Telephone Interference from A-C. Generators Feeding Directly on Line with Neutral Grounded

The problem of telephone interference from a-c. generators feeding directly on the line with neutral grounded is discussed. It is shown that the triple and non-triple harmonics in the voltage wave shape cause currents which flow in different paths on the power system. As a result of this, the induction on paralleling telephone lines per ampere of triple harmonic is greater than the induction per ampere for a non-triple harmonic on a balanced power system. A method of measurement which would allow a rough comparison of the effects to be expected from both types of harmonics is suggested. This is based upon using the apparatus already available and setting up special connections for test. Data are given showing the results of such measurements made in the factory on a number of machines while they were being tested. Analyses are given of the wave shape of machines which gave rise to cases of interference. References are given to some of the data already published on methods which have been applied to relieve such situations.