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- Illinois
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- retired electrician
The 25 ohm ground rod is in parallel with the 0.1 ohm neutral conductor. That changes the path resistance from 0.1 ohm to 0.0996 ohms. How does that change reduce the shock hazard?
Mike Holt's video on grounding & bonding
- Thread starter jeff48356
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That would be true only when the 25 ohm ground rod is in series with the parallel combination of POCO's neutral ground resistances. But this is not the case as all system ground resistances including the 25 ohm ground rod are in parallel and there is no series parallel combination as you think.
So what would be the effect of this?
With only 25 ohm ground rod at one end and POCO's neutral ground resistances on the other end, the 60V voltage drop along the neutral wire between these two ends during a ground fault would be divided almost equally between the two ends, if the resistance areas of the two ends do not overlap.
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Even if that were true, it would not change the touch voltage at the equipment. There is still 60 volts between the equipment and the earth outside of the local influence of a grounding electrode.
You are treating this circuit like a voltage divider, but it is a current divider. The paths are in parallel and the voltage drop is the same across each path.
mbrooke
Batteries Included
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Even then, its cost prohibitive to upgrade it all.
Also, I have to disagree. A lot of electronics have a ground pin. Perhaps not for fault clearing being most are double insulated but more on surge requirements. Often the hot and neutral has an MOV to ground that helps shunt phase to ground surges over.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I have no idea why you are bringing in the transformers neutral to earth reference, for all purpose of modeling the above said circuit it should not even be considered a part of the circuit, this is because for the most part because of all the MGN to earth connections through out the grid system this connection can be about as close to 0 ohms as you could get, that is the reason I just drew the connection from the SOI of the 25 ohm resistor to the transformer neutral as if you had ever measured the voltage drop from an energized rod electrode to a point out 26 feet from it you would notice that you would have dropped 100% of the voltage supplied to the rod, so for those of us who know how to model earth grounding electrodes we do not include the very very very small loss that may occur at the many earth connections of the utility grid.
also including this very low resistance in the model will not add a load to the circuit at hand but quite the opposite, as putting this resistance in series with the 25 ohm resistor will increase the resistance of this path making the 25 ohm resistance even less effective!!!
So I have no idea why you are trying to even go this route other then to continue an end less argument that has no bases of fact or math to back it up, we gave you the reasons a ground rod electrode will not add enough load to the circuit to reduce the voltage to a safe voltage, we have done the math that proves this, and even drew a diagram of the circuit model to make the circuit easier to understand, after all this if you still wish to continue then I'm am done, because I have no other way to get you to understand the model you should be looking at.
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hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I have always believed that for the most part a hot to ground or a neutral to ground MOV's is and always will be a waste of money, surges do not want to go to earth or ground, like any electrical circuit they want to return to source, most all surges are developed between the hot and neutral or line to line in the case of 240 volt equipment, also surges are a high frequency event and clamping has to take place close to the load you wish to protect, or at the source of the surge, also most loads are only between the hot and neutral not ground, older networks used to reference the grounding system but Ethernet does not, most computers today have isolating SM power supplies that only use the ground as a safety ground, any electronics today that use the grounding system as a reference is a poorly design system that will be wrought with noise problems as well as voltage problems cause by the voltage drop on almost any neutral which will produce a voltage difference between the neutral and the EGC.
I have always thought that surge strip manufactures put MOV's between the hot to ground and neutral to ground to boost their clamping level claims, there are a few manufactures who do not do this.
I have lived in a couple houses that had no grounding at all, and never had any problem with any of my computers, just a few years ago most laptops didn't even have a ground pin in their cords they worked just fine,there are a few countries that don't use a grounded system again their electronics work just fine.
There many myths out there, many are put out because of greed, so they can sell you more.
mivey
Senior Member
Hurk is discussing the ideal scenario of course but it might be useful down the road to remember that these shells are exact and round only in a uniform medium and where we don't have interference from other fields.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Correct, many times the rod will be next to a foundation or concrete basement wall, there maybe other conductive utilities such as metal water lines or gas piping within the SOI, in some cases you might have a mixture of sandy loom on one side of the 26' shell with clay on the other, all of which can distort the shape of the shell, but for the most part the shell out away from the buildings foundation holds true for the most part.
The definition of touch voltage states a voltage at about 3 feet from the grounding point. So if you move beyond the local influence of the GEC rod resistance and towards the POCO's transformer neutral ground resistance local influence, the fault voltage would naturally rise to 60V.
So if we measure the GEC ground rod resistance, a parallel path, its resistance would not be 25 ohms but close to zero as do its other parallel paths. Correct?
- Location
- Illinois
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- retired electrician
At 3' from the rod the touch voltage would be ~45 volts per the information in the IEEE Green Book. At 5' it would be about 52 volts.
You don't measure the resistance of a ground rod when it is connected to a circuit. After you find the resistance of the ground rod, you solve for the resistance of all of the parallel paths. If you can measure the resistance of the parallel paths, it will be less then the lowest resistance of any single path just like OHMs law tells us.
As pointed out in previous posts, if you put the 25 ohm ground rod in parallel with a 0.1 ohm grounded conductor, the resistance of the two paths in parallel becomes 0.0996 ohms. How does a reduction of 0.004% in the resistance change anything??
