Grounding Electrode System NEC 250-50

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peter d said:
petersonra said:
What makes you think the terminology is incorrect?
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If the GES is connected to an AC supply, then then it becomes a measure of impedance, not resistance.
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Pete if you measure the impedance you will not be meeting the requirement.

The requirements for any grounding specs I have seen are always ohms of resistance.

The two types of grounding meters I have used both measured resistance.

I don't know what to tell you but there must be a reason for this choice by the NEC and others.
 
Stupid question time: Is lightning DC or AC?

Would't impedance change with what system it were connected to?

Frequencies can vary, right? Aren't there some high-frequency systems out there for work wildly beyond my comprehension?

Does impedance change with voltage?

These could be reasons for using resistance over impedance. Resistance won't change, right?
 
George it is beyond me as well but I think your onto it.

If I recall frequency does effect impedance and I believe the current also effects it as well.

But I am way out of my depth here. :shock: :lol:

Charlie?

Jim?

Anyone?
 
A ground rod stuck in the ground with nothing connected to it will have some value of resistance. Maybe that's the measurement they're speaking of.

But ground rods are mostly connected to AC systems, and any opposition to current flow in an AC system is impedance, not resistance.

That's all I'm trying to say. I just don't think the NEC is being technically correct.
 
georgestolz said:
These could be reasons for using resistance over impedance. Resistance won't change, right?
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AC current introduces all kinds of complexity into the mix. Put simply, impedance is the vectoral sum of the resistance, capacitance and inductance, measured in ohms.

Granted, at 60Hz the inductive and capactiave effects aren't generally an issue.

But inductance is also the reason why we can't put a single hot wire through a single hole in a metal enclosure.
 
hello, guys
Don, Bob, Bryan, n other Gents,
I'm grateful for all your opinions however with regards to your views on the "earth issue" , they're useful but not "acceptable" to me.
As for my views, no need to push it further, they can stand on their own.
Let them rest on the shelves for a while so we can move on. :wink:
I hope we didn't lost anything here. thanks

davidv
es
 
Still on the subject of Ground resistance,
I agree with George's deduction,
Running an low frequency, 60 hz AC current through a straight wire would bring about a resistance R ,and a very weak inductance ( very very much smaller compared to the R ) , (too impractical to measure).
By coiling the same piece of wire will then make a considerable increase in the inductance, however the R will still be d same.( same lenght)

Now until some wiseguy starts driving a spiral ground rods. I don't think that NEC will change the term.

But with all those bonding here & there, grids, rings rebars etc. loops are created so with a strong ac current or high frequency, the GES becomes an impedance but when & how big , theres a lot of possibilties.

davidv
es
 
davidv said:
Don, Bob, Bryan, n other Gents,
I'm grateful for all your opinions however with regards to your views on the "earth issue" , they're useful but not "acceptable" to me.
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Interesting.

Than I have to assume basic electrical theory is also not acceptable to you either.

davidv said:
Don,
For current to flow there must be a complete circuit, unless the earth is generating the current in question, it cannot be returned to the earth.
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The earth is a source & sink for all energy forms, sizing its reaction depending on the energy coming to it, not a conductor. it is part of the whole energy circuit, currents can be returned
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You above post is simply not true.
 
David,
I would really like to understand your reasons and sources. The grounding subject is so filled with misconceptions that there are many unsafe installations as a result of these misconceptions.
Don
 
davidv said:
Running an AC current ( like lightning ) through a straight wire would bring about a resistance R ,and a very weak inductance ( very very much smaller compared to the R ) , (too impractical to measure).
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This isn't exactly true either. The resistance of the circuit is fixed based on the installation specifics. The amount of current in the return stroke of lightning is fixed based on the physics of the lighting. The only variable will be the imposed voltage across the circuit of discharge.

The larger the lightning current the larger the inductive field around the conductor(s) lightning current flows. This is the aspect of lightning that creates flashover to other "unbonded" system and materials. This magnetic field is quite large in most cases as flashover can gap a distance of over 6'.

So again, it matters little how much (or little) ground resistance is obtained if all systems and metallic materials are bonded together.
 
Does impedance change with voltage, and does it change with frequency?
No and Yes.
The physical characteristics of the circuit do not change. The resultant impedance changes with frequency, it is not affected by voltage.

The impedance of an inductor (coil) is given by
Z=j2(pi)FL
where j is the complex number operator (sqrt(-1)) (this just gives the direction of the phase displacement), pi is 3.14..., F is the frequency in question, and L is the value of the inductance (in Henrys).

