Good earth ground, what is your definition?

[gar]

050601-1325 EST USA

I do not expect that all of you are able to answer all of these questions. But answer those that you can.

How do you define a good earth ground?

What resistance do you measure at 60 Hz and 100 kiloHz?

How would you measure the resistance?

Do you know what the NEC ( National Electrical Code ) requires for equipment grounding?

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[Al_The_Man]

It is usually the resposiblility of whoever installs the supply into a building to connect up to the earth conductor provided by the Electric Service provider, which originates from the grounded star point of the three phase supply, this should be carried through and connected to all electrical equipment in the building, some manufacturers also recommend a ground rod or stake be driven in at each machine location and bonded to the incoming service ground.
To measure the actual value of the ground at a building with respect to the actual star point, (never often done), the resistance by current drop and is measured from the incoming point to a ground rod or good source of ground as far as practically possible at least 5 times the depth of the test ground rod etc, The (Planet) Earth is considered to be zero resistance, the code I believe says anything above 25ohms is not passable, recommended 2 to 3 ohms tops.
If an incoming ground is not available, Ground rods or electrodes are allowed.
Now there is available sophisticated equipment that is used to measure the GR.
There is probabally a more definitive explanation if you google.
Al.

[ghyman]

Originally Posted by rc-cellar Once again... "THE MAN" spouts off on a subject he knows absolutely nothing about.

Bad form, pal.

This is an adults-only forum. Please try to act like one.

Furthermore, if the answer that was offered is incorrect, or if you have a different solution, then please share it with the rest of us.

[Karl_T]

When I was an engineer (3M) one great fear was ground loops. Really does weird things to control equipment. All ground and sheild wires in a machine were bonded at only one point, other end of wire left free. The bond point was attached to the machine case and a ground went back to the main ground point at the building electrical entrance. Other ground rods, tying to other ground points was a HUGE NO NO.

I'm not the expert here, but the control guys were adment about this.

Karl

[2muchstuff]

ghyman,

Well put, I second it.

[OCNC]

Originally Posted by Karl_T When I was an engineer (3M) one great fear was ground loops. Really does weird things to control equipment. All ground and sheild wires in a machine were bonded at only one point, other end of wire left free. The bond point was attached to the machine case and a ground went back to the main ground point at the building electrical entrance. Other ground rods, tying to other ground points was a HUGE NO NO. I'm not the expert here, but the control guys were adment about this. Karl

Did this apply to control wiring and power wiring or only control wiring? It seems like you would want to protect the control wiring from small potentials that could develop between points in a ground loop and cause false triggering but that power wiring might be safer with 'local' grounds in the event of short circuits.

Chris

[miljnor]

i have the benefit of having this explained to me recently. (still means i don't know what i am talking about. ) but if you connect the ground rod to the incoming ground wire there is no "Loop" thus still "bonded at one point".

and i think if you read the post by AL he says that. So.....

[gar]

050602-0524 EST USA

Thanks folks:

I am glad to see some response on this subject.

I would like to see more quantitive data from all of you on ground resistance.

Al did provide some values, his NEC 25 ohms is correct. But I would like to know the construction of the ground system and soil conditions that produced 2 to 3 ohms ground resistance.

Al I know of one large plant ( million sq-ft ) where their primary 480 delta was completely floated from ground except for leakage impedance, of which there would be considerable. How the code reads on this I do not know. I also do not know if they had large capacitors to ground. Their building structure and safety ground wiring provided for personnel safety.

In general the code requires a protective earth ground at the service entrance.

More input please.

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[trilect]

Ground needs depend on what your protecting the equipment from.

Medical grounds would be hated by the audiophile where ground loop avoidence would be unacceptable to medical and industrial applications (where copper data control wiring is present)


Lighting protection is also a different monster and can cause ground loop problems when machinery on a roof is electrically grounded and also has the metal case tied to the lighting protection ground.


also depends on the electrical code of the state where the ground in question is located and how the inspector interprets that code.

[Scott_bob]

Interesting,

Personally, I know enough about electrical to stay away from it...
From experience though, the ground issue has been very problematic for reliable CNC controls. Of the CNC machines I have worked on the Fanuc has been the most susceptible to damage due to spikes, feed back, surges what ever they are, I don't know. By ensuring that a good low resistance ground rod is provided at "each" CNC, reliability has been improved.
The ohms resistance that I remember was to no more than 9 ohms...
In Colorado where the soil is mostly decomposed granite, the ground rod had to be 6 feet deep minimum, and the hole thru the concrete slab had to be kept wet in order to enhance the conductivity of the ground rod. No small task considering what it takes to get a copper rod driven into 6 feet of dg. In that part of the country, electrical storms are a serious issue. We had to shut down in the afternoons or risk major disasters with sensitive CNC equipment. They do go haywire when voltage comes in where output should be going.
This really is a serious problem for a lot of folks, I'm glad to see some informed discussion...

[gar]

050602-1232 EST USA

Scott_bob:

Can you clarify the wiring of the separate ground rod.

When a separate ground rod was used at each machine was there still a heavy safety ground wire running back to the service entrance, or was the machine isolated and solely dependent upon the ground rod for grounding?

Do you know how the ground rod resistance to ground was measured?

How often did you pour water into the ground rod hole?

What resistance values were obtained with and without added water?

When you speak of problems from electrical storms, what were the specific problems? Were these mostly momentary loss of power, or large voltage transients, or close by lightening strikes ( very large transient, maybe a million volts or more )?

.

[Al_The_Man]

Originally Posted by trilect Medical grounds would be hated by the audiophile where ground loop avoidence would be unacceptable

THATs why the audiophile wasn't in today, he was off due to Medical Grounds.
Al.