[Trade Journal]
Publication: The Railroad Gazette
New York, NY, United States
vol. 44, no. 6, p. 187-188, col. 1-2,1
Transmission Line Crossings Over Railroads.
BY RALPH D. MERSHON.
The rapid increase in the number of electric transmission lines and the distributing circuits therefrom, is rendering every day more important and more perplexing the question so frequently encountered as to the protection, if any, which should be provided at railroad crossings.
The fear which the steam railroad operator has in regard to such crossings arises from two sources. He fears the damage which might result through mechanical agencies, in case the transmission line should fall upon the track. He fears, also, the damage which he thinks might result through electrical agencies. The fear of the electric current is usually the greater, probably because of the fact that most steam railroad operators have had little, if any, experience with electricity, and they attach to it more or less of the mysterious dread which people have for it generally.
The protection for which the railroad man generally asks, is a steel bridge constructed underneath the transmission line for that portion of its length which is across the railroad company's right-of-way; or, at least for that portion of its length which is across the tracks. In some cases, it has been required that the transmission line be carried on insulators attached to this structure; such a requirement is an undesirable one from every standpoint.
There can be no question that a transmission line can be made as strong as any steel protecting bridge that can be installed, and, in general, strength can be obtained at a great deal less expense than is involved in installation of the bridge. A simple span of wire or cable supported at each side of the right-of-way, on steel structures if necessary, can be made as strong, both as to supporting structures and as to the cables, as any bridge which can be erected. It seems foolish, therefor& to insist upon the installation of a bridge below a transmission line for fear that through mechanical agencies, the transmission line may be thrown down.
Probably no railroad operator would make special objection to have built across his tracks a construction similar to that of a well designed transmission line, if he knew no current was to be put upon it: but, in some instances, the mere idea of having current on the wires, seems to introduce immediately a fear of the construction—both mechanically and electrically. In such cases, the ultimate source of the fear and objection to the transmission line crossings may be said to lie in the fact that the lines carry an electric current; and it is well, therefore, to examine into the possible ways in which the existence of an electric current on the lines may affect the safety of the property or employees of the railroad.
Assume a crossing with the same side clearance, overhead clearance and factor of safety as would be allowed in the case of a bridge. Assume that there is no electric current on the wires. Under such conditions, no damage except such as is equally likely in the case of a bridge can result to the property or employees of the railroad company.
Now assume that the electric current is put on the crossing. The added possibilities of danger are as follows:
(1) The possibility of overhead contact either direct or through some conducting object.
(2) The presence of the electric current in case the structure is thrown down by a train wreck, by wind, or other mechanical forces.
(3) The possible effect of the electric current in causing the line wires to fall.
The probability of (1), overhead contact, can be indefinitely reduced by increasing the overhead clearance, if such clearance as would be allowed for a bridge is not considered a sufficient insurance.
The probability of (2) can, so far as wrecks are concerned, be indefinitely reduced by sufficient overhead and side clearance; and, so far as wind or other forces are concerned, by proper design and construction with reference to ice and wind loads. The practice in these matters in the case of bridges would seem to be amply sufficient, especially in view of the fact that, as mentioned later on, in a well designed and properly operated transmission system, anything which would cause the line wires to be thrown down, would cause the power to be cut off by reason of the resulting short circuit.
The question, therefore, finally resolves itself into an examination of (3), the effect which the presence of the electric current might have in causing the line wires to fall upon the tracks.
The only conceivable way in which the presence of the electric current could be the cause of the contingency mentioned is that of the establishment of an arc which should burn off one or more of the line wires. Such an arc might be established between two or more of the line wires, or between one or more of the wires and the structures supporting the transmission line. An arc between a line wire and the supporting structure could occur only in case the structure is metal, or in case the insulators are supported upon metal pins connected to each other or to ground.
An arc between the line wires, if it be instituted at a point distant from the supporting structures, can hardly be conceived of as due to anything other than the swinging together of the wires, or to their having thrown across them, either maliciously or by accident, some conducting object. The chance for an arc to be started by the wires swinging together can be removed entirely by putting them sufficiently far apart. The chance for an arc to be established by something being thrown across the line wires can be made very small by recourse to the same expedient.
The occurrence of an arc between one or more of the line wires and the insulator pins or the steel structures carrying them, might be due to a direct puncture of the insulator by electrical means, or to breakage of the insulator by mechanical means, or to the establishment of an arc around the insulator to the pin by an initial creepage of current over the insulator surface, when the insulating value of such surface had been reduced by a film of moisture, dirt or other conducting or semi-conducting substance. It should be noted that the first and last of the three possible causes mentioned would be extremely unlikely except in the case of high voltage lines, and, even then, not at all likely, if the insulator be well designed, except in the case of lightning.
