Impressed Current Systems, Rectifiers, and Groundbeds
The system
An impressed current system uses an external DC source - nearly always a rectifier fed from AC - to drive current from a groundbed through the electrolyte to the structure. Because the driving voltage can be tens of volts, groundbed resistance is less limiting and the system can protect large, poorly coated structures in high-resistivity soil.
Rectifiers
A rectifier contains a transformer, a tap or auto-transformer for coarse voltage adjustment, and a rectifying stack of diodes (silicon, or older selenium). You should know how to confirm AC input and DC output with a meter, how tap changes shift output, what a blown fuse, failed diode, or open DC circuit looks like, and that rectifier amp-hours are a useful record of performance. Ripple, efficiency, and constant-current versus constant-voltage control appear in troubleshooting questions.
Groundbeds
Anode materials include high-silicon cast iron, graphite, mixed metal oxide on titanium, and magnetite, usually in a column of metallurgical coke breeze that lowers resistance and spreads the anodic reaction. Configurations are vertical, horizontal, and deep well. Groundbed resistance uses the Dwight relationship plus a spacing factor for multiple anodes; deep wells need venting for the gases generated at the anodes.
Traps
Assuming rectifier current equals current reaching the pipe (leakage and foreign structures intercept it); ignoring back-EMF and cable resistance when computing resistance from volts and amps; sizing a groundbed without accounting for drying or coke-breeze consumption.
Worked example
A rectifier reads 24 V and 10 A. Total circuit resistance ≈ 24/10 = 2.4 Ω. If the pipe-to-soil back-EMF is about 1 V, the groundbed plus cable resistance is roughly 2.3 Ω, and adding an anode to the bed should raise current noticeably.