Grounding rod philosophy

[rlitman]

...The purpose of the ground rods is to shunt lightning current to "the ground," and to ensure that anything that is electrically grounded is at the same potential as "the ground." ...

No, and yes. UL listed lightning protection systems have completely different grounding electrode requirements. They call for a minimum 3/4" copper clad rod that is 10' long and is a minimum of 2' from your foundation.

But yes, your grounding rod is there to ensure that your system ground is at the same potential as your building and the earth it sits on.

 

[sberry]

UL has different systems but code doesn't require it. Same purpose.

 

[checkthisout]

If you're only going to look at it schematically, you could step back 60 years and do away with grounds entirely, since the neutral is by definition the same potential.

The only point being that a schematic doesn't necessarily illustrate the entire issue.

Might be better to think of the "ground" as in "earth" as a common connection point.

Connecting two separate earthing/ground systems in different structures would not change anything. It's redundant and serves no purpose.

 

[nelstomlinson]

Because it creates another pathway, doesn't require a fault to take the one true path.

Parallel conductors, but still leading to the same place. What's vital is having a complete circuit for fault current so the breaker will see enough current to trip. You don't want a fault to electrify something, then you touch it and complete the circuit.

 

[Raisedonadeere]

Might be better to think of the "ground" as in "earth" as a common connection point.

Connecting two separate earthing/ground systems in different structures would not change anything. It's redundant and serves no purpose.

My simple mind sees it that way, - ground is a common connection point, but there is resistance and the 12 feet of copper wire connecting the two grounding systems could awaken dragons there. I think of lightening as some kind of spook and I do not mess with it. High frequencies act in ways far beyond simple ohms law circuit theory.

 

[dave*99]

Might be better to think of the "ground" as in "earth" as a common connection point.

Connecting two separate earthing/ground systems in different structures would not change anything. It's redundant and serves no purpose.

Actually if you read up on ground rod resistance you will see that increasing the distance between the rods beyond the 6’ minimum in the electric code has benefit. The resistance to earth of the grounding system is lowered when the rods are spaced further apart. So connecting two earthing systems together like the one at your shed and the one at your house has benefit. Assuming you have ground rods at both.

EDIT: I changed the wording to resistance from impedance as I see it caused some confusion in the next post.

 

[SGKent]

Ok so maybe to an electrician impedance might mean something different than it does to someone in electronics. In electronics impedance is the resistance to changing electrical currents. That is measured in henries and not ohms unless the frequency of a circuit is known to put into the equation to convert it to ohms. Resistance is measured in ohms. Resistance and impedance are two totally different things although both impair the flow of electrons. A coil with many turns of very heavy copper wire might have very little resistance to dc electrical current flowing thru it, but it might have a great deal of resistance to fluctuating current. So when the definition of 25 ohms resistance is used it is not referring to impedance in an electronic sense. Impedance and resistance are not interchangeable words, although similar. To convert henries to ohms one must have a frequency in the equation to make the conversion. Also a single phase and a triple phase circuit would use different calculations to measure impedance depending whether a transformer core is shared or not. There is no transformer core in a ground rod. So I'll read the link and look at the video but is it 25 ohms IMPEDANCE at 60 cycles, or 25 ohms RESISTANCE? And what does lightning grounding care about impedance unless the windings of a coil have insulation that will handle several million volts before breaking down. The sentence from the NEC that was quoted said 25 OHMS RESISTANCE. No frequency is mentioned to test for impedance. Say a florescent driver, a LED driver, or nearby electrical transmitter tower caused potential, and leakage thru the grounding system. Each of those situations would see a different resistance than 60 cycles would to ground if we are talking impedance. With resistance it would be the same.

Exception: If a single rod, pipe, or plate grounding electrode has a
resistance to earth of 25 ohms or less, the supplemental electrode shall
not be required.

 

[sberry]

Parallel conductors, but still leading to the same place. What's vital is having a complete circuit for fault current so the breaker will see enough current to trip. You don't want a fault to electrify something, then you touch it and complete the circuit.