In the case of the occurrence of an arc from any of the causes mentioned, the arc would not, with a properly designed transmission system, exist for any considerable portion of time, since its occurrence would cause the automatic circuit breaking appliances in the generating station to operate, thus opening the circuit and stopping the arc. The amount which such an arc could do towards burning a line wire asunder, or otherwise damaging the crossing, would depend upon the amount of power behind the arc, the length of time it lasted, and the size or mass of the metal from which the arc was drawn. That is to say, it would depend upon the amount of heat generated in the arc tending to melt the wire or other metal concerned, and the amount of metal there was present to conduct away and absorb this heat, and thus diminish the melting action of the arc. The size and weight of the wires and other metal parts involved could be so proportioned relative to the destructive potency of any arc which might be formed that they would easily withstand the melting action, without being burned off, until such time as the automatic protective devices at the generating station opened and the arc ceased.
It would even be possible to make such provision that, in case an arc formed, it would rupture itself, even if the automatic protective devices at the generating station did not operate; and so that it would rupture itself before any serious damage to the crossing could occur. To accomplish this, it would be necessary to properly proportion not only the metal parts, but also the distances separating those parts between which an arc could occur; that is, the distances between the line cables and between the line cables and the supporting structures. Such proportioning should be done with reference to the voltage and power capacity of the transmission system; the less the voltage, the less the distances would have to be; and the less the power capacity of the system, the lighter could be the metal parts.
It would appear, therefore, perfectly possible to design a transmission line crossing so that it could not constitute a source of danger from either a mechanical or electrical standpoint. Such a crossing would preferably be one along the following lines:
(a) It should be so constructed that the line conductors (line wires or cables) and the supporting structures at each side of the track would be of proper strength to withstand the ice and wind loads which might come upon them. It should be self-sustaining; that is, should be capable of standing up under the action of wind and ice without reference to the remainder of the line, so that the line, on one or both sides of the crossing, might break without interfering with the crossing itself.
(b) There should be sufficient overhead clearance between the line and the track, so that there would be no possibility of contact except by deliberate Intent.
(c) The line conductors should be far enough apart so that they could not swing together.
(d) The line conductors should be sufficiently massive so that an arc might exist between them for several seconds, without danger of burning or melting them off.
(e) If the supporting structures are of steel, or the insulator pins are of metal and the pins connected to each other, or to ground, the insulators should have cast metal caps cemented upon them. These caps, or extensions of them, should extend out on each side of the insulator for some distance along and underneath the conductor, in order to further protect the conductor, or else the conductors should have, in addition to the caps, a protection from arcs in the form of a serving of wire upon them for some distance on each side of the insulator. The result of such protection will be that an arc formed near the insulator will expend itself upon the serving wire, or metal casting, instead of upon the conductor itself.
It is not to be understood from the above that a crossing would necessarily consist of a single span. It might consist of a series of spans, meeting the above requirements, where several spans were necessary for crossing a number of tracks, such as would be found in a railroad yard. A crossing constructed on the above lines would be as safe as any reasonable individual could ask, and at the same time would be a great deal cheaper than the steel bridges sometimes insisted upon.
The matter of steel protective bridges is a very serious one to the transmission companies, not only on the score of expense of the bridges themselves, but because of the difficulty often met in installing them or getting permission to install them, especially when they are required in towns and are objected to by the municipality. If they are insisted upon, it will greatly retard the development of those sections of the country fortunate enough to be within electric reach of cheaply generated power, since the requirement of a bridge for a railroad crossing will, in many cases, prohibit the supplying of small customers with power. This, of itself, is a matter of importance to the railroads in that on the welfare and progress of the community depend the amount of, and increase in, the railroad company's revenue, but there is an aspect of the question aside from this which should be the subject of much thought by railroad men. The time is undoubtedly near at hand when all of the principal railroads of the country will equip a portion at least of their lines for electric traction. If this is done, the railroads cannot hope to be free from the necessity of carrying their line conductors across not only their own rights-of-way, but the rights-of-way of other roads. If the railroad men now insist upon the elaborate means of protection which have been insisted upon in some cases, and crystallize public opinion to the idea that such elaborate protection is necessary, they themselves will be confronted with the necessity of making use of the elaborate and expensive protection in question.
It behooves the railroad man, therefore, to consider this matter of crossings very carefully and with the utmost spirit of fairness, since in so doing he will not merely serve the interests of the transmission companies, but also the interests of his own and other railroad companies in the future.
