDF
Click to expand...

Careful....read the fine print:

Stakeless measurement

The Fluke 1625 earth ground tester is able to
measure earth ground loop resistances for multigrounded
systems using only current clamps.
Click to expand...

Key words: 'ground loop resistances'. Not the same as electrode to earth resistance. I think the nomenclature is a bit misleading.
 
jxofaltrds said:
Understood.

What I am asking is, if I have the metal waterline and add a rod, first is it one system?

Second must I take my reading 'at' the rod?
Click to expand...

The reading must be taken from several points. There will be two stakes that come with the tester that need to be driven a pre-determined distance apart. An earth resistance tester will have (typically) three ports. One port is connected to the electrode being tested and the other two to the standardized rods that need to be driven. The electrode being tested must be disconnected from the power and other electrodes. Electrode systems need to be intact for testing, but still must be disconnected from the AC mains for accurate testing. The system will be connected to the electrode testing port and will appear to be one single electrode to the tester.

This isn't something you just whip out a DVOM or a Megger and test in five seconds.
 
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Not to drift away from the original subject, but the question is whether a measurement is needed at all.

In the original scenario, an underground water pipe is presented as the primary grounding electrode. This requires a supplemental grounding electrode.

Jxofaltrds stated that a ground rod was chosen as the supplemental electrode which is perfectly fine per 250.53(D)(2) and refers us back to 250.53(A).

If you carefully read 250.53(A) you will see that they are talking about rod, pipe, and plate electrodes as the only electrode present, now we need a supplemental electrode unless.

Only if you have a rod, pipe, or plate and have no other supplemental electrodes, do you need to prove the 25 ohm limit.

When single rods, pipes, or plates are combined with other grounding electrodes, no ohm measurements are needed or required.

At least this is the way I read 250.53 as a whole.
 
jxofaltrds said:
This brings up another point. Where does the NEC require the EC to prove to the AHJ that the 25 ohms was meet? Does the EC also have to prove voltage drop? True 120 or any other voltage?

I say if the AHJ wants to verify 25 ohms then they must perform the testing.

Does the EC have to stand there with a tape and show proper wall or counter top spacing?
Click to expand...

I have only had one AHJ that actually tested the ground rods, he had one of the clamp on testers. I don't remember what state or jurisdiction it was though. Local thing though.
 
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