Noise induced from power lines in voice-frequency communications

Noise voltages in communication lines usually originate from primary distribution power lines parallel to the copper communication cables. The communication industry always faces a tough challenge to resolve the noise problems associated with the local loop. The power distribution lines are generally three phase grounded wye with primary line to ground voltages of 7200 to 14400 volts. Shunt capacitor banks, for power factor correction are located in these lines at the tail end. The article analyzes the power line harmonic frequency interference within the voice-frequency spectrum of 300-3400 Hz. A discussion of noise harmonics begins with the three ingredients `necessary´ for noise to take place; (i) power influence causing hum on the telephone lines; (ii) coupling (electromagnetic) between power and telephone lines; and (iii) susceptibility (unbalance) of copper telephone cable pair or terminating equipment. The problems and possible solutions discussed in this article include the following: (i) the 60 Hz-fundamental frequency problem; (ii) the 540 Hz-9^th harmonic frequency problem; (iii) the 420/660 Hz-adjacent odd harmonics of 9^th harmonic problem; (iv) and harmonic anomalies related to specific loads. The interference of power line harmonics into communication circuits is a very complex phenomenon. It is important to have an overall perception of the interference before any remedial action is attempted. A guideline of possible inductive coordination solutions applicable to both power and telephone lines are proposed to mitigate the harmonic-frequency interference