Shield wires or shielding are wires used in high voltage power transmission lines to protect the cable insulation and also people and equipment. They are used primarily for protection from direct hit of lightning, and its corresponding surges.

When a stroke terminates on the phase conductor (Figure 1a), most of the current will discharge to ground through the nearest line arrester. Adjacent arresters will discharge some of the energy, limited by the span inductance.

The sharing is most effective on the tail of the surge, which is the portion of greatest energy content. If the struck pole or tower ground resistance is reduced due to soil ionization, the energy sharing is less effective.

An overhead shield wire is designed to intercept most lightning strokes that would otherwise hit the phase conductors. Most of the current will discharge through the tower and pole grounds, with relatively little current flowing through any line arresters.

This reduces the energy duty on the line arresters. Some lightning strokes may terminate directly terminate on a phase conductor, but these should have low current magnitudes (5 to 20 kA) according to the present theory of shielding.

Therefore, line arresters may be applied to shielded lines to improve the back flashover performance, with little concern for energy duty on the arresters.

Lightning Protection Performance Assessment of Shielding Failure for EHV Transmission Lines 
The operation results show that the shielding failure is the main cause of lightning accidents of EHV transmission lines in North China. The shielding failure performances of six 500kV transmission lines are evaluated by the Electric Geometry Method (EGM) in this paper. Read more...

Magnetic Field Cancellation (Active Shielding)
Our active magnetic shielding systems are used primarily for reducing powerline magnetic fields. They are effective for both overhead and underground lines. Within a defined area, the magnetic field can often be reduced to a very low level. Read more...

Optimal Design of Active Shielding for Power Lines
This paper analyzes the design of cost effective active loop field shielding for power lines. In this paper, a mathematical model is developed. Read more...

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