Complications in the Analysis of the Frequency Spectrum of Structure-To-Soil Potentials for Assessment of Stray Currents

Abstract The subject of this paper is complications in the analysis of the frequency spectrums of structure-to-soil potentials as it relates to the assessment of cathodic protection (CP) criteria, which, as per AMPP SP0169, require the interruption of all CP current influences for measurement of polarized potentials. The subject method requires collecting structure-to-soil voltage readings at high sample rates, and applying the Fourier Transform methodology to acquire the signal magnitude in various frequency bands.1 The measured magnitudes in the 120 Hz or 360 Hz bands during the ‘off’ portion of a CP interruption cycle are then interpreted to assess external influence on the structure. This paper evaluates the effectiveness of this methodology in the presence of multiple CP current sources, AC power harmonics, and reactance in CP system components. It is demonstrated that the presence of interfering signals within the same frequency can alter the spectral readings in unpredictable ways. The ability of this methodology to produce reliable information regarding the presence of CP currents during the ‘off’ portion of the interruption cycle is brought into question.