New Non-Destructive Condition Monitoring Techniques for On-Site Assessment of Low-Voltage Cables

The number of techniques available for on-site monitoring of low-voltage cables at nuclear plants is limited because of the requirement from station personnel to use non-destructive and non-intrusive techniques. This paper summarizes the results obtained to date using two new non-destructive methods. The first technique is based on the indentation of the cable insulation or cable jacket material and the study of post-indentation parameters to characterize the visco-elastic properties of the material tested. The novelty of the technique consists of measuring the time taken by the polymeric material to recover a set portion of the initial deformation and using this duration as a material degradation indicator. The technique can be used locally on the insulation of hook-up cables, on the insulation at the termination of jacketed cables, or directly on cable jackets. The second technique is based on the measurement of electrical dissipation factors (or tan delta) in the insulating material. A broadband frequency tan delta analyser was used to measure electrical dissipation factors at various frequencies, and identify the frequency ranges showing increased sensitivity to cable degradation. Specific electrodes and measurement methods were developed for practical on-site condition monitoring work. The measurement of electrical dissipation factors can be used to assess the local degradation of cable insulation in hook-up cables and global degradation of multi-pair conductor cables. When used on multi-pair conductor cables, the technique presents the advantage of providing a global indication of the cable condition.