When an external source of power with associated ground bed is used to provide cathodic protection to an underground structure, the possibility is presented of creating accelerated corrosion on neighboring structures which are not a part of the cathodic protection system due to the flow of interference currents on these neighboring structurs. This article describes a method of obtaining data which will permit the extent of interference to be determined before permanent cathodic protection installations are made. And, if the need for a bond between structures to prevent interference is indicated the bond resistance can be calculated on an engineering basis. Data obtained by the use of this method are of engineering accuracy which can be obtained in minimum of field time, thus eliminating cut and try methods and the need for extensive surveys made before and after permanent installations are completed.
The method was evolved approximately 20 years ago by the late Dr. J. M. Pearson of the Sun Oil Co. in connection with his development of engineering methods for the investigation and control of stray currents on underground structures. Since cathodic protection interference currents are of the same nature as stray currents with the exception that they are controllable and of steady value, the method has been found to be very useful in cathodic protection work. In developing this method Dr. Pearson based his work upon the fact that the resistance through the earth between structures and the resistance between a structure and earth in any given location is constant and thus can be treated as such mathematically. Upon this basis underground structures can be considered as an electrical network permitting solution of problems relating to cathodic protection, interference, etc., by measuring the network constants at the proper location. Simple mathematical treatment of the data thus obtained will provide required solutions most easily and quickly.
A brief discussion is given concerning the proper location of a ground bed in order to minimize the effect of interference on foreign structures. It is explained that when the ground bed is improperly located, interference currents may be set up on the foreign structure which cannot be eliminated by the use of a bond between structures. There is generally a location at which a ground bed can be placed that will result in minimum interference current flow on foreign structures and which is in the direction permitting resolution by bonding.
