The essential
additional component is shielding. However, where is it placed, what materials
are used, and what does it do to the dielectric field? Let us start from the
conductor again and move outward fiom the center of the cable.
Conductor - Nothing unusual as compared
to a non-shielded cable.
Conductor Shield - A conducting
material is placed over the conductor circumference to screen (shield) out
irregularities of the conductor contours. The dielectric field will not be
af5ected or “see” the shape of the outer strands (or other conductor contours)
due to the presence of the conductor shield (screen).
Electrical Insulation (Dielectric) - The
differences between insulation for a non-shielded cable as compared to a
shielded cable are in material, quality, cleanliness, and application. The
thickness applied is primarily influenced by considerations of electrical
stress (voltage gradients).
Insulation Shield - his is a two-part
system, consisting of an auxiliary shield and a primary shield.
Auxiliary Shield - A conducting
material that is placed over the outer diameter of the cable insulation. This
material must be capable of conducting “leakage” current radially through its
wall without creating an abnormal voltage drop.
Primary Shield - A metallic layer of
tapes, wires, or a tube that is placed over the circumference of the underlying
auxiliary shield. This must be capable of conducting the summation of “leakage”
currents and carry them to the nearest ground without creation of an abnormal
voltage drop.
Dielectric Field - A dielectric field,
composed of electrostatic flux and equi-potential lines, exists when the conductor
is energized. There is no distortion in this dielectric field because of the
shielding of insulation and conductor. Electrostatic flux lines are symmetrically
spaced and equi-potential lines are concentric.
However,
observe features not previously noted; the electrostatic flux and equipotential
lines are spaced closer together near the conductor shield as compared to their
spacing near the insulation shield. This is why we are cognizant of maximum
stresses at areas of minimum radii (and diameters). Insulation voids at the
conductor shield are more critical than voids at the insulation shield.
Also these
lines are spaced closer together at the minimum diameter (or radii).
This
substantiates the maximum radial stress theory.
Insulation Thickness - The use of
shielded cable permits using cables that are more economic to manufacture and
install as compared to non-shielded cables that would require very heavy
insulation thickness.
Jacket or Outer Coverings - Over the insulation
shielding system, the cable contains components that provide environmental
protection for the cable. This can be extruded jacket (of synthetic material), metal
sheath or wires, annoring, or a combination of these items.
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