Ground Rod at a construction trailer Y or NO

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I recall a job done last summer involving about a hundred construction trailers ~ all with 2 rods.
 
well i am seeing it as the electrodes in the originating service location. if i am going to run a egc out to the building. what good does the rod at the secondary location do? it surely wont clear a fault. what i would expect to see is the secondary building bonded to the panel in/at that building. then the egc would carry back to the originating service location to that electrode. with the speed of light at 100 some thousand feet per second how long does it go a couple hundred feet.
 
vinster888 said:
well i am seeing it as the electrodes in the originating service location. if i am going to run a egc out to the building. what good does the rod at the secondary location do? it surely wont clear a fault. what i would expect to see is the secondary building bonded to the panel in/at that building. then the egc would carry back to the originating service location to that electrode. with the speed of light at 100 some thousand feet per second how long does it go a couple hundred feet.
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250.32 is two separate requirements, (A) is the requirement for grounding electrodes, at separate buildings, and (B) is the requirement for an equipment grounding conductor to be ran with the feeders to the separate building. these are two very distinct requirements. not all rolled into one.

(B) has nothing to do with the (A) other than it states you must bond the Egc required by (B) to the grounding electrode required by (A) at the separate structure.
 
OK i see the light. only had to reread another 50 times. only problem i have with it now is, in lightning prone areas, this is asking for trouble. i don't need job security. there is enough idiots around the world today that job security is always there.
 
yeah i get all that but sticking your head in an alligators mouth isnt the greatest idea but yet people keep on doing it. the more buildings you have, the more rods you have, the increased likelihood that the current can go across buildings. for example: lightning strikes the premises system in building C. as luck would have it building A actually provides the least resistance to earth through its electrode. the main service is located in building B. the current could then travel through all the buildings to get to the one that provides the least resistance. there's nothing that says the closest one would be the one that's utilized. so why have so many points of entry, when one defined exit would be more efficient?
 
vinster888 said:
yeah i get all that but sticking your head in an alligators mouth isnt the greatest idea but yet people keep on doing it. the more buildings you have, the more rods you have, the increased likelihood that the current can go across buildings. for example: lightning strikes the premises system in building C. as luck would have it building A actually provides the least resistance to earth through its electrode. the main service is located in building B. the current could then travel through all the buildings to get to the one that provides the least resistance. there's nothing that says the closest one would be the one that's utilized. so why have so many points of entry, when one defined exit would be more efficient?
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In most cases, lightning wont even see the GES, as lightning is a high frequency event and most of the time if it even hits a building, it will jump off to the cable system or in many cases the phone system. here we had it jump to a phone line and run about 60' around the outside of a house to then jump to a chain link fence, never even going to the two installed ground rod or the water pipe electrode. so to even say that a small rod electrode will attract a lightning strike is a very loose statement with all the variables that determine what lightning will strike at any point in time. trees can attract many more lightning strikes because they act like a very broad band antenna with many branches of different lengths.
 
090529-2049 EST

Following are data from my backyard test setup:

My house has a large, 1.25" OD, copper water pipe that runs from one end of the house and under the basement floor for about 55' and then another 100' to the street. Pretty much a straight line. The GEC from the water line to the main panel is about 8' long.

I have an experimental vertical ground rod, 8' long, about 75' from the back of the house. This is connected to nothing else except when I run experiments.

My soil is a moist clay loam. Very solid when dry.

Generally any place in my yard the voltage of the earth is less than 0.100 V relative to the said water pipe. This was true today.

From prior knowledge I know the earth resistance from this test rod to the water pipe is greater than 6 ohms. I did not measure it today. However, I connected the test rod to 120 V and did not trip a 20 A breaker.

With 120 V applied to the rod I got the following results for earth potential relative to the water pipe and at distances of X ft from the test rod.

3 ft ... 30.5 V
15 ft ... 12.0 V
30 ft ... 6.8 V
60 ft ... 3.5 V
75 ft ... 2.5 V this is about 3 ft from basement wall.

If I were to go to the front side, then the voltage would be the residual voltage from normal ground currents.

