This article was posted 11/04/2005 and is most likely outdated.

Grounding vs Bonding Online Training

Grounding vs Bonding: 250.32 Buildings or Structures Supplied by a Feeder or Branch Circuit

November 4, 2005 

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Grounding versus Bonding
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250.32 Buildings or Structures Supplied by a Feeder or Branch Circuit.

(A) Grounding Electrode. To provide a path to earth for lightning, each building or structure must have its disconnecting means [225.31] grounded (earthed) to one of the following electrodes [250.50 and 250.52(A)]:
  • Underground metal water pipe [250.52(A)(1)]
  • Metal frame of the building or structure [250.52(A)(2)]
  • Concrete-encased steel [250.52(A)(3)]
  • Ground ring [250.52(A)(4)]

Author’s Comment: See Article 100 for the definitions of “Building” and “Structure.”
Where none of the above grounding electrodes are available at a building or structure, then one or more of the following must be used:
  • Ground rod [250.52(A)(5)]
  • Metal underground systems [250.52(A)(7)]
Author’s Comment: Grounding the building or structure disconnecting means to the earth:
• Is intended to limit elevated voltages on the metal parts from lightning [250.4(A)(1)]. Figure 250–81
• It doesn’t serve as a low-impedance fault-current path to clear ground faults. In fact, the Code prohibits the use of the earth as the sole return path since it’s such a poor conductor of current [250.4(A)(5) and 250.4(B)(4)].
• It doesn’t protect electrical or electronic equipment from lightning voltage transients.
Exception: A grounding electrode isn’t required where only one branch circuit serves the building or structure. For the purpose of this section, a multiwire branch circuit is considered to be a single branch circuit. Figure 250–82

(B) Bonding Requirements. To quickly clear a ground fault and remove dangerous voltage from metal parts, the building or structure disconnecting means must be grounded (bonded) to an effective ground-fault current path in accordance with (1) or (2) [250.4(A)(3)]. Figure 250–83

(1) Equipment Grounding (Bonding) Conductor. The building or structure disconnecting means can be bonded to an equipment grounding (bonding) conductor, as described in 250.118, installed with the feeder conductors. Figure 250–84

Figure 250–81
(Click on image to enlarge)

Figure 250–82
(Click on image to enlarge)

Figure 250–83
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Figure 250–84
(Click on image to enlarge)


Grounding versus Bonding Library - DVDs
Grounding and bonding problems are at epidemic levels. Surveys repeatedly show a high percentage of power quality problems are due to poor grounding and bonding. Electrical theory has been applied to this difficult to understand Article, making it easier for students to grasp the concepts of grounding and bonding. Additionally, Mike has color coded the graphics so you can easily differentiate between grounding and bonding. For a limited time, you can order the Entire Grounding versus Bonding Library including the textbook, 2 videos, MP3 Audio CD and the Online Program for this great price. You save over $200!

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  • If grounding at the seperate building is mostly for lightning events, how does lightning know not to strike a building that is fed by only one branch circuit?

    Don Haskin
    Reply to this comment

  • So how does the temporary service with one or two rods expected to clear a fault? If the first rod on a temporary service can’t meet the 25 ohm rule why add a second rod if it is only to protect against lighting, would two or more rods really make a difference?

    Reply to this comment

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