IN AREAS where grounded d-c electric railways are operated, some of the return current strays from the rails to underground metallic structures, such as lead covered telephone cables. These parallel structures provide low resistance paths for the current to return toward its source of supply, near which point it leaves the cables. This interchange of current from one grounded structure to another presents serious anodic corrosion problems.
For many years protection from this type of corrosion has been provided by electrolysis drains, which consist of low resistance copper conductors connecting cables to the negative return of the railyway system. This method of protection, where several structures are drained to a common point, has a disadvantage in that various structures have different potential gradients, which result in differences of potential between the various structures at points remote from the substation.
Figure 1 illustrates a typical case, where a potential difference exists...
