The Potential of Earth Ground

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As usual, I'm late. However, I spent a lot of time looking for a 'true grounding theory', so I am quite intoxicated with usual information from NEC ,IEEE 142,IEEE 80, BS 7430, IEC 60364, IEC EN 50522 and other.
The Earth absolute potential it is not so important but only the voltage drop and the potential gradient since E[Electric Field Intensity]=-grad(V).
If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.
The resistance between these 2 points depends on grounding resistance of the grounding electrode in each point [or grounding grid]- which is just a contact resistance between the electrode and the ground. The ground resistance between electrodes is considered 0 [somewhere I found this equation rE=π^2.freq/10^4 Ω/km].More important is the “depth” of this ground considered a conductor buried at this depth [according to Carson] in order to calculate the reactance from a wire to ground.
If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].
 

jim hardy

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If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].
That becomes your reference point and you're measuring potential difference with respect to it instead of with respect to infinity. .

If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.
Hence the concept of "step potential".

http://www.esgroundingsolutions.com/what-is-step-and-touch-potential/
upload_2018-7-22_8-23-23.png


They design a switchyard grounding grid (network in that image) to hold voltage drop along 'ground' during a fault to a small enough number of volts per meter that it won't electrocute that unlucky fellow standing underneath the line. I don't know about transmission line towers..

3 phase HV lines are generally delta connected and don't need a neutral,
That's a misleading statement that i found posted on Stackexchange...
The lines themselves are just wires. It's the transformers at their ends that are connected wye or delta.
At my plant the main generator stepup transformer had a 24KV delta primary fed by the generator, and a 240 kv wye secondary feeding the switchyard.
So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.

any help ?

old jim
 

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Svein

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So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.
The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...
 
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Thank you for your remarks Jim and I agree with you.
About the essenmein post: usually HV Systems are Y connected as it is indicated in Stackexchange post.
However, a phase-to-ground short-circuit at HV appears as phase-to-phase short-circuit at 24 kV Delta. No neutral so no neutral current at generator side.
Usually, if the transmission line it is an overhead line then the overhead grounding wire it is also static wire protecting the live conductors and insulators against lightning strokes.
If lightning strikes an overhead ground wire, a traveling current wave will be set up which will
induce a traveling voltage wave. This voltage wave will generally increase in magnitude as it
travels down the wire, until it reaches a structure where the reflection of the traveling wave from
the ground prevents the voltage from further increasing. (The overhead ground wire is grounded
at every structure). If the traveling voltage wave at the structure is sufficiently high, a "back flashover" across the insulation from the structure ground wire or from the overhead ground wire
to the phase conductor will occur. The factors that determine if a back flashover will occur are:
the amount of insulation, the footing resistance (the higher the footing resistance, the higher the
voltage rise at the structure) and the span length.
 

jim hardy

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The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...
Since the neutral need only carry the unbalance, which is small(or zero if the line feeds a delta transfomer load), it is often a much smaller wire that's run atop the towers to intercept lightning before it reaches the phase conductors. It'll be connected to the structure and to earth at every tower. Since it doesn't need insulators it's pretty cheap to install. Power company can put a fiber optic cable inside it and earn revenue from a communication company...
TransmissionNeutral.jpg


Another consideration is that a Delta connected transformer winding nearly eliminates third harmonic distortion.

There's a long discussion at Stackexchange .
 

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I'm curious why this is a deadly misunderstanding? Aside from the working voltages (eg 120/230 etc) which will get you even if the ground is correctly connected, generally speaking if the thing you are worried about getting a shock from is grounded and you are also grounded then there is no potential difference between the thing and you to give you a shock?
If there is no potential difference then there's no "shock", I agree. This does not necessarily mean if two things are grounded you're safe. Any voltage applied to earth is going to create a gradient in the soil. When there's a voltage gradient there's a risk of step-potential. By my research as little as 60V AC from hand-to-foot can be lethal and current so low that the threshold of sensation isn't met can still cause serious arrhythmia of the heart. This means for someone with less optimal heart health, even 10V AC from hand-to-hand or hand-to-foot could be life threatening. These figures are well inside a threshold for possible step-potential for anyone working around live equipment. The best course of action is to assume anything not cut-off from the supply is deadly. Overkill? Probably. I'd rather be safe than sorry. Put on the rubber shoes and go home to your family.

There's a consideration to be made for the impedance of the ground material. Gradient will rely on this factor, mostly. Conductance of earth is getting hairier the more I'm looking at it but generally you're right.

I'm arriving at a place where I think it's very difficult to explain through literature what grounding (or anything much more complicated than Ohm's Law) "is." Trusting the physics and creating pictures/animations might be the best option to come close to "getting it." This opinion may change if I can find or derive a good explanation. So far, all I do is wave my hands frantically and say "Kirchhoff and Conservation of energy!" This isn't going to help anyone, myself included.
 
