Certification tests in accordance with the requirements of the recent IEC 61259 standard for switching of bus-charging current by disconnecters of a (three-phase enclosed) 145 kV gas insulated switchgear (GIS) are described. These tests are primarily aimed at verifying that the discharge between the disconnector contacts does not introduce internal arc faults. Special emphasis is given to the high-frequency measuring system, used to verify a sufficiently severe waveshape of switching transients in the MHz range. Three-phase tests were performed, showing phenomena that were not demonstrated in practice before: first, it was shown that phase-to-phase voltage transients with a higher amplitude than the phase-to-ground values do actually occur. Second, the phase-to-phase capacitive coupling of the busbar system could raise the trapped charge, left after at the load side of the installation after disconnection, to values above 1 pu. In addition, measurements of induced voltages in the secondary system are reported.