.
 
iwire said:
Lightning protection is really the only reason (beyond being required to) to have electrodes out at separate structures.
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hurk27 said:
In most cases, lightning wont even see the GES, as lightning is a high frequency event and most of the time if it even hits a building, it will jump off to the cable system or in many cases the phone system. here we had it jump to a phone line and run about 60' around the outside of a house to then jump to a chain link fence, never even going to the two installed ground rod or the water pipe electrode. so to even say that a small rod electrode will attract a lightning strike is a very loose statement with all the variables that determine what lightning will strike at any point in time. trees can attract many more lightning strikes because they act like a very broad band antenna with many branches of different lengths.
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whats the point of the rod again? again i see no reason to just go throwing rods at the ground that are most likely ineffective. florida sand is not very conductive and take away the moisture, all were really doing is stabbing at the ground hoping not to hit a rock. i drive 95% of the dumb things by hand. heck ive even had some fall into who knows where. just disappear into the ground.
 
gar said:
090529-2049 EST


With 120 V applied to the rod I got the following results for earth potential relative to the water pipe and at distances of X ft from the test rod.

3 ft ... 30.5 V
15 ft ... 12.0 V
30 ft ... 6.8 V
60 ft ... 3.5 V
75 ft ... 2.5 V this is about 3 ft from basement wall.

If I were to go to the front side, then the voltage would be the residual voltage from normal ground currents.

.
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this is kind of a back wards way to test touch/step potential as with the test I have seen you would measure from the ground rod to the earth at different distance out from the rod to determine what a person would feel if they came in contact with a live ground rod, I think it turned out at 3' you would have about 90 volts potential between the rod and Earth thus showing that the rod does nothing to stop someone from being electrocuted. I think there is a graph here on Mike's site showing this.

measuring from a grounded source to the earth area around the rod only show how far the voltage can travel before dropping off, and from your expermint is not far, as you show a 70 volt drop just 3' from the rod.

Also mesure the current on the line feeding the rod, with ohms law this will give you the impedance of the rod to earth.

Just don't get bit:grin:
 
090531-0825 EST

hurk27:

I believe you misunderstood the point of my experiment.

In an earlier post I stated that a long distributed grounding electrode, such as a conductive water pipe, would be a much better grounding electrode than a vertical ground rod.

Yes, my experiment does demonstrate a large step potential at the vertical ground rod.

But the experiment demonstrates a low step potential relative to the water pipe. At my point 3 ft from the rear of the house I am still 15 to 20 ft from the water pipe, 7 ft down and 8 to 13 ft horizontally. How much conductivity the block wall and cement floor provide is hard to determine. The floor is insulated from the ground by a layer of plastic. Thus, I believe most of the conductivity is thru the earth.

A straight water pipe could be simulated by a number of vertical ground rods and get much the same result if the distance from the string of ground rods to a point source of current injection was maybe 10 times the spacing between the ground rods.


Now consider the farm problem that has been discussed in other threads. If you ringed the barn, and watering and feed area, with a grounding electrode, and all conductive items within this area were bonded to this electrode, then within the ringed area there would be very little difference in potential between different points if the current injection point was outside the ringed area.


Next consider the post by LawnGuyLandSparky. I did not read the whole document. It appears that the trailer body was not grounded. It was probably on rubber tires. There was apparently a fairly low resistance from the hot supply to the trailer body. Suppose that low resistance was 10 ohms. If the trailer had a single ground rod connected to the trailer body and we assume its resistance to ground is 10 ohms, and most of this resistance is near the rod. then the step potential can still be quite large. However, a ground rod or larger electrode still may not be a good solution. A GFCI at the outlet where the trailer was connected in addition to a ground rod at the trailer would probably have been effective.

.
 
gar said:
090531-0825 EST

hurk27:

I believe you misunderstood the point of my experiment.

In an earlier post I stated that a long distributed grounding electrode, such as a conductive water pipe, would be a much better grounding electrode than a vertical ground rod.

Yes, my experiment does demonstrate a large step potential at the vertical ground rod.

But the experiment demonstrates a low step potential relative to the water pipe. At my point 3 ft from the rear of the house I am still 15 to 20 ft from the water pipe, 7 ft down and 8 to 13 ft horizontally. How much conductivity the block wall and cement floor provide is hard to determine. The floor is insulated from the ground by a layer of plastic. Thus, I believe most of the conductivity is thru the earth.

A straight water pipe could be simulated by a number of vertical ground rods and get much the same result if the distance from the string of ground rods to a point source of current injection was maybe 10 times the spacing between the ground rods.


