Velocity Factor - Megger BTDR1500 Guia Del Usuario

RANGE, EXIT, VF, VF Units and CURSOR position Units. To change a highlighted option use
the CURSOR LEFT and CURSOR RIGHT keys (#3 & #5, table 1) to decrement / increment
the current living process. The only exceptions to this are the EXIT command confirm with the
Up/Down Gain key (#6, table 1) and GAIN control that normally selects the gain from one of
four levels.
Ensure the test leads are firmly fitted into the sockets of the instrument. Connect the test lead
to the cable under test. If working on live power cables a blocking filter must be used to isolate
the instrument from the live line.
The BTDR will then display a trace. The instrument will have powered up, set to the last used
range and velocity factor. If these settings are different for the cable under test (C.U.T) then
use the menu and cursor keys to set the correct values. With the Gain, set it at the lowest
level required to easily identify the cable feature, e.g. an open or closed circuit, and move the
cursor to the very beginning of the reflection. This is done by using the Menu key to set the
instrument into Cursor mode and then using the left and right cursor keys to set the cursor
position. The distance is then directly read from the display. The distance calculation is
performed using the current velocity factor. If this velocity factor is not correct, the displayed
distance will be incorrect.
To enable partial cable faults to be identified, the gain of the instrument can be adjusted. With
the gain at minimum the end of the cable should be seen on the trace, if a minor fault is
suspected then increase the gain until the fault is more visible.
NOTE: The test lead length is automatically removed to give a direct reading of the cable
length, therefore the test leads supplied with the instrument must always be used.
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6.2 TX NULL
Without TX Null (#2, table 1) the transmitted pulse would be visible at the beginning of the
trace, swamping any reflections within the pulse length (the dead zone). The balancing circuit
attempts to match the characteristic impedance of the cable under test to produce an
equivalent pulse. Subtracting this equivalent pulse from the transmitted pulse effectively
removes the dead zone and allows cable features much closer in to be detected.
NOTE: In many cases, it will be impossible to completely null the transmitted pulse.

6.3 VELOCITY FACTOR

The velocity factor is the scalar that is used to convert the measured time interval into an
actual length of cable. It can be displayed in one of two ways: a ratio of the transmitted pulse
speed to the speed of light, or as a distance per microsecond. When it is displayed as the
distance per µs (either m/µs or ft/µs) the velocity factor will be indicated as half the speed of
the pulse in the cable. This is because the pulse in fact has to go along the cable to the cable
feature and back again which is twice the distance to the feature.
If the exact length of a piece of cable of the same type as the C.U.T is known and the reflection
from the cable end is visible then a more accurate value for the velocity factor can be
determined:
1. Locate the reflection caused by the end of the known length of cable with the instrument
set on the shortest possible range to see the end of the cable.
2. Locate the start of this reflection as described in the Operation section of this manual.
3. Adjust the velocity factor until the correct cable length is shown.