Vop Variations - Fluke TS 100 PRO User Manual

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, the start and end of a bridge tap cause a
negative and a positive reflection, as shown at the top
of Figure 5. The tester uses the time taken to receive
the first reflection to calculate the distance to the
bridge tap. Then it uses the time between the two
reflections to calculate the length of the bridge tap.
A connection causes a small, S-shaped reflection, as
shown at the bottom of Figure 5. The tester ignores
small reflections because they do not usually indicate a
problem on the cable.
If there is more than one problem on the cable, the
TS100 PRO shows only the first problem. If that
problem is a bridge tap, you can turn on the BT Filter to
ignore the first bridge tap and find a second bridge
tap. The TS100 PRO can always see past the bridge tap
to measure the cable length.
The actual result of the measurement is the time to the
fault. The software in the tester converts the measured
time to a length by multiplying the time by the speed
of the electrical signal in that particular cable. That
speed is represented as a percentage of the speed of
light and is called the Velocity of Propagation (VOP).
The actual formula used is as follows:
Length= Time in billionths of a second
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]).
VOP
X
2
0.9835
Time Domain Reflectometry (TDR) Technology

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 PRO measures time
within specified tolerances, but the displayed length is
the result of a calculation with the user-selected VOP,
and is only as accurate as the 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.
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 (±2
meters) from the value listed in Table 2.
Note
15