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| Figure
1 |
| Typical example of soil resistance variation with depth |
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IAEI
News>Issue Listing>January/February 2003 >Other Code—NESC Substation Grounding — Part 3
Other Code 
NESC Substation Grounding — Part 3 |
After completing the soil resistance measurements at the proposed substation site, the next step is the development of a mathematical equivalent soil model that is a good approximation of the actual soil resistance data. The most common models are the uniform soil model and the two-layer soil model. |
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Interpretation of Soil
Measurements
After completing the soil resistance measurements at the
proposed substation site, the next step is the
development of a mathematical equivalent soil model that
is a good approximation of the actual soil resistance
data. The most common models are the uniform soil model
and the two-layer soil model. The uniform soil model
should only be used when there is very little variation
in the average resistivity with position and depth. The
average resistivity referred to in IEEE Standard 81-1983
is the same as the apparent resistivity referred to in
IEEE Standard 80-2000. Further discussion of the uniform
soil model can be found in Clause 13.4.1, page 56 of
Std. 80. More commonly, the resistivity varies greatly
with depth. The following plot of soil resistivity
versus rod spacing is a more typical example of soil
resistance variation with depth. [See Figure
1]
There are three ways to develop the two-layer model.
Clause 13.4.2, page 57 of Std 80, describes a graphical
method for approximating a two-layer soil model.
Appendix B, page 38 of Std 81, presents a mathematical
method for determining the two-layer soil model. The
mathematical method described in Appendix B is the basis
for several computer programs that can be used to
develop a two-layer soil model that best fits the
resistance data. One such program is available from the
Electric Power Research Institute.
Design Procedure
Clause 16.4, page 88 of Std 80, presents a step-by-step
procedure for the design of a substation grounding
system. The forty design parameters, which must be
calculated to verify the design meets the safety
requirements of Clause 4, are listed in Table 12, page
89 of Std 80. Annex B, page 129 of Std 80, presents
sample calculations. Computer programs based upon Std 80
and 81 are also available to do most of the
calculations.
If you have general questions
about the NESC®, please call me at 302-454-4910 or
e-mail me at dave.young@conectiv.com.
National Electrical Safety Code® and NESC® are
registered trademarks of the Institute of Electrical and
Electronics Engineers.
Dave Young is a senior engineer with Conectiv Power
Delivery of Wilmington, Delaware, where he has been
working with and teaching the NESC® for over 31 years.
He is a member of the NESC Interpretations Subcommittee
and the NESC Overhead Line Clearances Subcommittee 4.
Dave is also vice-chairman of the Delmarva Division of
the IAEI Chesapeake Chapter.
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