22
TS90 Cable Fault Finder
Users Guide
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).
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 tester 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, and you can see a
junction box at about 70 feet, 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.
Length
Time in billionths of a second
2
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VOP
0.9835
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=
