250.4 General Requirements for Grounding and Bonding Continued
(4) Bonding Conductive Materials to an Effective Ground-Fault Current Path. To remove dangerous voltage from ground faults, electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, and other metal-piping systems, as well as exposed structural steel members that are likely to become energized, must be bonded to an effective ground-fault current path. Figure 250.21 The phrase a€ślikely to become energizeda€ť is subject to interpretation by the authority having jurisdiction.
(5) Effective Ground-Fault Current Path. Metal raceways, cables, enclosures, and equipment, as well as other electrically conductive materials that are likely to become energized, must be installed in a manner that creates a permanent, low-impedance fault-current path that facilitates the operation of the circuit overcurrent device. Figure 250.22
Figure 250.23The earth is not considered an effective ground-fault current path.
To assure a low-impedance ground-fault current path, all circuit conductors must be grouped together in the same raceway, cable, or trench [300.3(B), 300.5(I), and 300.20(A)].
DANGER: Because the resistance of the earth is so high, very little current returns to the electrical supply source via the earth. If a ground rod is used as the ground-fault current path, the circuit overcurrent protection device will not open and metal parts will remain energized.
For example, the maximum current flow to the power supply from a 120V ground fault to a 25Ω ground rod would only be 4.8A. Figure 250.24
I = E/R
I = 120V/25Ω
I = 4.8A
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