Stray Voltage

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ohmhead said:
Well no i disagree you must be lower than 25 ohms if it takes more than two or three or four ground rods thats how we see it .
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I can show you a code-making-panel statement to refute that as well, if the language of 250.56 does not prove that to you. There is no requirement for more than two rods.

ohmhead said:
we never said he would be alive we just limit the voltage to him ?
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And the point of that would be...? :confused:

ohmhead said:
and its 1000 volt touch voltage not 5000 volts that is the line voltage
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1. Dead is dead. 1000V is 940V more than acceptable.

2. Show me a circumstance where the victim would touch less than 5000V during the fault, assuming conductors energized to 5000V to earth under normal operation.
 
high voltage or low it all the same

high voltage or low it all the same

Well let me explain in detail you have a high voltage tower made of metal each phase is 5000 volts one phase shorts to the tower by conduction spark or cross over now impedance of this connection is 40 ohms resistance , now the ground rod at base of that tower is 10 ohms, I= E/R&Z 5000/50 =100 amps short circuit current a person touching tower only gets 1000 volts as the 10 ohms is to ground earth not the 40 ohms of the impedance the ground rod is the key to the touch voltage 100 amps at 10 ohms if you drop or lower resistance of ground rod to one ohm it would be 100 volts touch voltage , i didnt make this up its a fact earth ground is earth itself that voltage is earth to tower . comments ? best to ya
 
ohmhead said:
can one tell me the advantage of not having a ground rod on your service ? iam open for discussion best to ya
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No one has said that a service should not have grounding electrodes.

In this post lets forget about substations, lets forget about Tel Com. Lets focus on building services 600 volts or less.

With that in mind if all I had was two rods that had a combined resistance of a 50 ohms to ground that would still be enough of a connection to earth to help dissipate a lighting strike.

Now in my area the primaries are 13.8KV so that is about 7.9 KV to ground. Lets say a primary falls on the neutral to my home and at the same time the neutral to the utility was open. The only fault path being through my two grounding rods.

7,900 volts / 50 ohms = 158 amps, that should be far above the over current protection of the primary that fell on the line. So the over current device should open.

So I believe a grounding electrode system at a home is needed but we do not have to get hung up on the resistance value of it.
 
ohmhead said:
Well let me explain in detail you have a high voltage tower made of metal each phase is 5000 volts one phase shorts to the tower by conduction spark or cross over now impedance of this connection is 40 ohms resistance , now the ground rod at base of that tower is 10 ohms, I= E/R&Z 5000/50 =100 amps short circuit current a person touching tower only gets 1000 volts as the 10 ohms is to ground earth not the 40 ohms of the impedance the ground rod is the key to the touch voltage 100 amps at 10 ohms if you drop or lower resistance of ground rod to one ohm it would be 100 volts touch voltage , i didnt make this up its a fact earth ground is earth itself that voltage is earth to tower . comments ? best to ya
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Again it would be imposable to add enough rods to keep somebody alive if they got between the 'hot' tower and the earth. It is very likely that the concrete footing provide less resistance to earth then any added grounding rods.
 
Well we were just trying to make one understand that a ground rod its important and 25 ohms is not good enough , and it doesnt matter a house its all the same if line shorted to a homes ground the rod resistance means less of a fault damage to ones home than a higher resistance to earth ground . let it go to earth and not thur your house . What we were say about the tower was the resistance of the person touching the tower is series resistance think about it he gets a lower voltage yes hes dead but its just a example to show how a lower resistance rod works, when fault happens anywhere .Sorry for my post ill let you go. Were not trying to offend anyone just speaking out on the subject , best to ya
 
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ohmhead said:
Well we were just trying to make one understand that a ground rod its important and 25 ohms is not good enough , ...
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Then I guess you need to submit proposal for the 2014 code to correct this problem.
Lowering the resistance of the grounding electrode system for normal building electrical system, one that is operating at 600 volts or less, provides almost no change in the safety of the system. Unless the fault is cleared, you have a voltage that is about 85% of the system voltage, when you are standing on the earth 3' away from the grounding electrode and touching the hot wire or faulted equipment.
It does not provide equal potential as many seem to think it does. This is why you use grounding mats in the substation...to provide reduced step and touch voltage. It doesn't work by lowering the voltage to earth, it works by raising everything to the same voltage....often thousands of volts above earth.
 
ohmhead said:
Well we were just trying to make one understand that a ground rod its important and 25 ohms is not good enough
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For building services 25 ohms is more then 'good enough', 5 ohms will not be better then 50 ohms for services operating under 600 volts.

Substations and tel com requirements are very different and specific installations.


Were not trying to offend anyone just speaking out on the subject
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I am not offended and I don't think anyone else is but many of the things you have posted in this thread are simply not true in my opinion.
 
George Stolz said:
Ohmhead, the grounding electrodes and the man have equal access to the voltage. They are in parallel, not series.

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Well yes we do agree he would be paralled with the rod resistance of 10 ohms so voltage on touch would be and in series with the tower from earth or his feet to hands to tower is that what you are saying ? Rod to earth person to earth same voltage different currents paralled circuit at 100 amps at ten ohms = 1000 volts he doesnt get 5000 volts comments
 
iwire said:
. . . many of the things you have posted in this thread are simply not true in my opinion.
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I agree, you really should re-read all of the posts instead of arguing so hard. Then take a look at theMike Holt Grounding vs. Bonding Textbook or the 2008 Soares Book on Grounding for more information.
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Ohmread earth is a very poor conductor whose resistivity is around 1 billion times that of copper. Lets say you drive enough rods to achieve your 10 ohms to earth at a tower structure. The resistance is made up of the collective resistance of series of equal-thickness concentric shells of earth. The inner shell will of course represent the largest incremental value of resistance, since the resistance is inversely proportional to the shell diameter. Thus the central small diameter shells of earth constitute the bulk of the earthing terminal resistance. Half of the 10 ohms resistance value will be contained within a 1 foot diameter cylinder.

