Ground Rod at a construction trailer Y or NO

[crossman gary]

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.

I agree completely and will/would install a proper EGC every time.

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.

At the very worst, the electrode would do nothing to lessen the severity of the shock. At the very best, the electrode will reduce the potential. Therefore, it seems prudent to install the electrode. Certainly the electrode will not make the shock danger even worse?

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.

Precisely the experiment I had in mind. How do you suggest I attach the voltmeter probe to the earth? To me, this could be critical as to the "touch potential" that someone would experience. Maybe a 12" x 12" flat metal plate with a heavy weight on it layed on the ground?

I will probably drive a one foot pipe in the ground for the voltage probe attachment.

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

Not one of the choices!

 

[benaround]

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

Wayne,

" the neutral is referenced to earth in many places" , This is why I chose #1 to Gary's

question, in #1 there was no connection to the earth by the GEC or the EGC, so in my mind

the reference would be further away equalling less shock, connecting the GEC,EGC, back

up would put it closer to the location equalling more shock.

note: yeah, I used the small slot.

 

[hurk27]

but Frank I think you missed the part that the trailer has 120 volts on it because of a fault to the frame from a ungrounded conductor, this means it has a potential of 120 volts to earth, and your standing on earth with bare feet, so your feet are at Earth potential and your hand is at 120 volts

this is because of the neutral to earth reference, at the service, at the transformer, and every pole 4 time a mile down the road.

 

[crossman gary]

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

I appreciate that. And that applies to all of us even if it seems that some of us forget it sometimes!

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

If you go back and read all the posts, that has been the basic assumption of all the posters in this thread. You are completely correct in thinking about this. There are other resistances involved which have been neglected and will certainly have an effect on the shock.

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.

First, understand that this is not MY claim. The other posters on this thread made that assumption. I am simply following there examples.

Second, concerning your experiment. You got 14 volts. This is like the trailer without an electrode.

As a thought experiment, a person could mimick the trailer with electrode by taking the exact same measurement, but this time with a 5 foot piece of metal pipe, EMT, whatever, and drive it into the ground near the voltmeter. Take a small piece of copper wire and attach it from your extension cord hot wire to the pipe. Then take the voltage reading. Notice that the voltmeter will be in parallel to the small copper wire and the pipe. The pipe represents the electrode attached to the trailer.

This should cause your voltage reading to be at least a little bit lower based on ohm's law and dividing of current along two paths and all that stuff.

If a person was to actually perform the thought experiment above, I would recommend they wear non-conductive shoes and just be oh so careful. I cannot condone any experimentation with actual live electricity and highly advise against it. I am in no way condoning such hazardous actions.

 

[benaround]

Gary, Wayne,

Just want to say THANKS for your time and for your mind !!!

 

[crossman gary]

Frank,

I just wanted to make sure yuo understand my point. It isn't about the actual voltage from the hot wire to ground. It is about whether an electrode can lower the potential from the hot to the ground.

In your example, you got 14 volts with no connection to the earth except for the meter leads. If there was a parallel path around your meter leads, the voltage most likely would be less.

This means that with a ground rod, the potential shock a person would receive would be less than the shock without the electrode. Certainly there may be a shock even with the ground rod, but most likely it will be less than without an electrode.

Is there anyone here that would disagree with that?

 

[tryinghard]

This is a good read Grouding Vs Bonding The Big Picture for the topic. The equipment grounding really is the proper and safest path for fault current, not earth.

As it has been pointed out a few times the NEC addresses the need of an electrode system in 250.4(A)(1), and the most likely description to be used in this article is lightning control.

 

[gar]

090801-1740 EST

hurk27:

I was doing ground voltage measurements this morning. During that time most were in the 50 to 100 MV from either my pole transformer ground rod or my other reference point of the water line where it enters the basement. At one or two points I measured up to about 135 MV.

I was going to comment on some of your previous comments, but I am off on other experiments.

