Lesson 2/Learning Event 3
plant cables carry the trunk circuits between switching centers, and inside
plant cables carry the circuits between the main frames and in-house
equipment. However, cables are seldom interconnected directly on terminal
blocks because such an arrangement lacks flexibility; jumpers on main frames
are used to achieve interconnection flexibility.
Most of the information
concerning telephone cables in this lesson is limited to the inside plant
variety.
a. Cable Characteristics. The wires in multipair telephone cables are
normally single conductor and are color coded. Single-conductor wires break
easily when moved, making mechanical rigidity of the main-frame assembly a
prime requirement.
Further, the cables must be firmly lashed to the
framework.
In most inside plant cables, wires are terminal for ease of
b. Cable Routing.
Most installations specify a cable vault located
immediately below the main frame, with the cable feeding upward from the
floor along the framework. This arrangement gives the combined advantages
of safety, convenience, and physical security of the cabling.
Trenches
under the floor leading to equipment items complete the cable routing.
Where water or excessive moisture is a problem, the outside cables can be
brought through the cable vault, but inside plant cables must travel from
the main frame to equipment through overhead racks. Wiremen will bring in
and connect the outside plant cable to the line side of the protectors on
the main frame, and there, their work ends. All in-house wiring usually is
accomplished by technicians assigned to the station.
c. Color Coding. If all conductors in a cable were of the same color,
it would obviously be impossible to correctly connect them to apparatus
without making continuity tests to identify the ends.
The use of
differently colored conductors thus facilitates the location of wires for
making
connections
and
performing
maintenance.
In
inside
plant
(switchboard) cables, the colors of the pairs and the sequence of their
arrangement have been standardized.
Insulation on each wire in the cable
carries an identifying color derived from five solid colors.
The
combinations appear in the form of varicolored tracers (streaks) in the
solid-color insulation.
Five solid colors and fifteen color combinations
(called twenty basic colors) are used for the ring wires, with further
combinations for the tip wires. Ring and tip refers to switchboard patching
plugs.
Although color codes have been standardized, this does not
necessarily mean that every
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