Line Unbalances

As was also shown in the preceding discussion, if either the voltages induced in series or those to ground are not equal, then noise currents will flow through the connected telephone sets. In the following paragraphs it will be assumed that the telephone wires are raised to equal voltages above ground, and the way these equal voltages cause noise will be considered. Later in the section the effect of equal series voltages will be discussed.

Suppose that the two telephone wires of Fig. 6 are raised to equal potentials E_g1 and E_g2 above ground, that the series impedances Z₁ and Z₂ of each section of line wire are exactly the same, and that the impedances to ground also are identical. Then the telephone line is balanced, and the equal voltages to ground will produce no noise currents in the telephone receivers. In commercial systems it is impractical, if not impossible, to keep the telephone lines exactly balanced, and thus equal voltages induced to ground will produce noise currents in the connected telephone sets.

Figure 6. An approximate simplified circuit for a balanced telephone line on which equal voltages Eg1 and Eg2 exist to ground. The series impedance Z of each elemental section is the same. Also, the impedance to ground of each elemental section is the same. In these simplified circuits the shunt elements (Fig. 3, page 196) and leakage paths to ground are not shown.

Figure 7 shows a line with a series unbalance, such as would be caused by a poor joint in the telephone line wire. The capacitance to ground of each elemental section is represented approximately by the lumped capacitances C_g1 and C_g2. The equal voltages to ground E_g1 and E_g2 acting on the telephone wires send currents to ground through these capacitances. But the current flowing in wire 2 must flow through the unbalance R, and the two points X and Y will accordingly be at different potentials. As a result, noise currents will flow through the telephone sets.

Figure 7. Illustrating the way in which a series unbalance, such as a poor splice, causes noise currents to flow when the telephone wires are raised to equal voltages Eg1 and Eg2 above ground by a power parallel. The telephone wires are assumed to be balanced to ground. Points X and Y will not be at the same potential, and a noise current will flow through the connected telephone sets.

Unbalances to ground originate in many ways. The most common causes are tree "leaks," defective or dirty insulators, and poorly designed or maintained terminal equipment. An unbalance to ground causes noise as shown in Fig. 8; the "leak" R_g is assumed to be of high resistance. With this arrangement point Y is at a lower potential than point X, and noise currents will flow through the connected telephone sets even when the voltages induced to ground, E_g1 and E_g2, are equal.

Figure 8. Illustrating the way in which an unbalance to ground, such as would result from a poor insulator or a tree "leak," causes noise currents to flow. The two voltages Eg1 and Eg2 to which the telephone line is raised above ground are equal. The resistance of the ground is assumed negligible. The potential of point Y approaches as a limit that of ground, and noise currents accordingly flow through the telephone set. The series impedances of the two telephone wires are assumed equal.

In addition to causing noise, a high induced voltage to ground may offer a serious electrical hazard to employees and to equipment. When a telephone line is very close to a power line, the telephone line may assume potentials of hundreds or even thousands of volts above ground. Voltages to ground on a telephone line necessitate a high quality of maintenance of both lines and equipment if excessive noise is to be prevented. Thus, even if the noise is not actually produced, the presence of the induced voltage necessitates higher maintenance expenditures. In many instances excessive noise on telephone circuits has been reduced when the balance of the lines, both series and to ground; was improved by removing faulty line joints and by replacing broken insulators and removing tree and brush "leaks."

Leaving now the effect of voltages to ground, the effect of equal series voltages will be briefly considered. In Fig. 3 it was shown that voltages induced in series would act to ground. Thus, even if the induced voltages are equal, unbalances either in series or to ground may cause the currents which flow to be unequal. Then, the ends of the two line wires to which the telephone sets are connected may be at different potentials, and noise currents may flow.