TM 5-811-7
(6) Calculate the number of anodes required to meet maximum anode groundbed resistance
requirements from equation 2-11:
(D K)
DP
Ra '
%
LN
S
DK
N'
D
L (Ra &
P
S
2000 ohm&cm (0.016S)
N'
(2000 ohm/cm (0.
7 ft (20 ohm &
20 ft
(Values for K and P
from tables 2-6 and
2-7, respectively.)
N = 2.75 . 3 anodes.
(7) Select the number of anodes to be used. Since the last calculation resulted in the largest number of
anodes, it will be used. The groundbed resistance, Ra, using three anodes, would equal 1.86 ohms; to insure
compliance with the manufacturer's limitations, four anodes will be used.
(8) Select an area for anode bed placement. The area of lowest resistivity will be used, which is 100
feet from the pipeline.
(9) Determine the total circuit resistance.
(a) Calculate the anode groundbed resistance using equation 2-11:
(D K)
DP
Ra '
%
LN
S
2000 ohm&cm (0.0165)
(2000
Ra '
%
(4 anodes)(7 ft)
(Values for K and P
are from tables 2-6 and
2-7, respectively.)
Ra = 1.46 ohm.
(b) Calculate the groundbed resistance for a 50-foot header cable using equation 2-12. The resistance
specified by the manufacturer is 0.0159 ohm per 100 ft of No.2 AWG cable:
Rw = (ohms/ft)(L)
Rw = (0.0159 ohm/100 ft)(500 ft) = 0.0795 ohm.
(c) Calculate the structure-to-electrolyte resistance from equation 2-14:
R
Rc '
N
2500 ohm/sq ft
Rc '
11,800 sq ft
= 0.212 ohm
D-3
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