For the same reason, half of the voltage drop resulting from current injection into this grounding electrode would appear across the first ? foot of earth radially away from the ground electrode. If a fault current of 1000 amps were injected into the grounding electrode, the electrode would be forced to rise above mean earth potential by 1000 * 10 = 10,000 volts. Half of this voltage, 5,000 volts, would appear as voltage drop between the electrode and the earth spaced only ? foot away from the electrode. While the current is flowing, a person standing on earth a ? foot away from the ground electrode and touching the structure where the fault occurred would be spanning a potential difference of 5000 volts.

This is why substations have ground grids buried just beneath the surface to form a equipotentail ground plane to minimize gradients and step potential, and platforms installed around all switch gear and structures for personnel to stand on. This is done because there is no resistance low enough to protect you period.
 
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ohmhead said:
Well yes we do agree he would be paralled with the rod resistance of 10 ohms so voltage on touch would be and in series with the tower from earth or his feet to hands to tower is that what you are saying ? Rod to earth person to earth same voltage different currents paralled circuit at 100 amps at ten ohms = 1000 volts he doesnt get 5000 volts comments
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Wrong. Your hypothetical 10Ω ground rod system would see 5000V and deliver 500A back to the source ( I = E / R ), and the man would see 5000V and deliver 5A back to the source. Since 5A is 1000 times what the man can survive, he dies.
 
Well this is not something which i just came up with on my own . Its out of a associated research inc , vibroground book called Earth Resistance Tests .As follows it reads assuming a power source impedance Z of 40 ohms a short circuit between the 5000 volt power line and the steel tower would produce 100 amps of short circuit current and ground rod has 10 ohms earth resistance . I= E/ Z&R=100 amps 5000/40&10 a person touching the tower would be subjected to the voltage E developed across the ground resistance E=IXR=100X10=1000 volts between tower and ground . this depends on body resistance this is touch voltage . I wrote this per word for word out of book so think about what you are telling me if iam wrong then the people who make our tester for ground testing have a big problem come monday . This is a training manual from vibroground test equipment. the best part it shows a man stand by the tower getting zapped and the ground rod at 10 ohms it also shows the 1000 volts on the man ? comments i will copy this if need be best to ya
 
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So let me get this straight OK? You want to take the word of a equipment manufacture, over the word of thousands of Engineers who have developed all the standards and have sworn oaths to protect the public? :confused:
 
090125-1555 EST

dereckbc:

Look at the assumptions that were made. The 5000 v high voltage source impedance was assumed to be 40 ohms, and earth resistance back to the common point of the 5000 V source is 10 ohms. Based on these assumptions the current is approximately 100 A and the voltage drop across the 10 ohms is approximately 1000 V. Note there is probably a greater inductive component in the high voltage wire than in the diffuse earth.

Either way the guy that touches the tower is probably dead.

.
 
gar said:
Look at the assumptions that were made.
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Assumptions will get you killed. 40 ohm source impedence is really high. Th ewhole point of a low impedence ground in high voltage applications is to clear the over current protection device, and that is it. No amount of impdence is going to save your hide. And in low voltage applications of 600 volts or less which is what this forum is dedicated too, the ground impedence is of no concern.


gar said:
Either way the guy that touches the tower is probably dead.
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Which brings us right back to what we are trying to say. Low impedence ground systems will not save your hide, only operate the OCPD in high voltage applications.
 
Well i find it hard to believe a vibroground book is not correct ? Next Gar the text is correct and we know the man is dead we never said it will protect .all were trying to do is say if your ground rod resistance is low its better then high in some areas of electrical work . now this is out of a publication from associated research ,inc skokie ill manual 18507 it comes with our tester. again we never said the man is alive this is step voltage or touch voltage in a power plant . comments ? best to ya
 
dereckbc said:
Assumptions will get you killed. 40 ohm source impedence is really high. Th ewhole point of a low impedence ground in high voltage applications is to clear the over current protection device, and that is it. No amount of impdence is going to save your hide. And in low voltage applications of 600 volts or less which is what this forum is dedicated too, the ground impedence is of no concern.


Which brings us right back to what we are trying to say. Low impedence ground systems will not save your hide, only operate the OCPD in high voltage applications.
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you are not understanding the 40 ohms Z impedance is in the line the transmission line not the ground rod the short to tower ground the line . the ground rod is 10 ohms so the man touching the tower gets to share the voltage of the 10 ohm rod voltage in the earth ground rod not the z impedance it just controls the current in the circuit do you understand now . yes we know hes dead were just explaining touch voltage its a standard in power plant work we are dead but we have to know how far we can stand from objects . and at lower resistance to ground its a lot safer than high resistance . comments
 
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The man touching the tower and standing on the earth 5' away from the grounding electrode will see the almost the same voltage no matter what the resistance of the grounding electrode is. The only way the grounding electrode can provide any shock protection is if it flows enough current to open the protective device. If it doesn't the man is just a parallel path with the grounding electrode conductor and the grounding electrode really doesn't even enter into the issue.
 
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