Abstract
Corrosion of underground structures, applications of cathodic protection and testing procedures are discussed. Piping, cable sheaths and other structures at electric generating stations are solidly connected (for safety of personnel during power fault conditions and for reduction of the station ground resistance) to the station grounding system, which generally is an extensive grid of heavy bare copper cable and ground rods. The resulting galvanic cell is an important factor in corrosion, particularly when the structures are in low resistivity soil. Current requirements for cathodic protection range from 50 amperes to several hundred amperes, supplied from rectifiers and graphite anode-coke breeze ground beds. Choice of ground bed locations is restricted by the need of avoiding proximity to underground structures. The station grounding system generally is connected to transmission line shield wires or counterpoises, or to transmission cable sheaths which take a part of the cathodic protection current. Precaution to avoid shock hazards during power faults is necessary in isolating incoming pipe lines. Pronounced shielding effects result from large pipe lines, multiple cable runs and reinforcing steel in concrete mats. Testing procedure includes checking continuity of the grounding system and tests of current distribution, potential changes and polarization effects to determine protection. Circulating water intake screens, spillway, gates and other isolated structures require separate; additional protective installations.
