Co-ordination of Power and communication Circuits for Low-Frequency Induction

We recapitulate here principal developments of the past decade in the problem of low-frequency induction, with remarks on the resulting present status and future plans for study of different phases of the problem. GENERAL 1. Studies of effects in actual low-frequency inductive exposures have been useful in developing general procedural methods, but have indicated the need of further work on certain items, with reference particularly to facilitating the appraisal of proposed situations in advance of construction. INFLUENCE 2. In the calculation of residual currents, the more important gaps have been filled by the experimental determinations of the impedance of ground-faults on operating systems, and of the impedance and admittances of lines (especially cables). 3. In residual-current limitation, probably the most interesting development is the installation, within the last three years, of about 30 Petersen coils. The Petersen coil generally limits residual currents to small values. Little or on change in the status of other methods of fault-current limitation has taken place. As heretofore, the application of fault-current limitation is primarily a matter of its reaction on power-systems operation, although developments have somewhat simplified this question. 4. Frequency of occurrence of ground faults has been reduced by improvements in the insulation of power lines, including improved ground-wire application and features of structural design. In the reduction of duration of ground faults, the protector tube is probably the most important development from the standpoint of low-frequency induction.