8' ground rod

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DrSparks said:
I dare you to try this experiment:

Run a 300 ft long feeder out to a detached garage, which is about 500 ft away from the transformer at the pole (which has it's neutral grounded). Now disconnect all your grounding electrodes from your system.

Now that those useless buggers are gone, wet your hands and feet and touch the neutral bus if your sub panel.

Just make sure you have a buddy with a defibrillator nearby.

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You can have a dangerous voltage situation doing that even if the transformer is only 5 feet away and/or if you never disconnected any the local grounding electrodes.
 
infinity said:
Sounds like one of those we always did it that way. I took a peek at the 1965 NEC and that still only required an 8' rod or pipe electrode.
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That or bust the chops of the greenhorn. He did eventually go with the 8ft rods, with no explanation.
 
LarryFine said:
Nobody here would expect a neutral in a sub-panel to be at zero volts to earth.

A rod does not "force" the nearby earth to be at the same potential as the bus to which it's connected.

Do a bit of research on step potential.
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Actually it does, but there voltage gradients change pretty rapidly in a short distance. Apply 120 volts to that rod and take a measurement just a few feet away from the rod and you likely have already dropped all 120 volts.
 
kwired said:
Actually it does, but there voltage gradients change pretty rapidly in a short distance. Apply 120 volts to that rod and take a measurement just a few feet away from the rod and you likely have already dropped all 120 volts.
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Agreed. I meant beyond the sphere of influence. For example, the earth around an entire house.
 
Voltage gradients are mostly about trying to keep the potential between one's left and right feet non-lethal.would be easy if the earth was a good conductor.
 
DrSparks said:
Run a 300 ft long feeder out to a detached garage, which is about 500 ft away from the transformer at the pole (which has it's neutral grounded). Now disconnect all your grounding electrodes from your system.

Now that those useless buggers are gone, wet your hands and feet and touch the neutral bus if your sub panel.
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The detached garage should be supplied by a feeder with EGC, and the neutral and grounding bars are kept separate in the subpanel, with the grounding electrodes connected to the grounding bars. So detaching the grounding electrodes is going to have no effect on the neutral-to-earth to voltage at the subpanel.

[Edit: OK, maybe it has a effect very close to the grounding electrodes at the garage, as the EGC will bond those to the grounding electrodes at the supplying building, so if there is an earth voltage gradient between the two building, some current would be flowing on the EGC.]

Cheers, Wayne
 
DrSparks said:
Multi-grounded neutral helps to reduce the dangerous step potential and touch potential during line-to-ground faults. By creating low impedance path back to the source, faults are quickly cleared by fault interrupters. Multiple grounding electrodes tied together helps to reduce the touch potentials at the fault point. With multiple neutrals, step potentials are usually not dangerous since fault currents spreads between several grounding electrodes



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It just really doesn't work that way. For the duration of the fault, all the connection to earth does is to raise the voltage for a small area around the electrode, but only for a very small area...more than a couple of feet away, there is no protection for a person and the electrodes actually create a possibly dangerous step potential around them under fault conditions.
For substations installations, a grounding grid under the substation is used to limit the step potential, because just a connection to earth doesn't work.
 
LarryFine said:
Agreed. I meant beyond the sphere of influence. For example, the earth around an entire house.
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The sphere influence is very small around a ground rod.

There was a table in the IEEE Green Book that says at one foot the voltage is 68%, at 3', 75% and at 5', 86%. So if you put 120 on the rod, you have 82 volts at 1' 90 volts at 3' and 108 volts at 5'. The only shock protection you get from a rod is if you are standing directly on top of the rod.
 
There is a question of an effective grounding electrode (rod). The effectiveness of one rod is proportional to its length. So other than Code reasons if it’s below 25 ohms it’s good no matter whether it’s 4 feet or 25 feet. In sand mines we use threaded rods and just keep driving it in until we reach the desired resistance. They often dewater the sand so it might have to go 50-60 feet to get low resistance.

As to 2+ rods the second rod is only 70% effective and the third is only around 40% effective. It continues to drop so that after 5-6 rods it doesn’t matter how many you use.

Also in welding you have to place the ground far enough away from the electrode for a reason and grounding in soil works the same way. As we expect the resistance through the Earth on any single path is proportional to the distance between points. However as the distance grows the number of available paths grows with the square of the distance. As we know electricity follows all paths and current is proportional to the conductivity (inverse of resistance). So one of the oddities of grounding is that we divide R (=K x distance)!by the square of the distance, so curiously resistance is inversely proportional to distance.
 
kwired said:
Telephone companies used to use them around here. Only 3/8" diameter as well

I have one that I welded a T handle to one end and use as a probe when excavating to help find other buried utilities and such.
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I doubt they use 4’ rods.
NEC allows 5’ ground rods in chapter 8
 
Fred B said:
Don't know if it was code, I was just starting in the field and just a grunt back in the early 80's and my boss told to us to drive in these 10 ft rods. Also used 10ft X 3/4in galvanized pipe. End would get smashed pretty good and have to cut off the smashed end with a hacksaw before clamping cable on.
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The company I work for, quotes work with 10’x3/4” rods, I refuse them every time I go to the supply house to pick the material up. They quote the 10’ rods because they make more money off them.
 
Hv&Lv said:
I doubt they use 4’ rods.
NEC allows 5’ ground rods in chapter 8
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But do they have to follow NEC? They certainly don't nor have they in the past around here.

One time they ran their bonding conductor to the main electric panel, landed it on an EMT set screw fitting entering that cabinet. (this long before intersystem bond terminal was required). Also seen them run bond conductor to a sill cock, but if you went inside you would see the thing was supplied with non metallic pipe.
 
kwired said:
But do they have to follow NEC? They certainly don't nor have they in the past around here.

One time they ran their bonding conductor to the main electric panel, landed it on an EMT set screw fitting entering that cabinet. (this long before intersystem bond terminal was required). Also seen them run bond conductor to a sill cock, but if you went inside you would see the thing was supplied with non metallic pipe.
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Depends on who they are and where. RUS borrowers follow RUS guidelines. (Phone co-op)

4799229D-B165-49FE-B7CA-92C834D8F687.png
 
Hv&Lv said:
Depends on who they are and where. RUS borrowers follow RUS guidelines. (Phone co-op)

View attachment 2557135
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I haven't seen a rod on telephone on anything I can recall other than some pretty old existing installs anyway, they have always run a bonding conductor to an electrical enclosure of some type or 30+ years ago maybe to metal water pipes.
 
electrofelon said:
Correct. Take a look at this link and the graphic in post #6:

Step potential????

First off give me a laymens definition of Step Potential. When grounding inplant industrial substations,when does step potential come in to play and how do you combat it? ie: How do you calculate the fault current and how do you design a safe system???? dick
forums.mikeholt.com

Dirt is such a poor conductor that that it forms a steep voltage gradient when voltage imposed on it. If you try to "bond dirt" you will literally only bond a very small area like a square foot or less and then you are back to square one again. In high risk areas like substations, you essentially need a continuous conductor everywhere which is why they often employ these grids or mats so you are essentially always standing on a bonding conductor.
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What about bonding the rebar in the slab?

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DrSparks said:
What about bonding the rebar in the slab?

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That works for step potential on the slab. You will get much steeper voltage gradient in the dirt at the edges of the slab though similar to what you get around a single rod electrode.
 
kwired said:
That works for step potential on the slab. You will get much steeper voltage gradient in the dirt at the edges of the slab though similar to what you get around a single rod electrode.
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Which is why sensitive areas like dairy barns use an equipotential ramp with the grid sloped to be further from the surface as you move away from the edge of the slab.

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