| Let’s separate out the two questions for
ease of explanation; firstly, changing the VOP (also known as VP)
can dramatically affect a two-wire test result because the calculated
fault distance is directly proportional to VOP. For example, when
testing BELDEN 8262 M17/155-00001 MIL-C-17 16428 RX coax cable that
is 28’ 4” in length the VP presented after a “Learn
VP” test is performed is 0.642 VP (includes a coax connector
crimped on to facilitate this test). Impedance setting is preset to
74.
Once the VP is changed to .800 the length results is now 35’
3”, a variance of approximately 11 feet.
Changes in Impedance settings can be presented as follows: With
the above VP set (0.642) we can then adjust the Impedance from 74
to 400. The result is an output distance to failure at 28’
4” (no change in distance measurement).
Changing VP does have an effect on distance measurement. Summary?
Changing Impedance has much less effect on distance measurement.
These statements apply to length of wires. Components in-line can
have various effects on VP and Impedance settings. Also, corrosion
and other types of environmental effects can also have impact on
VP or Impedance settings to the LCD output on the Model ESP.
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