Sections
250.4 General Requirements for Grounding and Bonding Continued
(A) Solidly Grounded Systems.
(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
25021
Authors Comment: The phrase likely to become energized is subject to interpretation by the authority having jurisdiction.
(5) Effective Ground-Fault Current Path. To remove dangerous voltage from ground faults, metal parts of electrical raceways, cables, enclosures, and equipment must be bonded to an effective ground-fault current path with an equipment grounding (bonding) conductor of a type specified in 250.118. Figure 25022
Authors Comment: 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)]. Figure 25023
The earth is not considered an effective ground-fault current path.
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 25? ground rod would only be 4.8A. Figure 25024
I = E/R
I = 120V/25?
I = 4.8A
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Figure 250–21
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Figure 250–22
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Figure 250–23
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Figure 250–24
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