Publication: The Electrician & Electrical Engineer
New York, NY, United States
THE CONSTRUCTION OF LINES FOR ELECTRIC CIRCUITS.
BY THOMAS D. LOCKWOOD.
(Continued from page 135.)
IF poles are strong enough to do their work, and if good wires are employed there is little danger of wires breaking across a highway ; but at the same time it is wise to avoid all unnecessary crossings as accidents will happen, and a sharp frost or a heavy sleet storm may break down the best of lines. In some countries this is regarded as such an important point, that oblique crossings are not permitted at all, and where crossings are absolutely necessary, the poles on the opposite sides of the road are required to be immediately opposite one another. This seems to be rather an excessive caution.
Another question to be considered is the height of poles. Through the country for a line of but few wires—say one to six, a 25 foot pole seems to be quite long enough, and a good rule is to make 20 feet the minimum distance between the lowest wire and the ground. If the poles have to be set unusually wide apart, it will be better to add a couple of feet to the length as the dip at the centre of the span will be considerable. In cities, and at road or railway crossings as also where there are trees, the length will necessarily depend on the surroundings, and the judgment of the constructor must here be called into play. It is good practice to avoid sudden extremes in passing from long to short poles or vice versa, and the rise or fall should be made gradual so as to present a symmetrical appearance.
How many poles should be used per mile? This again depends absolutely on the conditions to be complied with. For one or two wires through a straight and clear section of country I would not use more than 25 poles to the mile, while if the surface of the country is rolling, and the curves and crossings abundant, it may be necessary to increase to as high a number as 35 to the mile. The average number throughout the country is probably about 30 to the mile. Ordinary poles such as these, should never have a less top diameter than five inches, and the longer a pole is the stouter it must of course be.
Supposing now that the line of route is decided upon, and the poles purchased and prepared ; the next thing is to have them delivered and distributed. If the line is to have 25 poles to the mile, a pole must be dropped about every 70 yards. If men are plentiful, it may be well to have one set of men go ahead to dig the holes, while another squad follows close to do the planting. It may seem to be a very simple thing to dig a hole properly, but none the less it is one of those minor operations which have to be more closely watched than greater ones. Not long since a case came under my notice wherein it was found that after a section of line was built by contract, the poles, which were at least 30 feet high, were planted in holes scarcely two feet deep; a circumstance which proves that even a hole for a telegraph pole should not be trusted to a contractor. Holes should always be dug with their length in the line of the wires, so that the smallest part of the loose earth after filling the hole, shall be in the line of lateral strain. If work is let out to contractors, a proper inspector should in all cases be present to see that it is properly done. How deep then should poles be set? I should say, under no circumstances to a depth less than five feet. 1 am aware that it is customary to make four foot holes serve for 25 or 30 foot poles, but it is not good practice. A hole five feet deep will do for all poles 35 feet long or under (of course for temporary work, no such depth is essential), and as the poles increase in length the holes should increase in depth. All poles before being set should have a mark put on them a definite distance from the butt end, so as to indicate when set the depth of the hole.
In digging, the tools which are found the most universally convenient, are the pick, the ordinary shovel, the spoon shovel, the digging bar ; and in some localities where the soil is soft and friable a variety of spoon like a short funnel, or a suction pump bucket is also very useful. This, as well as a two bladed scoop or spoon, rigged up with a swivel, ice-tongs fashion, are much used in the south. Through a rocky country, blasting appliances are also found necessary. In addition to all these, where the soil is light, time can often be gained by using a boring tool, or post auger. This tool has a cross handle and is worked by two men who walk round pushing the handle in front of them. To work it easily it must be lifted out and cleared of the loose earth, quite frequently. It has also the additional disadvantage that it cannot be worked near a wall or fence, but needs a clear sweep for the operators to work, but it makes a nice compact hole. Holes must be kept as near the size of the pole butt as possible, but if deep, may be widened in one direction—that of the line—at the surface, or even dug in steps on one side, but this is simply a concession for the sake of greater ease in setting the pole. On the whole my individual preference is simply for the pick, shovel, digging bar and spoon. Poles of ordinary size can easily be lifted into the holes ; but for very large ones, a shears or derrick is required. As soon as a pole is placed in the hole, it must be turned round, to the position it is required to stand in ; if only brackets are attached, these should be on the side away from the road. If crossarms, they should be sighted so as to stand directly at right angles to the line of the roadway.
