LCR, TDR, and FDR are cable testing techniques that measure imped­ance in cables in order to detect anomalies. There are two basic types of impedance tests — lumped data and distributed measurement — based on the ability of the test to localize its measurement. Lumped data tests like LCR typically identify anomalies in cables with greater accuracy than dis­tributed measurement methods. But once the fault is detected, the distrib­uted measurement methods (TDR and FDR) can determine the distance to the fault.

The LCR test uses an LCR instrument or meter at specific frequencies to make impedance measurements along the cable at specific frequencies to verify the characteristics of the cable conductor, insulating material, and the end device. The results are evaluated to determine if they are as expected for the type of circuit being tested. Imbalances, mismatches, or unexpectedly high or low impedances between the cable leads indicate problems caused by cable degradation and ageing, faulty connections and splices, or physi­cal damage. For example, abnormal capacitance measurements indicate a change in cable dielectric or insulation. In addition to providing information about cables, connectors, and end devices, LCR measurements can identify circuit problems such as moisture or loose connections (Hashemian, 2010; IAEA, 2011).

The most popular and effective cable testing technique today, TDR, is used to locate problems along a cable, in a connector, or at passive devices at the cable end by sending a test signal through the conductors in the cable and measuring its reflection. It works on the same principle as radar. A pulsed or swept DC signal is sent through the cable, and its reflection is measured to identify the location of any impedance discontinuity or change in the cable and the end device (load). It measures the time taken for the signal to travel down the cable to where the impedance change is located, and return. This propagation time for a known distance is then converted, and depending on the type of display used, the information can be presented as a waveform and/or a distance reading (IAEA, 2011).

Any significant change in impedance along the cable will cause a reflec­tion that will appear on the TDR signature as a peak or valley whose ampli­tude depends on the characteristics of the cable impedance. Depending on the impedance of the load, the TDR trace representing the end of the cable may step up or step down. That is, reflected voltage waves occur when the transmitted signal encounters an impedance mismatch or discontinuity (fault) in the cable, connector, or end device. Any such change in impedance along the cable due to a short, open, shunt, or other electrical effect can thus be identified and located using the TDR test. A rise in the reflected wave is indicative of an increase in impedance, and a decrease in the reflected wave is indicative of a decrease in impedance. Thus, the peaks and dips in a TDR plot are used to identify the location of normal and abnormal electri­cal effects throughout the cable (Hashemian, 2010).

The TDR test is typically performed using a pulse generator, which produces a step pulse, and a recorder, oscilloscope, or automated computer-controlled data acquisition system, which captures the reflected wave. The test signal is applied between pairs of lead wires, a cable shield, and a ground plane, and the results are displayed as a plot of the reflected wave versus time or distance (Hashemian, 2010).

Yielding diagnostic information about the cable conductor and any con­nector or connection, the TDR method relies on comparisons with a base­line TDR. Its success therefore typically shows significant improvement if there is a baseline TDR for comparison (IAEA, 2011). In light water reactors, the TDR method is useful for testing instrumentation circuits, motor and transformer windings, pressurizer heater coils, thermocouples, RTDs, motor-operated valve cables, neutron detector cables, and other components that are normally inaccessible, such as in high temperature and high radiation zones. The simplest and perhaps most important appli­cation of TDR is to locate an open or short lead, moisture, or problems such as erratic behavior along a cable or in an end device (e. g. resistance temperature detector).

The TDR test non-destructively identifies and locates cable defects and dis­continuities on an installed cable in-situ, providing trendable measurements. However, end terminations of the cable must be disconnected in order to per­form 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 reflectometry (FDR) technique can measure the distance to and severity of a fault in a cable conductor, connec­tors, 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 mea­sure — 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 reflected 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 fre­quency voltage magnitudes. Using pulses of discrete frequencies can iden­tify and locate small faults in connectors or cables, making possible a more realistic picture of cable condition than TDR provides.

FDR measures reflection 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 reflected 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 specified low value to a specified 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 sig­nal through miles of cable without attenuation as long as the cable under test is shorter than the signal wavelength (IAEA, 2011).

Reverse time domain reflectometry (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 conduc­tor of a coaxial or triaxial electrical cable. The RTDR method estimates the distance to a fault in a cable by coupling a repetitive DC pulse to the shield and allowing the pulse to travel the length of it. The time delay between the DC pulse and when the signal is received can be measured by simul­taneously monitoring the cable signal path (Hashemian, 2010). These time delays make it possible to identify the point at which the electromagnetic interference (EMI) couples into the cable system. This reveals the location of degraded connectors or cable shields because such interference usually couples at cable connections or terminations that tend to degrade through ageing or damage (IAEA, 2011).