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That becomes your reference point and you're measuring potential difference with respect to it instead of with respect to infinity. .

They design a switchyard grounding grid (network in that image) to hold voltage drop along 'ground' during a fault to a small enough number of volts per meter that it won't electrocute that unlucky fellow standing underneath the line. I don't know about transmission line towers..

old jim
If there is no potential difference then there's no "shock", I agree. This does not necessarily mean if two things are grounded you're safe. Any voltage applied to earth is going to create a gradient in the soil. When there's a voltage gradient there's a risk of step-potential. By my research as little as 60V AC from hand-to-foot can be lethal and current so low that the threshold of sensation isn't met can still cause serious arrhythmia of the heart. This means for someone with less optimal heart health, even 10V AC from hand-to-hand or hand-to-foot could be life threatening. These figures are well inside a threshold for possible step-potential for anyone working around live equipment. The best course of action is to assume anything not cut-off from the supply is deadly. Overkill? Probably. I'd rather be safe than sorry. Put on the rubber shoes and go home to your family.

There's a consideration to be made for the impedance of the ground material. Gradient will rely on this factor, mostly. Conductance of earth is getting hairier the more I'm looking at it but generally you're right.

I'm arriving at a place where I think it's very difficult to explain through literature what grounding (or anything much more complicated than Ohm's Law) "is." Trusting the physics and creating pictures/animations might be the best option to come close to "getting it." This opinion may change if I can find or derive a good explanation. So far, all I do is wave my hands frantically and say "Kirchhoff and Conservation of energy!" This isn't going to help anyone, myself included.
Here's an interesting thing relating likelihood of ventricular fibrillation to the time and magnitude of electric current exposure.
upload_2018-7-24_16-2-16.png
 

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jim hardy

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We usually think of "Ground" as conducting like a wire with no voltage drop. But earth is actually a pretty poor conductor compared to copper. Since it's so big the current density we encounter from household wiring is low enough to not produce much voltage drop(potential difference) per meter. Utility lines and lightning bolts are another matter, though..

That's why i tell people "Ground is just another wire and it goes most everywhere."
Like any other wire it'll have voltage drop in proportion to the current flowing through it. More accurately, in proportion to the local current density .
That's why at the bottom of a lightning bolt there's a large number of volts per meter along the ground as charge flows outward from the strike. People or cows standing underneath a tree that's hit by lightning often get electrocuted from the ground voltage drop between their feet.
Myself i think that's why tropical wading birds sleep on one foot.

old jim
 

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jim hardy

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The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...
Wye connection does not necessarily need 4 wires. A transformer connected in wye om the primary
 
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The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...
Wye connection does not necessarily require 4 wires. If a transformer has a wye primary, with delta secondary, the primary feed only need be 3 wires. Triple harmonic current circulates in the delta secondary, as well as zero sequence currents due to secondary load unbalance. The wye neutral is connected to earth for safety reasons, but 3 wires carry full load current balances or unbalanced.
If the transformer has both primary & secondary wye connected, without a 3 legged E core, then a 4th wire is needed. To avoid 4 wires, power company avoids Y-Y connection. A tertiary winding is added, connected in delta. Or a core type construction is used.
Claude
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EE 39 years
 
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That's a misleading statement that i found posted on Stackexchange...
The lines themselves are just wires.
old jim
Correct, poor choice of words on my part.
 
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Wye connection does not necessarily need 4 wires. A transformer connected in wye om the primary
This is only true if you have a balanced load. For example all the machines I've worked with do not have a neutral connection from the winding star point when they are Y, the stator (should) be balanced. An unbalanced Y system with no neutral connection will unbalance the Line to neutral voltage on the phases. A Y-delta transformer does this balance with the flux in the core. If its a Y-Y transformer then the construction needs to be considered, this can be coupled core (Eg E), or literally three mechanically separated single phase transformers, where the flux cannot help to balance EMF.
 
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This is only true if you have a balanced load. For example all the machines I've worked with do not have a neutral connection from the winding star point when they are Y, the stator (should) be balanced. An unbalanced Y system with no neutral connection will unbalance the Line to neutral voltage on the phases. A Y-delta transformer does this balance with the flux in the core. If its a Y-Y transformer then the construction needs to be considered, this can be coupled core (Eg E), or literally three mechanically separated single phase transformers, where the flux cannot help to balance EMF.
If a xfmr has a wye primary, with delta secondary, the primary needs only 3 wires even if secondary load is unbalanced. The reason for avoiding wye-wye xfmr is the desire to use only 3 wires.
Do you agree or disagree with the following:

A wye-delta xfmr needs only 3 wires on the primary side & will support an unbalanced secondary load with 3 wires.
 
