AbstractGIS for operation voltages ≧ 245 kV can be tested with oscillating impulse voltages on site advantageously, because this H. V. test can be performed with easily mountable impulse generators. The IEC recommendation for on‐site testing of GIS which is now under discussion doesn't fix the time parameters of the oscillating impulse voltages, but these influence the breakdown behaviour of slightly non‐uniform fields with disturbances (sharpe edges, fixed or free‐moving particles) in SF2.For the mentioned GIS (≧ 245 kV) fixed disturbances are more dangerous than free‐moving particles. Therefore the breakdown voltages of model insulations with a disturbance at the H.V. electrode (needle with length l=0…5 mm) in SF6 (p20=0.1…0.4 MPa) were investigated at oscillating lightning (OLI) and oscillating switching (OSI) impulse voltages with different times to crest.From the results, which were compared with those at aperiodic lightning (LI) or switching (SI) impulse voltages, permissible time parameters for OLI and OSI can be derived. On the basis of these parameters an impulse generator was designed which enables on‐site testing for a wide range of the capacity of the tested GIS.