[Trade Journal]
Publication: Proceedings of the National Electric Light Association 43rd Convention
New York, NY, United States
p. 677-679, col. 1-2
Ground Wires on Transmission Lines
The answers to the questionnaire regarding the use and operation of ground wires on transmission lines indicated that there is a considerable difference of opinion concerning their performance of the sixty companies operating transmission lines which answered the questionnaire, twenty-seven make it a practice to install ground wires.
Opinions regarding the protection afforded by ground wires are conflicting, some operators believing that they are of little or no value, while others feel they are quite indispensable. Very few operators, if any, hold that the ground wire will protect the lines from severe direct strokes of lightning, although some state their experience has shown that the lighter strokes are carried off by it. There is a more pronounced difference of opinion regarding the protection secured from induced lightning charges. The theory as to how a ground wire acts as an electro-static shield to the wires below it may be generally accepted. but there is a belief among a number of companies that the ground wire attracts direct strokes of lightning which might not otherwise strike a line, and to this extent it may cause rather than prevent trouble.
Subsequent correspondence and discussion in the meetings of the Committee have brought out, how-ever, that many of the companies operating large transmission systems favor the installation of ground wires, not only for protection from lightning, but also to protect the poles where grounded metal crossarms are used, as otherwise the failure of an insulator or the development of any fault bringing a conductor in contact with the pole or arm is very likely to burn off the pole top, particularly on grounded neutral systems. Of course, the grounding of the crossarms by running a ground wire vertically on each pole would protect the pole tops to a certain extent, but the installation of an overhead ground wire, properly and frequently grounded, gives much better protection, owing to the lower resistance of the resultant ground. Furthermore, a ground wire rigidly attached to the pole gives additional stability to a line because it acts as a continuous head guy, and when tightly strung tends to distribute the transverse stresses.
The methods of construction used by the companies having ground wires differ greatly. Fig 13 shows a ground wire bolted to the pole with a through bolt and clamp. Fig. 14 shows a similar installation where the ground wire is supported on a steel extension. Fig. 15 illustrates a ground wire supported on insulators and bracketed to the top of the pole. Fig. 16 shows a two circuit tower line which carries two ground wires, one over each circuit.
In order to get the best results the ground wire should be grounded frequently. The intervals between grounds naturally vary with the soil conditions, 400 to 500 feet spacing being satisfactory in wet or marshy ground; but in extremely dry soil, ground connections on every pole are required.
Some companies find it advisable to test the resistance of all grounds when installed, and to check periodically at least the ones in dry or unsatisfactory locations.
Galvanized steel strand is commonly used for the ground wire, although a number of companies use copper clad steel strand. The copper clad wire is adopted when the atmospheric conditions are such that galvanizing does not adequately protect the steel strand from rusting; also where a high conductivity is desired, although this advantage is generally a secondary consideration.
