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)]
Where none of the above grounding electrodes are available at a building or structure, then one or more of the following must be used: See Article 100 for the definitions of a€śBuildinga€ť and a€śStructure.a€ť
- Ground rod [250.52(A)(5)]
- Metal underground systems [250.52(A)(7)]
Grounding the building or structure disconnecting means to the earth:
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
a€˘ Is intended to limit elevated voltages on the metal parts from lightning [250.4(A)(1)]. Figure 250.81
a€˘ 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)].
a€˘ It doesn't protect electrical or electronic equipment from lightning voltage transients.
(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
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