Grounding and Bonding
250.4 General Requirements for Grounding and Bonding
Change clarifies that swift operation of an overcurrent protection device in the presence of a ground-fault depends upon the existence of an effective ground fault current path from the point of the fault to the power-supply neutral.
The last sentence was revised to make it clear that the earth cannot be used as the effective ground-fault current path. It will not facilitate the opening of the circuit protection device from a ground fault.
(A) Solidly-Grounded Systems.
(5) Effective Ground-Fault Current Path. Electrical raceways, cables, enclosures and equipment, as well as other electrically conductive material that are “likely to become energized,” must be installed in a manner that creates a permanent, low-impedance path that facilitates the operation of the circuit overcurrent device or ground-fault detector for high-impedance grounded systems. Figure 250-3
The effective ground-fault current path must be capable of safely carrying the maximum fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source neutral [110.10].
Clearing ground faults (line-to-case faults) is accomplished by bonding all metal parts of electrical equipment and conductive material likely to become energized to the power-supply grounded neutral terminal.
Another factor necessary to help ensure an effective ground-fault current path is that all circuit conductors (ungrounded, grounded and the equipment grounding (bonding) conductor) must be grouped together in the same raceway, cable, or trench [300.3(B), 300.5(I), and 300.20(A)]. Figure 250-4
The earth is not considered an effective ground-fault current path. Figure 250-5
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