Consider the induced interference situation of Figure 11, with a zoom around the protected tank in Figure 11b. A structure A that passes close to the groundbed does not always approach the cathode (protected tank) but it may pass close to another foreign structure B that, itself, passes close to the cathode. In this case, current picked up by the first foreign structure A in the neighbourhood of the groundbed, will be transferred at the crossing to the second foreign body B and discharged by the latter one, close to the cathode.
This is called induced interference since at the crossing of the two foreign bodies A and B, the current will flow from one to the other. Note that this point might be situated very far from the cathodic protection system.
In Figure 12, the current density pattern along the second foreign pipe B is plotted. Note the large peak at about the crossing with the other foreign pipe, where a substantial amount of current enters the pipe B with a high current density. A little bit further, at the shortest distance with the protected tank, this current is discharged to the tank, with a lower current density. Due to the fact that the point of induction can be situated far away from the cathodic protection system, this kind of interference is very difficult to detect. The best prevention techniques are those used for the cathodic interference cases.
See also: Anode interference, Cathode Interference, Combined Interference, DC Traction Interference, FPSO ICCP , Induced Interference
Study and Evaluation of Stray Current Influences on Cathodic Protection Systems of Buried Pipelines, L. Bortels, ELSYCA - Kranenberg 6 - 1731 - BELGIUM, ELSYCA