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IndustryArena Forum > CNC Electronics > CNC Machine Related Electronics > Good earth ground, what is your definition?
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  1. #1
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    Good earth ground, what is your definition?

    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?

    .

  2. #2
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    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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
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    Quote Originally Posted by rc-cellar
    Once again... "THE MAN" spouts off on a subject he knows absolutely nothing about.
    :bs: :bs: :bs:
    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.

  4. #4
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    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

  5. #5
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    ghyman,

    Well put, I second it.
    If it's not nailed down, it's mine.
    If I can pry it loose, it's not nailed down.

  6. #6
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    Quote 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

  7. #7
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    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.....
    thanks
    Michael T.
    "If you don't stand for something, chances are, you'll fall for anything!"

  8. #8
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    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.

    .

  9. #9
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    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.

  10. #10
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    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...
    Scott_bob

  11. #11
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    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 )?

    .

  12. #12
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    Quote 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.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  13. #13
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    Gar, I subscribe to this engineering forum where you might find some answers, here is a couple regarding grounding technique for delta supplies etc.
    http://www.electrical-contractor.net...ML/000078.html
    http://www.eng-tips.com/viewthread.cfm?qid=123595
    Grounding is probably one of the most mis-understood subjects in the electrical field, In Europe and in N. America the methods seem to be constantly changing. The NE Code does little to clear up this misunderstanding.
    For example most of the world uses grounding as a form of safety protection, an example is the UK used to be no exception, and although, similar to N. America where the star point or the 'Neutral' point was grounded at the source transformer the supply company discouraged using their ground, the insistance was to provide a tested local ground, which was until the advent of non-metalic water supplies was the metalic water company supply , the ground resistance was measured from the supply companies incoming ground to the metalic water pipe, this measured the path from the supply ground back to the transformer ground, through earth ground back to the sytem water pipe . An AC megger was used measure the resistance path which had to conform to a certain resistance value, if not Ground rods had to be sunk and an earth leakage trip device had to be supplied. The tendency now in this part of the N. America at least, is that whether or not a service supply ground is provided, not only is the incoming service ground bonded to the incoming water supply, that ground rods are manditory at the service entry.
    The main object of the NEC is safety, and that means that when an operator stands at a machine the ground he is standing on and the machine he is operating is at the same potential as the grounded neutral electrode back at the supply transformer which may be some distance away from where his machine is.
    So, not only do we have safety, in the case of a CNC machine, there is a noise issue, where all kinds of sources of power supplies and circuitry ranging from 5v logic to High voltage, high Frequency switching drives, coupled with the fact that there can be adjacent machinery on the same power and ground buss like Plasma machines and MIG welding robots.
    This is where the practice of local ground rod can be an advantage, as long as it is applied in the right manner, The machine should have one main single point where all power supply commons, shields , ground rod and incoming service ground conductor, should be connected, if the ground rod is applied effectively, this should result in the machine ground plate being at or near the same potential as the source transformer ground electrode.
    Some mention that ground loops are introduced by a ground rod, the fact is that often a machine has un-intentional grounds all the time, the pipe fitter that runs an air line to the machine and clips the pipe to the machine and the pipe is fastened in turn to a steel building support that goes 5ft into the ground provides a ground path, so isn't it better to provide a known low impedance path to earth?

    NEC excerpts.
    The safety ground must be completely isolated from neutral all the way back to the power transformer.
    The safety ground must never be dependent on a conduit alone.
    A wire must be run into the electrical feed along with the power wiring and neutral for use as safety ground.
    The safety ground and neutral must be firmly connected together at the power transformer.
    The safety ground must be firmly wired to the unit for personnel safety and to ensure that the AC line filters properly function.
    To actually measure earth impedance of a ground electrode systems you need a three-point, or four-point "fall-of-potential method.
    As for what should the ground impedance should be is undefined by NEC. It only requires a single rod, pipe, or plate that does not have a resistance of 25 ohms or less shall be augmented by one additional electrode of any of types specified by 250.52(A)(2). So if one rod produces say 500 ohms, you are only required to add another rod which should give you 250 ohms. The 2 ohms quoted are specifications used by some industries, not any NFPA requirement.
    (Of course, on the other hand, it might be a load of BS) (nuts)
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  14. #14
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    Dont bother wasting time at this one...
    www.eng-tips.com <<<<<<<<<LOUSY WEBSITE... TOTAL LOOSERS
    Murphy

  15. #15
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    Ground is, by definition, zero potential. There are a million ways to skin a cat, but generaly as stated previously 6 ft of exposed copper to conductive earth minimum. The questions your asking involve some serious calculus to describe and there are engineers that spend lifetimes studying the effects of transients and pulse circuit theory. Way outta our league. As far as measuring "resistance" of an AC source, it is also very complicated and no simple answer will do. AC circuits "resistance" is actualy calculated as a reflected impedance and requires the consideration of many variables, including phase angle, phase shift, phase lag, load, inductance, capacitance, frequency wire size and the calculus. Allot of this rambling may not apply to all situations of course but the gist is that you cant hook up a DVM and measure the resistance and have it provide a meaningfull value as far as an AC source is concerned. This stuff is way over my head, so Im gonna shut up and refer you to Al, who can probably elaborate more and correct my mistakes.
    Halfnutz

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  16. #16
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    050602-2003 EST USA

    To my original question I need to add the following:

    How do you earth ground your CNC machines?

    Do you only use the NEC safety ground wire back to the service entrance?

    Or do you use this safety ground wire plus a ground rod at the machine?

    Or do you not use the NEC safety ground, but only use a ground rod at the CNC?

    What is the resistance of 100 ft of a #6 copper wire?
    What practical effect does paralleling this #6 wire with 1 ohm of ground resistance have? If you inject 1 amp of current into this parallel path how much flows in the #6 wire, and how much in the 1 ohm ground rod path?

    .

  17. #17
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    Are you trying to get us to do your homework?
    Halfnutz

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)

  18. #18
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    See there, I was right! I do know enough about sparks, to stay away from them...
    The above confussion and risk associated with not properly grounding equipment is why we would not even chance operating our CNC during a lightning storm... If you have these in your area, you know what I mean.

    I cannot even begin to ansewer some of these questions,
    Scott_bob

  19. #19
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    #6 gauge is .0013 Ohms/metre.
    Al
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  20. #20
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    the best way to ground your machine. Well time or money... mines with money. Call a localy certified electrician and see what the code is (most of them like to talk, maybe they are lonely ) you could end up getting your info. for free. Or, better yet, pay him, to make the grounding to code.
    thanks
    Michael T.
    "If you don't stand for something, chances are, you'll fall for anything!"

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