Ground Rod at a construction trailer Y or NO

[hurk27]

At the same site you posted here is the page that supports the touch potential http://wiki.myelectrical.com/index.php?title=Earth_Electrode_Resistance

That supports this graphic from Mike holt:

and this is what would had happen at the trailer in the Coke a Cola post, if there was a ground rod at the trailer and no EGC.

 

[hurk27]

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 water pipe is bonded to the return path for the 120 volt source you connected to the rod, yes you could have reference the voltage between two isolated points but, this would only show the voltage across these two points and not the true voltage of a energized point and Earth at a given distance from this energized point. this type of measurement would be mute as one could not contact these two points as they would two far apart, unless there was a conductive pathway to this point, which would still reference the energized voltage to earth. the voltages you measured at 3' from the rod to the water pipe only reflect the voltage drop from the rod, in this case 120 volts minus 30.2 volts would equal a 69.8 voltage potential at the 3' mark, from the rod, still a dangerous shock hazard. next lets say a person was to contact a conductive path such as the metal siding of the afore mention trailer at a distance of 5' from this rod, the voltage would be much higher somewhere around 100 volts for your rod which is some what lower in Resistance that the average conductivity of earth. this will only go higher as a person moves out from the rod.

090531-2325 ESTThe 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.

Same answer as above


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.

090531-2325 EST 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 Thur the sheet. This does not address what happens as you exit the conductive sheet.

This is an equal potential ground plane, and yes this is a true statement, But a ground rod or a horizontal pipe or even a ring electrode will not create this, just because like it said in the link you provided 67% of the resistance is in the first .3 meters from the electrode, this would mean that the ground ring would have to be smaller then .3 meters in diameter to form a equal potential plane within the ring, too small to be of use. and then if it is buried at the 24" normally required it would not protect a person standing on top of it. this is because of the spherical of influence is all encompassing of the ground ring conductor at the same distance from it as mention in the web site you posted.

090531-2325 ESTNow 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.

again this would act just as above

090531-2325 ESTNow 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 Thur the earth outside the box will flow in the conductive box but extremely little Thur the earth inside the box.

again same as above

090531-2325 ESTNow Using suitable dimensions you can start putting holes in the conductive sheets and have little change in the equal potential earth in side the box. Then the sheets can be turned into rods.

same as above, just like the "hole between the wire mesh used in equal potential ground planes. but if the holes were larger then a given size, again there would be a potential if a person could stand in the middle of one of these holes and touch the energized ground plane. .3 meters is not very big.

I wish I had some graphic program on this puter to show like in Mike Holt example, of how the sphere of influence looks like and how it will go right passed other metal objects in its sphere. in the case of a vertical rod if you imagine a set of equally smaller bowls placed in the Earth with the top level with the Earth and filled with earth then place the rod in the middle of the smallest bowl, you would have the same image that in in the previous post but with the image of what it looks like underground. with other electrodes that are buried below earth this sphere takes a more ball shape elongated to the shape of the electrode. and in a ring it would be like a big donut.

 

[gar]

090601-1306 EST

This discussion has become confusing. At the moment rather than continue some of these points I want to go back to the original post.

Before that I want to make it clear that in general I am not a proponent that ground rods are a great safety feature. In general I do not think they solve many problems where they are claimed to be a solution.

Now consider a trailer on rubber tires that is basically well insulated from the earth on which it rests. This trailer has a good EGC connection back to the main panel, and therefore to the ground rod at the main panel. The main panel is the reference point. For this discussion I do not much care whether the hot and neutral wires are even connected. By implication of the function of the EGC all conductive parts of the trailer have good continuity to the EGC.

For some reason we have stray current in the earth. This current is of such a magnitude that 1 A will flow from a rod in the ground to a connection on the trailer chassis. Just for convenience assume that the source voltage in an equivalent circuit for this stray current is 120 V. That means that the equivalent resistance of the stray current to the rod is 120 ohms including the rod resistance to the earth, and the rod resistance to earth is 20 ohms. This means well away from the rod there is a resistance of 100 ohms.

Next with the rod disconnected from the trailer, very wet soil, and a barefoot person with wet hands walks to the trailer and grabs the door handle. This person will have 120 V applied across their body, receive a sever shock and maybe die. This is because there was a good EGC connection.

Next connect the ground rod to the trailer. Because of the description I gave above there is now only about 20 V from the earth around the trailer to the trailer chassis. The above described barefoot wet person can still get a substantial shock, but there will be considerably less current, about 1/6 th. Put a few more rods around and the voltage will be less. Or better yet provide a continuous ground conductor around the trailer and maybe 6 or 8 ft away.

