A capacitive test method for cable insulation degradation assessment

Integrity of nuclear power plant cable insulation is crucial for successful reactor shut-down following a loss-of-coolant event in containment, for example. Present acceptance criteria for cable insulation in NPP relates to destructive, mechanical, elongation-at-break (EAB) measurements, requiring EAB > 50 % for acceptance. Through extensive correlation studies, the EAB acceptance criterion has been previously related to measurements of indenter modulus (IM), which can be performed on cables in situ and are nondestructive. In this paper, measurements of IM are compared with values of dissipation factor measured by an interdigital capacitive clamp sensor. Measurements are made on cables insulated with flame-retardant ethylene-propylene rubber. Cable samples are prepared by accelerated aging in various thermal and ionizing radiation environments. The measured IM and dielectric loss tangent show a high degree of correlation, whereas the dielectric measurements are easier and quicker to perform than the indenter measurements.