This afternoon I cut my load to about 260 W from about 2500 W to check on the change of neutral drop measured from the pole ground rod to my entry point copper water line. With 2500 W I had about 54 MV, unknown amount of current on the neutral. Under these conditions adding 12 A load the drop increased to about 88 MV, a 34 MV change. This implies a neutral resistance of 0.0028 ohms. This is consistent with a calculated estimate of 0.0034 ohms.

When I dropped my load power to 260 W and no additional test of 12 A the neutral voltage dropped to 22 MV. Implies some other current than would result from the 260 W load. The most possible current from a totally unbalance 260 W load would be 2.16 A. This should produce no more than 2.16*0.0028 = 6 MV.

Next I got side tracked on the following observation. A higher ground voltage. With my reference point the pole ground rod and a measurement point several feet away the voltage difference was in the 50 MV range and then jumped to 215 or so. This had a periodic nature.

Changed ref point to my entry water pipe. Also changed measurement point to my 8 ft test ground rod in the middle of the back yard. Then I saw
217 MV 7 sec
053 MV 19 sec
225 MV 10 sec
restart
211 MV 16 sec
063 MV 22 sec
201 MV 6 sec
050 MV 28 sec

There is some unknown source of this stray ground current. It is not caused by my loads.

Back to the discussion on ground current from the primary side. For all practical purposes my primary is an ungrounded delta anywhere along the primary. The source might be a Y transformer with neutral grounded. Or it could be a delta transformer with a synthetic ground. Either way there is no resulting ground current other than resistive leakage, and transformer and line capacitance to ground, unless a primary wire shorts to ground. Thus, normally I do not expect ground currents from the primary of my supply.

To attempt to see something of the possible source of the stray source I setup a different experiment. Two screwdrivers spaced 12 feet apart.

In the backyard about 22 deg east of north gave a maximum (19.2 MV), 90 deg from north was minimum (3.8 MV).

In the front yard maximum 112 deg east of north was about maximum (32.8 MV), and 45 deg east of north was about minimum (9.4 MV).

Between these two is the obstruction of my basement.

I need to get back to my tests on "Kill A Watt".

.

 

[RUWired]

You need grounding electrodes at the transformer, 250.30(A) the guard shack and the trailers. 250.32(A).

You can add two additional rods at the outdoor panel board 10' from the trailers as well.

 

[crossman gary]

I did the "earth shell" experiment today concerning just how much a ground rod really can reduce the potential shock hazard....

The results are in the following thread:

http://forums.mikeholt.com/showthread.php?p=1083916#post1083916post1083916

 

[gar]

090802-1402 EST

I conducted some more measurements in my yard on the vector direction of current flow.

My house runs north and south and the garage is at the south end. Further south is open grass. My copper water line runs from the street at the south end of the lot, by the front of the garage, and then under the house to the north east corner where it enters the basement. Total length about 150 ft.

The city water line runs east and west under the street at the south end of my property. No city water line in front of the house. However, the gas line runs north and south under the street on the front side. My gas line to the street is copper.

Over the water line on the south grass the current vector seems to point perpendicular to the water line. I believe the water line is in the range of 9 to 12 ft below the ground surface. As I move to a point near the street on the west side and perpendicular to the south west corner of the house the vector is turning slightly south. At the north west corner of my lot the current vector is about south east.

At the north side of the house the vector is north south. All along the back side the vector is north south.

I do not know if the vector angle is changing much when the large ground current changes occur that I previously mentioned.

When I measured over the path in line with the pole transformer ground rod and my meter the magnitude diminished. Thus, the area was being shielded by the two ground connections and their connection by the neutral wire.

This experiment would be much better with an octagon or larger array of probes and fast automatic voltage and phase measurement.

.

 

[art82]

ground rods

ground rods

if you feed any structure with more than 3 current carring conductors or a 3 wire branch cirtuit you have to put ground rods but only to the body of the pnl not connnected to the neutral

 

[wawireguy]

This is how a basic trailer is done here:

Outside meter and pedestal. Two ground rods. POCO hits the ped. From PED you feed the trailer with your two phases, neut and a ground. The trailer frame is already bonded at the factory. All you "should" have to do is feed the panel. Float the neutral.

