The basic cable failure modes resulting from exposure to stressors include: short circuits between cable conductors, short circuits between one or more cable conductors or the shield and ground (ground fault), open circuits in the cable conductors, and breakdown of the cable insulation (AMS Corp., 2010). The most common failure mode is ground fault, in which the cable faults to ground from one or multiple conductors (U. S. NRC, 2001). Ninety-five per cent of cable problems occur at the cable connector where age factors are combined with mechanical damage and wear (AMS Corp., 2010).

For power cables, such failure modes can cause circuit protection devices to trip or partially discharge, resulting in excessive heating and degrada­tion of the cable insulation and ionization of the air around the discharge. This failure mode — degraded insulation resistance — can lead to a conduc­tor short-circuit to ground failure, conductor to conductor short circuit, or potentially both. Power cable failures have resulted in reactor trips, weak­ened engineered safety features, loss of redundancy, and reduced power operation (U. S. NRC, 2001).

For I&C cables, conductor short-circuit to ground failure and conductor — to-conductor-short-circuit failure interrupt the transmission of control sig­nals through the cable.

Degraded insulation resistance failure can impair the functioning of I&C cable and/or increase the rate of error (U. S. NRC, 2010a). The transmit­ted signal may become erratic, causing errors in measurement, spikes, noise, and other problems. When cables become bare, shunting and short circuits can occur, and if the cable insulator is degraded, the insulation material can become brittle and flammable (Hashemian, 2010). I&C cables are the most susceptible to ageing degradation (U. S. NRC, 2001).

In addition to signal anomalies and problems with plant control and safety systems, cable ageing has resulted in loss of critical functions and fire (AMS Corp., 2011). In light water reactors, the most severe cable failure scenario is loss of normal function during a LOCA when hot steam under pressure can cause cables to malfunction if insulation ageing, cracks, or other damage allow moisture to enter the cable. Hot steam combined with high pressure is the primary cause for cable malfunction in a LOCA, because steam pen­etrates smaller cracks more easily than water. Such consequences explain why the Hungarian Paks Nuclear Power Plant has described cable ageing as ‘the most significant I&C ageing issue’ in its plant (Hashemian, 2010).

A 2007 U. S. NRC report found that 93% of reported cable failures occurred in normally energized power cables: ‘More than 46% of the fail­ures were reported to have occurred while the cable was in service and more than 42% were identified as ‘testing failures’ in which cables failed to meet testing or inspection acceptance criteria’ (U. S. NRC, 2010a). The majority of these cable failures occurred between 11 and 30 years of service — less than the typical 40-year licensing period of a plant (U. S. NRC, 2010a).

While many cases of cable failure are identified through routine cable testing, some occur before a failure is identified (e. g. on cables that are not normally tested or powered). This fact underscores the importance of imple­menting a cable condition monitoring program (U. S. NRC, 2010b).