11
Time Domain Reflectometry (TDR) Technology
The actual formula used is as follows: The time is divided by two because the signal traveled
the length of the cable twice. Once when it left the
tester and went to the failure point, and again when it
reflected back to the tester to be detected. The speed
of light expressed in billionths of a second per foot is
0.9835 (about a billion feet per second) (0.2998 [about
300 million meters per second]).
bfz04.eps
Figure 4. TDR Waveform
VOP Variations
This characteristic speed of the signal for a particular
cable is not normally a tightly controlled part of the
cable manufacturing process and can vary widely from
one manufacturer to another as well as from one box
of cable to the next. As with all TDR-based cable
measurement tools, the TS100 measures TIME within
specified tolerances, but the displayed LENGTH is the
result of the calculation with the user-selected VOP and
is only as accurate as that selected VOP.
For most uses, a length reading with an incorrectly set
VOP is sufficiently accurate to locate the fault in the
cable. After all, an installed cable is hardly ever run in a
straight line. It can be stapled along the 2x4, laid
diagonally in the ceiling, and coiled behind the
junction box, all of which is not visible.
Also, common sense should prevail. For example, if the
tester reports an open at 80 feet (25 meters), and you
can see a junction box at about 70 feet (20 meters),
your first step should be to check at the junction box.
However, for some uses such as measuring the
remaining cable in a box, it is important to set the VOP
correctly in order to achieve the accuracy desired.
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 metal conduit
has a VOP of 82, while that same wire in a smaller
conduit filled with other wires has a VOP of 72.
0.98352
Length= Time in billionths of a second
X
VOP
To Next Page
