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For 20,000 volts an insulator made entirely of porcelain was designed. Fig. 196 affords a very good idea of this type. In
Fig. 197 is the porcelain pin base employed in combination with the insulator of Fig. 196.
The insulator used on the transmission line from Niagara to Buffalo, together with its wooden pin, and section of the cross-arm, is show in Fig. 198. The eaves tend to shed the water at two points, where it will not drip on the cross-arm.
An interesting insulator is the one used by the Telluride Power Trans
Fig. 795. Built-up Porcelain Insulator with Iron Pin.
mission Company to transmit power 35 miles, from Provo to Mercur, Utah. The pressure, 40,000 volts, was the highest employed up to that time (1898) for commercial use. Fig. 199 shows the Provo insulator.
A still newer type is that shown in Fig. 200. It is of brown china ware with a glass or porcelain cone extending down around the pin, which is of wood with a porcelain sleeve and base. The idea of this sleeve is to make the striking distance greater, this being of as much importance as that the length of the path from the cross-arm to the wire be made long. This insulator is in diameter, about 15" high, and weighs about 12 pounds.
Another feature is the beveled trough around the top, which catches all the water at the periphery, and carries it off to one side of the cross-arm.
Insulated Wire for Overhead Lines.— For long-distance transmission bare conductors, described at the beginning of this chapter, are generally employed, even with very high voltages. For local distribution, especially within the limits of cities and towns,
electric light and power overhead wires are covered throughout with insulating material, to reduce the danger of accidental contact with persons or with other wires or conducting bodies.
The insulation of overhead wires is in two parts. One of insulating material impervious to moisture, placed next to the wire, and the other of some substance fitted to resist abrasion or like mechanical injury. The inner coating is a rubber compound, or for lower grades some cheaper substitute. Before this is laid on the wire it is first tinned to prevent the sulphur contained in the rubber compound from corroding the wire. This inner coating is then covered with a hard braid of cotton or hemp, woven on to the wire, or the wire is served with a tape and insulating compound. Where the wire is to be continually moist, gutta percha is better than rubber, but it is more costly.
In the more expensive grades of wire the coatings are greater than two in number, and they alternate, insulating compound and then braid or tape.
In Figs. 201-208 are shown "* 200- i00*8 m p"tent,al lnsu"aorthe manner of application of the insulation and the braid.
Joints in Overhead Lines. — Whether an electrical conductor is bare or insulated it is necessary that any joint made in it shall be nearly equal in conductivity and in mechanical strength to the
Fig. 201. Insulated Line Wire.
rest of the conductor. The ordinary "lineman's splice" (Fig. 204) has been the standard practice for galvanized wire iron, in telegraph lines; but the use of copper wire, both hard and soft drawn, and the necessity for better connection with heavy currents, has resulted in the adoption of various special forms of joint. Of these the Mclntire joint illustrated in Fig. 205 is a prominent example.
202. Standard Conductor, Insulated for Outside Work.
Fig. 203. Overhead Wire with Weatherproof Insulation.
This joint is made by use of a "connector" which consists of two tubes drawn side by side out of one piece of copper. The internal diameter of each of these tubes corresponds to the ex
Fig. 204, Lineman's Splice.
ternal diameter of tne wire to be spliced. The two wires need not be of the same size.
Fig. 205. Mclntyre Wire Joint.
The joint is made by slipping the wires inside the tubes, and then by means of special pliers, twisting the tubes one on the other; thus by friction the two wires are bound firmly together. Unless required by Insurance Rules, they need not be soldered, a great advantage with hard-drawn copper wire as it avoids annealing the wire, and the joint more nearly retains the full strength of the wire.
In the "lineman's splice" the actual area of contact wire to wire is small, and unless well soldered the crevices will afford places for starting corrosion, and the resistance will be high.
The Mclntire joint affords plenty of contact area, giving a low resistance and being impervious to moisture. This form of joint is especially valuable with the aluminum wire that is now coming into use.
All joints made in insulated wire lines should be taped and painted with an insulating compound till the insulation over the joint is as good as that on the wire of the line.
Method of Attaching the Line Wire to the Insulators. —The ordinary plan is to take a simple U-shaped tie-wire, place the curve of it around the insulator, and wrap up the projecting ends around
the line-wire. This puts a side pull on the line-wire which objectionable feature of this tie is indicated in Fig. 206. This might, in the case of hard-drawn copper wire, cause breakage, because it is quite brittle.
The standard method now in use is shown in a completed form in Fig. 207. A soft copper tie-wire is laid in and around the insulator groove, in such a manner that one end comes over, and the other end under the line-wire; the ends are then wrapped around the line-wire. A method of making the tie is shown in Fig. 208. When properly made in this way the line-wire is anchored to the insulator with no side pull.
Tie-wires should be the same size as, or slightly smaller than, the conductors themselves. This is true even when the line-wires