250.24 Grounding and Bonding at Service Equipment Continued
(C) Grounded Neutral Conductor Required. Because electric utilities aren't required to provide an equipment grounding (bonding) conductor to service equipment, a grounded neutral service conductor must be run from the electric utility transformer to each service disconnecting means and it must be bonded to each service disconnecting means as required by 250.24(B) [250.130(A)]. Figures 250.54 and 250.55
The grounded neutral service conductor provides the effective ground-fault current path to the power source to ensure that dangerous voltage from a ground fault will be quickly removed by opening the circuit-protection device [250.4(A)(3) and 250.4(A)(5)].
DANGER: Because the resistance of the earth, when used as a ground-fault current path is so great, often as much as 500 ohms, very little fault current returns to the power source if it is the only fault-current return path. The result.the circuit overcurrent protection device will not open and clear the ground fault and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized by circuit voltage. Figure 250.57
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