Information Technology Reference
In-Depth Information
installer should do a good job of routing and terminating each cable, and should
verify that the link is properly connected and passes the performance measures for
the desired category. However, if a failure occurs, you must examine the cable and
connections to find the problem. Even if the cable link was once in perfect condi-
tion, cable runs can be physically damaged, and poorly made connections can fail.
Occasionally, even cabling components such as connectors can fail.
Wire Break.
One of the most common wiring failures is a broken wire. Most
testers will report the wire as an “open,” meaning that the circuit is open. Some
testers may actually declare the entire pair bad, although only one wire might be
open. A wire that was never connected will also test open, even though the failure
is not a result of wire damage. If only one wire of a multiwire cable is open, you
should probably look for a bad connection at one of the termination points.
Connections in insulation-displacement connectors, such as 66M or 110
blocks, sometimes are not driven home, resulting in a bad or intermittent connec-
tion. What happens is that the insulation is not quite stripped away, and an appar-
ently good connection will actually have a thin layer of insulating plastic remaining.
You should never reuse old 66M blocks.
The contact jaws can deform outward just enough to cause a problem when
new wires are punched. This can also happen if a larger-gauge wire was previously
punched down. These blocks may also have the same problem with jumper wires
that are frequently moved. You may be able to “rehabilitate” a contact by squeez-
ing the empty contact jaws together with sharp-nosed pliers, or even by rocking the
punchdown tool back and forth on the contact. The 110-type blocks are much less
prone to this problem, as they use a cutting-displacement method of termination,
where the connector contact actually slices through the wire's insulation.
Stranded wire must never be used in a 66 or 110-type connecting block. These
blocks require solid wire, and connections made with stranded wire will often fail.
The insulation-displacement contact (IDC) needs to actually cut into the solid con-
ductor slightly, and there is simply no way to do that with stranded wire. The
strands move out of the way and deform their shape when pushed down into an
IDC terminal. The connection now depends on very slight pressure and low contact
resistance. These stranded-wire connections will often fail over time, and temporar-
ily heal when the wire is touched. Although this is not technically a break, the symp-
toms are the same.
If inspection of the cable termination ends does not reveal the open wire, use a
cable scanner to measure the length to the open wire. Mid-cable open wires may be
very hard to find visually, unless there is obvious mechanical damage to the cable.
You never want to find out that an installed cable has an open due to a manufac-