 

[conshyboy5]

wawireguy , thats how I've been doing it for 20 years , now some new young code inspector thinks a manufactured building on wheels , is a structure or building ,even if i can pull my truck up to it and tow it away.
When we go camping and hook up trailer to recpt [branch circuit , feeder ?] 50 amp plug 4 wire 1ph 240v [ which goes to 8 circuit panel in trailer ] I don"t see a ground rod to hook up to at post [ are they required to be there ? ] , or maybe i should buy a whole lot of ground rods , a coil of #6 solid copper wire , box of acorn clamps , bring ground rod driver and start driving them in where ever we go camping , then i'll be code compliant [250.32 A ] where ever we go camping .
manufactured building = structure ?
When you start driving ground rods all over the place ,and connecting grounding electrodes together thru GEC , should it be considered ground grid and follow code rules ?

 

[DEM]

ground rods at job shack

ground rods at job shack

I became a member of this forum for the same question, and after reading alot of the replies here is my take. the shack falls under article 550. The article states it needs a 4 wire feeder, and makes no mention of supplementials. Mr. Baker in another discussion mentions article 90. Chapter 5,6 and 7 are modified from chapter 1 thru 4. Therefore no supplemential are mentioned so none need driven. Please correct this if it is wrong.

 

[tryinghard]

It helps to remember grounding and bonding have two distinctive and different purposes. If these separate buildings were not mobile Article 250 is clear to ground them because they would be considered separate structures (250.50).

To error on the side of grounding is fine because in doing so the purpose of grounding will exist at each trailer, (250.4(A)(1)). If this were the application chosen bonding the neutral at the trailers would NOT take place because this would create a parallel return path to its source by allowing return current where it does not belong, bonding the neutral would only happen once either at the source or first disconnect.

Equipment grounding is extremely important because it is the fault path, not grounding, and equipment grounding only works with the neutral bond in place; again at the source or first disconnect.

 

[hurk27]

I became a member of this forum for the same question, and after reading alot of the replies here is my take. the shack falls under article 550. The article states it needs a 4 wire feeder, and makes no mention of supplementials. Mr. Baker in another discussion mentions article 90. Chapter 5,6 and 7 are modified from chapter 1 thru 4. Therefore no supplemential are mentioned so none need driven. Please correct this if it is wrong.

A job site building is a article NEC 545 structure, it is not a dwelling, a trailer, manufactured housing, or a modular home.

545.3 Definitions. Manufactured Building. Any building that is of closed construction and is made or assembled in manufacturing facilities on or off the building site for installation, or for assembly and installation on the building site, other than manufactured homes, mobile homes, park trailers, or recreational vehicles.

the GEC is allowed to be ran with the supply to the service GE.
See 545.12

 

[DEM]

I am not sure that I understand what you are saying? I understand that a GEC has to be run with the other conductors. The original post asked whether or not code required supplemental ground rods. Even in article 545 the rules are very similiar when it comes to grounding. So do you need supplemental ground rods in addition to your ungrounded, grounded, and grounding electrode conductor?

 

[hurk27]

I am not sure that I understand what you are saying? I understand that a GEC has to be run with the other conductors. The original post asked whether or not code required supplemental ground rods. Even in article 545 the rules are very similiar when it comes to grounding. So do you need supplemental ground rods in addition to your ungrounded, grounded, and grounding electrode conductor?

Well I can't figure why this would have been put in the code if you couldn't use it? I could be wrong

545.12 Grounding Electrode Conductor.
Provisions shall be made to route a grounding electrode conductor from the service, feeder, or branch-circuit supply to the point of attachment to the grounding electrode.

it from the "2002" not sure if it changed in the "2008"

 

[hurk27]

I think I can answer my own question, 250.32(B):roll: so I guess your effectively bonding both electrodes together.

just doesn't make sense why they had to put in 545.12 if the above covers it already.