What is the difference between Earth ground and circuit common?

[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