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This article was posted 01/13/2006 and is most likely outdated.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

Sample Pages

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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 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.

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