This is not a solution for a bad EGC connection and internal leakage in the trailer from its hot supply wire.

.

 

[hurk27]

I now understand what you getting at and if this injected stray voltage was from a isolated source that had the return path bonded to the trailer chassis but isolated from the power grid, this could be passable, but since we are using power system that has miles of multiple grounded neutrals through out the grid this alone will apply a canceling effect on any source injected into Earth, thus the reason you only had 30.5 volts from your ground rod referenced to the XO of the transformer through the water pipe, and as you got farther from the rod, it dropped off exponentially as the distance increased from this injected point. the injection point would have to be right next to the trailer to have this potential, here is what happens, the Earth resistance over all is about .05 ohms enough to shunt any voltages from sources bonded to the grid MGN, it is at the point of connection that the sphere of influence to the electrode will have the high resistance, to the point outside the sphere at which point the injected current is nullified, or brought to the same potential of the grid neutral. as you have shown this sphere at 3' from your rod has reduced the voltage to a point of 30.5 volts, that is a voltage drop of 89.5 volts that right at what mike shows in his diagram. this is science fact as it can be repeated, it is not theory.

The problem is many believe that stray currents come into an area through the earth from the grid, but in reality is comes into the area by a low impedance path such as the neutral. this has baffled many engineers, and mike has a PDF on here of this that the fact the neutral is bonded across the transformer causes most of stray current problems. the rest is from sources isolated from the grid Earthing like in the SWER system.

But your right we are off topic, I thought the OP was answered
Ok Bob don't beat up on me too bad

 

[conshyboy5]

Sorry to bring up this post again, I read it from the beginning , but I'm having a hard time trying to figure out the definition of structure . Is a job trailer built on site , or is it considered a mobile home. Local code enforcer says if it is a structure than it needs fire alarm, exit signs , emergency lights and sprinkler system installed by their code , so my ? , is a temp. job trailer a structure or considered a mobile home in nec.
if I drive 2 ground rods at trailer , does this make it a structure . Service [ 4 wire 240 volts single ph 100a, panel has #6 copper bond to frame from ground bar in panel ] comes from a metered distribution center about 10 feet away with 2 ground rods driven at 6 ft apart,and is fed from 100 amp breaker in meter center, thanks

 

[mistabass]

I have a question is ser rated for outdoors?

 

[gar]

090730-1956 EST

hurk27:

At my home we have a three phase ungrounded primary supply. From my perspective it is an ungrounded delta. It might be a Y at the substation and grounded there. My transformer is bridged across two wires of the three phase source. Only the center tap of my pole transformer is grounded at the pole.

Ground voltage differences within my entire yard are basically less than about 0.1 V if I do not inject a current into the yard. 0.1 volt is an insignificant error in any measurements I make when injecting current into my yard.

If I were to use a different test frequency than 60 Hz and filter out the 60 Hz my results would be the same.

.

 

[hurk27]

090730-1956 EST

hurk27:

At my home we have a three phase ungrounded primary supply. From my perspective it is an ungrounded delta. It might be a Y at the substation and grounded there. My transformer is bridged across two wires of the three phase source. Only the center tap of my pole transformer is grounded at the pole.

Ground voltage differences within my entire yard are basically less than about 0.1 V if I do not inject a current into the yard. 0.1 volt is an insignificant error in any measurements I make when injecting current into my yard.

If I were to use a different test frequency than 60 Hz and filter out the 60 Hz my results would be the same.

.

It doesn't matter if the primary is isolated from earth such as in a delta primary system, they still install center tap or zig-zags to give the Earth a reference, and to limit the amount of primary fault current on the generators if one should occur. but even if the delta primary's was totally isolated from Earth, it wouldn't matter as you are using the potential from the secondaries that does reference Earth, at the X-0 bond, so if you were to inject a 120 volts into the Earth at a distance of lets say 30' from the trailer, and the trailer frame is still bonded to the EGC that also references the X-0 of the transformer your injected voltage source is from, the EGC will force the trailer frame closer to the X-0 voltage above Earth more than the injected voltage.

Do this test, place a metal object (simulate the trailer) that is isolated from Earth, but connected to the EGC system of the power source of your injected voltage, put it about 30' from your injection rod, now stick a short metal object into earth and measure the voltage between this earth object and this EGC bonded metal object, I'll bet it wont be more than a couple volts. move it farther away or closer to your injected point, it won't vary all that much until you get so close to the injected point that you are in the spherical of influence (less than 3' away) at this point the break down in the resistance of the earth will drive the voltage up but it will also bring up the current as well, so you might be surprised as how little earth will actually drive this voltage up even as close as 2' to this EGC bonded metal object.