Now consider the farm problem that has been discussed in other threads. If you ringed the barn, and watering and feed area, with a grounding electrode, and all conductive items within this area were bonded to this electrode, then within the ringed area there would be very little difference in potential between different points if the current injection point was outside the ringed area.
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To do a experiment testing the step potential of this water pipe would require the applied voltage be connected to the pipe, of course this would require that this water pipe to be isolated from the electrical system which it is likely not.
Many experiments have been done using many kinds of electrodes and the results have all been the same. Earth has too high of an impedance to carry enough current to bring voltage down (or up in this case) enough to create a equal potential plane within the touch area of the electrode a person might come into contact with. even the ring electrode if connected to a 120 volt source will have a step potential from within the circle, toward the ring. this has been proved many times, it was tried at dairy farms to eliminate stray voltage but it fail to do so, it will not bring the earth within this circle to the same plane as the electrode, this is because of the resistance of the spherical influence that surrounds the electrode, the voltage from the ring will rise toward the center of the ring then fall as you get closer to the other side of the ring. experiments in body's of water were done that showed the spherical of influence clearly.
Even with a long pipe, rod, or wire the touch potential was just elongated to the shape of the pipe, but will still the same touch potential 3' from the pipe along its length. the only thing that changes this is the impedance of the soil which can not be depended upon as it has to many variables to be a constant. I have some of these experiments and results on my other computer sitting in a closet, and when I get it back on line I'll send them to you. but you might find them on line. not sure though.

as for the Coke A Cola case the only input I would say is the lack of the ground connection at the receptacle in the shed which did not provide a low impedance return path of the fault current that happened in the trailer was the sole failure in this case, no ground rod would have ever prevented this accident. GFCI at the shed would have lesson the likelihood of an injury but was the receptacle located inside of this shed even required to be GFCI protected. they spoke of a 50 amp cord which I have seen on many of these style trailers at carnivals, if that is the case then it wasn't required to be connected to a GFCI protected circuit, so the only thing that was evident was the fact there was no low impedance return path back to source, and earth can not be this path. the statments from these so called expert wittness that the ground rod would have prevented this is in error.

IMO:rolleyes::rolleyes::rolleyes:
 
gar said:
090531-0825 EST


A GFCI at the outlet where the trailer was connected in addition to a ground rod at the trailer would probably have been effective.

.
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So would have been the NEC required EGC, the fact the law suit was concentrating on the lack of ground rod has very little to do with science and much more to do with escaping liability.
 
Gar I have to say it is very discouraging to have a guy like yourself with such a high level of intelligence trying to say a grounding electrode would protect people even in the slightest.

There are countless injuries and fatalities due to the fact many believe a grounding electrode is a 'people protector.'
 
If I remember right the over all resistance of Earth as a whole is .05 ohms but it is the connection point to Earth at any one point on earth that is the weakest point. this is why a SWER system works as long as the connection point is spread out to create a return plane large enough to provide a low impedance return path. (SWER= Single wire Earth return) is a power transmission system that is used in rural areas to bring electrical power to isolated ares. it comprises of a single phase line to a transformer and the return is connected to a system of engineered electrodes to provide a path through Earth back to the source.

I posted this to give you an idea of the problem of the connection point of a ground rod or ground rod systems at a single point on Earth.
The SWER system is at a higher voltage but it does not mater because the voltage drop across a given resistance is still the same for a given amount of current, so if you have 4.8 amps across 25 ohms you will have a voltage drop of 120 volts no matter if the applied voltage is 7200 volts or 120. this is why the SWER system uses a system of engineered electrodes spread out over a large area to create an equal potential plane that also lowers the resistance of the connection point to earth. even then they still have voltage gradients that have caused shock in towns of rural community's. I don't think there used in the US any more, but they are still in use in other ares of the world.
One thing is they use an isolated transformer so the voltage developed accross the resstance loss of the earth connection point is not passed on the the loads connected to the transformer. this is also the fix for many dairy farms with stray voltage problems.

goggle this and look for articles that talk about the resistance of Earth
Here was one return you can click on:
http://en.wikipedia.org/wiki/Single_wire_earth_return
 
090531-2111 EST

iwire:

I will try to clarify some points I was trying to make.

Yes, there should have been a proper EGC from the trailer to the main panel such that if the short was of a sufficiently low resistance the the breaker would trip.

If the short is not that low but was at the breaker rating, then it might never blow. Suppose we had 30 A thru 200 ft of #10 copper wire for the EGC, then the drop on the EGC would be 30*0.2 = 6 V. In most cases not too unsafe.

However, the problem was there was no EGC. Had the trailer manufacturer included a GFCI device as part of their system, then the person that was injured might have received only a minor shock.

Would a single vertical ground rod at the trailer have prevented the injury? No. This is because the source of the current was internal to the trailer and to the body of the trailer. The step potential would be very high between earth and the trailer body because of the nature of the field around the moderately short vertical rod, assume 8 ft.

Now change from a single vertical rod to a long horizontal rod that extends well beyond the trailer, maybe 100 ft long. This does not directly connect to the ground rod at the main panel. This long horizontal rod is connected to the trailer chassis, body, and all internal conductive material. The location of the main panel ground rod in relation to the long horizontal rod will determine the field map of the current between the two rods. To a large extent the results would approximate the values in my experiment of the earlier post.

