Environmental Engineering Reference
In-Depth Information
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 refl ection. It works on the same principle as radar. A pulsed or
swept DC signal is sent through the cable, and its refl ection 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 signifi cant change in impedance along the cable will cause a refl ec-
tion that will appear on the TDR signature as a peak or valley whose ampli-
tude 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, refl ected 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 identifi ed and located using the TDR test. A rise in the refl ected wave
is indicative of an increase in impedance, and a decrease in the refl ected
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 electri-
cal 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 refl ected
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 refl ected
wave versus time or distance (Hashemian, 2010).
Yielding diagnostic information about the cable conductor and any con-
nector or connection, the TDR method relies on comparisons with a base-
line TDR. Its success therefore typically shows signifi cant 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 appli-
cation of TDR is to locate an open or short lead, moisture, or problems
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