Abstract :
The overhead grounded wire is used for three purposes: lightning protection, mechanical support for towers, and a test circuit. The functions of the grounded wire are subdivided into at least four categories: First, the vertical grounded wire; second, the lightning rod extending above the ground; third the electrostatic induction in the horizontally situated wires, and fourth, electromagnetic induction. The vertical wire prevents splitting of the poles. The lightning rod is of mooted desirability. The electrostatic induction for a given cloud on wires under various conditions is worked out in this paper. There is given also the protective values of overhead grounded wires in different positions and in different numbers. The effects of electromagnetic inductions have been taken into account. Theory is given to show that the grounded wire introduces into the main wave of induced lightning surge a superposed high frequency of electromagnetic induction. The several factors to be taken into account in the process of determining the protective value of a grounded wire are as follows: 1. Strength of electric field in the neighborhood of the line wires. 2. The direction of the gathering charge in the cloud, that is the path of the discharge relative to the line, parallel or perpendicular to the line before it turns vertically downward to the earth. 3. The screening effect obtained by the use of several wires, with and without grounded wires. 4. The initial momentary potential induced on a wire at the instant the cloud discharges to earth. 5. An instant after the lightning discharge has taken place, the sudden increase in capacitance between the power wire and the adjacent parallel grounded wire. 6. The effect of the number and location of parallel grounded wires. 7. The effect of electromagnetic induction between the horizontal part of the grounded wire and the parallel power wires, in which the energy of the lightning charge on the grounded wire is more or less transferre- to the power wire, instead of being dissipated in the earth. High frequencies are produced in this transformation. 8. The gradual transference of the charge which travels along the power wire to the successive sections of the grounded wire and its dissipation in the earth. A cloud charge is chosen of such value as to produce corona potential on a No. 000 B. & S. wire, strung at a height of 1000 cm. (33 ft.) above the surface of the earth. This storm cloud is used as a standard in all cases for comparison. The induced voltage on any wire by lightning is directly proportional to the height of the wire above the earth. The induced quantity is not quite proportional due to the variations in the capacitance of the wire at different heights. For heights between 30 and 60 ft. (9.1 and 18.2 m.), however the quantity can be considered as approximately proportional to the height. The quantity induced on the wire is only slightly affected by the diameter of the wire. This leads to the conclusion that a small grounded wire is nearly as effective as a more expensive large one. The theory is given to show that even on a non-grounded circuit a charge can be induced by a cloud and produce practically the same potentials as when the circuit is grounded. The only exception is that of circuits of short length. The instantaneous value of induced potential on a circuit is independent of the number of wires used. Using a greater number of wires reduces the quantity per wire but does not decrease the instantaneous value of the potential at the instant the cloud discharges to earth. Even the grounded wire may take the full potential and give no relief at the first instant. Whether it does or not depends upon how quickly the discharge takes place from cloud to earth, and how frequently along the line the grounded wire is earthed. There is, however, a screening of electrical energy by increasing the number of power wires. In other words, each surge has less energy although it has n
