What is the ground rod for?

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I have a book at home written by ???????? (Martin Ullman MAYBE) from U of F Gainesville FLA. U. He has completed some wild experiments to determine frequency and voltage of a stroke. But at best any given stroke the magnitude would be a SWAG (I believe).
 
...(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.
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What's a line surge? What causes them? And how does a grounded system get rid of them? I've never seen a definition.

carl
 
al hildenbrand said:
Electromagnetic pulse induced voltage and current from any number of sources ...
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Okay. An atmospheric nuke will generate an EMP - at least that is what I have been told - I've never seen one.

What are some other sources?

How would I measure one (a line surge)?

carl
 
Switching loads, on and off result in "SURGES" or voltage transients. Measured with high speed line disturbance analyzers.

I think in the case of a NUKE, the line disturbance analyze might get damaged, maybe even us?
 
brian john said:
Switching loads, on and off result in "SURGES" or voltage transients. ...
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Alright. Surge = voltage transient. One cause is switching loads. Okay, that sounds good.

Now, how does a ground rod mitigate this?

Truly, the science/physics of this is eluding me.
 
Had an incedent locally where the poco crew mixed the line and loads on a substation transformer. There was a brief "unintentional contact" when it was first energized. Many tv sets and the like had to be replaced. Can't really say if any of the ground rods in that area helped anything or not. Who knows???
 
coulter said:
Alright. Surge = voltage transient. One cause is switching loads. Okay, that sounds good.

Now, how does a ground rod mitigate this?

Truly, the science/physics of this is eluding me.
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Ground rods do not get rid of switching surges.T.V.S.S. devices will attenuate these surges.This forum has lots of discussion on these devices--do a search.

An excellent discussion on grounding is contained in FIPS94(M.H. free stuff).I have included the link
http://www.mikeholt.com/documents/calculations/formulas/fips94.pdf

Good Luck:)
 
ghostbuster said:
Ground rods do not get rid of switching surges. ...
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I would not have not thought so either. Maybe I mis-understood what the NEC said about line surges - or, maybe I still don't understand what a line surge is.

ghostbuster said:
...T.V.S.S. devices will attenuate these surges.This forum has lots of discussion on these devices--do a search. ...
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Good suggestion. I may if the spirit (or ghost;)) moves me. But right now I'm concentrating on: What exactly is a line surge? How does a ground rod mitigate one - as the NEC alludes one should/will?

ghostbuster said:
... An excellent discussion on grounding is contained in FIPS94 ...
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I recall reading that a few years back. Likely time to reread. Thanks for the link.

carl
 
I reviewed FIPS94 section on grounding and the section on lightning protection. It was pretty much as I recalled. It talks about surges from switching transients (sect 4.6) and it discusses surges from lightning (sect 7). As ghostbuster said, surge suppression devices will divert the current. In 1983 I don't think they knew about TVSS devices. No help here on how a grounded system assists in mittigating.

carl
 
brian -
Here is what I am after:
The NEC alludes to a phenomena called "line surge" as separate from lightning, or unintentional contact with higher-voltage lines. It also alludes to a grounded system will mitigate this phenomena, "Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by ... line surges ...".

So I am sitting here thinking, "For the first 25 years of my electrical career, I knew exactly what the NEC meant on this issue. For the last 15 years, I have realized I don't even have a clue about some of the things they are talking about. This 'line surge' thing is one."

Is the NEC talking about a voltage transient where the line conductors are suddenly raised above the grounded conductor? I don?t see a grounded system helping on that. Or are they talking about a common mode transient where both the un-grounded conductors and the grounded conductor are raised above ground. A grounded system might help out here. Of course, surge suppressors may well help out more in both cases.

So, I am thinking, maybe the problem is I just don?t know (as I asked in post 22):
What is a line surge?
What causes them?
How does a grounded system mitigate?

Then I figured I?d ask how would one measure one (line surge). My theory is if I can measure it, then I have some clues as to what it is.

If anyone has some insight here, I?m listening.

carl
 
coulter said:
brian -
So, I am thinking, maybe the problem is I just don?t know (as I asked in post 22):
What is a line surge?
What causes them?
How does a grounded system mitigate?

Then I figured I?d ask how would one measure one (line surge). My theory is if I can measure it, then I have some clues as to what it is.

If anyone has some insight here, I?m listening.

carl
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Carl
We have been measuring voltage transients(which I believe you are calling line surges) for over 30 years.They come in all flavours.
Phase to Phase
Phase to Ground
Neutral to Ground

What flavour do you want to discuss to-day??:)
 
ghostbuster said:
...We have been measuring voltage transients(which I believe you are calling line surges) for over 30 years. ...
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That's great. I'm very interested in someone with experience to understand the reference to "line surge" in 250.4A1. But no, I'm not equating "voltage transient" with "line surge". That particular equivalency was suggested by another poster in response to me asking for a definition of "line surge".

ghostbuster said:
...What flavour do you want to discuss to-day??:)
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Flavor of "voltage transient"? Absolutely none - unless it relates to the "line surge" referred to in 250.4A1.

I have really tried to be clear on this - I not sure how I can explain my specific lack of knowledge any better than I already have, but I'll try again.

NEC 250.4A1 discusses limiting the voltage imposed by "... lightning, line surge, or incidental contact with higher-voltage lines ...". NEC 250.4A1 also alludes to a grounded system helping to mitigate this voltage.

From this NEC article, I infer the NEC considers voltage anomalies from "lightning", separate from "line surge", separate from "incidental contact with higher-voltage lines"

I think I understand voltage anomalies caused by lightning. I think I understand voltage anomalies caused by incidental contact with HV lines.

********* Please listen to this piece *************
I don't know what NEC 250.4A1 is referring to when it uses the term "line surge"

So, the questions are:
What is line surge (as used in NEC 250.4A1)?
What are some causes of line surge?
How would a grounded system mitigate the effects of a line surge?

Alternate question only if the first three are not answered:
How would one measure this undefined line surge?

**** Hope you got clear to here ***************

This is not just a debate thing for me. I really am interested in what the NEC is trying to save us from.

carl
 
Carl, I just read this thread (I've been in New Zealand) and I sympathize with your perseverance despite not getting clear answers to your clear question. Maybe it is because you are asking the NEC for what they mean and no one here is representing the NEC. So you get various answers.

OK. How would I answer it? By saying that I have always been unclear about what "line surge" meant. I always pictured a high voltage line falling on a distribution line in a storm and energizing the neutral/ ground conductor.

Is Mr. NEC in the house?

Karl
 
but 2???

but 2???

I have been doing service upgrades for many years, but finally been told by many inspectors that two ground rods spaced 6 feet apart are required. I complied by driving two rods(in which one was enough pain in the tookess), but after doing this in different municipalities i was told only one was fine. can we come up with a uniform code so we don't have to second guess the inspectors or at least give us a UL rod that will comply for all areas?
 
Ruwiredrite,
Take a look at 250.56. This is the reason that many inspectors will require two rods.
Don
 
spaced 6 feet apart are required
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They are not required to be 6 feet apart, the minimum spaciong is 6 feet. setting them a 100' apart is not a violation.

One rod you are suppose to test for a minimum of 25 OHMS (a magical number that comes from???? {oh Bryan})

Drive two rods and forget the magic mystical number, your done.
 
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