Grounding Electrode System NEC 250-50

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davidv said:
Running an low frequency, 60 hz AC current through a straight wire would bring about a resistance R ,and a very weak inductance ( very very much smaller compared to the R ) , (too impractical to measure).
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There will also be a slight amount of capacitance. It will be small enough to ignore, but it's still there.



But with all those bonding here & there, grids, rings rebars etc. loops are created so with a strong ac current or high frequency, the GES becomes an impedance but when & how big , theres a lot of possibilties.
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No, the GES became an impedance the instant it was connected to an AC supply.
 
peter d said:
Show me any electrical theory book where the term "resistance" is used to measure total opposition to current flow in an AC system and I will change my mind. :)
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Electrical theory books are not relevant at inspection time.

The NEC is what is relevant and they call it resistance, the people who make the testing equipment call it resistance testing.

Try Googling (with the quotations)

"Ground impedance testing"

You will get about 100 hits mostly from the UK.

Then Google

"Ground Resistance testing"

You get about 800 hits.

Majority rules. 8) :lol:

peter d said:
I'm not going to change my mind, you're not going to change yours, so we will have to agree to disagree.
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Fair enough so I assume you are writing your proposal for a change from resistance to impedance? :wink:
 
iwire said:
The NEC is what is relevant and they call it resistance, the people who make the testing equipment call it resistance testing.
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Bob, as you well know the NEC is not infallible. The very existence of this forum depends on the confusion that is generated by the NEC.

I agree, that a ground rod stuck in the ground and being tested with one of those fancy testers will have some value of resistance. I think that's what the NEC is referring to. But once we connect an AC system to that ground rod, the game changes.
 
Peter,
You're absolutely correct about the Impedance thing. Don't let these guys try to convince you otherwise. Could it be that you are simply more 'reactive' than they are?

Food for thought. The NEC refers to "Impedance Grounded Neutral Systems" not "Resistance Grounded Neutral Systems." 250.20(E)

Eric
 
eric stromberg said:
Peter,
Food for thought. The NEC refers to "Impedance Grounded Neutral Systems" not "Resistance Grounded Neutral Systems." 250.20(E)
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Yes I was aware of that article.

That proves to me that the NEC knows of the term impedance and choses not to use it in regards to grounding electrode testing.

It matters not what the NEC calls it, it's their ball game, they make the rules. :p

But as I said Pete as you are so bent on this write a proposal and see what they say, sadly you will have a long wait at this point.
 
eric stromberg said:
Peter,
Could it be that you are simply more 'reactive' than they are?
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:lol: :lol: I love electrical humor.

Thanks for the support Eric. Stick around here and you will see that we get pretty hard headed sometimes. :shock: :D
 
Well, let me add my 2 cents worth here. I'm not an engineer, and its been a long time since I took college physics, so my theoretical knowledge is pretty weak, but here's my logic on this.

A given ground rod will have various properties. One property is resistance. Another is impedance. After we connect that ground rod to an AC system, it does not loose its resistance property. Resistance and impedance are not mutualy exclusive. The rod will have a resistance to ground and an impedance to ground. My understanding is that impedance varies with the frequency of the current. The NEC could have required an impedance at 60 hz of 25 ohms or less, but how relavant would that be? Would it be any more relevant that a resistance of 25 ohms? A likely purpose of the rod is to conduct lightning energy to the earth. What is the fequency of a lightning strike? If the NEC were to specify an impedance, at what frequency should that imepdance be measured? It would not be practically possible for the electrician to measure the impedance of a rod under lightning conditions. The resistance of the rod is a much more measureable property, and that is what the NEC requires to be measured.

The NEC speaks of Impedance Grounded Neutral Systems, because in that context, the relevant frequency is 60 hz. and it makes sense to consider the impedance with that known fequency.
 
Here's how I see it. The ground resistance of a ground rod is an installation value. It only indicates a physical aspect of the rod and earth connection. The surge impedance of a ground rod is more of a performance based quantity. With this, you may be able to determine how the grounding system will perform under surge and transient currents.
 
Since were on the subject, Every time I connect lightning down conductors to the main grouding loop or the common GES. I get a little discomfort by the thought that lightning current will one day decide to enter the building power sys. though the bonds or interconnections and toast all the connected gizmos or the people on it .

(this was compunded when I read on Beemans book that GES of transmission lines should be at least 25' from stepdown X'fmrs secondary winding GES and that the two GES should have no interconnection.)

What precautions does the electronic & communications industry used against this condition, given the sensitivities of euipments.

Can somebody help me put my fears to rest.?

davidv
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