I think IEEE considered a network of closely spaced ground rods (as in an outdoor substation) where independent, non-overlapping resistance areas of individual ground rods do not exist in contrast to the case being considered here.
Well, if you now measure the resistance of GEC ground rod, it is close to 0.0996 ohms. Similarly, if you measure the POCO's neutral ground resistance at the transformer end, it is also close to 0.0996 ohms. So there may be two ground resistances with non-overlapping resistance areas and of almost equal resistance values. These two resistances are interconnected by the neutral wire during a ground fault. Correct?
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- Retired PV System Designer
When a number of points are interconnected by a network of different metallic and earth paths, you have to be very careful about what you call the various components of resistance. In the case that don proposed there is a metallic path between customer bond point and the POCO neutral at the point where it joins the transformer secondary and the (possibly present) POCO ground local ground electrode wire.
If that resistance is .1 ohms, it would be very confusing and totally misleading to say that the ground resistance at each end is .1 ohms.
The portion of the total resistance between the two points that corresponds to the GEC to customer electrode, through earth to POCO electrode and POCO GEC is likely to be much higher than that. At the customer end, we might have a GEC directly to earth (with all other metallic connections temporarily removed) of 25 ohms. (Just a randomly chosen value out of the likely range from 100 ohms to 10 ohms for a ground rod.)
At the POCO end the resistance between the transformer secondary neutral point and remote earth may be as little as 1 ohm because of the metallic connection to other POCO electrodes and other customer electrodes. But if you isolate the local GEC at the pole, you might have 10 ohms to remote earth.
If you are careless, you will call all of the "ground" resistances .1 ohm, but that would be wrong! It would, however correspond to a reading you might get on an ohmmeter when trying to use an incorrect method to measure the ground resistance.
In any case, in the example above, the fault clearing resistance from the customer's neutral/GEC to the utility transformer secondary would be ~.1 ohm. And the current would be high enough to trip secondary side OCPD pretty rapidly. What would happen if the fault was in one of the service conductors would depend on the primary OCPD on the POCO side, but I would not count on it being quick.
And both fault clearing time and the potential to earth on the customer EGC network will not be particularly affected by the presence or absence of the 25 ohm plus 10 ohm path (or the 25 ohm plus 1 ohm path if counting the effects of the MGN) through the earth back to the POCO transformer.
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Consider the parallel resistance i.e 0.1 ohm to ascertain the voltage drop across each parallel resistance is same.
The gist of of the argument is not an incorrect method to measure the ground resistance but to ascertain that the voltage across each parallel ground resistance is same.
The problem is how this ~.1 ohm is divided between the two ground resistances.
Exactly. Unless there is one or more neutral to ground resistances between the 'two points', that would be the case.
- Location
- Illinois
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- retired electrician
It is a parallel circuit, how would the voltage drop across each resistance that is connected in parallel not be the same? It doesn't matter what the values of each parallel path may be. The voltage drop is determined by the resistance of all of the paths connected in parallel. Even is one path is 0.001 and the other path is 10,000, both paths will have the same voltage drop.
Again, this is a current divider and not a voltage divider. In the above example while the voltage drop is the same for both paths, the current is not. Almost all of the current would be one the 0.001 ohm path.
The addition of a 25 ohm grounding electrode would do nothing to improve the safety of the system under fault conditions, unless you are standing on or very close to the ground rod...the area where the voltage of the earth has been raised to a value that is closer to the total voltage drop.
As far as the IEEE table, it is not for a system of interconnected rods...it is area around the rod where the voltage of the earth is raised. If there are multiple electrodes there will be interaction between them and if they are close enough, you can raise the voltage of an area to the point where the step and touch potential is reduced to a safe level.
Don:
The bonding of GEC ground rod with MGN is actually a method of reducing its ground resistance. Its reduced ground resistance after its bonding with MGN is as real as its ground resistance before its bonding with MGN. Hence the validity of my point:
The bonding of GEC ground rod with MGN is actually a method of reducing its ground resistance. Its reduced ground resistance after its bonding with MGN is as real as its ground resistance before its bonding with MGN. Hence the validity of my point:
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- retired electrician
The bonding of one resistor to another one does not change the resistance of either resistor. You point is not valid.
There is a way out. Here is the web link:
http://ecmweb.com/site-files/ecmweb.com/files/uploads/2014/03/4346628a_en_EarthGround-tutorial-e.pdf
on page 9, measurement of individual ground resistance in a (parallel) system of ground resistances without disconnecting its connection to the site is mentioned. So it is possible to calculate individual voltage drop across each ground resistance in a parallel system of ground resistances.
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- Placerville, CA, USA
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- Retired PV System Designer
The question asked earlier was about measuring, not calculating. And clamp injection devices for directly measuring individual earth resistances have been around for years. The techniques do, however, require the existence of a second earth access path to enable current flow, as well as access to a remote ground voltage reference.
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Aren't you ashamed or at least tired of these adolescent arguments?
I am neither ashamed nor tired because I want to be certain if I am right or wrong. I advise you to have that guts.
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