additional ground rod?

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Besides, it never helps if you electrocute the inspector[/QUOTE]

I don't think he would aprove either.
 
additional ground rod

additional ground rod

Sounds to me like someone is trying to give the inspector a hard time. What he is asking for is a NEC requierment. Document the 25ohm or drive a second ground rod,it is your choice.

Inspectors have a lot more complicated issues to deal with than this very simple and clear requierment. Why agrivate him ?
 
Regardless of the NEC MINIMUM, or what the inspector wants. It can be flat out dangerous to have a resistance to ground at the rod greater then 25 Ohm.

References to follow should be IEEE 142 and IEEE 80.
 
I sense alot of negativity and inspector hatred here and am not sure why! We have a big red book that tells us almost exactly what to do. For the record....I have been a Journeyman electrician for ten years and an inspector for only a year.

Go easy guys.
 
acrwc10 said:
Now my van took your comment very personally and is sulking in the driveway, It is a ford and they have very fradgile egos.:)
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My fords don't... usually take over 300,000 miles of heavy abuse without complaining... although some got some unusual modifications to do it.
 
kingpb said:
Regardless of the NEC MINIMUM, or what the inspector wants. It can be flat out dangerous to have a resistance to ground at the rod greater then 25 Ohm.

References to follow should be IEEE 142 and IEEE 80.
Click to expand...

I believe it's unrealistic to apply IEEE 80 to commercial grounding. It is a substation standard which concerns itself with among other things, step and touch potentials neither of which is addressed in the NEC or even really applicable to most commercial installations.

In the 40+ years I've been in the electrical trade I've never seen a ground rod test to 25 ohms or less. Even if it did, I don't know how or why it would be safer than one that tested to 50 ohms.

The utility neutral is the effective ground fault path from the service point back to the transformers in almost all grounded services, not a grounding electrode system.

Jim T
 
barbeer said:
I sense alot of negativity and inspector hatred here and am not sure why! We have a big red book that tells us almost exactly what to do. For the record....I have been a Journeyman electrician for ten years and an inspector for only a year.
Go easy guys.
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I don't know if I'd call it hatred... more of a playful banter. :D
There is of course a little hostility to inspectors enforcing codes that obviously don't exist, and even more to electricians doing things below the code especially if dangerous or unwilling to change their ways.
 
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jtester said:
I believe it's unrealistic to apply IEEE 80 to commercial grounding.

In the 40+ years I've been in the electrical trade I've never seen a ground rod test to 25 ohms or less. Even if it did, I don't know how or why it would be safer than one that tested to 50 ohms.

Jim T
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I said the references were 142 AND 80. Rightfully so, that 80 is for substations, but the techincal information in the standard provides a lot of insight into the how's and why's of grounding.

I assume you are then familiar with 142, which in fact, is were I believe the NEC takes it's que, and If familiar with it, then there should not be a question as to why 25 ohm is safer then 50 ohm.

The fact that you have not seen one test out to 25 or less, tells me there is a lot of out of compliance installations. That's a shame.
 
King,
Can you explain how the resistance to ground makes the installation safer?
The fact that you have not seen one test out to 25 or less, tells me there is a lot of out of compliance installations. That's a shame.
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As long as there is a second ground rod, there is no code requirement to reach the magic 25 ohm barrier.
Don
 
barbeer said:
I sense alot of negativity and inspector hatred here and am not sure why! We have a big red book that tells us almost exactly what to do.
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I haven't looked back to see what the scuffle was, I'll just say this: generally, with inspector rant threads around here, the anger revolves around two attributes of the inspector: unwillingness to read and/or enforce the code as written (which goes along with making up their own code), or a questionable interpretation.

I feel that among the majority of the membership here, an inspector who reads and enjoys the code and enforces as written, seeking opinions or official commentary as necessary, is welcomed with open arms. And chances are, the folks inspected under them are never compelled to go online and rant about them. It hasn't happened yet that I've seen. :D

For the record....I have been a Journeyman electrician for ten years and an inspector for only a year.

Go easy guys.
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Your skin will get thicker with more experience as an inspector, I reckon. :D ;)
 
kingpb said:
Regardless of the NEC MINIMUM, or what the inspector wants. It can be flat out dangerous to have a resistance to ground at the rod greater then 25 Ohm.

