Questions on the Economic Value of the Overhead Grounded Wire

Overhead grounded wires have been in extensive use since the construction of the earliest transmission circuits. The fundamental theory of their protective value is based on Faraday´s ice-pail experiment. As the resulting law goes, there is no electrostatic field emanating from the inner surface of a charged hollow conductor. The parallel grounded wires do not surround the power wires. Consequently the protection of these grounded wires against induced electric charges by thunderclouds, is only partial??usually of the order of 25 to 40 per cent. It might be erroneously inferred that several decades of use of the overhead grounded wire had established by practise its value. The several factors involved in its use do not lend themselves easily to experimental observations. For example, power lines extend over hundreds of mile while any particular induced charge is localized at some point in these vast distances. Taking into account the broef period of a lightning stroke. the unwilling observer stands a small chance of being near the point of discharge. Furthermore, thunder-clouds differ from one another. Still further, at the instant the lightning bolt takes place the distance from the thunder-cloud to the power wires varies quite indefinitely. In fact, there is a long list of difficulties involved in experimental observation of the effect of cloud lightning on power wires. As a result, except for a few small-scale experiments performed in the laboratory, knowledge of the subject is confined almost entirely to theoretical analyses.