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This article was posted 01/13/2006 and is most likely outdated.

250.58 Common Grounding (Earthing) Electrode.

January 13, 2006

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Grounding versus Bonding

PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR

Page 8 of 14

Sections

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 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 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 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.

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