Abstract :
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 miles, while any particular induced charge is localized at some point in these vast distances. Taking into account the brief 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. This paper is an addition to the theory but it is not of a mathematical nature. Conditions of protection have changed in recent times. Therefore, in this paper the definite conclusion is drawn that the expense of overhead grounded wires on wooden pole lines is, in general, an economic waste. In particular cases it may be justified. On metal tower construction the use of the overhead grounded wire is, in general, fully justifiab- e. The analyses in this paper were made for presentation to a Public Service Commission. This Commission, on reconsideration, reversed its order that an overhead grounded wire should be installed on a 13-kv. transmission circuit supported on a wooden structure. Review of New Material. — References to the technical literature on the subject of overhead grounded wires are given in the bibliography which follows the paper. For those familiar with the subject there is given below a brief review of several parts of the paper which emphasize the recent additions to the knowledge of the subject. 1. Analysis of the functions of the overhead grounded wire under nine distinct parts, where previously only three functions were classified. 2. Recognition that the requirements of the overhead grounded wire are less than formerly. In the early days the overhead grounded wire was needed to assist lightning arresters, but today the arresters have sufficient discharge rate not to require the assistance of the grounded wire. 3. Analysis which points out that the overhead grounded wire protects only for a specifically limited range of voltage. It is no protection for induced voltage below the normal arc-over value of the insulator, and no protection when the induced lightning voltage is sufficient to arc-over the insulator in spite of the presecen of the grounded wire. 4. Appraisal of the weight to be given to each of the nine functions of the overhead grounded wire and considerations of its cost lead to the conclusion that, barring exceptional cases, it is an actual detriment when placed on semi-insulating structures, such as wooden pole lines. Used here the overhead grounded wire lowers the arc-over voltage of lightning. 5. The overhead grounded wire with considerable sag cannot be considered as a mechanical support to rigid tower structures. 6. Analysis is given to show that the overhead grounded wire on a metal tower line loses its function in protecting arc-over of insula
