Breakdown caused by a geomagnetically induced current in the Finnish telesystem in 1958

Abstract

At the Earth's surface, a “space weather” event is seen as a geomagnetic storm, accompanied by a geoelectric field, and “geomagnetically induced currents” (GIC) in technological systems, like electric power transmission grids, pipelines, telecommunication networks and railway equipment. In general, GIC are a potential source of problems to the system. Although Finland is located at high latitudes with intense geomagnetic storms the country has not suffered from major GIC harm. This paper presents the only event with noticeable GIC consequences in Finland, namely a sudden interruption of two coaxial phone cable systems in the southern part of the country during the great magnetic storm of February 11, 1958. Blown fuses associated with the ac power feed at the repeaters were the reason for the problems. As inferred from analogue magnetograms of the Nurmijärvi Geophysical Observatory in southern Finland, the largest time derivatives of the magnetic field were about 6.5 nT/ s . A rough estimation indicates a strength of the GIC of about 0.5 A , which would not be a high value in power lines, but obviously critical in a telecommunication system.