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If a xfmr has a wye primary, with delta secondary, the primary needs only 3 wires even if secondary load is unbalanced. The reason for avoiding wye-wye xfmr is the desire to use only 3 wires.
Do you agree or disagree with the following:

A wye-delta xfmr needs only 3 wires on the primary side & will support an unbalanced secondary load with 3 wires.

I believe I agree with this already:
" A Y-delta transformer does this balance with the flux in the core. "
 
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I believe I agree with this already:
" A Y-delta transformer does this balance with the flux in the core. "
I did not say that. If a wye-delta or delta-wye is built by combining 3 single phase xfmrs, then the core flux does not maintain balance.
A 3 phase core type xfmr does that regardless of wye or delta connection. With 3 phase core construction, a Y-Y connection stays balanced with unbalanced loads due to core flux. Likewise for other connections.
Claude
 
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I did not say that. If a wye-delta or delta-wye is built by combining 3 single phase xfmrs, then the core flux does not maintain balance.
A 3 phase core type xfmr does that regardless of wye or delta connection. With 3 phase core construction, a Y-Y connection stays balanced with unbalanced loads due to core flux. Likewise for other connections.
Claude
Had to think about that, don't normally deal with 3ph transformers. With three single phase transformers with one side in Y the other in delta the EMFs still force the other phases to balance via the transformers, I kind of loosely look at it from the perspective that on the Y side you have two windings in series line to line, on the delta you have one winding line to line, so any line to line voltage change on the Y, forces the same EMF change on two windings in the delta side. So indirectly the transformer flux is still doing the work via the linked EMFs.
 
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Had to think about that, don't normally deal with 3ph transformers. With three single phase transformers with one side in Y the other in delta the EMFs still force the other phases to balance via the transformers, I kind of loosely look at it from the perspective that on the Y side you have two windings in series line to line, on the delta you have one winding line to line, so any line to line voltage change on the Y, forces the same EMF change on two windings in the delta side. So indirectly the transformer flux is still doing the work via the linked EMFs.
The linked emf's is correct, but the fluxes are NOT linked. It so happens that flux is related to emf per Faraday's Law. A delta forces the 3 emf's to sum to zero. A 3 legged E core pair for es the 3 flutes to sum to zero. Different means, but same end result.

Claude
 
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My point is that many electricians and lineman I talk to are convinced that, once something touches earth reference, it becomes safe. That's a deadly misunderstanding.
No it is mandatory standard practice. We use grounding straps on power lines to protect linemen when they are working on them. The power should be shut off before starting work, but just in case the power is mistakenly turned on when the men are working it assures that the current will go to ground through the strap rather than through the workers.

By the way, in power system analysis, exactly that mistake, closing the breaker on a line grounded by straps close to the power plant is the worst case scenario for short circuits. It results in higher short circuit currents than any other scenario.
 

Svein

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We usually think of "Ground" as conducting like a wire with no voltage drop. But earth is actually a pretty poor conductor compared to copper.
And at that, the conductance of "Ground" varies very much with the composition of the ground in question. If you live in the mountains and the only ground available is bare rock, you will be hard pressed to measure any conductance at all. This used to be a real problem with short-wave transmitters in Norway, because there are just three places in the whole country where the "Ground" conductance is good enough for that kind of appliance. That is also why the traditional power grid is delta-connected with "protective ground" being a local responsibility.
 

jim hardy

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That is also why the traditional power grid is delta-connected with "protective ground" being a local responsibility.
Be circumspect of that statement. As discussed earlier the transmission lines are usually connected at one end to a wye transformer and at the other end to a delta transformer.
 
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Svein

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Be circumspect of that statement. As discussed earlier the transmission lines are usually connected at one end to a wye transformer and at the other end to a delta transformer.
Yes, and the center of the wye is connected to "Ground". Assuming that the ground resistance (between the wye transformer and a power consumer) is ≈100kΩ, a ground fault current of 10mA will try to create "ground" potential difference of 1000V.

It sounds crazy, but I once worked in a research institute where one of the guys measured the three delta voltages and phases relative to "ground" and concluded that "ground" was outside the delta...
 

jim hardy

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Assuming that the ground resistance (between the wye transformer and a power consumer) is ≈100kΩ, a ground fault current of 10mA will try to create "ground" potential difference of 1000V.
Between the wye and the consumer is a transformer.
Fault current is constrained to one side of that transformer.


On the transmission side of most systems the neutral that runs along the top of the towers provides a metal path through which fault current can get back to its wye source (if there is one, and usually there is).
Annotating the previous illustration
GROUNDEDXMISSIONLINES.jpg


On the consumer's side IEEE 142 and NEC require a metal path for fault current back to the transformer or generator winding from whence it came.

Not arguing there's no such thing as a "high impedance grounded system" like you describe
just they're the exception not the rule.

old jim
 

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