Porcelain strain insulator

AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS.

MEETING UNDER AUSPICES OF RAILWAY COMMITTEE.

A special meeting of the American Institute of Electrical Engineers was held in the auditorium of the Engineering Societies Building, New York city, on Friday evening, May 27,1910. William MeClellan, the chairman of Railway Committee, presided at the meeting and two papers were read, dealing with subjects connected with electric railways.

PORCELAIN STRAIN INSULATORS.

The first paper was “The Application of Porcelain to Strain Insulators,” by W. H. Kempton.

Porcelain has the advantages that it can be made in any shape, does not deteriorate. has high electric strength, and is not affected by frost. Its disadvantages are low tensile strength and lack of resiliency. It has fair shearing strength and good compressive strength and these make it available for strain insulators. Tests were made on actual insulators to determine breaking strength, ten samples being used. The average breaking stresses were, for tension, 654 pounds per square inch; shear. 2,400 pounds per square inch; compression, two forms of specimen, 12,690 and 16,370 pounds per square inch. respectively. The individual variations were large, the minimum values being about three-fourths of the average values. The spool, loop and barrel types were next considered individually, and the design of an ideal insulator given. The load-bearing portion of this has two exactly parallel surfaces held between two exactly parallel and rigid plates. A spool form of insulator has been made of 5.5 inches diameter and twenty-four inches long which withstands a mechanical stress of 34,000 pounds, and an electrical test of 80,000 volts. It has been shown that high mechanical stress does not weaken the porcelain electrically. For low-voltage work, the "goose-egg" insulator has proved reliable. This is an oval mass with grooves in each end at right angles to each other. Moulded insulators have proven more popular on low-voltage work, but are not suited to high voltage. They have also the dis-advantage of depreciation from exposure. Wood strain-insulators have also become popular.

CATENARY TROLLEY

This was followed by a paper by W. N. Smith on "Electric Railway Catenary Trolley Construction."

Since 1904 catenary construction has come into extensive use for electrified trunk lines and interurban railways. Although originally installed for single-phase operation at 2,200 volts, it is now used even for direct-current lines and at various voltages from 600 to 11,000. The principal features combined in this construction are the flat and smooth alinement of the trolley wire for high speed, the prevention of a trolley wire falling to the ground when broken, which is imperative with high potentials and the use of a pneumatically-operated sliding bow for making contact, which dispenses with the trolley rope. avoids the necessity of frogs at switches and will stay on the wire at any speed regardless of the swaying of the car. However, there has been a tendency to retain the use of the wheel trolley even in 3,300-and 6,600-volt lines. If the catenary line is put up with too much slack in the trolley wire it causes sufficient sag in ten-foot sections to make kinks at the hanger points and thus cause flashing and sparking with the sliding bow. There are three common types of catenary construction: 1. The original and most common is the plain single catenary, with the trolley wire hung directly from the messenger wire. 2. The compound or three-wire single catenary, in which the trolley wire is suspended by clips of uniform length from a tight secondary wire just above it, this secondary wire being supported by hanger rods of varying length from slack messenger wire above it. 3. Double catenary construction, consisting of two messenger wires and one trolley, the latter below and between the other two, the three wires being at the corners of an equilateral triangle and rigidly connected by triangular pipe spreaders varying from six inches to six feet on a side according to the sag of the messenger wire. This is the New York, New Haven & Hartford Railroad construction and, as it has been found too rigid, has been modified by the suspension of an auxiliary working conductor directly below the trolley wire and supported from it at points midway of each ten-foot section between hangers. The second type of construction is used considerably in Europe for single-phase lines Where wooden poles are used they should be invariably treated, at least at the butt end, with some good preservative agent. Steel poles are obtainable in three forms: the second or third section tubular pole, the "tripartite" sectional pole, and the "diamond" pole. The author thinks the tripartite poles have the advantage of being "paintable" at every point. Reinforced concrete poles have been used to a limited extent only. Porcelain petticoated insulators are now used almost invariably. The expense of steel bridges for supporting catenary trolley wires over more than two or three adjacent tracks can be greatly reduced by using span-wire construction. The trolley wire is supported by suspending from the span wire a stirrup of tee iron. Brackets and span wires should be entirely grounded. A light steel bridge construction has been used with spans of 300 feet and plain single catenary without any special adjustment or tension devices. Messenger cables of 500,000 circular-mil copper are used with these bridges and form the feeders of the line. This elimination of special messenger cables and making the feeders serve this purpose. along with the light bridges, has made this construction quite economical. For messenger wires seven-strand steel cables are generally used. The trolley wire is usually No. 000 or 0000 B. & S. hard-drawn copper. As the elastic limit of the latter is dangerously near the tension that is liable to be attained in winter, it is necessary to use "phono-electric," copper-clad steel or steel wire. These wires can easily stand the tension of 5,000 pounds to which they will be drawn in cold weather to insure absence of slack in the trolley wire during summer. In Europe, tension devices and flexible devices are generally used, but they have not found favor in this country. The excellent service rendered by the steel trolley wire on the New Haven system has suggested to the writer to use it with a copper messenger wire. Many designs of catenary hangers have been brought out. The desirable features that should be incorporated in them are simplicity, flexibility, certainty of grip, speed of application and adjustment and cheapness. A minimum number of lengths of hangers is also desirable to reduce the varieties to be kept in stock. There is a notable tendency to increase the distance between hanger points. Staggering the trolley wire from side to side of the center of track has not been found necessary to spread out the wear on sliding bows. The latest design of heavy catenary construction by the New Haven line has many marked advantages. Various types of steady strain insulators and pull-offs are used. Porcelain is becoming more common for strain insulators. The best form of section insulator is the overlapping break. The author of this paper is inclined to the opinion that the roller type of pantograph trolley has distinct advantages over the sliding bow and that it is quite likely to see such development in the not distant future as to permit its use in the heavy service of electrified trunk lines. In conclusion he points out that the complications in the overhead work with all double trolley systems have not only prevented their use in this country, but have hindered the development of three-phase lines as well.

