two ground rods negating the requirement to check for impedance

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kwired

kwired

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GoldDigger said:
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.
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True, but let's not forget we don't install grounding electrodes to serve the purpose of equipment grounding conductors either, they don't have to carry the required current to trip an overcurrent device.
 
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kwired said:
True, but let's not forget we don't install grounding electrodes to serve the purpose of equipment grounding conductors either, they don't have to carry the required current to trip an overcurrent device.
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And given that the electrodes do not carry fault current, it is not important for fault clearing that they have low contact resistance!
 
tom baker

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paulengr said:
NEC is a lousy reference for effective grounding systems. Note that the quoted section refers ONLY to ground rods and not any other type of grounding. In previous versions of the Code, 25 ohms was more or less suggested but not even given as a specific target. The reality is that the grounding electrode resistance has more to do with it's use. With building structures although lightning systems are a separate issue (and required by Codes to be a separate ground plane), that's generally the highest duty that the grounding electrode will ever experience. Maintaining low voltage between Earth and the electrical system only requires that it be less than roughly 1/3 of the system capacitive impedance, or it acts more like it is ungrounded. With substations, IEEE 80 specifies generally 1 ohm or less. With residential/commercial installations usually the rule of thumb is 25 ohms or less.

But the interpretation given above is flat out backwards. What NEC 250.53 is saying is that if you've already achieved <25 ohms, then no additional ground rods (or plates, etc.) are needed but that if it's above 25 ohms, keep adding ground rods until it gets below that point. As it says, it's an EXCEPTION, not a RULE. Note though that this is not only an exception but if for instance the installation used a flat plate, then the rule would not apply and only one ground electrode would be necessary. So clearly the purpose here is to require measurement. In previous Code editions where the "25 ohm rule" was more of a suggestion, it was up to the jurisdiction to establish a cutoff.

As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds. The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone. There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds. I don't see how this is "slower" than driving another ground rod, even if you have an electric vibrator to speed the process up.
Click to expand...

Drive two and go home is fine for most applications. The NEC is the minimum standard, someone wants an engineered system for a cell tower, the NEC does apply to the installation, but that's optional
 
Smart $

Smart $

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electrofelon said:
Well the NEC does have that "contact with higher voltage lines" phrase in the beginning of 250, so the writers clearly think it is an issue and that it would help.....? As far as documented number of cases I dont know if there is any data.
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Also, need I remind everyone that grounding electrodes have multiple purposes... the broader aspect of equipotential grounding is far from and doubtful it will be covered here in this discussion.
 
Fulthrotl

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paulengr said:
As to expense and time, are you kidding me? The clamp on ground testers on the market, if used correctly, give you a reading in seconds. The old days of driving a couple ground rods and connecting 100 feet or so of wire (not that this isn't the most accurate, best method) are gone. There are issues with the clamp on testers to be sure (and price tag is one of them), but a trained operator can use them successfully in seconds. I don't see how this is "slower" than driving another ground rod, even if you have an electric vibrator to speed the process up.
Click to expand...

fluke 1630 tester = $1,600
ground rod = $16.78

so, allowing for labor, you'll
probably break even at 75 ground rods.
 
don_resqcapt19

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Fulthrotl said:
fluke 1630 tester = $1,600
ground rod = $16.78

so, allowing for labor, you'll
probably break even at 75 ground rods.
Click to expand...
That would be assuming that using the tester will let you use a single rod....not very likely in most areas.

Also it won't work if the rod is not connected to other grounding electrodes, so if you are trying to use it for a single rod at a service and the service is not connected to the utility, the clamp-on tester won't work.
 
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