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ETL 1110-9-10(FR)
5 Jan 91
RA
p PF
%
RN '
N
Cc
Where:
RN =
Groundbed-to-earth resistance
RA
=
21.06 ohms (Resistance of a single
anode-to-earth
from
the
previous
calculation)
p
=
4500 ohm-cm (Soil resistivity from
item 1 of paragraph 2-3a)
CC
=
10 ft (Estimated approximate spacing
of anodes)
PF
=
0.00212
(Paralleling
factor
from
table 3-5; assume 9 anodes; reasoning
is the same as in step 5, paragraph
2-2b)
N
=
9 anodes (Estimated number of anodes
required; from step 3, paragraph
2-2b)
RN
=
21.06
+
4500 x 0.00212
9
10
RN
=
3.29 ohms.
Resistance is too high. Additional calculations using
an increasing number of anodes (i.e., 11, 12, 13, 14,
etc.) have to be made; these calculations show that
fourteen
anodes will yield
a groundbed-to-earth
resistance of 2.26 ohms.
6)
Select the number of anodes to be used.
The numbers
determined are:
For life = nine anodes maximum required
For resistance = fourteen anodes maximum required
Therefore, the larger number of anodes, fourteen, is
selected.
7)
Determine the
total
circuit
resistance
(RT)
from
equation 1-3:
36
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