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Time Domain Reflectometry (TDR) Technology
Depending on the cable construction (shielded, twisted,
etc.), insulating material (foam, air, fiber, etc.), and
conductors tested (wire-to-wire, wire-to-shield), coiling the
cable on a spool or in a box may alter its VOP.
Additionally, other conductors in close proximity to the
conductors being tested can affect the VOP. For example, a
solitary 12 gauge THHN in a 1 inch conduit has a VOP of 82,
while that same wire in a ½ inch conduit filled with other
wires has a VOP of 72.
Note
The actual VOP of any particular cable is dependent
on the conductor spacing and the material between
the conductors and could vary by as much as ±5 feet
from the value listed in Table 2.
For help with your measurements, see Table 2 and Table 3
for the VOP values for many cable types and conditions.
Maximum Length
The maximum length of cable that can correctly be
measured by the tester is determined by several factors. The
most significant is the signal loss of the cable itself. When the
signal loss in a particular cable is large enough, the tester
cannot “hear” the TDR echo and cannot determine the
length of that cable. In this situation, the tester displays
Err
on the readout. The amount of signal loss in a cable is
determined by the characteristics of that cable and its
length. The maximum length shown in Table 2 is the length
above which the tester is not expected to be able to make a
valid measurement. For lengths above those stated in Table
2, the tester’s accuracy is not specified.