They do go to the same place but the farther apart, the more path it takes it slows the trip. This is why no daisy chain of simply running a ground wire around to the different outlets, there are rules about adding grounds, it's very specific and I don't recall all the language but separating the conductors is a problem. Not saying it won't work, that it's an issue, I am not sure but it's a why ,,, why do it when there could be a couple issues and no gain?
It's not a deal breaker, probably no big deal. Most likely more theoretical than anything.

 

[dave*99]

Ok so maybe to an electrician impedance might mean something different than it does to someone in electronics. In electronics impedance is the resistance to changing electrical currents. That is measured in henries and not ohms unless the frequency of a circuit is known to put into the equation to convert it to ohms. Resistance is measured in ohms. Resistance and impedance are two totally different things although both impair the flow of electrons. A coil with many turns of very heavy copper wire might have very little resistance to dc electrical current flowing thru it, but it might have a great deal of resistance to fluctuating current. So when the definition of 25 ohms resistance is used it is not referring to impedance in an electronic sense. Impedance and resistance are not interchangeable words, although similar. To convert henries to ohms one must have a frequency in the equation to make the conversion. Also a single phase and a triple phase circuit would use different calculations to measure impedance depending whether a transformer core is shared or not. There is no transformer core in a ground rod. So I'll read the link and look at the video but is it 25 ohms IMPEDANCE at 60 cycles, or 25 ohms RESISTANCE? And what does lightning grounding care about impedance unless the windings of a coil have insulation that will handle several million volts before breaking down. The sentence from the NEC that was quoted said 25 OHMS RESISTANCE. No frequency is mentioned to test for impedance. Say a florescent driver, a LED driver, or nearby electrical transmitter tower caused potential, and leakage thru the grounding system. Each of those situations would see a different resistance than 60 cycles would to ground if we are talking impedance. With resistance it would be the same.

I appreciate and understand the concept of impedance, but why discuss it here?

A ground rod connected to earth is purely resistive. There is no inductance or capacitance associated with it. So the impedance is equal to the resistance for all frequencies for the ground rod.

 

[Raisedonadeere]

I appreciate and understand the concept of impedance, but why discuss it here?

A ground rod connected to earth is purely resistive. There is no inductance or capacitance associated with it. So the impedance is equal to the resistance for all frequencies for the ground rod.

From a purely resistive considerations I would not think twice about the ramifications that would come from connecting to as many ground rods I could find. Just being silly though, enough is enough. But in the case of lightening behavior, there is not enough theory to precisely predict what will happen in various scenarios because there, inductance and capacitance are included in the equation along with the resistance and they vary all along the path. Theory takes us part way to an answer but experience sets the boundary. That gave me pause as I eyed that ground rod about 12' away wondering, if since the wire would reach should I just go ahead and connect it. Thought maybe there was a clear answer someone knew about, that would clear up the matter.

 

[dave*99]

You can use equations with inductive and capacitive reactance all you want. But Xl And Xc for a Purely resistive element like a ground rod are not a factor. I was addressing the discussion about whether a ground rod is addressed in the code by impedance or resistance
The lightning discussion is another matter.

 

[Mattlt]

Somewhat off topic, but regarding 1 or 2 ground rods... Does the AHJ have much to say about that? Reason I ask is I don't think I've ever seen multiple ground rods in my area (not that I've gone hunting for them) Our area is typically clay / loam soil, so I would think you'd be good with one rod.

 

[dave*99]

See posts 4 and 6 in this thread.
Using 2 rods is the simplest way to meet current code requirements

 

[dscheidt]

Somewhat off topic, but regarding 1 or 2 ground rods... Does the AHJ have much to say about that? Reason I ask is I don't think I've ever seen multiple ground rods in my area (not that I've gone hunting for them) Our area is typically clay / loam soil, so I would think you'd be good with one rod.

there may be places that have adopted a single rod. But a single rod was acceptable by default until fairly recently, so older installations have a single rod, and installations done without a permit by people who don't care about the code changes are stil done with a single rod.

 

[SGKent]

I appreciate and understand the concept of impedance, but why discuss it here?

