Determination of Current Required for Protection. The first step
in the design of sacrificial anode type cathodic protection systems (refer to
para. 4.2.6) is the determination of the total current required for the
system. This fixes the current to be supplied by the sacrificial anodes.
Determination of Anode Output. The output of a single anode in the
environment is determined. This may be determined by a simplified method
which uses standard factors for the type and size of anode to be used and for
the structure-to-electrolyte potential desired. Single anode output can also
be determined by using the driving potential between the anode and the
structure and the total circuit resistance. The anode-to-electrolyte
resistance is a major factor in most cases. This method is essentially
identical to the design procedure for impressed current systems.
Simplified Method for Common Situations. The formula given in
para. 4.2.5 can be used to estimate the output of zinc or magnesium anodes in
environments where the resistivity is above 500 ohm-cm. The following formula
gives a good approximation of current output in many cases and can be used to
check the results of the more detailed procedure outlined in para. 7.4.2.
current output (mA)
Determination of Output Using Anode-to-Electrolyte Resistance. As
in the case of impressed current systems, this method determines the total
resistance of the cathodic protection circuit including anode-to-electrolyte
resistance, structure-to-electrolyte resistance, and the resistance of all
electrical connections and splices. Then, using the difference between the
anode potential and the protected structure potential, the current output is
determined using Ohm's Law.
Calculation of Anode-to-Electrolyte Resistance. As in the case of
impressed current systems, the resistance between the anode and the
environment is commonly the highest resistance in the cathodic protection
circuit. This is particularly true when the anodes are located a small
distance (10 feet or less) from the structure to be protected. The
anode-to-electrolyte resistance can be calculated using simplified equations
which are adapted to the most common situations, or the more complex but more
general basic equations.