DC Loop Resistance
DC Loop Resistance is the total resistance through two conductors
looped at one end of the link. This is usually a function of
the conductor diameter and varies only with distance. This measurement
is sometimes done to ensure there are no gross misconnections
which can add significant resistance to the link. Note that
the wire map test automatically isolates breaks but not high
resistance connections.
DC resistance is often confused with impedance, a term describing
the dynamic resistance to signal flow, usually at a specified
frequency. Both are measured in ohms because they define different
types of opposition to electrical current flow. DC resistance
increases proportionately with the length of the cable tested
while impedance remains "fairly" constant regardless
of length.
From a signal perspective, attenuation (sometimes called
insertion loss) is now a more useful measurement, and DC resistance
has become less important.
Results Interpretation
Variations in loop resistance between pairs can often be a
quick indication of a cabling problem. In a shorted loopback
test environment, the expected value is simply twice the sum
of the value expected for the given length. This is a simple
test for any advanced field tester.
Troubleshooting Recommendations
In the case of unexpected high DC resistance, compare the
failed pair against other pairs in the cable. This will determine
whether the issue is specific to the one failed pair or due
to a problem affecting the entire cable. If a single pair
is at fault, inspect termination points for a poorly made
or oxidized connection.
If all four pairs have unexpected high DC resistance, check
your assumptions. Did you allow for double the resistance
to include the loopback? Is the resistance assumption correct
for the gauge of wire used? 26 gauge has higher resistance
per foot than 24 gauge. Do you have an unusual patch cord
in the link that could have high resistance? Look for anything
unusual especially if adjacent cables appear to be normal.
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