Publication: American Institute Of Electrical Engineers
New York, NY, United States
TRANSPOSITION AND RELATIVE LOCATION OF POWER AND TELEPHONE WIRES.
BY P. M. LINCOLN.
A consideration of the causes of electrical disturbances in telephone lines which parallel high-tension lines, and of means for reducing these disturbances.
(1) The extraordinary sensitiveness of the telephone receiver makes this instrument peculiarly susceptible to electrical disturbances. One authority states that the energy used in a sixteen candle power incandescent lamp is sufficient to produce an audible sound in thirty billion receivers. The methods, therefore of shielding telephone wires from the inductive effects of neighboring wires become important. Particularly is this true in the case of a telephone line paralleling a high-tension transmission line, where the inductive disturbances are apt to be large, and uninterrupted service on the telephone line important.
(2) The remarks and discussion in this "Introduction to Discussion" apply particularly to telephone lines paralleling high-tension lines, but comments hereon need not be restricted to such cases.
(3) There are three ways in which disturbing current in telephone circuits may be caused by the high-tension circuit.
(1) Electromagnetic induction.
(2) Electrostatic induction.
It is the first two causes of disturbances which will claim particular attention in the following discussion.
(4) Electromagnetic induction may be briefly described as a transformer action. In Fig. 1 let a, b and c be the conductors of a three-phase line, and m and n the two wires of a paralleling telephone circuit. a and b may then be regarded as the primary and m and n as the secondary of a transformer. The e.m.f. in circuit mn will depend, among other things, upon the amount and frequency of the current in the inducing circuit. By transposing m and n in the well-known manner, the e m.f.'s set up in one part of the telephone circuit will be neutralized by equal and opposite e.m.f.'s set up in other parts. Thus, the electromagnetic effects between m and n may be entirely neutralized by transposing the telephone wires only, regardless of whether the transmission wires are transposed or not. It may be well to note, however, that while the e.m.f. between the two telephone wires may be reduced to zero by properly transposing the telephone wires only, the e.m.f. between the two telephone conductors considered as one side of a circuit and the earth as the other, can be reduced to zero only by transposing the power wires. This point is of little importance, however, as any electromagnetic e.m.f. between the telephone wires and ground is entirely overshadowed by the electrostatic which will be considered later.
(5) Electrostatic effects will also take place in m, n, due to transmission circuit a, b, c. If conductor a has a minus charge for instance, it will induce a certain plus charge on m and a smaller plus charge on n, on account of n's greater distance from a. If now the minus charge be removed from a, current will flow from m to n, proportional to the difference in the amounts of these charges. The electrostatic influence of b, being opposite a in sign, will reinforce the action of a. Transposition of the telephone wires will have the effect of neutralizing this tendency of setting up electrostatic currents between m and n. It is important to note that a system of transpositions designed to correct electromagnetic induction between the wires will also be correct for electrostatic induction.
(6) Considering the comparative electromagnetic and electrostatic disturbances in a section of untransposed telephone line, it may be interesting to observe that the first is in the nature of a constant potential effect and the second of a constant current effect. It is evident that induced electromagnetic e.m.f. is constant as long as the inducing current is constant. As for the electrostatic effect, it is evident that the amounts of the induced charges on in and n, and therefore the electrostatically induced current between them, will not become appreciably reduced until the current flowing between m and n makes a difference of potential between them appreciable, compared with the inducing difference of potential. With telephone receivers of varying resistance, therefore, the ampere-turns in the receiver due to electromagnetic induction are practically- constant, while those due to electrostatic induction increase with number of turns and therefore the resistance of receiver. The electrostatic and electromagnetic effect become roughly equal with an arrangement shown in Fig. 1, when a, b, c is a line carrying 50 amperes at 20,000 volts, and the telephone circuit contains a total resistance of 1,000 ohms, including receivers.
(7) The bridged telephone has almost universally taken the place of the series instrument for all telephone work. The series telephone is particularly objectionable for use on a circuit in which static induction takes place to any great extent. The reason for this is seen by an inspection of Fig. 2. The telephone wire m has between A and B a plus charge induced and between B and C a minus charge. There is, therefore, at the transposition point B a flow of current from one section of m to the other. If now a series telephone be placed in series with m at B, it not only gets the benefit of this charging current between the two sections of m, but it also creates a difference of potential and, therefore, disturbing currents in telephones at A and C as well.
(8) Although a proper system of transposition will prevent the establishment of an induced e.m.f. between the two telephone wires, it does not necessarily prevent the two wires from assuming a potential which differs from that of the earth. In a properly transposed system, each telephone wire is the same average distance from each power wire. The potential, therefore, which the telephone system tends to assume from the static induction of the power wires is that of the neutral point of the power system. By neutral point is meant that point between which and each of the power wires the average e.m.f. is the same. Under normal conditions this neutral point is at ground potential. If, however, leakage takes place from one of the power conductors to ground, this neutral point will differ in potential from the ground and the amount of this difference becomes greater as the resistance of the leak becomes less. In a three-phase system, when the resistance of this leak becomes zero, the maximum difference of potential between the neutral point and ground occurs, and is 58 per cent. of the power circuit voltage. In a 20,000 volt system, for instance, there may exist a potential of nearly 12 000 volts between the neutral point and ground. When the neutral point of the power line differs in potential from the ground, an electrostatic difference of potential tends to exist between the telephone wires and earth, and will exist if the insulation of the telephone circuit is perfect.
(9) The amount of this electrostatic potential between the telephone circuit and the earth will depend upon the relative capacities between power and telephone lines on one hand and between telephone line and earth on the other. The power and telephone lines may be considered as opposite plates of one condenser and the telephone line and ground as opposite plates of another condenser. These two condensers being in series, they will distribute the total e.m.f. in inverse ratio to their capacities. With usual construction, the capacity between telephone line and ground will not be less than that between telephone and power wires, so that the potential of the telephone wires above ground will be equal to at least one-half the potential of the power line neutral point above ground. A grounded power line may thus cause a potential between the telephone wires and ground which will reach well into thousands of volts and even a bad insulator may cause such an e.m.f. measured by hundreds of volts. In this connection it is significant to note that in the great majority of cases the telephones become inoperative when a ground occurs on the power lines. Is it any wonder? How many telephone lines are built to stand up under a strain of even 1,000 volts, let alone 5,000 or 10,000 volts to ground? It is hardly necessary to point out the path of the disturbing currents. The first voltage strain comes not between telephone wires, but between the two wires and ground. A break down of its insulation, either partial or complete, occurs at some