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

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