Pulse Widths - Megger BTDR1500 Bedienungsanleitung

The measurement of the distance to the fault can now be made with more confidence that the
measurement will be correct. The ability of the instrument to accurately measure the distance
to a cable feature relies on the velocity factor being correct; any errors in the velocity factor
are directly proportional to distance measurement errors. Please refer to the Quick Reference
Guide for a table of typical Velocity Factors.

6.4 PULSE WIDTHS

The BTDR1500 pulse widths range from 8 ns to 3 µs to overcome signal attenuation and
enable the instrument to see further down a length of cable. In distance terms for the size of
the transmitted pulse, this represents a transmitted pulse from as small as 1.5m to 600m!
(This assumes a velocity factor of 0.67.) Without TX Null, this would be an enormous dead
zone, but with the instrument correctly balanced, faults can be seen well within the pulse
width.
As the measured distance is taken at the start of the reflected pulse, the size of the pulse width
does not affect the accuracy of the measurement. However, if the first feature does not give
a complete reflection such that the instrument can see beyond it to a second feature, the
ability to discern between features is affected by the pulse widths. If there are multiple
features, the instrument can only fully discern between them if the features are more than the
pulse width apart. Hence, for discerning multiple features, the instrument should be used with
the shortest range, and so smallest pulse width, that can see both features (refer to the pulse
width table in the specification).
6.5 TECHNIQUES FOR TDR USE
To improve on the accuracy of the measurement and the ability to discern faults, numerous
techniques can be used, depending on the situation encountered. Here are a few for your
information:
6.5.1 Test the cable from both ends
When fault finding a cable it is good practice to shoot the cable from both ends. Particularly
in the case of open circuit faults, the true end of the cable is not visible. Thus, it is harder to
estimate whether the answer that is obtained is realistic. If the measurement is made from
both ends, then the combined answer should add up to the expected length of the cable. Even
in the case when the true end of the cable is still visible, 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
will usually be found at points were human intervention has occurred, junction boxes splices
etc.
6.5.2 Reflections caused by Mismatches
On very short faults, when there is a mismatch between the test lead impedance and the
cable under test a proportion of the reflected wave from cable fault "bounces" off this
impedance mismatch. This reflection generates an apparent second fault at double the first
fault's distance. If there is sufficient energy left in the wave a third and fourth reflection can
occur. The problem is more evident on 50 Ω and 25 Ω cables (i.e. power distribution cables)
as the impedance mismatch is greater and the signal attenuation is less. This will show on the
screen as multiple, equidistant faults of diminishing amplitude.
9