 

[hurk27]

Sorry to bring up this post again, I read it from the beginning , but I'm having a hard time trying to figure out the definition of structure . Is a job trailer built on site , or is it considered a mobile home. Local code enforcer says if it is a structure than it needs fire alarm, exit signs , emergency lights and sprinkler system installed by their code , so my ? , is a temp. job trailer a structure or considered a mobile home in nec. if I drive 2 ground rods at trailer , does this make it a structure . Service [ 4 wire 240 volts single ph 100a, panel has #6 copper bond to frame from ground bar in panel ] comes from a metered distribution center about 10 feet away with 2 ground rods driven at 6 ft apart,and is fed from 100 amp breaker in meter center, thanks

Wow, some inspector:-?

look at the definition of a structure in the NEC, if this inspectors goes by this, then even a pole would require fire alarm, exit signs , emergency lights and sprinkler system.

Fire codes like these are based upon the construction square footage in almost all cases, not whether or not it is a trailer or not.

But a Job trailer is a manufactured structure and falls under article 545 of the NEC, local requirements can vary from place to place, but I would assume it would deal with you UBC, codes in your area. here if even a office is less then a certain square footage we are note required to have fire alarm, exit signs , emergency lights and sprinkler system, like sprinkler systems are not required in a public building under 5k sf.

545.12 requires us to bring a GEC to the trailer from the service, but no where it requires us to drive one also at the trailer. IMO

 

[crossman gary]

If I was barefooted and had to touch the metal trailer skin in the following situations:

Trailer on rubber tires, no EGC, hot wire contacted to the metal skin at 120 volts, and no grounding electrode attached to the trailer metal

versus

the exact same thing with an electrode attached to the metal

I am going to choose the electrode version everytime.

What about the rest of you?

 

[hurk27]

If I was barefooted and had to touch the metal trailer skin in the following situations:

Trailer on rubber tires, no EGC, hot wire contacted to the metal skin at 120 volts, and no grounding electrode attached to the trailer metal

versus

the exact same thing with an electrode attached to the metal

I am going to choose the electrode version everytime.

What about the rest of you?

Your hypothetical is just what the code trys to prevent by requiring the EGC.

Gary no matter what, a driven ground rod will not prevent you from receiving a shock every time, that would be the same as playing Russian roulette,

now think about carnival trailers, many have 30 to 100 amp supply's, no EGC and what kind of earth resistance would you need to open a 100 amp breaker at 120 volts? would never happen even with your good soil.

remember the touch voltage between your feet and the trailer would be at least 75% of the source voltage, or 120 volts would = 90 volts running through your body, unless your standing on top of the GE

and that would be if your feet were within 3' of the GE, father away the higher it goes

 

[benaround]

If I was barefooted and had to touch the metal trailer skin in the following situations:

Trailer on rubber tires, no EGC, hot wire contacted to the metal skin at 120 volts, and no grounding electrode attached to the trailer metal

versus

the exact same thing with an electrode attached to the metal

I am going to choose the electrode version everytime.

What about the rest of you?

I think I would choose the 1st condition because there is no complete circuit !!

 

[hurk27]

even the first condition would have 120 volts to earth, so yes it would be a shock hazard, but if you look at the MH image in post 41 I posted, it shows that even at 3' from the rod you will still have 90 volts and at 5' you would have 105 volts, so like I said unless your standing on the grounding rod, there will be a potential, and what if the rod is on the other side of the trailer from where your standing, 10' away, you will have almost the full 120 volts.

If rods didn't have this small sphere of influence the NEC could just have us slam a bunch of rods around a pool and not require a bonding grid, but that is not the case.

 

[crossman gary]

If I have to choose between a 90 volt shock and a 120 volt shock, I prefer the former.

It seems that whenever these issues are brought up, there is an assumption made that the person with the "makes the earth and the object at the same potential" view is somehow saying that the electrode is the only thing needed, or that the EGC is not needed, or that the electrode is the panacea of all bad things electrical, whatever other bad things may be thought about the person.

That assumption is just wrong.

Nobody is saying the electrode is the be all and end all. Yes, the EGC is important, and all the other code rules too. Install the freaking EGC! Take care in its installation just as if you would with the hot and the neutral.... maybe even more care!

Now, in the "old days," many electricians thought the ground and the electrode was some type of magic carpet that solved everything and made the installation completely safe regardless of any other items in the installation.

So some educators who knew the above was wrong took the complete opposite view and said the electrode is useless and pointless and doesn't help protect people and on and on, and certain white lies and misinformation was perpetrated to convince these old-timers that they need to run an EGC, not just rely on the electrode. The ends justify the means I suppose?

