Using The Tdr2000; Connection To Cable Under Test; Measuring Distance To Fault - Megger Tdr2000/2 Benutzerhandbuch

USING THE TDR2000

Switch on the instrument. The TDR2000/2 will display the Megger logo
for a few seconds, followed by a trace. Set an appropriate RANGE and
Velocity Factor (keys 11 and 4) for the cable to be tested. Refer to
'Velocity factor' later.

Connection to Cable Under Test

Connect the test lead to the cable under test. Connection may be made
to a live system with a voltage to earth (ground) less than 300 V with an
installation (over voltage) category of III or lower. This means that the
instrument may be connected to any fixed wiring of a building
installation, but not to primary supply circuits such as overhead cables.
A Fused Lead Set must be used to connect to energised live systems.
Refer to the accessories section for options.

Measuring distance to fault

The Find key searches for the furthest major change of impedance.
This may be a fault or the end of the cable if this is a short or open
circuit.
When found the event is displayed and the cursor placed on the leading
edge. The gain, zoom and cursor position may need to be adjusted for
accurate measurement. Note that Find does not adjust the Tx Null or
Velocity Factor.
Ensure that the total length of the cable can be seen on the display, and
that the range selected is correct.
To enable 'difficult to see' faults (small reflection) to be identified, the
gain of the instrument can be adjusted, key (9). With the gain at
minimum small changes of cable impedance may be overlooked. by
increasing the gain the fault becomes more obvious. Adjust the gain
until all major reflections can be easily identified.
10
Using the CURSOR LEFT and CURSOR RIGHT keys (1 and 3) move the
cursor to the start of the first reflection. To position the cursor more
accurately, press the ZOOM key (6) to zoom into the current cursor
position. The cursor is now fixed and using the cursor keys, the trace
will move relative to that point. The current zoom location with respect
to the whole trace range is shown at the top of the display. The distance
is then directly read from the display.
Notes
Cursor: All distance measurements are made to the vertical cursor. By
moving the cursor left or right the distance displayed will increase or
decrease.
Dual cursor: Enabled by pressing the SHIFT and CURSOR LEFT key or
SHIFT and CURSOR RIGHT keys. In dual cursor mode, the distance to
the active cursor is displayed together with the difference between the
two cursors.
If no significant reflections can be seen use the "Tx Null" adjustment to
minimise the effect of the output pulse on any potential "near end"
faults. (If no reflections can be seen, try shorting (or earthing) the far
end of the cable to ensure that you are "seeing" the whole length of the
cable. )
The distance to the fault can then be directly read from the display.
NOTE: The distance calculation is performed using the velocity factor
(VF) set in the TDR. If this velocity factor is not correct for the cable
under test, the displayed distance will be incorrect.