Electrical Fundamentals and Circuit Theory for Cathodic Protection
The CP loop as a DC circuit
Cathodic protection is applied Ohm's law. Every CP circuit - galvanic or impressed current - consists of a source of driving voltage, an anode (current leaves into the electrolyte), the electrolyte (soil or water, which has resistance), the structure (current returns to the source), and metallic cables. Current flows only when the circuit is closed, and the amount is set by the total resistance: I = ΔE / R_total, where ΔE is the difference between the anode's open-circuit potential and the polarized potential of the structure.
You must be comfortable with series and parallel resistance, Kirchhoff's voltage law (the driving voltage equals the sum of all IR drops around the loop), and the difference between a potential (measured in volts, always measured between two points) and a current (amperes, measured through a conductor). Anodes in parallel do not simply divide resistance by n; mutual interference between adjacent anodes raises the total, so practical designs use a spacing factor of roughly 1.2–1.5 on the parallel value.
Soil resistivity is the dominant variable. Resistivity is quoted in ohm-cm or ohm-m; 1 ohm-m = 100 ohm-cm, a conversion that appears in nearly every calculation question. The Wenner four-pin method gives ρ (ohm-cm) = 2πaR with spacing a in cm and meter reading R in ohms.
Traps
Confusing potential with current; forgetting to convert ohm-m to ohm-cm; assuming the pipe-to-soil voltmeter reading is a pure structure potential when it also contains IR drop; treating cable resistance as negligible in long runs.
Worked example
A magnesium anode reads −1.55 V versus copper-copper sulfate; the pipe is polarized to −0.90 V. Driving voltage = 0.65 V. With anode-to-earth resistance of 6.0 Ω and 0.5 Ω of cable/soil path, I = 0.65 / 6.5 = 0.10 A (100 mA).