What is the difference between Earth ground and circuit common?

[Fisherman199]

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.

 

[Fisherman199]

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.

 

[jim hardy]

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

 

[Tom.G]

Additional fuzzy feathery info.
As @jim hardy said ...i think that's why tropical wading birds sleep on one foot.

Short version: https://www.allaboutbirds.org/why-do-some-birds-stand-on-one-foot/

Long version (click on "Full Transcript" button): https://birdnote.org/show/why-birds-stand-one-leg

 

[jim hardy]

Additional fuzzy feathery info.

Yes, I've not many takers on my theory as yet.

 

[cabraham]

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

 

[cabraham]

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
PhD student
EE 39 years

 

[essenmein]

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.

 

[essenmein]

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.

 

[cabraham]

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.

 

[essenmein]

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. "

 

[cabraham]

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

 

[essenmein]

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.

 

[cabraham]

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

 

[essenmein]

It so happens that flux is related to emf per Faraday's Law.

Claude

Almost like one can't happen without the other :D

 

[anorlunda]

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]

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]

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.

 

[Svein]

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]

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

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

 

[jim hardy]

Perhaps we should start a new thread on grounding practices. We've strayed from the original question.

 

[anorlunda]

Everyone, please be careful. There are many contexts, power transmission, distribution, industrial, residential, indoor and outdoor and consumer products. Plus there are normal and short-circuit cases that are very different. Those alone add up to 12 contexts, each of which may have different grounding practices.

So I think this thread is becoming very confused because assertions are being made without stating which context they apply to. That is one of the reasons why grounding is so devilishly difficult to discuss.

 

[jim hardy]

I admit to being beyond my area of expertise in Transmission and Distribution(T&D). It is a different environment from in plant distribution and there are nuances of which i am at best only marginally aware.

I was taught some fifty five years ago that T&D used "neutral" for the small residual unbalance currents and to intercept lightning. Since it needn't carry much current it can be small, and since it's near ground potential it needn't be insulated. Most but not all transmission lines I've seen in the years since indeed have that fourth conductor .and it's usually run above the power conductors .

So i am not asserting that the T&D neutral serves the same purpose as a NEC "Bonding Conductor"
only that the laws of Kirchoff and Ohm will prevail.

I am aware that T&D relay folks use means far more sophisticated than simple overcurrent to detect faults, and that's necessary because of the distances involved.
@Svein 's comments about ungrounded systems i am sure have basis in analytical methods for detecting power system faults.
Here's a 357 page book on the subject. I'm no expert, just a fellow who's aware of how little he i know of it. .
http://www.gegridsolutions.com/multilin/notes/artsci/artsci.pdf

I simply do not want this thread to convey to beginners the mistaken idea that power systems are ungrounded hence ground fault currents would be small.

 

[Fisherman199]

I admit to being beyond my area of expertise in Transmission and Distribution(T&D). It is a different environment from in plant distribution and there are nuances of which i am at best only marginally aware.

I was taught some fifty five years ago that T&D used "neutral" for the small residual unbalance currents and to intercept lightning. Since it needn't carry much current it can be small, and since it's near ground potential it needn't be insulated. Most but not all transmission lines I've seen in the years since indeed have that fourth conductor .and it's usually run above the power conductors .

So i am not asserting that the T&D neutral serves the same purpose as a NEC "Bonding Conductor"
only that the laws of Kirchoff and Ohm will prevail.

I am aware that T&D relay folks use means far more sophisticated than simple overcurrent to detect faults, and that's necessary because of the distances involved.
@Svein 's comments about ungrounded systems i am sure have basis in analytical methods for detecting power system faults.
Here's a 357 page book on the subject. I'm no expert, just a fellow who's aware of how little he i know of it. .
http://www.gegridsolutions.com/multilin/notes/artsci/artsci.pdf

I simply do not want this thread to convey to beginners the mistaken idea that power systems are ungrounded hence ground fault currents would be small.
View attachment 228891

The GE book is the real deal. Wealth of knowledge about a very complicated subject. It's almost as complicated as discussing grounding. ;)

 

[Fisherman199]

Everyone, please be careful. There are many contexts, power transmission, distribution, industrial, residential, indoor and outdoor and consumer products. Plus there are normal and short-circuit cases that are very different. Those alone add up to 12 contexts, each of which may have different grounding practices.

