The following information and criteria are from the 1983 revision of NACE
"Voltage measurements on pipelines are to be made with the
reference electrode located on the electrolyte surface as close as
practicable to the pipeline. Such measurements on all other structures a
to be made with the reference electrode positioned as close as feasible t
the structure surface being investigated. Consideration should be given
voltage (IR) drops other than those across the structure-electrolyte
boundary, the presence of dissimilar metals, and the influence of other
structures for valid interpretation of voltage measurements."
"No one criterion for evaluating the effectiveness of cathodic
protection has proved to be satisfactory for all conditions. Often a
combination of criteria is needed for a single structure."