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ETL 1110-9-10(FR)
5 Jan 91
This is the approximate resistance based on the law for
parallel circuits. (When equal resistance values are
joined together in a parallel circuit, the total resis-
tance value of the circuit is approximately equal to a
single resistance value divided by the number of resis-
tance values.)
To calculate the true anode resistance for five anodes,
we must perform the calculation from equation 1-11.
CC
=
20 ft (The spacing must be chosen
based on previous experience to solve
the equation.)
The spacing can
typically be from 10 to 50 ft.
For
this example, we will try 20 ft:
RN
=
8.53
+
2000 x 0.00268
5
20
RN
=
1.706 + 0.268
RN
=
1.974 ohms
This is within specification, but very close to
exceeding maximum specified circuit resistance.
Try
six anodes to allow for variations in soil resistivity
and to allow for wire and pipe-to-earth resistance.
Repeat calculation from equation 1-li,
Where:
N
=
6 anodes
PF
=
0.00252, paralleling factor from table
3-5
RN
-
8.53
+
2000 x 0.00252
6
20
RN
=
1.67 ohms
6)
Based on the previous selection of six anodes for the
number of anodes to be used,
the total circuit
resistance must be determined.
7)
Select an area for anode bed placement.
Here, the
selected area is 100 ft from the pipeline for improved
current distribution. The anode bed location for this
type design must be far enough away from the structure
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