This article was posted 01/13/2006 and is most likely outdated.

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250.58 Common Grounding (Earthing) Electrode.

January 13, 2006  

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250.58 Common Grounding (Earthing) Electrode.

Where a building or structure is supplied with multiple services or feeders as permitted by 225.30 and 230.2, the same electrode must be used to ground enclosures and equipment in or on that building.

Author’s Comments:
• Metal parts of the electrical installation are grounded to the earth to reduce voltage on the metal parts from lightning so as to prevent fires from a surface arc within the building or structure. Grounding electrical equipment to earth doesn’t serve the purpose of providing a low-impedance fault-current path to clear ground faults.
• The most practical method of meeting this requirement is to ground each of the disconnecting means to a common concrete-encased grounding electrode [250.52(A)(3)]. Figure 250–108

CAUTION: Potentially dangerous objectionable current flows on the grounding electrode conductor when multiple service disconnecting means are grounded to the same electrode. This is because neutral current from each service can return to the utility via the common grounding electrode and its conductors. This is especially a problem if one of the grounded neutral service conductors is opened. Figure 250–109



Figure 250–108
(Click on image to enlarge)


Figure 250–109
(Click on image to enlarge)

     

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Comments

  • I think the "cautionary" comment associated with Fig 250-109 and the drawing's representation of neutral current flow is somewhat misleading. Most of the neutral return current will flow back to each source thru the respective grounded(neutral) conductors, while a small amount will follow the grounding electrode thru the earth to the transformer ground. This will be the same whether there is one source or two. Yes, if a grounded service conductor is open the current will follow the grounding electrode path, but again, this potential exists whether there is one source or two.

    james wilson
    Reply to this comment

    • Reply from: Mike Holt   
    James, thanks for the feedback, but the potential between the metal parts and the earth is 100% dependent on the impedance of the return path. If the grounded (neutral) conductor is open, the potential can be at a level that will kill you (over 15V in a wet location or 30V in a dry location). If the grounded (neutral) conductor is not open, then the potential will be about 1 - 3V.

    To prove this, download a voltage potential calculator from my website. Go to www.MikeHolt.com, then click on the Free Stuff link, then click on the "Touch Voltage Calculator 3-Wire" link. Play with the impedances of the grounded (neutral) conductor and grounding electrode contact resistance.
    Reply to Mike Holt

  • When attaching the Grounding conductor to the equipment do we have to be concerned with dissimilar metals being bonded together and if so do we need to seperate them from each other by anapproved method.

    Gary Eide
    Reply to this comment

  • I agree 100% with Mike's reply, but I think the issue applies both to a single service and the dual service as illustrated in Fig's 250-108 and 250-109. I still don't understand the current arrows in Fig 250-109. They imply that part of the neutral return to one source flows back thru the other source. Again, that would be insignificant unless the neutral return of one source is interrupted and then the significant current return would be between the grounding electrode and the affected source. Very little would cross flow thru the other source unless they were located very close together.

    james wilson
    Reply to this comment

    • Reply from: Mike Holt   
    James, when multiple serves are grounded to the same electrode, neutral current will flow between the two. The reason is because the neutral for both services will be bonded to the primary utility neutral. This is way beyond the scope of an email respons, and requires the reader to understand the wiring practices of the electric utility. If the neutral from one service is opened, ALL of the neutral return current from that service will flow through the grounding electrode to the other service, then to the primary neutral then from the primary neutral to the secondary neutral of the transformer supplying the service where the neutral was opened.
    Reply to Mike Holt

    Reply from: james wilson   
    I recognize the utility and secondary neutrals are bonded together and grounded, so I would agree, as long as the primary neutral connects to both source transformers and is not too long, most, if not ALL, of the return current will flow thru the grounding electrode system to the other transformer and back thru the utility neutral tie to the transformer where the service neutral opened.
    Reply to james wilson

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