SHIELDED POWER CABLE BASIC COMPONENTS

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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