One of the most important problems in telephone interference at the present time is that of the dangerous voltages induced in communication circuits when an earth fault occurs on an adjacent high-tension transmission line with earthed neutral. Theoretical formulae have been developed which enable the coupling between the two circuits to be calculated, and measurements in the field have shown that, provided the resistivity of the earth is known, these formulae will enable an accurate forecast to be made of the induced voltage in the communication line, after steady fault con ditions have been established.The present paper examines the derivation of a mathematical expression for the coupling under the initial transient conditions. In order to see whether the variation between the initial and final values of the coupling is important, a number of practical cases have been worked out numerically for the first time. This has enabled curves to be drawn showing the induced voltage during the first few microseconds of a fault. The result of the investigation is to establish the sufficiency of the simpler steady-state methods of dealing with the problem for practical cases.