Cathodic Protection Current and Voltage Requirements in Plant
The electrical current required to achieve protection shall be calculated using the results of a current requirement test. If a current requirement test is used, it can only be performed on existing structures. Initial and mean current requirements shall be taken into account when designing the cathodic protection system. The impact of underground concrete structures and electrical system copper grounding grid on stray currents and current distribution shall be considered in the design.
The number of anodes in a particular cathodic protection design shall be determined by total current requirements of the structures to be protected and the optimum current density of the anode material selected. The design shall address the material insulation and installations methods of the following structures:
a. Tanks
b. Underground pipes
c. Buried metals structures
d. Electrical grounding system and electrical underground metals conduit
e. Existing cathodic protection system
To obtain the voltage necessary to drive the required amount of current, the resistance of the cathodic protection system can be estimated from the current requirement tests, soil resistivity tests, and, if applicable, the existing impressed current systems similar to the one to be installed.
The anode-to-earth resistance is generally the dominant part of the overall circuit resistance in an impressed current system. All impressed current cathodic protection designs shall include system circuit resistance calculations. Rectifiers with 50% excess capacity shall be selected to allow for adjustments during the life of the cathodic protection system and to prevent damage due to voltage overloads.