This article was posted 12/22/2006 and is most likely outdated.

Article 250 Grounding and Bonding 250.4
 

 
Subject - Article 250 Grounding and Bonding 250.4

December 22, 2006  

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Grounding and Bonding

250.4 General Requirements for Grounding and Bonding

Change clarifies that swift operation of an overcurrent protection device in the presence of a ground-fault depends upon the existence of an effective ground fault current path from the point of the fault to the power-supply neutral.

The last sentence was revised to make it clear that the earth cannot be used as the effective ground-fault current path. It will not facilitate the opening of the circuit protection device from a ground fault.


(A) Solidly-Grounded Systems.
(5) Effective Ground-Fault Current Path. Electrical raceways, cables, enclosures and equipment, as well as other electrically conductive material that are “likely to become energized,” must be installed in a manner that creates a permanent, low-impedance path that facilitates the operation of the circuit overcurrent device or ground-fault detector for high-impedance grounded systems. Figure 250-3

The effective ground-fault current path must be capable of safely carrying the maximum fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source neutral [110.10].


Author’s Comment: Clearing ground faults (line-to-case faults) is accomplished by bonding all metal parts of electrical equipment and conductive material likely to become energized to the power-supply grounded neutral terminal.

Author’s Comment: Another factor necessary to help ensure an effective ground-fault current path is that all circuit conductors (ungrounded, grounded and the equipment grounding (bonding) conductor) must be grouped together in the same raceway, cable, or trench [300.3(B), 300.5(I), and 300.20(A)]. Figure 250-4

The earth is not considered an effective ground-fault current path. Figure 250-5



Figure 250-3
(Click on image to enlarge)


Figure 250-4
(Click on image to enlarge)


Figure 250-5
(Click on image to enlarge)

   
 

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Comments
  • I understand the importance of the wires being grouped together but in a system preexisting with no ground wire in it wouldn't an external ground wire be better than nothing? If properly installed.

    tom glastra
    Reply to this comment

  • In the bottommost image no overcurrent device is shown.Whether it was delibrately omitted to emphasise that earth is too poor a conductor to operate an overcurrent device?

    Sozharajan
    Reply to this comment

  • Other code isses....

    All (THHN etc.) shall be in a raceway.

    So if it's a ground for lets say the telco system you strip it and you can do it.

    No.

    Can't use the pipe for support. Tie wraps need to be listed.

    Somebody just did that exact same thing the other day on a job I went to.

    AHJ opinion was.....

    You could strip it and strap it w/listed straps to... lets say the wall.

    electoman
    Reply to this comment

  • Isn't there a provision in 250.102 and 501.16 for running an external bonding jumper along a short (<6ft) run of conduit? That would seem to be an exception to your figure 250-4.

    Matt
    Reply to this comment


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