The design of any current-carrying cable system involves certain basic factors, which may be listed briefly as follows:
1. Ability to meet mechanical stresses and other environmental requirements; 2. Ability to meet voltage attenuation limits; 3. Ability of the insulation to withstand applied voltages; 4. Ability to meet limitations on current due to temperature and insulation stability; 5. Conformance to Kelvin’s Economic Law; 6. Conformance to legal codes, safety rules, and similar requirements.
The importance of each of these factors is discussed in the light of cathodic protection requirements and installations. Emphasis is placed on cable systems which connect the direct current output terminals of the rectifier to ground beds and to the protected metal structures.
Frequently engineers disregard one or more of the six factors in designing cathodic protection cable connections. For example, the tendency is to design to fit the current-carrying capacity of Cables as set forth in the various handbooks and to neglect a consideration of Kelvin’s Law, which shows that the choice should be toward much larger cables and lower current densities. Also because cables buried underground are subject to concentrated electrochemical attack the type of insulation is important. Ordinary insulations may not be sufficient and normal voltage ratings may be unimportant.
A method of determining annual fixed costs of cables is outlined and charts are given which enable the engineer to balance fixed charges against cost of losses and thus select the optimum cable size based on economy. Consideration also is given to the effect on the economic picture of selecting cable sizes somewhat off of the theoretical optimum.
