20
TS90 Cable Fault Finder
Users Guide
For example, if one or both wires of the pair are broken
(open), or they are shorted to each other, or they become
sufficiently separated from each other, their impedance will
change. The tester looks for these changes in impedance. If
the impedance change is large enough, (e.g., such as that
caused by a break in one of the wires of the pair), the tester
will detect the impedance change and will display the length
of the wire up to the impedance change.
From the previous information, it should be easy to deduce
that the tester can measure the length of a pair of un-
terminated wires, because, the open circuit at the far end
causes a very large impedance change.
The TS90 Cable Fault finder uses Time Domain Reflectometry
(TDR) to determine the length of the target cable. A TDR,
much like RADAR, sends a pulse down the pair of wires. Part
of that pulse reflects off any impedance variations in the pair
of wires. All of the reflections, together with the original
pulse, combine to make an electrical signal (TDR waveform)
that has various flat and bumpy sections that represent the
start, the impedance changes, and the end of the cable. The
size and shape of the flat and bumpy sections depend on the
distance to the impedance changes and the magnitude of
the impedance changes.
For example, two runs of 12/2 ac wire joined with a splice
will have a TDR waveform with 2 flat sections separated by a
bump. The two flat sections represent the lengths of the two
sections of wire. The small bump in the middle represents
the small impedance change at the splice point. The large
bump at the end represents the large impedance change at
the end of the wire run (see Figure 4).
