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Illustrated Changes to the NEC, 2005 Edition - Part 1

Article 250

Grounding and Bonding

250.4 General Requirements for Grounding and Bonding

The last sentence was revised to make it clear that the earth isnít 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.

Authorís Comment: A ground fault cannot be cleared on an ungrounded system because thereís 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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