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 ML-HDBK-419A
Figure 1-4. Nomograph for Determining the Resistance to Earth
of a Single Ground Rod (1-1)
Change number of ground rods. The above example shows that fewer rods could be used and still
a.
meet the 10-ohm goal. Thus, if rock outcroppings were present at certain points around the perimeter, it would
be permissible to omit some of the rods. Since 10 ohms (the net effective resistance desired) is 31 percent of
32 ohms (the resistance of one lo-foot rod in soil of 10,000 ohm-cm), Figure 1-5 shows that as few as 3 rods
would be acceptable. On the other hand, if the soil resistivity is very high more rods will be necessary.
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