VON MODEL SST15-832 ARC REFLECTION SECTIONALIZING SYSTEM Page 16
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RADAR THEORY
Cable radar has been available for over thirty five years. Cable radar is also called the
Pulse-Reflection Method, the Pulse-Echo Method, and Time-Domain Reflectometry.
Because it works well only on shorts (less than 150 ohms at 10 volts) and opens, it has
mainly been used in the telephone industry on communication cable. Radar can be
successfully used to locate faults on electric power cable faults by permanently lowering the
fault resistance by burning or temporarily lowering the fault resistance using the arc reflection
method.
Short duration pulses are transmitted along a cable by a radar. When these pulses reach a
discontinuity such as a splice or fault in the cable, a reflection occurs peculiar to the type of
discontinuity. By observing these reflections on a CRT or scope and knowing the
propagation velocity or the speed at which the pulse travels on the cable, the distance to the
discontinuity can be determined. The cable radar is essentially a pulse generator and a
cathode-ray oscilloscope. Special circuitry is normally provided with the oscilloscope for
determining the distance and for changing the pulse length for different distance ranges.
Pulses are generated and put on a cable that must have consistent distributed capacitance.
A reflection will result when a discontinuity or significant change of impedance occurs. An
upward reflection or blip would indicate a higher-impedance discontinuity such as the cable
ends, or a place where the cable neutral is missing. A downward reflection or blip will result
from a lower-impedance discontinuity such a cable fault. The reflection is upwards when the
impedance of the discontinuity is above the characteristic impedance of the cable. The
reflection is downwards when the impedance of the discontinuity is below the characteristic
impedance of the cable.
The characteristic impedance of a transmission line is important since it affects what types of
discontinuities will show up on the radar. However it cannot be measured directly with an
impedance bridge for a finite length of line. It can be calculated from the distributed-circuit
co-efficients of the line at any frequency using the following basic equation.
The equation contains the parameters of resistance, conductance, inductance, and
capacitance and is also related to frequency. As the frequency is increased above 1
megahertz, the above equation will reduce to a simplified equation based on the distributed
inductance and capacitance; and this simplified equation is shown as follows.
In the case of primary underground cable which acts as a coaxial line we have the published
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