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Grounding versus Bonding



250.54 Supplementary Electrodes.

A supplementary electrode is an electrode that is not required by the NEC. This electrode is not required to be bonded to the building or structure grounding electrode (earthing) system. Figure 250–103

The supplementary electrode is not required to be sized to 250.66, and it is not required to comply with the 25 ohm resistance requirement of 250.56. Figure 250–104

The earth cannot be used as an effective ground-fault current path as required by 250.4(A)(4).

Author’s Comment: Because the resistance of the earth is so high, very little current will return 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.

CAUTION: The requirements contained in 250.54 for a “supplementary” electrode should not be confused with the requirements contained in 250.53(D)(2) for the underground metal water pipe “supplemental” electrode.

Author’s Comment: Typically, a supplementary electrode serves no useful purpose, and in some cases it may actually create equipment or performance failure. However, in a few cases, the supplementary electrode is used to help reduce static charges on metal parts. For information on protection against static electricity in hazardous (classified) locations, see NFPA 77, Recommended Practice on Static Electricity.

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