MIL-HDBK-1004/10
Table 4
Structure Potential Factor
STRUCTURE-TO-ELECTROLYTE
STRUCTURE FACTOR
STRUCTURE FACTOR
POTENTIAL (VERSUS COPPER/
(Y)
(Y)
COPPER SULFATE)
FOR MAGNESIUM
FOR ZINC
-0.70
1.14
1.60
-0.80
1.07
1.20
-0.85
1.00
1.00
-0.90
0.93
0.80
-1.00
0.79
0.40
-1.10
0.64
0.00
-1.20
0.50
0.00
Total Resistance Determination. The next step in the design of
4.2.6.2
impressed current systems is the determination of the total circuit
resistance. This value is dependent on many factors as are described in
Section 6. In the majority of systems, the resistance of the anode or group
of anodes (anode bed) is the controlling factor in the total circuit
resistance and is the factor most easily controlled by increasing the number
of anodes used. This is primarily a function of soil resistivity. The cost
of the number of anodes to be used is balanced against the cost of power
required and the cost of the rectifier which is determined by the current and
voltage requirements of the system. An anode bed resistance less than 2 ohms
is highly desirable. Also, high voltages can result in premature failure of
system components such as anode lead wires and should be avoided where
possible.
A simplified expression for estimating the resistance of standard
60-inch-long graphite anodes installed either vertically in 8- to 10-inch
diameter, 10-foot-deep backfilled holes or horizontally in 1-foot cross
section, backfilled with coke breeze, 10-foot-long trenches, 6 feet deep is
given below. When more than one anode is used in parallel to reduce circuit
resistance, the adjusting factor must be used to determine the total anode bed
resistance:
P
PF
EQUATION:
R
=
or
R
=
F
(4)
537
H
v
483
where
R
=
resistance of the vertical anode or bed
V
R
=
resistance of the horizontal anode or bed
H
P
=
electrolyte resistivity in ohms-cm
F
=
adjusting factor for multiple anodes (F = 1.0 for a single
anode and for multiple anodes refer to Table 5).
26
Previous Page