In opening the discussion. P. H. Thomas spoke of the great value of definite figures for porcelain, as given in the first paper. In the suspension type of insulator, porcelain is used in its weakest form, and this matter should receive attention. Another point requiring attention is as to the relative behavior of porcelain from different manufacturers. Security in railway work can be obtained by allowing a proper margin of safety. On some high tension lines it has been noticed that the petticoat type of insulator was shattered by lightning without puncturing the material. It seemed like a mechanical shock.

C. J. Hixon stated that while the date 1904 might apply to the first catenary construction in this country, there was some in operation in Germany in 1903. Vertical flexibility is the keynote of successful collection of current from a trolley wire. The collector, in passing along the wire, should not only take up the sag but also raise the hanger a couple of inches to a floating position, thus eliminating the blow which would be given to a rigid support. This flexibility compensates somewhat for the inherent sluggishness of the roller pantagraph. With this flexibility, low tension in the wire and low pressure of the trolley, all strains are reduced and the cost of maintenance is decreased. On curves the pole spacing may be greater than given by Mr. Smith if pull offs are properly located.

R. D. Coombs considered it unnecessary to keep the trolley wire horizontal, since the track itself is not a plane. The question of flexibility is as yet open. The Germans find n secondary catenary entirely successful, hut their speeds and train-loads are less than our own. He has found a single catenary with a very light hanger satisfactory. A stiff and strong trolley wire, say steel with copper feed wire, would be worth trial in this connection. Such wire has been used on the New Haven installation.

Thurston told of troubles with slack wire and breaks and Charles Hart discussed rigid and flexible track and trolley wire. It is undesirable to have hard spots in a soft line or soft spots in a rigid line, for either will make trouble. He described a modified pantagraph, and a special hanger. Speaking of clearance, he declared eight feet to too little.

W. H. Kempton, replying to Mr. Thomas, expressed the opinion that the broken insulators were not caused by electrical stresses, but by heat from an arc. He also advocated large clearance and lining up trolleys on curves.

William McClellan predicted that practice would finally resolve itself into the use of a cable suspension between poles carrying the catenary and the elimination of all hard spots.

W. N. Smith, in closing, expressed a preference for the roller pantagraph over the sliding pantagraph.