So I think this thread is becoming very confused because assertions are being made without stating which context they apply to. That is one of the reasons why grounding is so devilishly difficult to discuss.

Would it be fair to say frame of reference is important when discussing grounding? Generally, when I say "ground" I mean a wire somewhere with negligible resistance returning to the source. When I say "earth ground" I mean a metal rod stuck in the ground somewhere with non-negligible resistance to the earth.

The people who told me writing skills weren't important to engineering were completely daft. Semantics are everything.

 

[jim hardy]

The people who told me writing skills weren't important to engineering were completely daft. Semantics are everything.

Amen. That Laviosier article i quote so often says at the beginning:

"We think only through the medium of words. --Languages are true analytical methods. --Algebra, which is adapted to its purpose in every species of expression, in the most simple, most exact, and best manner possible, is at the same time a language and an analytical method. --The art of reasoning is nothing more than a language well arranged."

Not bad for 1787 !
It's well worth the half hour it takes to digest it.
https://web.lemoyne.edu/giunta/lavpref.html

old jim

 

[essenmein]

Would it be fair to say frame of reference is important when discussing grounding? Generally, when I say "ground" I mean a wire somewhere with negligible resistance returning to the source. When I say "earth ground" I mean a metal rod stuck in the ground somewhere with non-negligible resistance to the earth.

We also tend to inadvertently create confusion, label the internal pcb "returns" as digital or analog ground, when in fact it is connected to battery negative, then to further confuse we AC couple our internal pcb ground to actual chassis ground, then, battery negative and chassis are connected together at the battery so if you were to continuity check between the two "in system", you'd get a beep. Then the hilarity is off course our chassis ground is isolated from actual Earth ground by four rubber hoops...

 

[Windadct]

Understanding the layout and purpose of Digital, Analog and Power grounds in a mixed environment ( like a robot) is critical, they are all connected, yet need completely separate distribution through the system. IMO the use of the word GROUND trivializes the issue and makes the layperson think it is simply a connection point.

 

[Fisherman199]

Understanding the layout and purpose of Digital, Analog and Power grounds in a mixed environment ( like a robot) is critical, they are all connected, yet need completely separate distribution through the system. IMO the use of the word GROUND trivializes the issue and makes the layperson think it is simply a connection point.

This is precisely the case. Since it's a topic of particular interest as of late, I've mentioned in passing to some piers the complexity of grounding throughout power systems and they looked at me as if I were crazy. The subject was always side-stepped in college as well; seen as a matter of physics rather than engineering.

This is not to say the practice of what "should" be done regarding grounded systems hasn't been established, but that's a different discussion. ;)

 

[jim hardy]

We also tend to inadvertently create confusion, label the internal pcb "returns" as digital or analog ground, when in fact it is connected to battery negative, then to further confuse we AC couple our internal pcb ground to actual chassis ground, then, battery negative and chassis are connected together at the battery so if you were to continuity check between the two "in system", you'd get a beep. Then the hilarity is off course our chassis ground is isolated from actual Earth ground by four rubber hoops...

Bravo !
I try to consistently refer to them as "Signal Common" .

Whenever i do say 'Ground" i try always to put it in quotation marks. And i use that term only to stay in context with the conversation already in progress.

In my alleged brain :

1. "Ground" refers to Mother Earth.
2. "Circuit Common" refers to the selected reference point for voltage measurements , ie where you should connect your voltmeter's negative lead..

British motorcycle manuals refer to that as "Earth" (even though it's separated from Mother Earth by the tyres)
Guided Missile manuals I've seen refer to that as "Vehicle Skin" which makes sense, if aluminum it amounts to a Faraday shield too.
Car manuals I've seen refer to it as "Chassis".
​

I smile when i see it referred to as "Common" , and say to myself "Bless that draftsman".

old jim