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DOE-HDBK-1092-98
complexity and number of interconnections of most grounding systems, the NEC allows modifications
of the grounding system and connections in order to address such problems. Those permitted:
1) Arrangement to prevent objectionable current. Grounding of electric systems, circuit conductors,
surge arresters, and conductive noncurrent-carrying materials and equipment shall be installed
and arranged in a manner that will prevent an objectionable flow of current over the grounding
conductors or grounding paths.
Use of a single-point grounding system, as well as meeting the other requirements of Article 250,
will usually overcome problems.
2) Alterations to stop objectionable current. If the use of multiple grounding connections results in
an objectionable flow of current, one or more of the following alterations are permitted to be
made, provided that the requirements of NEC Section 250-51, are met. Such permitted alterations
are:
1. Discontinue one or more, but not all, of the grounding connections;
2. Change the locations of the grounding connections;
3. Interrupt the continuity of the conductor or conductive path interconnecting the grounding
connections; and/or
4. Take other suitable remedial action satisfactory to the authority having jurisdiction.
3) Temporary currents resulting from accidental conditions, such as ground-fault currents, that
occur only while the grounding conductors are performing their intended protective functions
shall not be classified as objectionable. This does not prohibit changes in the system to correct
excessive current during a fault condition.
4) Limitations of permissible alterations. The intent of NEC Section 250-21 is not that of permitting
electronic equipment to be operated on AC systems or branch circuits that are not grounded as
required by Article 250. Currents that introduce noise or data errors in electronic equipment are
not considered to be the objectionable currents addressed in this Section.
Voltage differences do exist because impedances to ground are not equal throughout a grounding
system due to variations of the resistance of the earth, improper connections, or other problems.
Even though voltage differences allow unwanted currents to flow in the grounding conductors,
and induced noise may travel over this path, it is not to be used as a reason to disconnect the
grounding connections.
9.3.2 EQUIPMENT GROUNDING CONDUCTOR
The equipment grounding conductor of a power-supply cord or interconnecting cable should be at
least the size of the largest circuit conductor in the power-supply cord or interconnecting cable.
(ANSI/ISA-S82.01-1988 - Section 9.5.4.2)
9-2


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