Pulse Widths - Megger TDR1000/2 Guide De L'utilisateur

Operation

Pulse Widths

As the RANGE and IMPEDANCE matching of
the TDR are adjusted so the duration of the
transmitted pulse changes. The pulse widths range
from 7ns to 3µs to overcome signal attenuation
and enable the instrument to see further down a
length of cable. The greater the range selected on
the TDR, the wider the transmitted pulse.
The accuracy of the "Distance to fault" is not
affected by the length of the pulse. However,
if two or more features exist close together
(excluding open or short circuits), then the second
or subsequent feature may be partially masked
by the reflection from the first fault. Hence, for
potential multiple features, the instrument should
be used with the shortest suitable range, and
so the smallest pulse width, that can see both
features.
For output pulse characteristics, refer to output
pulse data in the TDR Specification at the end of
this guide.
Techniques
To improve on the accuracy of the measurement,
numerous techniques can be used, depending on
the situation encountered. Not every situation can
be described, but the following points are effective
14
and the most common and easily implemented
methods.
Test the cable from both ends
When fault finding a cable it is good practice to
take measurements from both ends. Particularly in
the case of open circuit faults, when the true end of
the cable is not visible. If the measurement is made
from both ends, then the combined answer should
equal the expected length of the cable. Even when
the true end of the cable is visible on the display,
the reflections after the fault may be too obscure to
analyse clearly. In this case, measurement from both
ends yields a clearer picture as well as improved
accuracy.
It is also good practice to follow the cable route
with a cable tracer, as not all cable runs will be
straight. It can save a great deal of time if the exact
route of the cable is known as faults are often
found at these points and can be accredited to
third party intervention.