Environmental Engineering Reference
In-Depth Information
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 pro-
viding diagnostic insights (Hashemian, 2010).
The diffi culty 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.
6.3.2 Electrical measurements
The IAEA has stated that electrical properties - voltage withstand, insula-
tion resistance, capacitance, attenuation, and/or signal propagation - are the
'most important functional properties' of cables (IAEA, 2011). Such prop-
erties provide a direct measure of the loss of cable resistance or dielectric
parameters and therefore its loss of functionality (IAEA, 2011). Electrical
measurements are primarily suitable for cable conductors, connectors,
splices and penetrations. Most electrical measurement techniques are less
sensitive to problems with cable insulation material though they can reveal
them (IAEA, 2011).
Two types of electrical tests are available: destructive methods, which iden-
tify cable failure locations before the cable is installed, and non-destructive
methods, which are better suited to identifying cable degradation (U.S. NRC,
2001). Non-destructive tests can be categorized by direct current (DC) and
alternating current (AC) methods. DC tests generally require the least expen-
sive test equipment, but may be less appropriate for some power cables and
cables used in AC applications (U.S. NRC, 2001). The primary advantage
of electrical techniques is that they can be used
in-situ
on installed and less
accessible cables, providing information on the entire length of a cable, not
just those points which are tested (IAEA, 2011). Because some methods
also enable trending based on baseline measurements, electrical techniques
can be used to note changes over time for ageing management purposes
(IAEA, 2011). The most important electrical parameters in cables are insu-
lation resistance, leakage current, loss factor, permittivity, and breakdown
voltage. Provided that one or both ends of the cable are accessible to mea-
surement and the cable can be de-energized, these electrical parameters can
usually be measured on any cable (Hashemian, 2010).
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