Lesson 2/Learning Event 5
at intervals along all or part of the unsheathed portion.
The conductors
brought out of the form are called skinners. When skinners are separated,
care must be exercised to prevent splitting pairs.
After skinners are
brought out from the cable, the exposed wires are formed and laced by the
crew.
a. Attachment to Distributing Frame. When the cable form is attached
to a distributing frame, the skinners can readily be terminated on lugs of
terminal strips.
(1) Each cable pair must be individually tested.
The best time for
testing cable pairs is immediately upon wrapping wires or soldering wires on
terminals. If the installer connects all cable wires prior to testing, he
may spend many hours clearing troubles such as split pairs and isolating
defective lines. He must also make sure all grounds are removed from lines.
The purpose of the test is to verify continuity. This can be done by either
direct current (DC) or voice-frequency test instruments. In DC testing, a
simple ohmmeter is needed at one end of the line, and a short lead equipped
with alligator clips at the other end. The short lead is used to connect
the wires in the pair being tested.
In VF testing the complement of
equipment may be as simple a common doorbell buzzer and a headset, or as
sophisticated as a VF signal generator and a decibel meter. The VF signal
generator method is preferred since there is less possibility for damaging
sensitive circuit elements that may be connected in the circuit, and the
power loss of the line can be measured or recorded at the same time.
(2) All spare lines must be connected to terminals.
Unused pairs in
outside cables are grounded because heat coils are not installed until the
line is to be used. The protector automatically grounds the line as long as
the heat coil is absent.
Inside spare lines should also be grounded or
shorted except when connected to equipment in which case the lines should
terminate in the proper impedance value.
For example, unused lines that
carry 600-ohm circuits should be terminated in 600-ohm resistors to provide
a load to equipment; a shorted or grounded lien may cause damage to
sensitive equipment.
(3) When the installers are separated by greater than talking distance,
their first act is to connect themselves by field telephones, and they must
maintain that circuit throughout the entire installation. They should never
be out of communication while installation is proceeding.
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