Ageing of electric cables in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 235

such as erratic behavior along a cable or in an end device (e.g. resistance
temperature detector).
The TDR test non-destructively identifi es and locates cable defects and dis-
continuities on an installed cable in-situ , providing trendable measurements.
However, end terminations of the cable must be disconnected in order to per-
form the test (U.S. NRC, 2001; 2010b). In addition, the size of the wave that
the TDR method sends down a tested cable is limited by the bandwidth of the
pulse and sampling circuitry. Because it sends only very broad DC pulses, the
TDR method can locate only DC open- or short-circuit conditions.
Like TDR, the frequency domain refl ectometry (FDR) technique can
measure the distance to and severity of a fault in a cable conductor, connec-
tors, and end device. However, because the FDR technique uses a selected
set of much smaller or narrower bandwidth frequencies, it is also able to
locate RF faults in cables, unlike the TDR test. The FDR technique can also
help identify degradation in cable insulation material. There are three types
of FDR which calculate distance based on the sine wave property they mea-
sure - namely, frequency, magnitude, and phase.
In FDR, a stepped (or variable) frequency sine wave generator sends
stepped-frequency sine waves down the cable. These waves are refl ected
back from the cable end as well as from any faults encountered along the
cable, and are sensed by either a frequency counter, received signal strength
indicator, or another technology for measuring high or intermediate fre-
quency voltage magnitudes. Using pulses of discrete frequencies can iden-
tify and locate small faults in connectors or cables, making possible a more
realistic picture of cable condition than TDR provides.
FDR measures refl ection responses in the frequency domain and then
converts the data into the time domain using an inverse Fourier transform.
Similarly, FDR data can be acquired by using a TDR to measure the refl ected
wave over the large bandwidth and then using Fourier transform to convert
from time to frequency domains. Decreasing the time required for a signal
to change from a specifi ed low value to a specifi ed high value (rise time) of
a TDR test will increase its accuracy, as will increasing the bandwidth of an
FDR test. Similarly, increasing the number of frequency samples in an FDR
test increases its maximum range, as does increasing the period between the
rise and fall of the pulse in a TDR pulse (Hashemian, 2010).
Like TDR, FDR is a non-destructive technique that can send a swept sig-
nal through miles of cable without attenuation as long as the cable under
test is shorter than the signal wavelength (IAEA, 2011).
Reverse time domain refl ectometry (RTDR) is a technique developed
by CHAR Services Inc., a division of Analysis Measurement Services
Corporation (AMS). It tests the quality of the shielding around the conduc-
tor of a coaxial or triaxial electrical cable. The RTDR method estimates the

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