References to follow should be IEEE 142 and IEEE 80.
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I too would welcome an explanation. Although I am not familiar with those standards, word of their proven protection methods have not graced this forum in the two years I've been here, that I've noticed.

As Don noted, the NEC does not have any resistance requirements and is considered "adequate for a safe installation."

If the IEEE documents have developed a standard from evidence that resistances over 25 ohms are dangerous, please enlighten us. I can still vividly recall when the difference between grounding and bonding were first explained to me, I'm ready for a new paradigm shift in thinking. Lay it on us. :)
 
follow up

follow up

infinity said:
I've done it. What happens? Nothing. Connect the conductor going to the rod to a 20 amp breaker and read the current flow with an ammeter. If you had 12 ohms the meter will read 10 amps. 6 ohms reads 20 amps.
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I went back to the job and curiousity got the best of me. I tested the ground rod using an amprobe GP-1 tester, two point test ground rod to water main and got a 8.46ohms reading. Then I tried the 20amp breaker test, 118.4 volts 15.47 amps=7.65ohms a .81ohm differance. Not bad.
 
Bob NH said:
If I clear the area around the ground rod and apply 120 volts through a 100 watt bulb, through an insulated wire connected to the ground rod, by pressing a momentary switch while I measure voltage relative to the grounded conductor and current through the insulated wire, it is not dangerous except to earthworms and grubs.
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How do you know what area to clear? The voltage gradient could be highest across someone's inground pool on the other side of a privacy fence.

I don't see how this is any different than the devices people make to get fishing worms. And I know those have killed people.

Steve
 
25 ohm ground resistance meter

25 ohm ground resistance meter

Since we are on the subject of the famous 25 ohm rule... the guys who sell the ground resistance meters say that your regular ohm meter will not read the ohms to ground accurately and that you need a real ground resistance meter. Is this true, if so please explain... at $1200 to $2500 for this type of meter I would like to see why etc...

Student in code class...
Bob
 
One of the moderators, Dereck, had a good explanation for it. If you do a search you might find it.

Essentially, a regular DMM can't hear through the "noise" of the earth. It's like the DMM is listening for it's resistance signal "whisper" in a busy conference hall.

A ground resistance tester is designed to hear itself over the din. :)
 
Not quite sure about the analogy George, then again I can't offer up a better one right now - coffee still flowing in the maker...

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Coffee full, the tester produces a specific sine wave to measure against.
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Also note that the tester here is ~$700, then look at the kits, its them leads that get you:
www.groundtesterstore.com/index.php?cPath=190
 
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Bob Anchorite said:
Since we are on the subject of the famous 25 ohm rule... the guys who sell the ground resistance meters say that your regular ohm meter will not read the ohms to ground accurately and that you need a real ground resistance meter. Is this true, if so please explain... at $1200 to $2500 for this type of meter I would like to see why etc...
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I think one big reason is a matter of source impedance of the meter itself. I could be wrong, but here's my theory.

A VOM uses only a battery and a relatively-high-impedance meter in series with the resistance being measured, and will be influenced by any voltage added by the 'circuit' under test.

Here's the part I'm not sure about, but I think I read it somewhere:

A ground-resistance meter uses an exciting current induced by one coil in the clamping jaw mechanism, and this current is much less influenced by externally-introduced voltages.

Make sense? :D
 
The problem is the code only says 25 ohms, it doesn't specify AC, DC, 60Hz, 1 GHz, the applied voltage, or anything. In fact, since it says resistance and not impedance, you could argue that a battery test would be sufficient. It should be sufficient to clip a 120V wire to a ground rod and use a clamp on meter to measure the current flowing through the rod. This will allow you to derive a resistance for 60Hz -- 4.8 amps or more and you're home free. I doubt most inspectors would allow that to fly, but how could he fail you if the 25 ohm testing methodology is not specified in the code?
 
I agree, Mark. I have an old electrical textbook that describes putting 120volts to the ground rod through a 5 amp fuse. If the fuse blows, you have 25 ohms or less. Not exactly today's recommended procedure, but interesting nostalgia, nonetheless. This test necessarily relies on a sufficiently low impedance utility multigrounded neutral rod or another path such as a grounded underground water line. The three point method removes those variables.
 
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