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Grounding versus Bonding

PART VII.

Sections

250.142 Use of Grounded Neutral Conductor for Equipment Grounding (Bonding).

Authorís Comment: To remove dangerous voltage on metal parts from a ground fault, the metal parts of electrical raceways, cables, enclosures, and equipment must be bonded to an effective ground-fault current path in accordance with 250.4(A)(3).
(A) Supply Side of Service Equipment. A grounded neutral conductor can be used as the effective ground-fault current path for metal parts of equipment, raceways, and other enclosures.

(1) Service Equipment. Because an equipment grounding (bonding) conductor isnít run from the utility to electrical services, the grounded neutral service conductor can serve as the effective ground-fault current path to the utility power source. Figure 250Ė183

Authorís Comment: The effective ground-fault current path for service equipment is provided by the installation of the main bonding jumper at service equipment in accordance with 250.24(B) [250.28]. Figure 250Ė184
(2) Separate Buildings and Structures. Where no equipment grounding (bonding) conductor is run to a building or structure disconnect, the grounded neutral conductor can serve as the effective ground-fault current path to the power source.

Authorís Comment: This is accomplished by bonding the grounded neutral conductor to the equipment grounding (bonding) conductor at the separate building or structure building disconnecting means in accordance with 250.32(B)(2).

CAUTION: Using the grounded neutral conductor as the effective ground-fault current path poses potentially dangerous consequences and should only be done after careful consideration.

Authorís Comment: The safest practice is to install an equipment grounding (bonding) conductor with the feeder conductors to the building or structure to serve as the effective ground-fault current path, as provided by 250.32(B)(1).
(3) Separately Derived Systems. The effective ground-fault current path is established for a separately derived system when the system bonding jumper is installed between the metal enclosure of the separately derived system and the grounded neutral terminal in accordance with 250.30(A)(1).

DANGER: Failure to install the system bonding jumper as required by 250.30(A)(1) will create a condition where dangerous touch voltage from a ground fault will remain on the metal parts of electrical equipment.
(B) Load-Side Equipment. To prevent dangerous voltage on metal parts, the grounded neutral conductor must not be bonded to the equipment grounding (bonding) conductor on the load side of service equipment, except as permitted by 250.142(A).

Exception 1: The grounded neutral conductor can serve as the effective ground-fault current path for existing ranges, dryers, and ovens [250.140 Ex].

Exception 2: The grounded neutral conductor can be bonded to the meter enclosure on the load side of the service disconnecting means if:
Figure 250Ė185

(a) No service ground-fault protection is installed,

(b) Meter enclosures are located immediately adjacent to the service disconnecting means, and

(c) The grounded neutral conductor is sized not smaller than specified in Table 250.122.


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