Grounding and Bonding
250.4 General Requirements for Grounding and Bonding
The last sentence was revised to make it clear that the earth isn't to be used as the effective ground-fault current path needed to facilitate the opening of the circuit protection device in the event of a ground fault.
(B) Ungrounded System
(4) Path for Fault Current. Electrical equipment, wiring, and other electrically conductive material likely to become energized must be installed in a manner that creates a permanent, low-impedance circuit from any point on the wiring system to the electrical supply source neutral to facilitate the operation of overcurrent devices should a second ground fault occur.
A ground fault cannot be cleared on an ungrounded system because there's no low-impedance fault-current path to the source neutral. However, all metal parts of an ungrounded system must be bonded together so that in the event of a second ground fault (line-to-line fault), the bonding path will provide a low-impedance path for fault current so that the circuit protection device will open to clear the fault [250.4(B)(4)].
The earth is not considered an effective ground-fault current path.
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