MIL-HDBK-1004/10
Section 6:
IMPRESSED CURRENT SYSTEM DESIGN
Advantages of Impressed Current Cathodic Protection Systems. The
6.1
primary advantage of impressed current cathodic protection systems over
sacrificial anode cathodic protection systems is that the driving potential of
the impressed current systems is not limited by the corrosion potential of an
active metal. The ability to select appropriate driving potentials, and to
adjust the driving potential after system installation, gives the designer and
operator of impressed current cathodic protection systems additional
flexibility to compensate for changing environmental conditions. The primary
advantage of this variable driving potential in the design of impressed
current cathodic protection systems is the ability to select the location of
the anode beds for an optimum distribution of protective current with a
minimum of interference. The variable driving potential available in
impressed current systems also allows the protection of structures in high
resistivity environments where the output of sacrificial anodes is severely
limited. The primary operational benefit of variable driving potential is the
ability to adjust the system for changes in soil resistivity, anode condition,
structure surface (coating) condition and additions to the structure.
Determination of Circuit Resistance. In the design of impressed
6.2
current cathodic protection systems (refer to para. 4.2.6), the first step is
the determination of the total current required for the system. This fixes
the output current required for the system power supply. The next step is the
determination of the required output or driving potential that will be
required. As the output current is fixed, the required driving potential will
be determined by the total circuit resistance and the back potential offered
by the structure-to-anode potential. The equivalent circuit is shown in
Figure 31. In most impressed current systems, the major factor in the
determination of the total circuit resistance is the anode-to-electrolyte
resistance.
Anode-to-Electrolyte Resistance. Also known as "ground bed
6.2.1
resistance," this is often the highest resistance in the impressed current
cathodic protection system circuit.
Effect on System Design and Performance. As shown in Figure 31,
6.2.1.1
the anode-to-electrolyte resistance, if high, is the most important factor in
the determination of the driving potential required to provide the current
required for effective cathodic protection in impressed current cathodic
protection systems. Anode-to-electrolyte resistance can be varied within wide
limits by the use of different sized anodes and the use of multiple anodes.
The lowest anode-to-electrolyte resistance commensurate with total system cost
is desirable since it will reduce the power costs by lowering the output
potential of the power supply. This lower power supply output potential also
results in higher reliability for other system components, particularly the
insulation on cables, splices, and connections. In general, anode bed
resistances below 2 ohms are desirable.
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