Chapter 7
Cable Testing Fundamentals
7-9
Length and NVP
Measuring the length of the cable requires that you know the Nominal Velocity of
Propagation (NVP) of the cable. Refer to the specification or the manufacturer of
the cable you are testing for the cable NVP. If the wire specification is not
available, use a known length of good cable (50 to 100 feet) and adjust the NVP
until the tester displays the known cable length.
Wire Length Errors
Lengths may differ slightly between pairs in the same cable, due to minor NVP
differences between the pairs and physical length differences due to twisting
patterns. When electrically measured cable length varies too much from actual
length, a problem exists.
Troubleshooting Wire Length Problems
Problem: Length between a pair of the same cables varies by more
than 10%.
Probable Causes Incorrect NVP.
Excessive cable length.
Installed matched terminator not functioning correctly.
Cable insulation damage to longer pairs.
Break or short in a pair.
Elevated capacitance on a pair.
Other Tests Affected Test Possible Result
DC Loop Resistance May be slightly high or fail.
Attenuation May be slightly high or fail.
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