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250.56 Revise or Delete

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    250.56 Revise or Delete

    Section 250.56 needs to be revised or completely deleted.

    This section makes no sense as currently worded. If 25-ohms is the magic number, then why does the addition of a second electrode negate the value?

    I propose that the section either require the 25-ohm value to be met regardless of the electrode count, or remove the requirement completely and simply make the requirement of two electrodes and forget about any resistance value.

    Proposals were submitted both ways for the 2005. None were accepted. The reason for rejection was that 25-ohms is not the actual intent of the section, yet that removing this section removes a minimum requirement. See Proposal 5-143 and 5-144.

    Well, which one is it? How can the panel say in one statment that its not the intention of the section to require 25-ohms, yet it makes the installer acheive that level or add a second electrode. Now of that second electrode still does not meet the requirement, nothing more needs to be done. And then one statement later the panel statement indicates that 25-ohms is a minium requirement.

    This section should probably go away.
    Bryan P. Holland, MCP
    NEMA - Codes & Standards

    Re: 250.56 Revise or Delete

    In at least one of the jurisdictions where I work, it is a moot point as we are required by local ammendment to install 2 rods anyway. BTW,can someone explain where the 25ohm figure comes from?
    John, Chair City, NC
    Technology: Mans best efforts to make things as good as they used to be


      Re: 250.56 Revise or Delete

      Bennie started a thread that gives a clue.
      A simple answer is; A static charge, on metallic objects, produced by a common high voltage surge,
      Will reach equilibrium with the earth within 10 seconds when connected by not more than 25 Ohms resistance in the path to the earth.

      The initial surge will have a time domain of 2 to 10 ms. The resultant electrostatic charge will not equalize in a short time, if there is a high resistance path between the negative and positive charges.

      The math is from Charles Coulomb and Johann Gauss.
      Take a look at the thread for more information.

      Bryan, if a second rod is driven in parallel with the first one, the resistance is not cut in half like resisters in parallel. Adding more rods is a losing proposition so the panel, very wisely, said two is enough. After that, the law of diminishing returns takes over.

      In my opinion, the rule is a good one and should stay as written.
      Charlie Eldridge, Indianapolis, Utility Power Guy


        Re: 250.56 Revise or Delete

        Adding a second rod reduces the resistance by 60%. The third is not much more. Its not all that important for most applications to have a low resistance ground.
        Moderator-Washington State
        Ancora Imparo