As an example, my experiment with the 4 foot pipe in the ground had a darned low resistance, and others on here chastised me with unbelief of how low it was. The just couldn't believe it because "those in the know" had told them otherwise. "It is impossible to achieve 25 ohms even with a rod driven 90 feet down!" And somehow this becomes a blanket statement that applies to every location on the earth. And everyone believes it as gospel.

But we are beyond all that now. An electrode is not useless. An electrode CAN reduce the dangers of electrical shock.

If I get to choose which trailer to touch, I want the electrode one.

If I am going to get shocked, I prefer the 90 volts to the 120 volts.

And, there are way way more things involved than those shells around the rod and the supposed voltage levels applied to someone standing on the earth around the rod.

Experiments are in order here.

 

[crossman gary]

I think I would choose the 1st condition because there is no complete circuit !!

Errr..... in the first condition, YOU become the complete path! And with nothing to reduce the potential in parallel with you.

1st condition = 120 volts on you

2nd condition with Hurk's claim of 75% voltage = 90 volts.

Care to choose again?

 

[crossman gary]

if you look at the MH image in post 41 I posted, it shows that even at 3' from the rod you will still have 90 volts and at 5' you would have 105 volts

I am not convinced that the image has universal application. What if the moisture level at the surface is neglible, but the bottom 5 feet of the rod is in the water table? Seems there would be neglible voltage drop anywhere along the surface because 99.9% of the current is flowing down deep in the earth.

The earth has different conductivities based on location and moisture content. If this resistance is low, and the electrode is in intimate contact with the earth, then the distances in your posted MH image will be different. Perhaps very very different. Perhaps even enough to render the shock neglible.

 

[benaround]

Errr..... in the first condition, YOU become the complete path! And with nothing to reduce the potential in parallel with you.

1st condition = 120 volts on you

2nd condition with Hurk's claim of 75% voltage = 90 volts.

Care to choose again?

So, are you saying that the earth is a "neutral" conductor to any hot wire from a 120vac

supply ? I just went outside with an extention cord and a digital meter, from hot wire in

cord to the dirt I got 14 volts, more than I expected but much less than the 120 volts you

are claiming.

Keep in mind here that I am trying to understand this stuff, not preach it, ok?

 

[hurk27]

I agree with you that even 90 volts could be a safer bet for me or you than 120 but there are people who would or could die with as little as 50 volts hitting them, a trailer at a fair, with many elderly and children around it, even if it had only 40 volts potential, I would not want to be the responsible electrician that caused it to happen.

the only thing I was trying to say is this sphere of influence is so small, even in the great soil you have, it would not provide a large enough circle of influence to safely bring the earth around the trailer up to a potential that would be safe for everyone that might come into contact with it.

you could do the same test I said to Gar, but in this case connect 120 volts to a piece of metal bonded to an 8' rod, with your soil you might have to use a 30 amp circuit, but the effect would be the same, set up to remotely measure the voltages at different intervals from the ground rod, lets say what an average size trailer would be, up to 30' this would give an indication of the voltage a person would come into contact with if they were to touch the trailer from the other side away from the rod.

It might help you understand how small this circle of influence really is.

But in the two choices in your previous post, knowing either one could kill me, I would chose neither

 

[hurk27]

So, are you saying that the earth is a "neutral" conductor to any hot wire from a 120vac

supply ? I just went outside with an extension cord and a digital meter, from hot wire in

cord to the dirt I got 14 volts, more than I expected but much less than the 120 volts you

are claiming.

Keep in mind here that I am trying to understand this stuff, not preach it, ok?

The measurement you get will depend upon how good of a connection to earth you have, try measuring to a screwdriver shoved in to the earth, it is the same with our bodies, if there is good contact then we get a good shock, like when were standing on concrete, also make sure you test from the small blade in the extension cord, not the wide. but yes the neutral is referenced to earth in many places.

Also it depends upon the resistance of your meter, even though its a didgital it might have a loading circuit that might place a load on it. some flukes have this

 

[hurk27]

I am not convinced that the image has universal application. What if the moisture level at the surface is neglible, but the bottom 5 feet of the rod is in the water table? Seems there would be neglible voltage drop anywhere along the surface because 99.9% of the current is flowing down deep in the earth.

The earth has different conductivities based on location and moisture content. If this resistance is low, and the electrode is in intimate contact with the earth, then the distances in your posted MH image will be different. Perhaps very very different. Perhaps even enough to render the shock neglible.

There will always be variables, a salt water table under you can act just like a equal potential ground plane, in a low resistance soil the fall of potential from the electrode will just be farther for the same voltage drop, the circle of influence will be larger, then get into even higher resistance soils and it gets even smaller than in the MH image.