Lesson 2/Learning Event 2
b. Protectors.
Protector groups or assemblies are always mounted on
the vertical side of the MDF and are designed to protect the equipment and
personnel from lightning or other foreign currents which may be conducted
over the outside lines and cables.
Each protector assembly serves an
external wire pair and generally consists of a carbon block air gap in
parallel with the wire, and a heat coil in series with it. Several types
are in use. One type is shown in A of figure 2-5, with the carbon blocks
shown disassembled in B of figure 2-5. Conventional mounting of a group of
protectors is shown in figure 2-6.
(1) Heat coil. A heat coil is like a delayed-action fuse; it operates
when current exceeds the rating of the coil for a period of time. Unlike a
fuse, however, a line coil has a dual function; when it operates it opens
the inside line leading toward the in-house equipment, and grounds the
outside line. Heat coils are normally removed from unused lines so that the
lines remain grounded through the coil holder.
(2) Carbon protector blocks.
One type of protector block assembly
consists of a carbon block and a porcelain block with a strip of carbon
inserted in it.
The porcelain block serves to hold the carbon parts
separated so as to form an air gap.
The outer block is slotted to
accommodate the spring which holds the air gap assembly together. Although
carbon blocks are essentially lightning arrestors, they also give protection
against high voltages from any source.
Carbon protector blocks are also
known as protector blocks, or open-space cutouts.
Regular maintenance is
required to keep the air gap clean. Failure to perform this maintenance may
result in the development of circuit noise.
(3) Operation.
Two cable pair terminals are connected to springs (A,
figure 2-5) resting on protector blocks which provide the air gap for
discharge of high potentials to ground. Two jumper terminals are connected
to springs resting against heat coils.
When a heat coil operates, it
permits an inner spring to release, opening the circuit to the in-house
equipment and grounding the line. In addition, spring contacts released by
the heat coils cause illumination of a warning lamp at the top of the frame
to alert the station personnel.
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