In practice, a rectifier or other power supply is normally selected with a
capacity in excess of the required voltage and current in order to allow for
deterioration of coatings on the system, additions to the system, aging of the
power supply, and inaccuracies in the system design.
More precise calculations for the design of impressed current
cathodic protection systems are given in Section 6. Examples of typical
designs using both sacrificial anode and impressed current systems are given
in Section 8.
Analysis of Design Factors. The following factors should always be
analyzed when designing either type of cathodic protection system:
a) Anode-to-electrolyte resistance (sacrificial anode output).
of a single anode, the effects of anode configuration and spacing, the effects
of anode orientation, and the location of the anodes with respect to both the
structure being protected and other metallic structures in the area.
b) Weight of anode to give the required anode life. This is most
important for both types of systems. One lb/A yr for high silicon cast iron
anodes and 2.5 lbs/A yr for graphite are typical consumption rates for
impressed current anodes.
c) The use of special backfill surrounding the anodes. This is
usually justified by increased anode efficiency and should be used unless it
is shown to not be economically justified. Backfill is not required when
anodes are hung in water or installed at the bottom of bodies of water.
d) Effect of seasonal variations in electrolyte resistivity from
variations in soil conductivity due to moisture or in seawater due to runoff.
e) Cable resistance. In impressed current systems, the size of
cable used should be determined based upon the economic analysis of cable size
given in Section 6.
Vulnerability to physical damage.
Location of structure to structure bonds and insulating joints.
h) Number, type, and location of test stations for initial
adjustment and periodic inspections for maintenance.
Availability and cost of maintenance.
Determination of Field Data. Data required for the design of
cathodic protection systems is normally obtained through a field survey. In
addition to field measurements, historical data such as soil resistivity
measurements, as well as plans, drawings, etc., concerning the site and other
structures located at or near the site of the system to be designed should be
reviewed. In general, however, specific field determinations of several
parameters will need to be determined in order to design an effective cathodic