Visual inspection involves examining the cable throughout its length during a formal plant walkdown, a useful practice when, as is often the case, degra­dation is visible to the naked, well-trained eye (IAEA, 2011). Visual inspec­tion can identify changes in physical/visual appearance, surface texture, and damage as a result of manufacturing or operation (U. S. NRC, 2010b). More sophisticated techniques can then be used to determine the degree of age­ing more accurately.

The advantages of visual inspection are that it is low cost and easy to perform, requires no specialized equipment, does not require that samples be removed from the cable, and can be performed on operating equipment in-situ. Its disadvantages include the requirement that the cable be accessi­ble and visible, inspectors must be trained to evaluate what they are looking at (subjectivity), it generally only provides information on the cable jacket, and it does not provide quantifiable results (no trending possible) (AMS Corp., 2010; IAEA, 2011).

Mechanical testing is a subset of life-testing techniques that involves inspecting cables for cracks or changes in color, texture or hardness, mass loss, visco-elasticity properties, or size (swelling, shrinkage, deformation). Among the most conventional and popular means of mechanical cable testing are mea­suring the elongation-at-break of the cable and its tensile strength when pulled apart. The elongation-at-break test measures the strain on the cable when it breaks and is a recognized standard for assessing the health, integrity, and func­tionality of a cable insulation material (IAEA, 2011). This test is performed by stretching a ‘dog bone’-shaped cable sample until it breaks. The elongation-at — break test yields information on the tensile strength and modulus of elasticity of the cable, but the percentage of elongation is the most important criterion in evaluating cable health. When the percentage elongation-at-break is less than 50%, the cable is considered to be unhealthy — potentially unable to survive DBA conditions (AMS Corp., 2010; IAEA, 2011).

The tensile test measures the stress needed to break the cable. For poly­meric materials like thermoplastics, tensile strength only begins to fall after substantial ageing has already occurred. Both the elongation-at-break and tensile strength tests can be performed using a tensile testing machine.

A third mechanical test, measuring compressive modulus, involves check­ing the ductility of the cable insulation or jacket material to determine if the cable has become dry, brittle, or prone to crack. Developed in the mid-1980s by the Electric Power Research Institute (EPRI), this test is performed with a device known as a cable indenter, which uses a small probe to press against the cable jacket or insulation. A PC-based system analyzes cable hardness by measuring the probe force and polymer deformation, thus pro­viding diagnostic insights (Hashemian, 2010).

The difficulty with these classic life-testing techniques is that they can check for problems only at the locations on the cable where the cable is tested. Such passive maintenance methods can thus fail to detect problems or hot spots in other areas. Similarly, the elongation-at-break and tensile strength test are also destructive to the tested material and require that the cable be removed from operation for testing (IAEA, 2011). For these reasons, mechanical life-testing techniques should be combined with other measurements, such as electrical or chemical functionality.