As the cable is thus held twelve inches from the terra cotta pipe all the way around, any leakage of current must pass over this length of glass surface at each cable or through the air.
A heavy coating of frost sometimes collects on these plates, and this increases the amount of current leakage over them. Surface leakage in a case of this sort, of course, varies with the size of the glass plate, and if a tile pipe is used the limit of size is soon reached.
There seems to be no good reason, however, why a glass plate of any desired dimensions should not be set directly into the brick wall of a station for each line wire and the tile pipes entirely omitted. This plan is followed on the system of the Utah Light & Power Company, which extends to Salt Lake City, Ogden, Provo, and a number of other points in that State.
On the 40,000-volt line of that system an entry for each wire is provided by setting two plates of glass into the brick wall, one plate being flush with the inner surface and the other with the outer surface of the wall.
In the centre of each plate there is a hole of about 2.5-inch diameter, into which a glass or porcelain tube fits. The line wire enters the station through this tube, and it does not appear that any shelter for the glass plates is located outside of the building. An entry of this type for the 40,000-volt line with glass plates in a brick wall at a gable end of the Murphy mill is said to have given satisfactory results during four years, though that wall faces the southwest, from which direction most of the storms come. At this entry each glass plate is not more than eighteen inches in diameter, and the wires are about four feet apart. On a 16,000-volt line of the same company, a glass plate twelve inches square with a three-quarter-inch hole at its centre, and the bare wire passing through without a tube, has given results that were entirely satisfactory.
Two quite different types of entry to stations are used on the 50,000-volt line between Cañon Ferry and Butte, Mont. One type, employed at the side wall of a corrugated iron building, consists of a thick bushing of paraffined wood carrying a glass tube two inches in diameter, four feet long, with a side wall of five-eighths to three-quarter-inch, through which the line conductor passes.
On the roof of the power-station at Cañon Ferry a vertical entry is made with the 50,000-volt circuit. For this purpose each line wire is brought to a dead end on three insulators carried by a timber fixture on the roof. A vertical tap drops from each line wire and passes through the roof and into the station. This roof is of wood, covered with tin outside and lined with asbestos inside. Each tap is an insulated wire, and elaborate methods are adopted in the way of further insulation, and to prevent water from following the wire down through the roof.
Over the point of entrance sits a large block of paraffined wood with a central hole, and down through this hole passes a long cylinder of paper that extends some distance above the block. Into the top end of this cylinder fits a wood bushing, and a length of the tap wire that has been served with a thick layer of rubber is tightly enclosed by this bushing. The rubber-covered portion of the tap wire also extends above the bushing, and has taped to it a paper cone that comes down over the top of the paper cylinder to keep out the water. On the outside of this paper cylinder, at a lower point, a still larger paper cone is attached to prevent water from following the cylinder down through the wooden block. At the lower end of the paper cylinder, within the station, there is another bushing of wood, and between this and the wooden bushing at the top of the cylinder and inside of the paper cylinder there is a long glass tube. Down through this tube and into the station the insulated tap wire passes.
From the experience thus far gained with high-voltage lines, it seems that their entrance into stations should always be at a side wall, unless there is some imperative reason for coming down through the roof. If climatic conditions permit, no form of entry can be more reliable than a plain, ample opening through the wall with a large air-space about each wire. If the opening must be closed, it had better be done with one or more large plates of thick glass set directly into the brickwork of the wall. Some additional insulation is obtained by placing a long glass or porcelain tube over each wire where it passes through the central hole in the glass plates. Each conductor should be bare at the entry, as it is on the line. Some of the above examples of existing practice in entries for transmission lines are taken from Vol. xxii., A. I. E. E.