Several hundred different cable types and sizes are used throughout a typ­ical light water plant. Plant conditions determine which type of cable is used; for example, cables for control rod drive mechanisms must withstand higher temperatures and have additional shielding capacity (IAEA, 2011). I&C cables are by far the most common cable type (Hashemian, 2010). Instrumentation cable, which includes thermocouple (T/C) extension wires, is a low-voltage (< 1 kV), low-ampacity cable used to transmit digital or ana­log measurement signals from transducers such as resistance temperature detectors (RTDs) and pressure transmitters. Control cable, also low-voltage and low-ampacity, is used in the circuits of control (rather than monitoring)

Wire insulation

Braided shield

Cable jacket

Foil shield

Conductor

6.1 Cable components.

components such as control switches, valve operators, relays, and contactors (Hashemian, 2010).

A complete cabling system (see Fig. 6.1) may include any or all of the following components: conductor, insulation, shield, jacket, terminations, penetrations, splices, connectors, and/or end devices (sensor, transmitter, detector, motor, etc.) (AMS Corp., 2010). However, the main components of an I&C or low-voltage power cable are conductors, electrical insulation or dielectric, shielding, and the outer jacket.

Power cables and I&C cables both operate by providing a conductive route for an electric circuit by using metallic conductors — typically cop­per or aluminum that are insulated with a polymer and have different configurations such as coaxial, triaxial, twisted pair, or multi-conductor arrangements of single-strand or bundled wires (AMS Corp., 2011 ; U. S. NRC, 2001). The cable insulation and jacket are made of different poly­mers, including polyethylene (PE), cross-linked polyethylene (XLPE), polyvinyl chloride (PVC), ethylene propylene diene-monomer (EPDM) rubber, ethylene propylene rubber (EPR), Hyplon, Lipalon, and others (AMS Corp., 2010 ; 2011). More than three-quarters of cable insulation and jacket used in nuclear plants is constructed from such polymers (U. S. NRC, 2010a). Another type of cable, fiber-optic, is used to transmit signals based on optical fiber technology. Though its outer jacket is similar to cop­per and aluminum cable, fiber-optic cables have unique ageing, degrada­tion, and failure characteristics (U. S. NRC, 2010a). As such, they are not covered here.

Connectors are also part of the conductor in a cable circuit. A multitude of connectors, terminations, terminals, splices, etc., join the conductor to other cables or electronic equipment. The failure of these components
may appear as a problem with the conductor in test data and may be the result of corrosion, loose terminations, and other faults (AMS Corp., 2010).

Cable manufacturers qualify cables for a specific service life (e. g. 40 years for nuclear power plant cables) and specific voltage class at a given max­imum ambient temperature (U. S. NRC, 2010a). Service life is affected by everything from voltage and temperature rating of the cable and the mate­rial and thickness of its insulation and conductor jacket, to the conductor size and construction (e. g. solid or stranded), the type of metal and coat­ings used in the conductor, the cable configuration (e. g. single or multiple), and the presence of ground conductors, shields, braids, or binding and filler material (U. S. NRC, 2001).

Because I&C cables are used at low current, their typical operating tem­peratures are between about 40°C and 65°C (IAEA, 2011). In contrast, power cables can operate at 80-90°C because of continuous current flow, which generates ohmic self-heating, and the higher voltages and currents used to power medium — and high-voltage equipment such as pump motors (IAEA, 2011 ).

Because of their typical length, cables can experience multiple operating environments as they travel through different areas of the plant, including harsh temperature, radiation, humidity, and moisture conditions, which may include submersion in water (IAEA, 2011; U. S. NRC, 2001).