Voltage measurements between the trailer body and the earth around the trailer within a step distance probably would be in the few voltage range. The magnitude will be a function of the orientation of the main panel ground rod and the long horizontal rod. Most of the voltage drop between the two rods is at the main panel ground rod. However, if you measured the trailer body relative to the main panel EGC-neutral point it would be about 120 V.


When you move to the farm problem the stray currents may originate someplace outside of the animal area. If the power company transformer is within this area, then move it outside the area so the currents can be excluded. In this case one is trying to shield or bleed off the stray currents before they reach the animal area and place everything in the animal area at an equipotential.

Internal shorts vs external current sources need to be treated somewhat differently.

.
 
gar said:
090531-2111 EST Voltage measurements between the trailer body and the earth around the trailer within a step distance probably would be in the few voltage range. The magnitude will be a function of the orientation of the main panel ground rod and the long horizontal rod. Most of the voltage drop between the two rods is at the main panel ground rod. However, if you measured the trailer body relative to the main panel EGC-neutral point it would be about 120 v.
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The problem with this is the gradient of differential voltage will still be as high as with just a single rod, it just now runs along the entire length of this longer rod, at 3' from any electrode at a given Earth resistance, the step potential will be the same, yes the over all resistance of the electrode will be lower to Earth, but not the step potential, at the point of contact to the area of the rod and Earth. if you understand the spherical of influence of Earth grounding systems, you should see the with touch potential you have two spherical of influence areas. one at the ground rod and one at the point of where your contacting earth, the problem is the resistance between these two points is two high to create a equal potential ground plane and in most test with average resistance of Earth will be at or about 90 volts 3' from the electrode, it doesn't matter if its a ring, a long pipe, or a simple rod driven 8' straight down. if this was the case then a ring electrode would be all we need to protect a person at a pool, or a power line person at a sub station, but this is why a electrode grid is needed to encompass the area needed to be protected, by installing a wire mesh under the entire area and to some extent even outside of it. this is the only method to bring a given area to the same potential in the event it becomes energized. I'll try to do some searches on sphere of influence of grounding systems because I'm not sure your understanding this, which is important in understanding Earth grounding systems and why they do so little in protecting people at low voltages.

If the short is not that low but was at the breaker rating, then it might never blow. Suppose we had 30 A thru 200 ft of #10 copper wire for the EGC, then the drop on the EGC would be 30*0.2 = 6 V. In most cases not too unsafe.
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Edited to add that a bolted fault at .2 ohms (supposing an infinite supply) at 120 volts would be about 600 amps, more then enough to clear a 30 amp breaker
 
Last edited:
090531-2325 EST

hurk27:

With respect to your comments in post #34.

Voltage measurements are always relative to some reference point. In my experiment of one 8 ft rod and long water pipe I can use any desired point for my measurement reference. I used the water pipe because I was interested in the voltage gradient relative to it. I could have used the 8 ft rod as the reference, or some place in the middle of the backyard, or somewhere half a mile away. The later creates problems if there are other current or voltage sources. This is one reason I did a residual voltage measurement in the yard with no current injected into the 8 ft rod.

The results of the experiment do not change if I go into the middle of an isolated field somewhere with no currents other than those I create. Whether I connect the center tap of the transformer to the 8 ft rod or to the water pipe makes no difference in the voltage gradient from the rod to the pipe.


Some values of resistivity are given at:
http://wiki.myelectrical.com/index.php?title=Earth_Resistivity
A measurement method is also described.

In your reference to the SWER system there are some parameters referenced like current and voltage gradient. 10 A ball park and 20 V/M. This is before you make any effort to shield a given area.

Suppose you cover the earth with a copper sheet in the area you want protected and connect this to your neutral, then anything on top of this sheet will be very close the voltage at your transformer neutral even with 10 A flowing under the sheet and some of it thru the sheet. This does not address what happens as you exit the conductive sheet.

Now add to the surface sheet some vertical sheets into the earth and connected to the surface sheet. This will reduce the current flowing under and up into the bottom side of the surface sheet.

Next put a bottom sheet in the earth and remove the surface sheet. All the earth inside this 5 sided box will will be at the potential of the sides of the box. The box is highly conductive compared to the earth. Thus, current flowing thru the earth outside the box will flow in the conductive box but extremely little thru the earth inside the box.

Using suitable dimensions you can start putting holes in the conductive sheets and have little change in the equipotential earth in side the box. Then the sheets can be turned into rods.

.
 
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