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Thread: Bonding of Isolated Grounding system to plant electrode system, i.e. ground loop

  1. #31
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    Quote Originally Posted by Dale001289 View Post
    The main concern here isn't exactly amps/voltage via the shielding, but rather several relative factors (electromagnetic induction, electrostatic coupling, or conduction), collectively known as EMI - Electromagnetic interference, i.e. "noise" feedback - into the DCS (or PLC) I/O, which controls plant operation. Specifically, regarding linear 4-20mA circuitry from critical, process-related transmitters, such as Flow, Temp, Pressure and Level.
    The concern is unwarranted as long as you ground only one end of shields. Disregard instructions which indicate otherwise. There are probably hundreds of thousands of plants out there operating this way with no problem.
    I will have achieved my life's goal if I die with a smile on my face.

  2. #32
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    Quote Originally Posted by Smart $ View Post
    The concern is unwarranted as long as you ground only one end of shields. Disregard instructions which indicate otherwise. There are probably hundreds of thousands of plants out there operating this way with no problem.

    Typically, in process related applications, the shield is grounded at the source end only (e.g. DCS cabinet) and left floating at the device (e.g. transmitter). But this is done to prevent circulating current and does not address the broader spectrum of EMI.
    The shield should be kept as 'clean' as possible, (as in free of outside interference, capacitance etc) and at the same ohmic level as the isolated equipotential plane, i.e. Triad. This is why bonding the two systems, as required by NEC 250.50, is regarded a negative factor by some Clients.

    There's nothing more subjective than grounding.

  3. #33
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    A few years ago there was an interesting paper presented somewhere that looked at the effect of grounding shields, either at one end or the other, both, or neither. While it was by no means a paper that "proved" much of anything, the results suggested that for the most part it just did not matter much if the shield was grounded or not, but that the best EMI performance was by grounding both ends. The real problem with grounding both ends of a shield is that in most plants there is a significant difference in the voltage between two ground points within the plant and this causes a current flow on the shield. In some cases this has been enough to melt the shield due to heat when a shield was inadvertently grounded at both ends.
    Bob

  4. #34
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    Quote Originally Posted by petersonra View Post
    A few years ago there was an interesting paper presented somewhere that looked at the effect of grounding shields, either at one end or the other, both, or neither. While it was by no means a paper that "proved" much of anything, the results suggested that for the most part it just did not matter much if the shield was grounded or not, but that the best EMI performance was by grounding both ends. The real problem with grounding both ends of a shield is that in most plants there is a significant difference in the voltage between two ground points within the plant and this causes a current flow on the shield. In some cases this has been enough to melt the shield due to heat when a shield was inadvertently grounded at both ends.

    Its sounds 'legit' to me.

  5. #35
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    Quote Originally Posted by Dale001289 View Post
    Typically, in process related applications, the shield is grounded at the source end only (e.g. DCS cabinet) and left floating at the device (e.g. transmitter). But this is done to prevent circulating current and does not address the broader spectrum of EMI.
    The shield should be kept as 'clean' as possible, (as in free of outside interference, capacitance etc) and at the same ohmic level as the isolated equipotential plane, i.e. Triad. This is why bonding the two systems, as required by NEC 250.50, is regarded a negative factor by some Clients.

    There's nothing more subjective than grounding.
    Actually having and connecting the shield is addressing rfi/emi rejection.

    Isolated equipment grounding systems under the NEC can originate at the main bonding jumper. This is as close to 'earth' as you can get with a compliant [I]EGC. The only point(s) closer is the along GEC(s) or the electrode(s) itself... but non-compliant unless for an SDS (separately-derived system). Achieving the lowest resistance to 'earth' as you possibly can is a better tactic than trying to skirt compliance. This is why many communications companies have a maximum R2E for their hubs. The only way you can buck compliance is to submit a public input to the cycle revision process.
    Last edited by Smart $; 10-06-17 at 05:47 PM.
    I will have achieved my life's goal if I die with a smile on my face.

  6. #36
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    Quote Originally Posted by Dale001289 View Post
    The sole connection to plant grounding system as required by 250.50 can result both common mode noise and objectionable currents. The removable link allows the field to 'trouble-shoot' the isolated grounding system and theoretically eliminate both.

    I run into this hoodoo voodoo thinking all the time, a properly installed grounding electrode system, MBJ and properly installed wiring with no downstream grounds on the neutrals are your best bet for a "CLEAN" grounding system.

    AND Not sure on your building type, but having a truly isolated ground system is difficult due to all the metallic components of a building, building steel, rebar, ducts, water pipes, every large scale IG system we have investigated has been compromised, somewhere in the distribution system, be it misunderstanding by installers, pinched IG conductors or unintentional connections.

    A lot of money is spent on additional copper with no real benefit IMO.

    As for Triads, no better or worse than a variety of driven electrodes.
    Brian John
    Leesburg, VA

  7. #37
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    Quote Originally Posted by brian john View Post

    As for Triads, no better or worse than a variety of driven electrodes.
    It's all about the triangle and the power that it holds in the universe.
    Once in a while you get shown the light
    In the strangest of places if you look at it right. Robert Hunter

  8. #38
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    Quote Originally Posted by ActionDave View Post
    It's all about the triangle and the power that it holds in the universe.

    For some, this statement is not far from the truth.
    Brian John
    Leesburg, VA

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