In gas-insulated substations (GIS), very fast transient overvoltages (VFT) with oscillationfrequencies in the MHz range may occur during disconnector switching because of the fastvoltage collapse within a few nano-seconds and the length and coaxial design of the GIS. Inthe vicinity of the operated disconnector frequencies of up to more than 100 MHz may arise.At more distant locations inside the GIS frequencies of still several MHz can be expected.The frequencies and the amplitudes of the VFT depend on the length and the design of theGIS and on the ratio of the source and the load side. Due to the travelling wave nature of thephenomenon, the voltages and frequencies differ from location to location within the GIS.High amplitudes occur if long sections are switched, if side buses exist at the feeding section,if the natural frequencies of the feeding and the switched side are similar and if the voltagedifference across the disconnector is high. Such a high voltage difference occurs for openingof the disconnector at the end of the switching operation (e.g. at one of the last restrikes) dueto the increasing dielectric strength of the gap and for closing at the first prestrike due to atrapped charge from the last opening operation. The highest amplitudes of the VFT normallyoccur at open GIS sections.