At crossings, curves and corners, the pole must be so twisted, that the strain of the wire will press against the pole and not away from it. Wherever the roadway is straight, the pole may and should be set quite upright, but on curves, they ought to be set a little aslant so as to lean back against the strain.
The next thing to consider is the "tamping" or as they call it in England, the "punning." This is another of the minor operations which is often done carelessly, but which really demands great care.
Only a little of the earth should be thrown in at once, and this is to be well tamped before any more is to be put in. Stones put in the hole close to the pole aid in supporting it. On the faithfulness exhibited and care expended in tamping, the permanency of the line in a great measure depends. The earth ought to be banked up round the pole, as this prevents the formation of depressions which would tend to hold water, at the earth line. It is always well to let the poles stand a while and settle, before the wire is strung, because their strain tends to pull the pole out of place.
LIGHTNING RODS AND LEAKAGE CONDUCTORS.
In some parts of the country, lines are when unprotected, continually exposed to danger from discharges of lightning. In such districts, the poles are fitted with lightning rods, which extend 18 inches or two feet above the top of the pole. No. 8 galvanized iron serves very well for this purpose, and it can be fastened directly to the pole by strong staples. Figure 10 shows the general arrangement adopted, which is found to answer well, although the side extensions, earth for these rods when they are used, is to procure a sheet of galvanized iron and thread the No. 8 wire, through holes prepared in it, soldering the connections thus formed at each hole, and then before putting any earth in the hole, bending the plate so as to form a loose sleeve around the pole, of considerable surface.
This can be still further improved by letting a loose leaf of the iron sheet lap over the bottom of the hole, so that the iron not only surrounds, but is also under the pole when set. In ordinary soil a good earth plate is thus made.
In England almost every pole is furnished with an earth wire, arranged as shown in figure 11.
The rod is attached to the pole before it is set, and the ground end is formed into a fiat helix or volute placed under the foot of the pole. I have measured the resistance of both of the earth connections which I have here described and have found the plate to be uniformly the lowest. In addition to the use of these devices as lightning guards, they have still another use. It is quite impossible especially in wet weather to so insulate wires, that no electricity shall escape from them. When two or more wires are stretched on the same poles, whatever current escapes from any of them, will of course divide itself among the routes open to it. The several routes, are first, by the pole itself direct to earth; and second by the way of the other wires to earth at other points. Now the portion escaping via the pole, does not cause any special inconvenience, as its only result is to weaken the signals on the leaky wire; but the portion which escapes on to the other wires sometimes proves a great nuisance, as it interferes with the signals passing over them. Much of the so-called "induction" on telephone lines is due most certainly to absolute leakage, and when this leakage is heavy, the contact conductor or earth wire is useful and its function is to intercept the leakage and conduct it direct to earth.
On short telephone lines —say for those not exceeding 15 miles in length, this device has proved useful. For longer circuits the capacity of the lines is so increased that retardation due to static induction is plainly perceptible, being due to the near proximity of the earth, which by means of close up to the line-wires. Therefore, while under certain conditions the earth wires are advantageous, I do not advocate their employment, except when rendered necessary by the prevalence of lightning storms. Moreover, unless a good ground can be obtained, it is far better to have none; because if a rod of metal runs down a pole, and does not make good earth, it only operates to attract the atmospheric discharge, which, finding difficulty in passing off, destroys the pole.
So far as the use of the rod as a contact or leakage conductor is concerned, it would seem that the method of preventing interference between conductors by carrying the interfering currents to earth, is but a makeshift, and we fully agree with Pope in his opinion written years ago, that "a much more economical and effective method of obtaining this result, is that of improving insulation." Poles as well as housetop fixtures, need guying against any side strain, such as occurs at corners and on sharp curves. For a line of from one to six wires, a guy or stay wire should be composed of about three No. 8 iron wires, and the distant end of the guy may be fastened to a tree if one is convenient, or to any adjacent fixed object.
(To be continued.)