Information Technology Reference
In-Depth Information
turing defect, because it is almost impossible to prove that the break did not occur
during installation.
It is a good idea to do a continuity check on each reel of cable prior to instal-
lation. During installation, be careful not to pull the cables over sharp edges, as the
cable might be cut. See the next two sections for advice on how to repair a twisted-
pair cable open or short.
Wire Short.
A wire short in some ways is easier to deal with than an open. A short
is rarely caused by a bad connection at a termination, so you can initially skip that
inspection. A cable scanner may be able to locate the short within a few feet. If you
do not have access to a scanner, use a sensitive ohmmeter and the specification for
cable resistance per linear foot (26 ohms per 1000 ft for AWG 26 wire) to find the
approximate location of the short. If more than two wires are shorted, this test may
be useless, as the linear resistance will be much less than expected.
Shorts are often caused by other contractors trapping the cable between sharp
objects during construction and remodeling. For example, the cable might be pinched
between metal studs as a wall is assembled. The ubiquitous plasterboard screw also
does a very nice job of piercing right through a cable, shorting the wires in the
process. Sometimes (very infrequently, we hope) cable installers will pull the cable
past a sharp edge, cutting through the insulation and shorting wires in the cable.
The method used for repair of a short or an open depends on the type of cable
and the performance requirements. Coax cable may be cut in half, the two ends con-
nectorized, and a barrel connector used to join the ends. Fiber may be spliced, as
mentioned later.
Twisted-pair cable repair methods depend on the category of performance
desired. Technically, the TIA-568-C standard allows no splices in a horizontal cable
run. However, practical considerations may dictate some variance with Category 3
cable runs. Category 3 has a more gentle twist than Category 5e or Category 6, and
it is possible to carefully perform a splice using a shrink-wrap solder splice available
from several sources. This hollow, shrinkable-plastic splice is a tiny tube about 1 in
long with a narrow ring of low-temperature solder in the middle. The insulation is
removed on the wires to be joined. About 1 cm is enough. The wires to be joined
are then placed into the splice from opposite ends and the splice heated with a
proper heat-gun tool to melt the solder and shrink the plastic. (A pocket lighter is
not a proper tool, but it works well just the same if used with care.) The next con-
ductor from the same pair is wrapped around the first wire in the same direction as
the natural twist and the wire ends are joined in the same manner. This approxi-
mately maintains the gentle Category 3 twist, but is not adequate for Category 5e/6
splices.