The impedance of a capacitor is given by
Z=-j/2(pi)FC

As you can see, the capacitor has a reactance that changes the phase in the opposite direction to an Inductor. (-j)

Nowhere, in either of these equations, is voltage. There are subtle correlations, however. Higher voltage requires a different geometry of the conductor arrangement, due to insulation requirements. This different geometry results in a different reactance.

I love this stuff! Now, if only i knew how to put together graphics, i could draw all sorts of impedance triangles.

Eric
 
This is off the op's topic but since the GES' purpose is being questioned I'm going to ask here.

I had two seperate delta-wye transformers that were connected without main bonding jumpers (bad deal). What brought attention to these mistakes was the voltage fluctuations between secondary phase wires to neutral when under load, 170v to n, 100v to n... Once the bonding jumpers were installed the voltage to neutral stabilized.

If earthing/grding has little to do with anything, which seems like what I am hearing from some on this forum, how did grounding the neutral correct the problem? What am I missing?

Brent
 
I've had a similar problem, albeit with a totally different system. I'll only elaborate because the specifics are relevant, in that the grounding system was very lax in my situation. In a standard utility setup, grounding is executed at every point; the power plant, the substation, the local transformer, the service, detached structures. We look at this expanse of grounding and say, 'Wow, that's a lot of grounding. The overall connection to the earth of the entire system is very compelling.'

The source of this house was a 48VDC PV system, with a pair of inverters configured to output 120/240 VAC. There was a ground rod at the panels, and a ground rod at the wind turbine. I had a water pipe and a ground rod at the house. This is a very small grounding system, so it's much less impressive. I don't know what the overall resistance to the earth was, but I'll bet it stunk.

I made an agreement with the solar installer to install the neutral-ground bond of the 120/240 VAC output in the panelboard of the inverter, and I would keep the grounds and neutral seperate at my panel. He forgot to follow through.

When I trimmed the house, we switched and plugged it with no problems, but when I went to hang a fixture, I got bit from the neutral of the lighting outlet. I whipped out my meter, and found 23V ground to neutral. No breakers were installed, save the temp'ed in Bath GFI circuit. I knew at once, the installer forgot to make the bond, so I installed the bond screw in my panel (that I wisely left in place, just in case).

The voltage disappeared. Was the grounding of the neutral key in this event, or the bonding of the ground to the neutral?

One of the promises of grounding made in 250.4(A)(1) is that it "will stabilize the voltage to earth during normal operation."

If the grounding is the hero of this circumstance, how good does it have to be to accomplish this? I'm not even sure that the different grounding electrodes on the premises were bonded together. For all I know, the water pipe and it's companion at the house were the only grounding electrodes I had at my disposal.

If the bonding was the act that zero'ed the voltage difference, then what causes the voltage to stop flowing through me, instead of having two conductors trying to flow voltage through me?
 
Brent,
What brought attention to these mistakes was the voltage fluctuations between secondary phase wires to neutral when under load, 170v to n, 100v to n... Once the bonding jumpers were installed the voltage to neutral stabilized.
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Something else happened to correct the problem. The phase to phase phase to neutral voltages are not affected by bonding or not bonding. Only the phase to ground and neutral to ground voltages are affected.
 
Don,

I knew you were going to say "something else happened..." :D .

After the first transformer was found without a bonding jumper I went back and checked all others. This is when I found a second transformer with the exact same problem. The coincidence of bonding xo seemed too great to ignore as it 'seemed' bonding corrected the problem. I even set up another transformer with unbalanced loads and no bonding jumper as an experiment and everything worked fine.

I sure wish I knew what the "something else" could have been as it's been driving me nuts.

Brent
 
For typical GESs my thoughts are that resistance is the term used, because the inductance and capacitance are mostly insignificant in the total measurement. Enough so, that for calculating GES conductor size it would not change the size of the conductors.
 
hello guys
Ive corrected my last post , I'm supposed to say normal current.
Its meant for Mr Peter d, as an illustration to using ground "resistance" term
Youre correct Bryan, thanks
As for the low resistance, I'll try to compose my thoughts on the matter again, while I enjoy my weekend.

and Bob please dont be so mean 2 me. Its just the Dirty Harry thing right?

:wink:
davidv
es
 
davidv said:
hello guys
Its meant for Mr Peter d, as an illustration to using ground "resistance" term
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Well, Ok then. If you or anybody else wants to apply the term "resistance" to AC systems, go right ahead. I maintain, and will maintain, that it's not technically correct. It makes no difference if the the non-resistive elements are small enough to ignore, it's still impedance.

Show me any electrical theory book where the term "resistance" is used to measure total opposition to current flow in an AC system and I will change my mind. :)

I'm not going to change my mind, you're not going to change yours, so we will have to agree to disagree.
 
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