Conductor Shields for Overhead Cables

When a bare conductor in air is energized, we all recognize that the electric field lines emanate from the
conductor surface and terminate at ground. If the conductor is round and smooth and the distance to
the ground plane is proportionately large, the electrical stresses at the conductor surface are quite evenly
distributed (Fig. 1). However, if there is a sharp burr or defect on the conductor surface, electrical stresses
will be concentrated at the burr, and the insulating air will be more highly stressed in this area (Fig. 2).
As the voltage on the conductor is increased, there will be a level at which the air can no longer endure
the increasing electrical stress and it will break down at that point. This breakdown causes the air in the
immediate vicinity of the conductor to ionize and appear as a "glow" of light around the conductor. This
condition is called CORONA.............

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