A ground rod connected to earth is purely resistive. There is no inductance or capacitance associated with it. So the impedance is equal to the resistance for all frequencies for the ground rod.

ohms refers to resistance and not impedance. Impedance is only converted to ohms when calculated in a formula at a given frequency, which we can assume to be 60 cycles but varies whether single phase vs triple phase, and transformer core construction. But I understand what you are saying, and you have helped me many times with electrical issues, for which I am always thankful.

 

[AntonLargiader]

The NEC uses the term “low-impedance” in the fault-clearing context so a lot of people just follow that. When talking specifically about ground rods, it says resistance and ohms.

Here’s some impedance vs resistance talk on this exact subject: https://www.nemasurge.org/wp-content/uploads/2015/01/grounding.pdf

 

[LOW1]

I hope a canadian sparky chimes in. My understanding is that canadian code (unless recently changed) does not allow outbuildings served by a subpanel to be separately grounded and instead requires the subpanel to be grounded by a conductor which goes back to the ground at the main panel. The theory is that potentially current could run from ground to ground creating a shock hazard if there were more than one ground location

 

[Raisedonadeere]

I hope a canadian sparky chimes in. My understanding is that canadian code (unless recently changed) does not allow outbuildings served by a subpanel to be separately grounded and instead requires the subpanel to be grounded by a conductor which goes back to the ground at the main panel. The theory is that potentially current could run from ground to ground creating a shock hazard if there were more than one ground location

Wouldn't that just be our 4 wire un bonded subpanel system without the ground rods? Or do they bond the neutral and ground?

I did a 50 amp subpanel in a tool shed last year and never got around driving the ground rod. It is laying there under the building next to the conduit riser just staring at me every time I go to the shed for something. I keep saying I will do it tomorrow. Shed is about 50 feet from service panel. Makes me feel sort of Canadian now.

 

[wyliesdiesels]

I hope a canadian sparky chimes in. My understanding is that canadian code (unless recently changed) does not allow outbuildings served by a subpanel to be separately grounded and instead requires the subpanel to be grounded by a conductor which goes back to the ground at the main panel. The theory is that potentially current could run from ground to ground creating a shock hazard if there were more than one ground location

Youre mixing things up here. Dont mix up ground rods with grounding conductors. They serve 2 different purposes.

Ground rods are NOT for clearing fault current. Theyre for shunting lightning to earth and limiting voltage potential to earth..

NEC code requires the same grounding conductor, you mentioned, to run from the subpanel to the main panel. This is in the 4 wire feeder.

 

[dave*99]

ohms refers to resistance and not impedance. Impedance is only converted to ohms when calculated in a formula at a given frequency, which we can assume to be 60 cycles but varies whether single phase vs triple phase, and transformer core construction. But I understand what you are saying, and you have helped me many times with electrical issues, for which I am always thankful.

Well sort of. Yes impedance is frequency dependent. But you don’t really convert impedance to ohms. Keep in mind resistance is specified in ohms. Reactance is specified in ohms. But resistance is not reactance.

We use R to represent resistance in ohms.
We use X to represent reactance in ohms.
We use Z to represent impedance in ohms.
And reactance can be capacitive or inductive.

Inductive reactance goes up with frequency
Capacitive reactance goes down with frequency.
Resistance is not frequency dependent.

There is a phase relationship between current and voltage.
It AC circuit analysis we use Eli the ice man to remember voltage (E) leads (L for inductor) current (I) and current (I) leads in a capacitor (C) the voltage (E). I have typed that out a bit clunky but I hope the point is made.

We can perform simple calculations at 60 Hz for ground rods using resistance only.

When we do transient analysis for lighting (which contains high frequency energy). The low values of capacitance in the contact with earth and the low values of inductance that occur when the ground wire makes bends become influential enough to warrant consideration. Again this is due to the higher frequency components of lightning and transient surges.

So ground rods get looked at differently for different analysis

 

[sberry]

I just hook the wires up, they don't know I don't know.

 

[dave*99]

I just hook the wires up, they don't know I don't know.

 

[sberry]

Ok, they don't care I don't know.