1-10 Option 100 Fault Location and SRL
Introduction and Measurement Theory
Cable Impedance and Structural Return Loss Measurement Theory
SRL and Periodic Cable Faults
SRL is the measure of the reflection of incident energy that is caused by
imperfections or disturbances (bumps) in the cable which are distributed
throughout the cable length. These bumps may take the form of a small
dent, or a change in diameter of the cable. These bumps are caused by
periodic effects on the cable while in the manufacturing process. For
example, consider a turn-around wheel with a rough spot on a bearing.
The rough spot can cause a slight tug for each rotation of the wheel. As
the cable is passed around the wheel, a small imperfection can be created
periodically corresponding to the tug from the bad bearing.
Each of these small variations within the cable causes a small amount of
energy to reflect back to the source due to the non-uniformity of the cable
diameter. Each bump reflects so little energy that it is too small to
observe with fault location techniques. However, reflections from the
individual bumps can sum up and reflect enough energy to be detected as
SRL. As the bumps get larger and larger, or as more of them are present,
the SRL values will also increase. The energy reflected by these bumps
can appear in the return loss measurement as a reflection spike at the
frequency that corresponds to the spacing of the bumps. The spacing
between the bumps is one half the wavelength of the reflection spike and
is described by equations 7 and 8.
Equation 7
Equation 8
The wavelength/2 spacing corresponds to the frequency at which down
and back reflections will add coherently (in-phase). The reflections
produce a very narrow response on the analyzer display that is directly
related to the spacing of the bumps. The amount of reflected energy is
observed as return loss. When this return loss measurement is
normalized to the cable impedance, the return loss becomes structural
return loss.
wavelength
c
f
---
≈
c
speed of light
= f frequency=
wavelength
2
------------------------------
spacing between the bumps
=
To Next Page
Loading...
Need help?
General
