Environmental Engineering Reference
In-Depth Information
most cables, but extended submersion is beyond the qualifi ed operating
conditions for most cables (U.S. NRC, 2010a).
Moisture can cause water treeing where voids or contamination are pre-
sent in the cable. This combination of water and electrical stress degrades
the insulation's dielectric properties (U.S. NRC, 2001). In fact, the U.S. NRC
(NUREG 6704) identifi ed wetting as the primary ageing-related cause of
failure (specifi cally, short circuit) for medium-voltage cables, in particu-
lar the insulation (U.S. NRC, 2001). Such failure could allow currents and
voltages to spread into the adjacent power distribution system, potentially
causing other degraded power cables to fail too (U.S. NRC, 2010b). For this
reason, cables in hard-to-access underground ducts and conduits, covered
trenches, bunkers, and manhole vaults are the subject of special concern
(U.S. NRC, 2010b).
Both power cables and I&C cables are directly affected by mechanical
stress including bending, abrasion, cutting, contact, deformation, and per-
foration, as a result of installation and maintenance, for example. Cables
connected to vibrating machines are also subjected to stress, leading to
chafi ng, cutting, or cracking of the cable insulation material (AMS Corp.,
2011). Cable jacket and insulation material as well as cable conductors can
be damaged by electromechanical forces caused by high levels of short cir-
cuit current passing through a power cable (U.S. NRC, 2010a).
Radiation is another signifi cant cause of cable degradation. During nor-
mal operation, gamma and neutron radiation cause oxidative degradation
in increasing (nonlinear) relation to the radiation dosage absorbed by the
cable. During accidents, beta radiation may also affect cables unprotected
by a conduit (IAEA, 2011).
6.2.2 Cable failure modes and consequences
￿ ￿ ￿ ￿ ￿ ￿
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-fi ve 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 degrada-
tion of the cable insulation and ionization of the air around the discharge.
This failure mode - degraded insulation resistance - can lead to a conduc-
tor short-circuit to ground failure, conductor to conductor short circuit, or
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